escobar wolf msc thesis

8/8/2019 Escobar Wolf MSc Thesis

1/66

1

Evolution of Volcn de Santa Mara, Guatemala:A 40Ar/ 39Ar, paleomagnetic and geochemical synthesis

By

Rdiger Packal Escobar Wolf

A THESIS

Submitted in partial fulfillment of the requirements

For the degree of

MASTER OF SCIENCE IN GEOLOGY

MICHIGAN TECHNOLOGICAL UNIVERSITY

2007


2/66

2

This thesis, Evolution of Volcn de Santa Mara, Guatemala: A 40Ar/ 39Ar,

paleomagnetic and geochemical synthesis, is hereby approved in partial fulfillment of

the requirements for the degree of Master of Science in Geology.

DEPARTMENT Geological Engineering and

Sciences.

Signatures:

Thesis advisor: ___________________________________________

Co-Advisor: ___________________________________________

Department Chair: ___________________________________________

Date: ___________________________________________


3/66

3

ABSTRACT

The Volcn de Santa Mara stratocone grew episodically through eruptions of basaltic to

andesitic lava. Based on 40Ar/ 39Ar dating of a dozen lava flows exposed in the 1902

eruption crater, we show that 60% (5 km 3) of the present day cone was emplaced over an

interval of 43 ka. Emplacement occurred mainly over two short periods of high eruptive

activity, the first between 76.7 and 60.2 ka, and the second between 37.2 and 33.6 ka.

During the two highly active periods 1.5 km 3 and 3 km 3 of mainly basaltic andesitic lava

flows and volcaniclastic material were emplaced; the eruptive eruptive activity wasgreatly reduced in between both peaks. After 36 ka there is no evidence of further

volcanic activity or cone-building until the great 1902 dacitic eruption. An average

eruptive rate of 0.1 km 3 /ky is calculated for the cone building phase, and a value of 0.2

km 3 /ky is obtained if the time period is extended to the present including the products of

the 1902 eruption and the Santiaguito dome. Peak eruptive rates during the high activity

periods may be an order of magnitude larger. Paleomagnetic directions obtained from

these lavas had previously been inferred to record the Mono Lake excursion. The data

reported here from 23 flow sites indicates that each of the three main periods of cone-

building corresponds broadly with groups of paleomagnetic directions and groupings of

40Ar/ 39Ar ages. No conclusive evidence for the Mono Lake excursion was found in this

new data. All Virtual Geomagnetic Pole (VGP) locations from these lava flows fall

within 30 of the earth spin axis, and do not show the characteristic clockwise and

counterclockwise loops associated to the Mono Lake event. However, abnormally large

paleosecular variation swings corresponding close in time to the Mono Lake event may


4/66

4

be linked to it. During the 43 ka period of growth, cone-building lavas evolved from

basaltic to andesitic (51 57% SiO 2) with time, but small regressions towards less

evolved compositions punctuate the two periods of high eruptive activity. The

geochemical evolution through time probably reflects crystallization and differentiation

of initially basaltic magma toward a residual andesitic magma, which may have further

fractionated to produce the 1902 dacite. Although the question of the origin of the dacite

remains open, changes in elemental ratios in the upper part of the cone over a very short

time period suggest that the last magmas that erupted effusively 33 ka ago may have had

more interaction with the crust, this could point towards the formation of a shallowmagma chamber where the dacite fractionated. Further exploration of the long term

activity patterns and the geochemical evolution could be important in assessing the

hazards at other nearby basaltic-andesitic stratovolcanoes.

Keywords: Santa Maria Volcano, Guatemala, Mono Lake paleomagnetic excursion,

stratovolcano growth, eruptive rates.

INTRODUCTION

The Santa Mara Santiaguito volcanic complex is located near the western end

of the Central American Volcanic Arc in the Guatemalan highlands (Fig. 1). The

complex includes the almost perfectly symmetrical Santa Mara stratocone and the E-W

elongated Santiaguito dome complex, covering of ca. 30 km 2 and ranging in elevation

from 900 to 3772 masl (Fig. 2). Santa Mara had been inactive with no historic eruptions

recorded until the October 1902 eruption. This eruption was the second largest of the 20 th


5/66

5

century and occurred from a vent on the southwest flank of the stratocone, killing

thousands of people and causing extensive damage to the Guatemalan economy (Sapper,

1904; Rose, 1972; 1987; Williams and Self, 1983).

Figure 1. Location of the Santa Mara Santiaguito volcanic complex within the CentralAmerican Volcanic Arc (highlighted as a lighter color band). Small triangles representmain volcanic centers (location after Carr et al., 2003). Two plate boundaries are shownwithin the map area: the left lateral strike slip Polochic - Motagua Fault Zone (NorthAmerican Caribbean plate boundary) labeled (MPFZ) and the Central Americanoceanic trench (Cocos Caribbean plate boundary), shown by 1000 m depth colorgradation. Convergence rates between Cocos and Caribbean plate calculated using platevelocities from NUVEL 1A (DeMets et al., 1994). Capital cities are shown by stars as

following: Guatemala City GC, San Salvador SS, Tegucigalpa T, Managua Mand San Jose SJ. Digital elevation model and ocean bathymetry data are from theSRTM30 PLUS dataset.


6/66

6

In 1922 the Santiaguito volcanic complex began to grow inside the crater formed

during the 1902 eruption, causing a second disaster in 1929, when a large dome collapse

killed hundreds or perhaps thousands of people (Sapper and Termer, 1930). The dome

complex has remained active continuously until the present, having erupted a cumulative

volume of material of more than 1 km 3 (Harris et al., 2003). The continuous activity at

Santiaguito and its derivative effects (e. g. river stream aggradation by volcaniclastic

material) pose a significant thread to the nearby communities.

The 1902 eruption formed a 0.5 km 3 crater in the volcano flank, centered ca. 2.5

km to the SW from the cones summit (Fig. 2). Sixty percent of the upper cones volume(~ 5 km 3) is exposed in the crater walls which consist of interbedded lava flows and

volcaniclastic rubble layers (Fig. 3). Rose et al. (1977) analyzed 26 lava flows from the

sequence exposed in the 1902 crater walls and discovered a possible excursion of the

geomagnetic field and a trend of increasingly evolved lavas with increasing stratigraphic

position. By correlating the apparent excursion with similar events in the Gulf of Mexico

and Lake Tlapacoya, Mexico, an age of 30 ka was assigned to the volcano. Conway et al.

(1994) correlated the apparent magnetic excursion with the Mono Lake event (Liddicoat

and Coe, 1979), for which they considered a duration of 1 to 3 ka, ending at ca. 25 ka.

The Mono Lake event is an excursion of the paleomagnetic field originally discovered in

the Wilson Creek sediments, near Mono Lake, California, USA (Denham and Cox, 1971

and Liddicoat and Coe, 1979). Since then it has been found in sedimentary records in

other places in the Western US (Lund, 1988; Liddicoat, 1992; 1996; Liddicoat et

al.,1982; Levi and Karlin,1989; Negrini et al,1984; Rieck et al., 1992; Mankinen and

Wentworth, 2004), Israel (Marco, 2002), India (Kotila et al., 1997; Bhalla et al., 1998)


7/66

7

and in sediments from the Atlantic and Arctic oceans and the Greenland Sea (Nowaczyk,

1997; Nowaczyk and Knies, 2000; Nowaczyk and Antonow, 1997). The excursion has

the potential to be used as a paleomagnetic time marker (Benson et al., 2003), although

its timing and even existence has been recently challenged (Kent et al., 2002). Correlation

of the excursion with a local minimum in paleomagnetic intensity records from ocean

floor sediments, allowed Laj et al. (2004) to assign an age of 34 to 35 ka to the Mono

Lake event. However the excursion has only been recorded in sediments, and therefore

finding it recorded in lava flows would help to get dissipate the skepticism about it being

a real feature of the paleomagnetic field. Given the modern dating techniques on younglava flows, a better age constraint could also be possible if such a record is found (e. g.

Guillou et al., 2004; Hu et al., 1994; Lanphere et al., 2007 and Renne et al., 1997). The

possibility to find the first record of the Mono Lake paleomagnetic excursion in a

sequence of lava flows and the possibility to better constrain the compositional and

volume emplacement history of the Volcn de Santa Mara stratocone, motivated us to

resample the lava flows in the 1902 crater, for new paleomagnetic, geochemical and

geochronological analysis, and to determine new eruption rates.

Detailed studies on the evolution history at some volcanic systems, combining

different dating techniques with field mapping of volcanic units and estimations of their

volume, has shown the highly episodic nature of arc volcanism, its complex

compositional variability through time and the differences in eruption rates (Hildreth and

Lanphere, 1994; 2003a; 2003b; Singer et al., 1997; Jicha and Singer, 2006 and Hora et

al., 2007).


8/66

8

Figure 2. Geologic map of the Santa Mara Santiaguito volcanic complex. Sources: Geology modified from Rose Cartography from Japanese International Cooperation Agency et al., 2003.


