Overview

Sampling Locations

References

Pumice blocks and ash matrix sampled from the 1980 pyroclastic flows at Mt. St. Helens and the 1912 Novar-upta eruption (Valley of Ten Thousand Smokes) enable us to demonstrate a novel geospeedometer based on nonconvergent cation ordering in titanomagnetites, as well as to further constrain emplacement tempera-tures based on the relative partitioning between two forms of titanomagnetite. The homogeneous titano-magnetites present in these samples undergo significant cation reordering at moderate (300-500°C) tempera-tures, with attendant changes in magnetic properties, including Curie temperature. Thus, Curie temperature in the samples evolves as a function of time and temperature, suggesting it could be used in constraining the thermal history of pyroclastic flows. Samples additionally contain a fraction of oxy-exsolved titanomagnetite which appears to vary as a function of emplacement temperature and cooling rate.

Julie A. Bowles1, Mike Jackson2, Jeffrey S. Gee3, Thelma Berquó4, and Peter Solheid2

PYROCLASTIC FLOW (POST-)EMPLACEMENT THERMAL HISTORY DERIVED FROM TITANOMAGNETITE CURIE TEMPERATURES:EFFECTS OF TEMPERATURE-DEPENDENT CATION ORDERING AND EXSOLUTION

1 Dept. of Geosciences, University of Wisconsin - Milwaukee, Milwaukee, WI 53201, bowlesj@uwm.edu; 2 Institute for Rock Magnetism, Dept. of Geology and Geophysics, University of Minnesota, Minneapolis, MN 554553 Scripps Institution of Oceanography, University of California, San Diego, La Jolla, CA 92093; 4 Physics Department, Concordia College, Moorhead, MN 56562

Conclusions

Banks, NG, and RP Hoblitt (1996). Direct Temperature Measurements of Deposits, Mount St. Helens, Washington, 1980-1981, USGS Prof. Paper 1387, 83pp.

Fierstein, J., and C.J.N. Wilson, (2005). Assembling and ignimbrite: Compositionally defined eruptive packages in the 1912 Valley of Ten Thousand Smokes ignimbrite, Alaska, GSA Bull., 117, 1094-1107.

Stratigraphic and cooling rate-derived changes in Curie temperature

Figure 4. (center) Thermomagnetic results from Mt. St. Helens May 18 pyroclastic flow (site 8). Low Curie temperatures measured on heating are associated with rapid cooling and a low degree of cation order. High Curie temperatures on heating are associated with slow cooling and a higher degree of cation order. (right) Direct temperature measurements made after the May 18, 1980 eruption, at a station ~2 km from our site 8. Red circles (and star) were measured 14 days post-eruption (Banks and Hoblitt, 1996). Blue squares were measured 94 days post-eruption. Red star represents a single temperature mea-surement at a location ~300 m from our site 8.

0

20

40

0 20 40 60 80

60

80

Magnetite(Fe3O4)

Ulvöspinel(Fe2TiO4)

Mg, Al, Mn endmembers

350 500

2.0

1.6

1.2

0.8

0.4

0.00 300200100

450400

Measured Temp. [°C]

Curie Temp. [°C]

Dep

th [m

]

2.0

1.6

1.2

0.8

0.4

0.0

14 days

94 days*

pumice blockheating/cooling

ash matrix

Abstract V41B-2782

Flow surface

Pyroclastic flow

Ash airfall

Modi�ed from Fierstein & Wilson [2005]

Figure 1. Samples of unwelded to poorly welded ash matrix were collected from 1912 Novarupta (NV) pyroclastic flow (left). Juvenile pumice blocks and unconsolidated ash matrix were collected from the 1980 pyro-clastic flows at Mt. St. Helens (MSH) (right).

Figure 3. Composition of homog-enous titanomagnetite grains from No-varupta (red X) and from Mt. St. Helens (blue triangles). Gray symbols from compilation of Ghiorso & Evans (2008): + = rhyolite; o = dacite; square = andes-ite.

-0.4

Exsolution constraints on thermal history

Conceptual model of geospeedometer

Homogeneous titanomagnetiteT

C < 500°C, varies with thermal history

Oxy-exsolved titanomagnetite (magnetite-rich host + ilmenite-rich lamellae)

TC > 550°C, reversible

Tem

per

atu

re

Time

Co

olin

g ra

te

Fast cooling

Slow cooling

Tem

per

atu

re

Order degree

Anneal/Equilibration Temp

Cu

rie

Tem

per

atu

re

Increasing temp

Fast

co

olin

g

Slo

w

coo

ling

Order degree

Order degree

Fast cooling

Slow cooling

Data from Banks and Hoblitt (1996)