9/66

9

Figure 3. Panoramic view of the NE wall of the 1902 Santa Mara eruption crater. Sampling site locations are labelebright yellow) according to their stratigraphic position. Ages for the upper and lower groups (in pale yellow) and fosamples (in pale yellow) bounding the intermediate transitional. Dashed lines separate the lower and upper lava flowintermediate transitional flows as defined by the age clustering (see text for discussion). Horizontal extension in vie


10/66

10

A better understanding of the long term evolution (over the entire time span of their

cone building phases) of stratovolcanoes that have a similar evolution history can contribute

to better hazard assessments, especially on volcanoes that have remained dormant for

extended periods of time and that could produce large volume explosive eruptions in the

future. 40Ar/ 39Ar dating, retrieval of paleomagnetic directions and XRF and ICP-MS

geochemical analysis of the lava flows exposed on the 1902 Santa Mara eruption crater

walls, coupled with volume calculations, allow us to reconstruct the volcanic history,

building upon previous efforts do describe and understand this complex, dangerous and very

interesting volcano (Sapper, 1904; Rose, 1972; 1987; Williams and Self, 1983, Sanchez-Bennett et al 1992).

GEOLOGY:

Santa Mara Santiaguito is one of 39 volcanic complexes that comprise the Central

American Volcanic Arc, resulting from the subduction of the Cocos plate underneath the

Caribbean plate (Carr et al., 2003) (Fig. 1). Breaking of the arc into different segments which

are offset and rotated with respect to each other has been proposed by Stoiber and Carr

(1973) to due to the segmentation of the subducting Cocos Plate. Quaternary volcanism in

the area is chemically bimodal, producing roughly the same order of magnitude volumes of

mafic and silicic products (Rose, 1981; 1987). Basaltic to andesitic magmas have been

erupted mainly from stratocones of the volcanic arc and dacitic to rhyolitic products have

been produced mainly in back arc calderas and dome complexes. The Santa Mara

Santiaguito complex is build upon crust that is 40-50 km thick (Carr et al., 2003; Ligorria and

Molina 1997 and Rebollar et al., 1999), belonging to the Chortis continental block (Donnelly


11/66

11

et al. 1990). On a regional scale, the Chortis block has a diverse pre-Mesozoic and Mesozoic

metamorphic basement, overlaid by Mesozoic limestones, sandstones, conglomerates and

shale. The upper section consists of Cenozoic silicic volcanic rocks, conglomerates and

sandstones, capped by the quaternary sequence of basaltic-andesitic volcanoes and dacitic-

rhyolitic calderas. Common occurrence of granitic and dioritic intrusive rocks, especially of

late Cretaceous and early Cenozoic has also been noted (Donnelly et al. 1990). However,

Case et al. (1990), considered the southern margin of the Chortis block to be composed of

Jurassic and younger acretionary crust. Based on aeromagnetic anomalies for Honduras,

Rogers (2003) considered this southern portion Chortis subterrane, to have a post-Paleozoicarc or ocean type basement. The location of the boundary between the older continental

central Chortis terrane and the presumably younger south Chortis terrane is not well know for

the Santa Mara region, and the volcano could be on either. Consistent with this

interpretation, Vogel et al. (2006) concluded from the analysis of geochemical and isotopic

signatures that silicic volcanic products from the Central American Volcanic Arc are

differentiates from basaltic magma and there is little or no involvement of old continental

crust in their generation; a result similar to the conclusion reached by Carr et al. (2003), and

interpreted in terms of a post-Paleozoic arc type basement under the currently active

volcanoes. However, Carr et al., (1990) reported evidence of moderate crustal contamination

for the volcanic centers in western Guatemala (e. g. Tajumulco and Tacan volcanoes). This

is seen as a systematic westward progression of higher 87Sr/ 86Sr and lower 143 Nd/ 144Nd ratios.

A change from relative young (post-Paleozoic arc type?) and relatively unradiogenic crust on

the east, to an older (pre-Paleozoic continental?) and more radiogenic crust to the west could

explain this systematic trend. Although absent in surface outcrops in the region surrounding


12/66

12

the volcanic complex, the presence of metamorphic and plutonic lithic fragments in the 1902

eruption deposit, and the intersection of shallow intrusive rocks at a drilling borehole near to

Santa Mara, show that at least the upper crust under Santa Mara has a large proportion of

metamorphic and plutonic rocks (Rose, 1987).

Santa Mara like the Quaternary volcanism in general is also geochemically bimodal,

having erupted approximately equal volumes of basalt-andesitic (~8 km 3) and dacitic (10

km 3) magmas. The stratocone lavas are calc-alkalic, medium-K and highly sodic, and consist

of two-pyroxene olivine-bearing basaltic andesites. The 1902 eruption and subsequentextrusion of lavas that formed the Santiaguito dome complex produced 10 km 3 of hornblende

dacite (Rose, 1987). Rose et al., (1977) proposed an evolutionary model, which was further

developed by Rose (1987) following the ideas of Carr (1984), in which the magmas feeding

the system are stored and evolved at least in two levels, in deep level ponds that behave as

open systems located at the base of the crust, and in shallower and smaller volume magma

bodies that are short lived and behave as closed systems. Magma became more evolved as

the volcano grew in elevation, resulting in progressively longer periods of dormancy between

periods of activity. Finally, during a prolonged repose period part of the magma in the deep

pond evolved to dacite, which erupted in the catastrophic event of 1902 and continues

erupting more effusively as the Santiaguito dome grows. Smaller scale variations and

reversals (i. e. small variations towards more mafic composition) in magma evolution,

could be explained mainly by the eruption of progressively more mafic magmas from

compositionally zoned shallow magmatic bodies, during each eruptive cycle. The correlation

of the apparent paleomagnetic excursion recorded in the Santa Mara lavas with the Mono


13/66

13

Lake event lead Conway et al. (1994) to derive an eruption rate of 4 km 3 /ky during the cones

emplacement phase.

METHODS AND PROCEDURES:

Sampling

Twenty three distinct lava flows (sites) exposed in the walls of the 1902 eruption

crater of Volcn de Santa Mara were sampled during field trips in 2005 and 2006. Guided by

photos from the Rose et al. (1977) study at Volcn de Santa Mara, we attempted to sample

the same lava flows as their study but unfortunately aggradation in the crater due to masswasting made it impossible to sample all of those flows. At each site, 4 to 7 one inch

diameter cores up to 3 inches long were drilled for paleomagnetic analysis; an inadequate

supply of water prevented more samples being drilled per site. Hand samples of fresh

unaltered rock were also collected for radioisotopic dating and geochemical analysis. Drilled

cores were oriented in situ using a magnetic compass and checked by sun compass readings.

Samples were collected only from the massive, non altered interior of undisturbed lava flows.

Relative stratigraphic positions were easily recognizable throughout the sequence. Sampling

site coordinates were obtained by hand-held GPS. Vertical distances between sites were

measured using a 50-meter tape measuring normal to the dip of the lava flows. Although no

major breaks in the general stratigraphy are evident in the crater walls, some minor

discontinuities were noted during our field campaigns. These unconformities correspond with

the longer time intervals between lava flows as seen from the 40Ar/ 39Ar geochronology

discussed later.


14/66

14

Paleomagnetic Work

Laboratory procedures and results:

One to two, one inch long specimens were cut from the core samples drilled at each

of the 23 lava flows. The natural remnant magnetization (NRM) of each specimen was

measured using a 2G Enterprises 755R superconducting rock magnetometer equipped with

an on-line AF demagnetization unit. Specimens were either subjected to AF (14-18 steps up

to 140 mT) or thermal (5-13 heating levels up to 575 oC) demagnetization to isolate the

characteristic direction of magnetization (ChRM) for each specimen. Secondary overprints

were usually small or non-existent and were easily removed in the first few steps of demagnetization, resulting in a linear decay of the magnetization vector to the origin on

vector endpoint diagrams (Fig. 4). At several sites samples displayed great circle paths with

increasing levels of demagnetization. These samples came from sites located high on the

crater wall suggesting their magnetization was partially reset by nearby lightening strikes.

ChRMs of all specimens were determined using principal component analysis (Kirshvink

1980). Site mean directions were calculated using either the statistical method of Fisher

(1953) or the method of McFadden and McElhinny (1988) in instances when the

demagnetized data defined one or more remagnetization circles.

As seen in Figure 4, AF and thermal demagnetization of sister specimens from the

same core sample isolate the same direction of magnetization. Maximum angular deviation

(MAD) values using AF data are usually less than 1 while MADs determined from thermal

demagnetization are usually less than 3 o. The difference in ChRMs determined by the two

demagnetization techniques is always less than 10 o and commonly less than 6 o. At four sites

where there were a sufficient number of two specimen samples, the difference in the mean


15/66

15

direction from thermal and AF demagnetized specimens is less than 5 o. Therefore, we

surmise that the ChRMs isolated using AF demagnetization represent a primary

magnetization acquired during the initial cooling of these lava flows.

Figure 4. Vector endpoint diagrams of 3 Santa Mara lava samples. Solidsymbols correspond to AFdemagnetization and open symbolscorrespond to thermaldemagnetization. Circles areprojections onto the horizontal plane;squares are projections onto the N-Svertical plane. Demagnetization stepsare as following; sample SM0501-3:

0, 5, 10, 15, 20, 25, 30, 35, 40, 45, 50,60, 70, 80, 90, 100, 120 and 140 mTand 0, 100, 300, 400, 450, 500 and550 C respectively. Sample SM0503-5: 0, 5, 10, 15, 20, 25, 30, 35, 40, 50,60, 70, 80, 90 and 100 mT and 0, 100,200, 300, 400, 450, 500, 525, 550, 560and 575 C respectively. SampleSM0505-5: 0, 5, 10, 15, 20, 25, 30, 35,40, 50, 60, 70, 80, 90 and 100 mT and0, 100, 300, 400, 450, 500 and 550 Crespectively. Both the AF and thethermal demagnetization yieldvirtually the same resultant direction,which confirms the validity of the AFdemagnetization approach.

Site mean directions, virtual geomagnetic poles, and associated statistical parameters

calculated from the 23 lava flows are given in Table 1. Also listed in Table 1 are the original

data from Conway et al. (1994) with VGP locations added. At several sites, a number of

specimens were eliminated before calculating site-mean directions and VGPs. These

specimens either had ChRMs that departed significantly (i.e. angular differences greater than

25o) from the average of the specimens from that site or were from samples that were


16/66

16

determined from the 2006 field season to have been drilled in rotated blocks, this resulted in

the removal of 4 from a total of 122 samples in our dataset. A stratigraphic plot of the site-

mean directions is shown in Figure 5. The distribution of the VGPs from this work and those

calculated from the data published by Conway et al (1994) is shown in Figure 6.

TABLE 1. PALEOMAGNETIC DIRECTIONSSample Inclination Declination n 95

VGPs AgeLatitude Longitude

(deg.) (deg.) (deg.) North West (ka)SM0501 34.5 8.0 6 6.2 154 81.28 327.98 75.011.0SM0502 32.6 2.7 7 6.5 88 86.06 309.54 70.5 7.3SM0503 29.8 2.6 6 3.7 334 87.22 331.23SM0504 26.7 1.5 6 3.8 312 88.46 22.50 75.0 13.0

SM0505 39.9 359.0 5 3.0 491 82.39 268.4 71.3 9.2SM0506 -1.8 16.5 6 3.5 302 68.16 42.67 60.2 8.1SM0507 8.6 17.0 6 4.8 199 70.25 28.67 53.9 6.8SM0508 1.9 17.8 7 4.1 218 67.63 34.93SM0509 4.2 11.1 5 5.4 200 73.27 46.54 46.0 5.0SM0510 0.7 16.3 5 6.9 122 68.40 38.79SM0511 -2.6 16.7 5 1.6 2642 66.99 41.28SM0512 -2.5 17.8 5 3.4 508 66.19 39.15SM0513 1.7 13.3 5 7.3 111 70.89 43.80 37.8 4.9SM0514 -8.3 23.1 3 2.8 1981 60.33 36.27SM0515 -12.0 18.4 5 3.9 380 62.35 45.98 35.4 5.6

SM0516 -7.1 15.3 5 4.6 275 66.25 47.68SM0517 -9.4 9.0 6 3.1 468 68.54 63.09SM0518 57.8 8.5 5 2.1 1358 65.18 284.43SM0520 33.3 20.6 4 8.4 221 69.96 345.74 33 3.7SM0521 23.4 17.7 5 4.2 389 72.61 4.67SM0522 35.5 16.8 6 3.8 412 73.19 339.11 36.5 4.6SM0523 28.1 17.9 5 9.6 84 72.70 355.44 36.8 4.4SM1 # 60.9 11.8 6 5.9 147 60.96 286.70SM2 # 60.9 20.6 6 4.2 289 57.51 297.59SM3 # 60.2 15.7 7 4.4 194 60.31 292.73SM4 # 57.4 8.9 5 4.9 240 65.43 285.48

SM5#

58.0 6.6 5 3.0 820 65.38 280.87SM6 # 28.7 13.8 5 3.4 781 76.66 354.23SM7 # 35.7 14.1 7 3.0 419 75.64 336.00SM8 # 6.1 344.0 7 3.4 314 70.37 143.43Notes: n is the number of specimens analyzed for each sample. 95 is the 95 % confidence interval (angular) around the mean value. is the Fisher precision parameter.# Data taken from Conway et al., 1994.


17/66

17

Figure 5. Paleomagnetic inclination anddeclination vs. stratigraphic elevation.The two main volcanic pulses are shownas grey bands with ages labeled. See textfor discussion about ages of these

groups.

Figure 6. VGP locationscorresponding to thepaleomagnetic directions.A) Solid circles show theVGP positions of the datapresented in this paper.Gray squares show theVGP positions of the dataoriginally published byConway et al (1994).Simple numbers give thestratigraphic order of oursamples (e. g. 1, 2, 3, etc.)and numbers followed bya capital C show thestratigraphic order of theConway et al., 1994dataset. Ages or rank of ages are shown for eachgroup. B) Mono Lakeexcursion VGPs path.Gray thick line shows thetwo loops followed bythe Mono Lake VGPs(shown as small dots).

Drawn after the data of Liddicoat and Coe (1979).Solid circles and graysquares as in A).

A

B


18/66

18

40Ar/ 39Ar Dating

Analytical procedures

Twelve of the 23 sampled lava flows shown in Figure 3 were 40Ar/ 39Ar-dated using

furnace incremental heating methods on groundmass separates at the Rare Gas

Geochronology Laboratory at the University of Wisconsin-Madison. Sample preparation and

analytical procedures are detailed in Singer et al. (2004). Three to four aliquotsca. 120 mg

eachof groundmass from each sample were degassed in 10 to 15 increments, from 620 C

to 1250 C. A total of 40 incremental heating experiments were done with 3 to 4 replicate

plateau and isochron ages calculated for all of the samples (Figure 7). Isochron regressionsindicate an atmospheric trapped component in all samples, so the inverse-variance weighted

mean plateau ages give the best estimate for time since eruption of these 12 lava flows. All

ages are calculated relative to 1.194 Ma Alder Creek Sanidine (Renne et al., 1998) and

uncertainties are reported at the 2 sigma level of analytical precision. Instrumental mass

discrimination was measured using an air pipette every 2 -3 days and varied between 1.0064

and 1.0076 per atomic mass unit, during the analytical. Furnace blanks were measured in

between samples at 200 C increments, covering the range of temperatures of the incremental

heating, and then were interpolated to the values of the actual temperature steps at which the

samples were measured; their values were at least one order of magnitude smaller than

sample signals, this blanks contribute little to overall age uncertainty.


19/66

19

Figure 7. Age spectra and isochron diagrams for 3 Santa Mara samples. Samples SM0502and SM0520 belong to the oldest and youngest groups respectively. Sample SM0509 has atransitional age in between these groups. Inset in the isochron diagrams shows a close up of the dispersion points around the best fit line and the associated error ellipses.


20/66

20

TABLE 2. SUMMARY OF 40 Ar/ 39Ar INCREMENTAL-HEATING EXPERIMENTS ON GROUNDM

Sample # Weight K/Ca Total fusion Increments Age Spectrum Isochron AnaExperiment Total Age 2 used 39Ar Age 2 MSWD N MSWD 40A

(mg) (ka) (C) % (ka) SM0520

UW55C2B 122 0.256 34.4 12.9 721 - 1250 88.6 31.7 8.1 0.04 8 of10 0.04 295.7 3.7 3UW55C1B 122 0.242 40.7 12.7 718 - 1250 76.7 35.8 11.8 0.05 6 of 10 0.07 295.6 10.0 3UW55C1T 128 0.255 39.9 10.2 650 - 1200 82.4 36.1 7.0 0.20 7 of 12 0.22 294.6 5.7 3UW55C2T 123 0.255 36.5 8.6 680 - 1200 74.2 31.3 5.6 0.33 5 of 10 0.42 296.5 6.8 2

Weighted mean plateau age: 33.1 3.7 0.49 26 of 42

SM0523UW55C5B 120 0.235 43.9 12.2 755 - 1265 81.3 33.0 6.8 0.12 7 of 10 0.08 296.7 4.3 2UW55C6T 122 0.235 54.8 14.6 771 - 1300 80.0 42.6 11.3 0.01 7 of 11 0.01 296.2 13.0 4UW55C5T 121 0.241 39.6 9.8 670 - 1230 92.1 38.4 6.6 0.04 8 of 11 0.04 295.9 4.3 3


SM0522UW55C3B 121 0.249 44.5 10.4 763 - 1230 90.6 37.7 6.8 0.04 8 of 10 0.03 296.1 4.0 3UW55C4T 121 0.245 35.9 13.0 778 - 1230 96.1 35.3 10.7 0.04 9 of 10 0.05 295.3 2.2 3UW55C3T 120 0.242 37.0 10.8 670 1230 99.6 35.6 7.7 0.07 10 of 11 0.06 295.8 1.8 3


SM0515UW55B6B 122 0.226 48.1 12.8 768 - 1250 86.8 37.5 9.3 0.09 7 of 10 0.11 295.9 4.4 3UW55B5B 121 0.213 46.3 12.8 768 - 825 85.4 32.4 9.3 0.04 7 of 10 0.05 295.5 5.1 UW55B5T 120 0.206 38.8 12.9 650 - 1310 92.1 36.7 10.4 0.03 8 of 12 0.03 295.8 2.9 3


SM0513UW55B3B 127 0.221 42.1 13.0 742 - 1210 91.6 40.0 11.2 0.08 8 of 10 0.08 296.4 6.4 3UW55B4T 126 0.213 43.0 12.6 766 - 1220 91.0 38.5 9.2 0.12 8 of 11 0.14 295.5 5.7 3UW55B3T 122 0.220 42.1 10.2 650 - 1220 97.1 36.5 7.0 0.12 10 of 12 0.13 295.7 3.4 3



21/66

21

TABLE 2. (Continued)Sample # Weight K/Ca Total fusion Increments Age Spectrum Isochron Ana

Experiment Total Age 2 used 39Ar Age 2 MSWD N MSWD 40A

(mg) (ka) (C) % (ka)

SM0509UW55D4B 124 0.266 56.3 13.6 734 - 1235 79.7 46.5 10.7 0.06 7 of 10 0.07 295.2 5.4 4UW55D3B 121 0.264 51.2 13.9 729 - 1235 93.9 46.1 10.9 0.01 8 of 10 0.01 295.7 4.5 4UW55D4T 124 0.259 58.9 13.1 680 - 1250 72.4 44.3 9.9 0.02 6 of 11 0.01 294.8 9.8 4UW55D3T 120 0.270 50.4 14.6 670 - 1220 99.6 46.4 8.9 0.08 9 of 10 0.08 296.2 3.6 4


SM0507UW53E3 120 0.193 75.1 14.2 650 - 1160 82.7 54.2 11.4 0.26 7 of 10 0.28 296.2 3.8 4UW57E2 120 0.192 84.1 12.5 650 - 1180 77.0 53.6 12.6 0.10 7 of 10 0.12 295.6 3.0 5UW57E2B 121 0.195 77.3 14.3 650 - 1160 77.7 53.7 12.4 0.09 7 of 10 0.1 295.4 4.5 5

Weighted mean plateau age: 53.9 6.8 0.00 21 of 30SM0506

UW55B1B 122 0.211 64.6 20.7 711 - 1210 66.9 60.4 13.6 0.09 5 of 10 0.12 295.6 7.4 6UW55B2T 121 0.207 67.9 34.5 704 - 1200 92.9 60.1 14.1 0.31 8 of 10 0.36 295.9 5.5 5UW55B1T 120 0.213 66.8 22.0 650 - 1200 94.6 60.0 14.7 0.27 10 of 12 0.29 295.8 2.1 5


Sm0505UW55D2B 120 0.186 75.4 25.0 722 - 1250 79.2 71.3 15.5 0.17 7 of 10 0.20 296.0 4.9 6UW55D1B 119 0.186 81.0 31.1 717 - 1220 83.9 72.3 21.5 0.01 7 of 10 0.01 295.3 6.5 7UW55D2T 121 0.191 70.8 39.9 680 - 1180 69.4 68.5 17.7 0.14 8 of 10 0.17 295.9 7.7 6UW55D1T 121 0.188 105 28.1 670 - 1220 92.1 74.0 20.7 0.07 8 of 12 0.03 296.2 2.4 6

Weighted mean plateau age: 71.3 9.2 0.06 30 of 42 0.02 296.0 2.0

SM0504UW55A5B 127 0.178 87.4 29.6 683 - 1220 82.5 74.0 21.4 0.08 7 of 10 0.1 295.5 4.2 UW55A6T 123 0.174 98.3 27.2 679 - 1220 77.9 78.2 19.5 0.09 6 of 10 0.11 295.3 4.6 8UW55A5T 122 0.182 72.3 26.8 670 - 1220 86.0 72.8 26.2 0.05 7 of 12 0.06 295.5 3.6 7



22/66

22

TABLE 2. (Continued)Sample # Weight K/Ca Total fusion Increments Age Spectrum Isochron Ana

Experiment Total Age 2 used39

Ar Age 2 MSWD N MSWD40

A(mg) (ka) (C) % (ka)

Weighted mean plateau age: 71.3 9.2 0.06 30 of 42 0.02 296.0 2.0

SM0502UW55A3B 139 0.176 65.2 18.6 717 - 1210 98.8 64.5 15.6 0.10 9 of 10 0.11 296.0 3.5 6UW55A4B 122 0.171 77.7 24.4 723 - 1223 90.3 71.6 17.6 0.03 8 of 10 0.01 296.4 5.2 6UW55A3T 118 0.173 76.4 24.3 640 - 1220 99.2 71.1 13.3 0.18 11 of 12 0.19 296.1 3.4 6UW55A4T 122 0.176 77.0 17.3 680 - 1200 92.0 73.5 13.2 0.02 8 of 10 0.03 295.7 3.9 7


SM0501

UW55A1B 122 0.173 65.5 25.1 632 - 1180 70.7 64.7 22.7 0.01 6 of 10 0.01 295.4 8.0 6UW55A1T 124 0.178 82.1 24.8 680 - 1250 92.2 81.5 19.7 0.04 6 of 11 0.03 294.8 4.8 8UW55A2T 123 0.198 74.6 26.8 620 - 1110 94.5 72.0 16.0 0.19 7 of 10 0.23 295.3 5.4 7

Weighted mean plateau age: 75.0 11.0 0.83 19 of 31Notes: All ages calculated relative to 1.194 0.012 Ma Alder Creek rhyolite sanidine (Renne et al., 1998).


23/66

23

Results

Plateau ages from individual lava flows range from 75.0 13.0 to 33.1 3.7 ka and

given the analytical precision are all in agreement with stratigraphic superposition (Table 2).

Most of the determinated ages fall within two groups, the oldest comprising 4 age

determinations between 75.0 13.0 ka and 70.5 7.3 ka , and the youngest defined by 6

ages between 40.4 ka 5.8 ka and 33.0 3.7 ka. These clustered age determinations

correspond with the previously mentioned breaks in stratigraphy Given the overlapping of

the 2 uncertainty among the flows within these intervals, we calculated the inverse-variance

weighted mean plateau ages for all the flows within each group, and take these as the bestestimates of the ages for these eruptive pulses, at 72.1 4.6 ka and 35.6 2.0 ka,

respectively (Fig. 3). For the purpose of bounding the time intervals during which the

different phases of activity happened, we use the extreme values given by the uncertainty

intervals for the grouped ages (e. g. 76.7 and 67.5 ka for the lavas grouped in the 72.1 4.6

ka).Three flows in the middle of the section have transitional ages between the two groups

discussed above, at 60.2 8.1 ka, 53.9 6.8 ka, and 46.0 5.0 ka, respectively. The presence

of a few lava flows with ages distributed over a long period of time (ca. 30 ka), and the

estimation of a low erupted volume for this period (see discussion later) indicate that

although the volcanic activity decreased and remained at low levels during an extended

period of time eruptive activity did not stop completely.

Major and Trace Element Geochemistry

Analytical procedures


24/66

24

Whole rock samples from the 23 flows, plus 9 flows from Conway et al. (1994) were

analyzed for major and trace elements by XRF and ICP-MS at Michigan State University.

Sample preparation and analysis followed procedures in Vogel et al. (2006).

TABLE 3. REPRESENTATIVE WHOLE ROCK CHEMICAL COMPOSITIONSample: SM0501 SM0509 SM0513 SM0518 SM0520 SM0523

Major elements (wt %):

SiO 2 50.8 52.91 51.82 52.22 56.87 53.89TiO 2 0.98 0.89 0.91 0.89 0.75 0.88Al2O2 18.12 18.63 18.38 18.31 18.36 18.37Fe 2O3 9.99 9.13 9.73 9.54 7.94 9.16MnO 0.15 0.14 0.14 0.16 0.14 0.15MgO 6.28 5.19 6.08 5.75 3.55 4.72CaO 9.2 8.09 8.29 8.55 6.68 7.7Na 2O 3.3 3.71 3.49 3.42 4.01 3.65

K2O 0.82 0.95 0.83 0.85 1.28 1.08P 2O5 0.21 0.22 0.19 0.20 0.24 0.23Totals 99.85 99.86 99.86 99.88 99.83 99.85LOI 2.06 0.61 2.26 0.91 1.16 2.53

Trace elements (ppm):

Ni 42.88 43.26 56.27 43.39 23.27 35.91Cu 78.62 75.46 53.2 68.63 40.47 71.82Zn 79.64 79.48 81.85 80.73 81.95 79Rb 11.23 13.08 10.23 12.11 22.26 17.44Zr 80.66 101.62 90.03 98.9 159.85 114.91Sr 591 534 538 485 506 521

Ba 407 455 401 414 592 523La 8.98 8.48 7.24 7.62 11.93 10.07Y 17.8 17.8 16.5 19.9 20.1 20.8V 244 197 210 205 139 195Cr 60 39 49 60 7 19Ce 19.9 19.1 16.7 17.4 25.7 21.8Pr 2.9 2.81 2.49 2.61 3.61 3.17Nd 13.1 3.11 11.7 12.3 15.8 14.6Sm 3.06 3.27 2.95 3.08 3.52 3.53Eu 0.96 1.01 0.95 0.98 1.05 1.05Gd 3.14 3.2 2.97 3.18 3.43 3.47Tb 0.49 0.5 0.47 0.51 0.53 0.55Dy 2.86 2.88 2.66 3.16 3.09 3.25Ho 0.58 0.58 0.54 0.65 0.64 0.68Er 1.72 1.69 1.52 1.94 1.89 2.02Yb 1.57 1.56 1.37 1.87 1.83 1.79Lu 0.24 0.23 0.21 0.28 0.29 0.28Nb 3.48 2.62 2.32 2.41 3.77 2.95Hf 2.17 2.57 2.3 2.51 3.64 2.8Ta 0.19 0.15 0.13 0.14 0.21 0.17Pb 3.28 3.88 2.22 3.24 5.28 4.62Th 1 0.9 0.79 0.76 1.41 1.48U 0.67 0.36 0.30 0.32 0.51 0.49


25/66

25

Results

The lavas (Table 3; Figure 8) range from basalt (51% SiO 2) to basaltic andesite (57%

SiO 2). Geochemistry results broadly agree with earlier analysis published by Rose et. al.

(1977) and Rose (1987), and show a clustering of the data in distinctive groups and trends

when plotted on variation diagrams (Fig. 8), with most of the chemical variation happening

in the upper part of the section.

Figure 8. Chemicalcomposition of SantaMara lavas. Errorbars showncorrespond to 2 . A)Variation diagrams of major and traceelements vs. silicacontent. Note thedistinctive differentcomposition of thefour lowermostsamples (enclosed ina line).


26/66

26

Figure 8 continued

B) Variation vs. thestratigraphic position. Eruptivepeaks indicated by gray bands

and age labels.

C) Elemental ratios. Note theclear shift in compositionbetween the first group of lavas at 72.1 4.6 ka and thelava flows with transitionalage, as well as the much higherdispersion of values for the lastgroup of lavas at 35.6 2.0 ka.Ages for some transitionalflows are labeled in gray. Graybands correspond to the twogroups of flows erupted duringthe high activity peaks.


27/66

27

DISCUSSION AND INTEGRATION OF RESULTS

Timing of activity and eruptive rates.

Figure 9 shows the estimated relationship between cumulative volume (see Table 4

and the description in the section on volume estimations) and age. Four main phases are

recognizable since 76.7 ka: the first corresponds to a relative high eruptive rate between 76.7

and 60.2 ka. The second phase is a period with a very low eruptive rate, from about 60.2 ka

to 37.2 ka. The third phase is another peak in eruption rate between 37.2 and 33.6 ka. Finally,

the period between 33.6 ka and 1902 A. D. was a period of very low eruption rate similar tothe period between the two eruptive peaks, during which no significant volume was added to

the edifice. From these data we can calculate values for the volume emplacement rates on the

volcanic edifice: an average eruptive rate of 0.1 km 3 /ky is calculated for the cone building

phase (i. e. 76.7 33.6 ka), and a value of 0.2 km 3 /ky is obtained if the time period is

extended to the present and the products of the 1902 eruption and the Santiaguito dome are

included. Peak eruptive rates during the high activity periods may be an order of magnitude

larger, and a rough estimate for the last peak gives a value of 0.6 km 3 /ka.

The average eruptive rates of 0.1-0.2 km 3 /ky calculated for the last 75 ky for the

Santa Mara Santiaguito complex fall on the lower end of eruption rates reported for other

arc volcanoes (Jicha and Singer, 2006 and White et al 2006). Table 5 and Figure 10, show the

eruptive rates and the cumulative volume addition through time at Santa Mara and other

volcanic systems. Differences in the tectonic settings may explain this, e. g. the crustal

thickness, the magnitude and direction of the local stress field, magma composition and melt

generation rate in the source (White et al 2006).


28/66

28

Figure 9. Edifice volume emplacement overtime. Note that the horizontal axis has twoscales, to describe the long term prehistoric andthe short term historic activity. Black regioncorresponds to the estimated value range

considering the uncertainty in time andvolume. Although there are two peaks of activity during the cone building phase (pre-historic) the largest contributions to the eruptedvolume occurred due to the 1902 eruption andthe ongoing Santiaguito dome growth. Timeintervals shaded in grey correspond to the twoage groups (ages label within shaded areas).

TABLE 4. CUMULATIVE VOLUME ESTIMATES:Samplesite:

Cumulativevolume

Lowerestimate

UpperEstimate

Age 2

(km 3) (km 3) (km 3) (ka)

SM0520 8.2 7.0 8.9 33.0 3.7SM0523 7.3 6.1 7.9 36.8 4.6SM0522 6.4 5.7 7.1 36.5 4.4SM0521 6.4 5.5 7.1SM0518 7.5 6.4 8.3SM0517 6.5 5.6 7.4SM0516 6.4 5.4 7.1SM0515 6.2 5.3 7.0 35.4 5.6SM0514 5.5 5.3 7.8SM0513 5.7 4.8 6.4 37.8 4.9SM0512 5.7 4.8 6.3SM0511 5.7 4.7 6.3SM0510 4.7 4.5 5.9SM0509 5.1 4.3 5.8 46.0 5.0SM0508 5.3 4.5 6.2

SM0507 4.9 4.1 5.6 53.9 6.8SM0506 4.8 2.0 7.6 60.2 8.1SM0505 3.4 3.2 4.9 71.3 9.2SM0504 3.6 3.3 3.8 75.0 13.0SM0503 3.5 2.9 3.9SM0502 3.3 2.8 3.9 70.5 7.3SM0501 3.3 2.8 3.8 75.0 11.0


29/66

29

Given the highly episodic nature of activity at arc stratovolcanoes, meaningful

comparisons of eruption rates can only be made for the values averaged over periods of time

long enough to minimize the influence of short lived variations. The delimitation of the

volcanic center or the volcanic field, in terms of its extent and the units and structures

included, also plays a fundamental role in the resulting rates. For the case of the Santa Mara

Santiaguito complex, although the information obtained for the last 75 ka is very detailed,

this information gives only a limited view of the evolution of the stratocone. Including the

nearby volcanic centers (e. g. Majadas and Cerro Quemado) in the analysis would help to putthe volcanic longer term and more extended volcanic activity in a broader perspective. Halsor

and Rose (1988) have suggested that paired volcanoes like Santa Mara and Cerro

Quemado may share a common deep source of magma, and the differences between them

arise from the different paths of the magma through the crust. Many of these volcanic centers

are located at a very close distance (i. e. less than 10 km) and were possibly active coevally

with Santa Mara (e. g. the Almolonga volcanic field, which includes the Cerro Quemado

volcano has been active from >84 ka to 1818 AD). Some of these volcanic centers are larger

than Santa Mara (e. g. Volcn Siete Orejas, with and estimated volume of between 40 and

65 km 3) and would add a considerable volume to the volcanic cluster, giving perhaps a

higher volcanic rate than that calculated just for the Santa Mara stratocone during the 75 ka

period.


30/66

30

Figure 10. Comparison of eruptive rates of Volcnde Santa Mara with otherwell documentedvolcanoes. Data for the

other volcanoes takenfrom Hora et al., 2007.

Estimates of the eruptive rates at other volcanoes in the Central American Volcanic

Arc are sparse (Table 5) and usually not as well constrained as the case presented in this

study. However, most of the estimated rates for other volcanoes are higher than for Santa

Mara. Eruptive rates calculated from the activity at other volcanoes during the last 500 years

are one to two orders of magnitude higher than the average value estimated for the Santa

Mara Santiaguito complex; however these rates are likely to be peak values that are not

sustained over the whole life time of the volcanic systems. This has important implications

for the previous estimates of the emplacement times for these volcanoes, suggesting that

these ages are highly underestimated, e. g. the age of Volcn de Fuego has been estimated by

Chesner and Rose (1984) to be 17 ka, assuming a constant eruptive rate as calculated from

the activity in the last 500 years and extrapolating for the time necessary to emplace the

complete volume of the cone.


31/66

31

TABLE 5. ERUPTIVE RATES A DIFFERENT VOLCANIC SYSTEMSVolcano Dominant Volume Eruption Comment Reference

rock type rateKm3 Km3 /ka

Amatitlncaldera Rhyolite 60-80

0.3 -0.4

Minimum DRE rate over 200 kafor pyroclastics.

Wunderman and Rose 1984;Rose et al 1999

Atitlnstratocone Andesite 50 0.6

Minimum rate assuming amaximum age of 84 ka. Halsor and Rose, 1988.

Atitln caldera Rhyolite 300 1.9 Minimum DRE rates over 161 ka. Rose et al., 1999Arenalstratocone

Basaltic -Andesitic 19 2.7

Average for the 7 ka conebuilding period. Soto and Alvarado, 2006.

Ceboruco San Pedro

From basalt todacitic 81 0.1 Over 819 ka. Frey et al., 2004.

Fuegostratocone (fullgrowth time)

Basaltic -andesitic 70 4.1

Using the an age of 17 kaobtained from extrapolating rates

from the historical period.

Age: Chesner and Rose,1984. Volume: Carr et al.,

2003.Fuegostratocone(historical time)

Basaltic -andesitic 50

3.7 7.0

Varies if only the last 100 or 475years are considered. Martin and Rose, 1981.

Izalcostratocone Basalt 2 5

During the 200 years it took toemplace the cone. Carr and Pointier, 1981.

Mount Katmaicenter Basalt - rhyolite 70 0.8 Over 89 ka. Hildreth et al., 2003a.Mount Katmaicluster Basalt - rhyolite 179 0.6 Over 292 ka. Hildreth et al., 2003a.Kluchevskoy 250 8 - 32 Over 7 ka. Fedotov et al., 1987.

MerapiBasalticandesite 0.1 1 Over the last 100 years. Siswowidjoyo et al., 1995.

Montserrat Andesite 26 0.15 Over the last 174 ka. Halford et al., 2000.Mount Adamsstratocone Andesite 200 0.38 During 520 ka. Hildreth and Lanphere, 1994.Mount Adamsvolcanic field Basalt to dacite 231 0.25 During 940 ka. Hildreth and Lanphere, 1994.

Mount Bakerstratocone Andesite 15 0.35 During 43 ka. Hildreth et al., 2003b.Mount Bakervolcanic field

Basalt torhyodacite 105 0.08 During 1300 ka. Hildreth et al., 2003b.

Mount Mageik Andesite 30 0.32 During 93 ka. Hildreth et al., 2003a.Mount St.Helens. Basalt to dacite 79 2 During 40 ka. Sherrod and Smith, 1990.

NgauruhoeBasaltic toandesitic 2.2 0.9 During 2.5 ka. Hobden et al., 2002.

Pacayastratocone

Basaltic toandesitic 6-7 0.6 -1.2 During 5 10 ka. Conway et al., 1994.

Parinacotastratocone

Basalticandesite to

dacite27.8 -45.3 0.53 -0.87 During 52 ka. Hora et al., 2007.

Parinacotacomplex

Basaltic

andesite todacite 46 0.28 During 163 ka. Hora et al., 2007.

Seguam IslandBasaltic -rhyolitic 79 0.25 During 318 ka. Jicha and Singer., 2006.

Tatara SanPedrostratocone

Basaltic -andesitic 22 0.25 During 90 ka. Singer et al., 1997.

Tatara SanPedro complex

Basaltic -rhyolitic 55 0.06 Durign 930 ka. Singer et al., 1997.


32/66

32

Long term eruption rates at Arenal and the Atitln volcanic cluster are one order of

magnitude higher, and eruptive rates for Amatiln caldera and the Atitln, Tolimn and

Pacaya stratocones are higher but within the same order of magnitude. Differences could be

due to the tectonic setting, with Santa Mara being located over a thicker crust than the other

considered volcanoes. In addition, local crustal tensional stresses have been postulated for the

Amatitln caldera, the Atitln region and the Pacaya stratocone. Cameron et al (2002) relates

the extension along the Guatemala City graben to the inferred decompression melting that

influence the magma generation bellow Pacaya. This would very likely be the case for the

Amatitln caldera too. Burkart and Self (1985) suggested that crustal extension is linked tothe voluminous volcanism at Atitln. In contrast, such an extensional setting is not evident

for the Santa Mara area. The extension regimes at other volcanoes would facilitate magma

ascent to the surface and could be one of the reasons why the eruptive rates at these

volcanoes are higher than at Santa Mara.

The higher eruptive rates at Arenal could be related in part to higher magma

generation rates due to faster subduction velocities (i. e. the convergence rate of ca. 6 cm/yr

near Santa Mara in contrast to the nearly 8 cm/yr convergence rate near Arenal, after

DeMets et al, 1994), but it seems unlikely that this alone could explain the order of

magnitude difference in rates (see for instance Wadge, 1984). Given the relatively short

duration of the time period used to estimate Arenals eruptive rate (7 ka), this is likely to

corresponds to a peak in the activity and does not represent a comparable long term rate to

the one estimated for Santa Mara (e. g. it could be viewed as equivalent to one of Santa

Maras peak eruptive rate during the cone building phase).


33/66

33

Implications of paleomagnetic data.

Do the directions/VGPs determined from the lava flows exposed in the crater wall of

Volcn de Santa Mara record an excursion of the geomagnetic field as first suggested by

Rose et al. (1977) and later by Conway et al. (1994)? The short answer is probably no.

Conway et al. (1994) based on addition sampling at Volcn de Santa Mara and reanalysis of

the Rose et al. (1977) data suggested the lava flows preserved a record of the Mono Lake

excursion. The Mono Lake event, discovered in sediments from the Wilson Creek Formation

near Mono Lake, California by Denham and Cox (1971), initially defined a large clockwise

excursion of paleomagnetic directions. Later work by Liddicoat and Coe (1979) onadditional sites in the Wilson Creek Formation showed that the original clockwise rotation of

paleomagnetic vectors was preceded by even a larger counterclockwise motion of

paleomagnetic directions. The VGP path defined by this clockwise and counterclockwise

looping of the paleomagnetic directions is superimposed on the plot of our VGP data in

Figure 6. The older and larger loop shows a progressive clockwise change in the position of

the VGP reaching latitudes as low as 32 N in the eastern hemisphere. The second and

smaller counterclockwise loop reaches latitudes as low as 37 N upon passing through the

western hemisphere. In directional space the counterclockwise older loop is defined by a

westerly swing in declination (D ~ 300 o) with inclinations changing from shallow positive to

shallow negative values (I ~ -23 o) and the younger counterclockwise loop is defined by an

easterly swing in declinations with increasingly steep positive inclinations (I~ 80 o). At

Santa Mara the inclination values vary between -12.0 and +57.8, while the declination

values vary between 0.3 and 23.1 (Fig. 5). The inclination waveform at Santa Mara looks

similar to the one corresponding to the Mono Lake excursion as recorded at Wilson Creek


34/66

34

(i.e. inclinations range from 34.5 at the bottom of the section to -12.0 at the middle of the

section and swings back to steeper inclinations up to 57.8, before ending at 33.3 in the

upper part of the section). This waveform was the feature interpreted first by Rose et al.

(1977) as a geomagnetic excursion and by Conway et al (1994) as the Mono Lake excursion.

However, the declination pattern shows little correlation to the Wilson Creek declinations.

Figure 6 shows the positions of the VGPs from our dataset and the VGPs calculated from the

paleomagnetic directions published by Conway et al. (1994). The largest angular departure

of a VGP position from the rotational axis in the combined dataset lies at a latitude of 57.5 N

(sample SM2 of the Conway et. al. 1994 dataset) while VGPs from the rest of the sites fallwithin 30 of the north geographic pole. VGP positions typically more than 45 from the

rotational axis are considered excursional (see for instance Verosub, 1977, Merrill and

McFadden, 1994, and Lund et al. 2005). In addition, the VGPs for the Santa Mara lavas do

not show the clockwise or counterclockwise rotation patterns that characterizes the Mono

Lake excursion. Therefore, the lava flows of Volcn Santa Maria appear not to record a

geomagnetic excursion as previously suggested by Rose et al. (1977) and Conway et al.

(1994).

However, the unique clustering of the VGPs determined from the lava flows from

Volcn de Santa Mara may indicate that these lava flows have in part captured the Mono

Lake event. Inspection of Figure 6 shows a distinct clustering of VGPs and several of these

clusters have the same mean 40Ar/ 39Ar age, i.e. 35.6 2.0 ka. The tight clustering of

paleomagnetic directions (VGPs) in distinctive groups suggests that the lava flows grouped

within these clusters were emplaced over relatively short time intervals with respect to


35/66

35

paleosecular variation, but more importantly, the large angular separation between clusters

made up of lava flows of essentially the same age indicate a rapidly changing geomagnetic

field. Figure 11 shows the high resolution global paleointensity stack for the last 75 ka

(Glopis-75) calibrated to absolute values (Laj et al., 2004) and the relative positions of the

dated lava flows from Volcn de Santa Mara. The youngest half of the lava flows from our

study have ages that at the 2 level overlap with the proposed time interval of the Mono

Lake event. Laj et al. (2004) estimate the duration of the Mono Lake event to be ca. 1.5 ky

while Kent et al. (2002) suggests 3 5 ky for the Mono Lake event. The absence of truly

excusional VGP positions in our dataset could be due to the incomplete sampling of the lava

flows exposed in the 1902 eruption crater wall, or more importantly to the absence of any

lava flows being emplaced during the narrow time window over which the paleomagnetic

field may have been clearly excursional. We suggest the clustering shown in Figure 6 may

be the result of the spot sampling of a rapidly changing paleomagnetic field by the lava flows

of Volcn de Santa Mara during the Mono Lake event. Conversely, Merrill and McFadden

(1994) have hypothesized that the 15 to 45 ka time interval was an excursional interval,

during which the dipole field was at times dominated locally by the non dipole field.

Similarly, Lund et al. (1988) concluded that during a period of more than 15 ka after the

Mono Lake Excursion, the PSV shows at least four very large amplitude swings, that mimic

(and could be related to) the Mono Lake Excursion. In this sense, our results could reflect

this erratic behavior of the paleomagnetic field, without actually correlating with any specificand timely narrow defined paleomagnetic excursion.


36/66

36

Figure 11. Comparison of the timing for the Mono Lake and Laschamp excursions with thetiming of emplacement of the Santa Mara analyzed lava flows. The lower panel shows thevirtual paleomagnetic intensity from the Glopis-75 paleointensity stack (solid line) andscatter envelope (dashed lines). The Mono Lake and Laschamp excursions are recorded asintensity lows (labels and indicated by shaded vertical bands) at ca. 34 and 41 ka (data fromLaj et al, 2004). The upper panel shows the points in time when the sampled lava flows atSanta Mara were (black circles over the dotted line). Also sown in the upper panel (as avertical gray band) and labeled with the corresponding age is the time interval that groups theupper lava flows in the Santa Mara sequences. Ages of the Glopis-75 record according to theGISP2 age model.

Additionally, the ages reported in this paper for the Santa Mara lava flows show that

half of what was first interpreted by Rose et al. (1977) and then by Conway et al. (1994) to

be the record of an excusional waveform in the paleomagnetic inclination data, is outside the

age range that could match either the Mono Lake or the Laschamp excursions, i. e. older than

40.4 2.0 ka (Guillou et al. 2004). This implies that at least the paleomagnetic directions in

the oldest part of the section can not be related to any of these events. The question remains

whether the paleomagnetic directions in the younger part of the section are related to the

Mono Lake excursion or not?


37/66

37

Geochemical variation through time

The geochemistry of the lava flows varies systematically with stratigraphy, and shows

clustering of the data in distinctive groups and trends (Fig 8), in broad correspondence with

the age and paleomagnetic directions. The variability in the composition increases with time,

being largest for the lava flows that were erupted during the last peak of activity. The four

oldest samples have distinctively different compositions than the rest of the samples, and lie

along a different evolutionary path. Besides the overall trend of lava flows to more evolved

compositions (i. e. higher silica and incompatible elements content) with higher stratigraphic

position as noted and discussed by Rose et al. (1977), there is a clear second order oscillatorytrend in the composition variation with stratigraphy (i. e. an alternation between more

evolved and less evolved compositions with increasing stratigraphic elevation, within

different stratigraphic groups). This could be caused by eruptions from a compositionally

zoned magma chamber, or by magma mixing and hybridization between magma batches of

different composition.

Volume estimations:

To calculate calculated the volume emplaced at different instants in time during the

evolution of Santa Mara, we consider the shape of the volcanic edifice to be a solid of

revolution with a characteristic profile. Given the highly symmetrical shape of the cone as

can be seen from its outer surface morphology and form its internal structure as exposed in

the 1902 eruption crater, we believe that the volcano grew mainly around an almost fixed

central vent position, and therefore we assume that the symmetry of the cone was maintained


38/66

38

throughout its construction history. We take the axis of the solid of revolution as the vertical

line through the present day summit. We assume that the location of each sampled site

represents the position of the volcanic edifice surface at the time when the lava flow was

emplaced. We calculate the volume of the edifice for each of those moments in time by

considering the solid of revolution surface going through the site and calculating the volume

as the intersected region between the solid of revolution surfaces and the pre-Santa Mara

basement.

To estimate the basement topography an approach similar to that described by Frey etal. (2004) was taken, defining a sloping surface that follows the trends of the regional

topography, e. g. the WNW trending ridges. The characteristic profile chosen for the

revolution surface is a parabolic curve, following the morphologic characterization by Bemis

(1995), adapted to a set of profiles obtained from the present day topography of the cone. The

areal extent of the Santa Mara cone was chosen according to the geologic map presented by

Rose (1987), without considering the lava flows that go far beyond the main cone structure

(Fig. 2). Table 4 shows the values of the cumulative volume. The total volume of the

volcanic edifice is ca. 8 km 3 and the sampled section covers the upper 5 km 3 (~ 60% of its

volume). Only the uncertainties in the position of the sites are considered and propagated into

the volume calculation results, other uncertainties are not accounted for, therefore these

values have to be taken with caution


39/66

39

Evolution of the volcanic system

Prior to 76.6 ka the volcano had already constructed a ca. 3 km 3 edifice. Between

76.6 and 60.2 8.1 ka a spurt of relatively high eruptive rate added ca. 1.5 km 3 to the edifice

volume. This volume addition was also accompanied by a change in the chemistry of the

lavas (e. g. a shift in the value of elemental ratios such as Ba/La, La/Yb, Zr/Y, Hf/Lu),

suggesting a change in the source. From 60.2 8.1 ka to 37.2 ka the activity remained at

relatively low levels, sporadically producing lava flows but without adding a significant

volume to the edifice. A sharp shift in the chemistry of the lavas with respect to the

composition of the previous group towards higher SiO 2 and other incompatible elements(Fig. 8) occurred during this low eruption activity. During this extended period of time the

composition of the lavas remained remarkably homogenous, including very low dispersion of

some elemental ratios (e. g. Ba/La, Hf/Lu, La/Yb and Zr/Yb), suggesting that a stable process

was delivering the magma from a homogenous composition source to the surface in a fast

and direct way. The presence of a reservoir at the base of the crust as hypothesized by Rose

(1987) could have acted as the MASH zone proposed by Hildreth and Moorbath (1988) for

the Andes of Central Chile, homogenizing the chemistry of the lavas over 36 ka, and would

also be consistent with the deep crust hot zone proposed by Anne et al. (2005) and

discussed later; a similar model in which magma is extracted from a deep crustal magma

reservoir has been proposed for Seguam (Jicha et al., 2005). In this context, no large shallow

magma chamber existed during that time. At 35.6 2.0 ka, a high eruptive rate peak added

another 3 km 3 to the edifice, this period of activity shows the largest variation in chemical

compositions of the lavas that form the stratocone. The pulse of high eruptive rate and

chemical variation of lava composition over a rather short period of time could be related to


40/66

40

repeated cycles of magma batches injection into the system, which undergo evolution and

mixing, accompanied by crustal contamination. However, the simultaneous and erratic

variation of elemental ratios like Ba/La and La/Yb, point to processes other than fractionation

and mixing to explain the variation in chemical composition; disruption of the deep crustal

reservoir or contamination of these lavas in an upper crustal reservoir could be relate to it.

After 33.0 3.7 ka, no significant volume was added to the edifice and eruptive activity, if

any, must have been at very low rates until the large eruption in 1902.

To accumulate and store the volume of dacitic magma that erupted in 1902, a largemagma chamber must have existed before the eruption. How the dacite originated is not

known, but the generation of the dacite by a process of crystal fractionation from a mafic

parent magma on an evolutionary path that includes the Santa Maria lavas, has been

considered as a plausible option (Rose 1972, Rose et al. 1977 and Williams and Self 1983);

such a mafic parent magma could have been stored in the deep crustal reservoir, and

evolution could either have happened in situ or at shallower levels, once the magma had left

this region. Annen et al. (2005) have proposed that deep crustal hot zones could be the

sources for intermediate and silicic magmas, in this context, a common source for the

magmas that underwent different processes to erupt first as the Santa Mara lavas and as the

1902 and Santiaguito dacites afterwards is possible. Rose et al. (1977) tried to model the

generation of the Santiaguito dacites by crystal fractionation from a parent magma with a

composition equal to the average composition of the lavas they sampled in the cone; although

they obtained good results for the major elements the trace elements turned out to be more

problematic to model.


41/66

41

If the dacite was derived from a basaltic magma through crystal fractionation, then

the question of where this happens remains open. Did the fractionation happen in the magma

chamber where the dacite accumulated and from where it erupted in 1902? Was this chamber

located in the deep crust or was it a shallow crustal reservoir? Rose (1987) noted that the

absence of wide scale subsidence or caldera collapse despite the large volume of the eruption

could indicate that the chamber was located at deep levels, however Barmin et al. (2002)

based on modeling of the cyclic eruptive activity at the Santiaguito Dome complex, estimated

that a shallow (~ 5 km depth) and large (~ 65 km3

) magma chamber has been feeding theSantiaguito dome activity since 1922. No further evidence has been presented so far to

support either hypothesis.

Regarding the possibility that the dacite magma differentiated at shallow levels, a

possible interpretation of the variation in chemistry and eruptive rate during the peak of

activity at 35.6 2.0 ka is to relate it to the onset of a magmatic reservoir in the upper crust.

Here different magma batches coming from depth could accumulate progressively increasing

the volume of the chamber, after the output of the system shut down at 33.0 3.7 ka. In this

shallow chamber the magma would reside over a relatively extended period of time,

evolving, and interacting with new magmas coming into the system and with the surrounding

crust. Further petrologic and geochemical research could help to answer this question.

If the fractionation happened in a shallow level magma chamber, the existence of a

mafic crystal cumulate left over in the chamber and perhaps the existence of a chemically


42/66

42

zoned magma body would be expected. Although part of the magma erupted in 1902 was of

basaltic composition, Williams (1979) presented evidence that this basaltic magma was a

juvenile component injected into the older dacite, such that both magmas are not directly

related through a process of crystal fractionation in situ . Therefore, no mafic tephra from a

zoned magma chamber was erupted in 1902. However the occurrence of mafic enclaves in

some of Santiaguitos lavas (Rose 1972) and a steady decline in silica content through time

for the lavas erupted since the 70s has been interpreted by Harris et al. (2003) to be the

progressive tapping of a zoned magma chamber. If this interpretation is correct, it would

constitute evidence that the fractionation from basalt to dacite happened in situ

at thelocation of the magma chamber, assuming that the magma comes from the same chamber as

the 1902 dacite came from.

If the above were the case, how could such a shallow chamber have been formed?

The repeated injection of hot mafic magmas over more than 36 ka could have heated the

conduit wallrock at shallow levels, leading to the melting of the surrounding crust and the

formation of a magma chamber. The presence of the growing edifice could have

progressively inhibited the eruptions, shutting off the systems output and forcing the

accumulation of the magma coming in from depth. At the same time (35.5 2.0 ka), an

apparent increase of the magma production happened, probably enhancing the

abovementioned effects.

If the accumulation of magma in a chamber started at 35.6 2.0 ka, and taking the

average eruptive rate of 0.75 km 3 /ky during the coeval eruptive peak as a proxy for the


43/66

43

average magma chamber recharge rate until the 1902 eruption (assuming a steady state mass

flux through the system during that peak), a volume of ca. 24.8 km 3 would have

accumulated during the intervening ca. 36 ky. This is less than the minimum values of 35 and

50 km 3 of parental basalticandesitic magma from which the dacite would have fractionated,

according to the calculations by Rose (1987) and Conway et al. (1994) respectively, and the

value of 65 km 3 obtained by Barmin et al. (2002). This suggests that the actual average

magma input rate from depth was higher. If we invert the abovementioned minimum

estimated magma chamber volumes, the corresponding minimum average rates of magma

input to the chamber before the 1902 eruption, are between 1 and 2 km3

/ky.

For a shallow magma body to maintain a significant melt fraction and grow over a

time period of 33 ka, a persistent heat supply is required to avoiding an excessive heat loss

that would lead to the complete crystallization of the magma before evolving to a more silicic

composition. This could have been achieved through recurrent injections of hotter mafic

magma from below. A more or less continuous supply of magma at an average rate of 1-2

km 3 could have provided this thermal input to the system. Although almost all the magma

erupted in 1902 was dacitic, a small portion had a basaltic composition, producing a

distinctive mingled texture where it came in contact with the dacite. Williams (1979)

presented evidence that this basaltic magma came in contact with the dacite residing in the

magma chamber only a short time before the eruption and proposed that this could actually

have triggered the eruption. The evidence of this late injection of mafic magma suggests that

such a process could have operated in a similar way many times over the prior 33 ka,

maintaining both the heat and mass flux budget.


44/66

44

Hardee (1982), applying a simple heat transfer model and the assumption of repeated

intrusions of mafic magma into cylindrical and dike conduits in a cold crust, concludes that a

magma injection rate higher than ca. 1 km 3 is required for the conduit walls to start to melt

and a magma chamber to form. This values agree with the observation by White et al (2006)

that most arc volcanoes maintain long term average eruptive rates that are above this value

(and therefore the input rates can be assumed as being also above the threshold), and are

accompanied usually by the formation of an open conduit and the development of a magma

chamber, as a characteristic of these long lived and spatially focused systems. White et al(2006) also estimated based on thermal flow considerations that most intermediate to silicic

systems are likely to remain as open systems, for which repeated/sustained magma input

maintains the condition of eruptibility (mentioned earlier). This is consistent with the inferred

increase in the magma input under Santa Mara above 1 km 3 at 35.5 2.0 ka and the

subsequent formation of a magma chamber.

What caused the eruptions to slow down or probably even cease after 33 ka? Pinel

and Jaupart (2000, 2004 and 2005) have shown that the presence of the volcanic edifice

changes the stress field in the upper crust and could strongly influence the capacity of dense

(i. e. more mafic) magmas to reach the surface. The progressive loading of the upper crust as

the volcanic edifice grows through time acts to inhibit the eruption of less silicic magmas,

reducing the eruption rate of the volcanic system. This could explain the changes in erupted

lava composition and eruption rate through time at Santa Mara. For a constant magma

supply rate from depth, the reduction and even cessation of eruptions implies that magma


45/66

45

would start to accumulate in the crust, and given favorable thermal conditions as discussed

above, could remain partially molten starting the formation of a magma chamber. Rose et al.,

(1977), considering only the change in lithostatic pressure due to the progressive increase in

the vent elevation through time at Santa Mara, suggested in a similar way that the observed

increase in silica and other incompatible elements with increasing stratigraphic position

could be explained by the inhibiting effect that a larger lithostatic column would have on the

rise of dense magmas. Jellinek and DePaolo (2003) have also shown that as a magma

chamber grows it becomes more difficult for magma from that chamber to reach the surface

in small eruptions, and that above a certain size (typically 100 km3

) the system effectivelyshuts off, until a vigorous enough intrusion of new magma destabilizes the system to

promote an eruption. For Santa Mara, the formation of a shallow dacitic magma chamber at

some time between 33 ka and 1902 AD, could have been related to the reduce or completely

absent volcanic activity after 33ka. The intrusion of basalt described by Williams (1979)

could have been that destabilizing factor that finally triggered the eruption of 1902.

So far the evidence for the formation of a large, shallow magma chamber as well as

its present day characteristics (mainly its depth) remain unknown, further petrological and

geophysical research could help to clarify these issues.

Analogies with other volcanoes and implications on volcanic hazard assessment.

From the perspective of the hazard assessment, two characteristics of the eruptions

are of foremost relevance: the size (usually measured by the VEI (Newhall and Self, 1982))

and the timing (or frequency of occurrence over time, in the case of considering multiple


46/66

46

recurring events). For the purpose of analysis, two different kinds of eruptions can be

considered: the extreme events, e. g. the 1902 Santa Mara eruption scenario, and the

common or expected events, e. g. the eruptions during the cone forming activity at Santa

Mara. Understanding how these events are distributed through time and how they relate to

other observable parameters, e. g. the chemistry of the lavas, is important for hazard

assessment purposes.

Extreme events may happen only a few times during the lifespan of a volcanic

system, and may be related to the overall evolution of the system, e. g. the system developingthe capacity to store a large volume of eruptible magma as happened at Santa Mara before

1902. The Santa Mara Santiaguito volcanic complex shares a similar evolutionary pattern

with other nearby volcanoes. The evolution of a stratocone that erupts progressively more

silicic lavas during the main cone building phase, and which subsequently undergoes

catastrophic cone destruction by a large silicic eruption accompanied by caldera collapse, has

been observed at nearby Siete Orejas (Gierzycki,1976) and Almolonga/Cerro Quemado

(Johns, 1975) volcanoes (Fig. 2). Gill (1981) observed the tendency at many andesitic

volcanoes around the world to erupt progressively more evolved magmas, and suggested that

this pattern could lead to more explosive eruptions. The evolution of the Santa Mara

Santiaguito complex, agrees with this idea and the notion that volcanoes that have had long

repose periods are more likely to produce larger explosive eruptions (Simkin and Siebert,

2000). Extending these ideas to other volcanic systems suggests that the possibility of large,

explosive (and potentially more hazardous) eruptions, even possibly involving silicic

magmas, happening at otherwise basaltic-andesitic stratovolcanoes similar to Santa Mara


47/66

47

shouldnt be disregarded as completely unlikely. This possibility should be evaluated

carefully if the recorded evolution of the geochemistry and repose intervals follows a similar

pattern as that observed at Santa Mara before the 1902 event, even if there is no evidence

that such an event has happened in the past at the considered volcano. This could be

especially critical for the hazard assessment of a volcano that has been dormant for an

extended period of time and is showing signs of reawakening. Other volcanoes that currently

have low eruption rates, like Atitln and Acatenango, or that has not had activity in historical

times, like Agua, may have the potential of producing a larger explosive eruption. In these

cases, a similar study of the volcanoes complete evolution, to define high eruptive activityepisodes and geochemical changes through time, may help to assess the likelihood of such an

event.

In the case of eruptions that are considered more likely to happen during the cone

building phase of a volcano, and which are limited to a narrower range of sizes, their

occurrence at volcanoes that have a persistent and steady level of activity may be considered

as a recurrent process that can be characterized in terms of their size and probability of

occurrence over time (De la Cruz-Reyna, 1991; 1996; Ho, 1990; 1991; 1996). Even more

important, an upper size limit bounding this category of events may be possible to estimate.

However, this approach would only be valid as long as the volcano remains in a well define

episode of activity (e. g. the cone building phases at Santa Mara). If we consider the case of

the historically active Volcn de Fuego which has had more than 60 recorded eruptions since

1524 (Rose et al., 1978; Martin and Rose, 1981), the frequency of relative large eruptions is

high (7 eruptions with a VEI of 7), but it is not known what the likelihood of a large eruption


48/66

48

would be. The important question that than arises is: could this be the maximum size of

eruptions at Volcn de Fuego, under the current volcanic system configuration? This

regularity in the activity may be due to Fuego being in one of its high eruptive cone building

activity episodes, behaving rather as an open conduit system where large volumes of magma

would not tend to accumulate. In that case, the approach of characterizing the size and

probability of occurrence of these eruptions based on their historical recurrence may be

appropriated, and derived from that, an upper limit to the size of Fuegos eruptions (VEI

escobar wolf msc thesis

Documents