A B

May 18June 12July 22Aug. 7Oct. 16

0 5 km

155°10'155°20'

58°20'

58°15'

58°10'N

155°W

NKatmaicaldera

= high-energy proximal facies

= valley-filling ignimbrite

from many flow packages

= all-rhyolite ignimbrite

Windy

Creek

Ukak River

Knife CreekRiverLethe

Griggs

Fork

BRBM

FM

MC

KP

Mageik Ck

M

GThree

Forks

VTTS

NKodiak

Anchorage164°W

60°N

58°

56°

160° 156°

0 100 200 km

UnimakIsland

HomerSeward

Kenai

King Salmon

KATMAIGROUP

13

B65

4B3

7

6

2

T [°C]

χ [x

10-6

m3 k

g-1]

2

3

6

0

8

200 400 600

MSH008-LT

emp ~ 350°C

C

MSH011-BT

emp > 450°C

200 400 600T [°C]

2

4

6

χ [x

10-6

m3 k

g-1]

0

Mixture of 2 phases

D

MSH003-GT

emp > 500°C

200 400 600T [°C]

2

4

8

6

χ [x

10-6

m3 k

g-1]

0

E

Figure 2. Samples at Mt. St. Helens contain two primary magnetic phases. (A) Homogeneous titanomagnetite (Fig. 2) and (B) oxy-exsolved titanomagne-tite with ilmenite-rich lamellae subdi-viding a magnetite-rich matrix. Homo-geneous samples annealed in the lab at T > 500°C for ~1hr or more rapidly begin to oxidize and undergo exsolution. In natural samples, sites with low emplace-ment temperature (C) have a relatively low fraction of the exsolved phase, while samples emplaced at higher tem-peratures (D, E) have a more variable contribution -- up to nearly 100% of the exsolved phase. The variability presum-ably results from variations in emplace-ment temperature and cooling rate.

Annealing experiments

Temperature [°C]300 350 400 450 500

dχ/d

T [m

3 kg-1

°C-1

* 1

0-7]

-2.4

0

-0.8

-1.6

B

0

50

100

150

101 100 1000Anneal Time [hrs]

T C hea

ting

- TC c

oolin

g [°

C]

MSH02MSH04MSH05MSH08

D

350

400

450

500

101 100 1000

T C [°C]

C

Anneal Time [hrs]Temperature [°C]100 200 300 400 500 600

χ [m

3 kg-1

* 1

0-6]

2

8

6

4

0

144 hr 70 hr 22 hr 4 hr

A

350°C

A

D

C

B Figure 6. (A) The equilibrium degree of cation ordering is a func-tion of temperature, with increas-ing disorder at higher T. The kinet-ics of the ordering process are also faster at higher temperatures. Combined, this means that a sample following a relatively rapid cooling path will depart from equi-librium at a higher temperature, freezing in a lower degree of order. A slower cooled sample will depart from equilibrium at a lower tem-perature, with a correspondingly higher degree of order. (C) The higher degree of order (slower cooling rate) is associated with a higher Curie temperature. (D) The order degree (and corresponding T

C) can be calculated for a particular

cooling rate or cooling path [e.g.

Harrison and Putnis, 1999]. Geospeedometers based on cation order have been developed for or-thopyroxene [e.g., Ganguly, 1982] and other silicates. A similar technique based on titanomagnetite has an advantage in that Curie temperature (as a proxy for order) is a fast and easy measurement. The temperatures and time scales of (re-)ordering are particularly well suited to pyroclastic flows.

heating/cooling350°C400°C

Curie temperature data from Mt. St. Helens and Novarupta are frequently strongly irreversible on warming and cooling (e.g., Fig. 2c), and the degree of irreversibility can be a function of strati-graphic depth and apparent cooling rate (Fig. 4).

The data are best explained by cation reordering as the cations (Fe2+, Fe3+, Ti, Mg, Mn) redistribute between the tetrahedral and octahedral sites in the cubic spinel structure.

Annealing experiments con�rm this and dem-onstrate that amount of reordering is signi�cant even at moderate temperature (300-500°C) and over time scales of minutes to months (Fig. 5).

This cation reordering can be exploited to con-strain thermal histories of pyroclastic �ows, once the kinetics are quanti�ed and TC is linked directly to order degree (Fig. 6).

Additionally, at MSH, the fraction of oxyexsolved titanomagnetite appears to be a function of both emplacement temperature and cooling rate.

The endmembers of the titanomagnetite solid solution (magnetite, Fe3+[Fe2+Fe3+]O4, and ulvospinel,

Fe2+[Fe2+Ti4+]O4) are both inverse spinels with cations distributed between tetrahedral and octahedral

coordination in the cubic spinel structure (square brackets denote octahedral coordination). Ap-proaching 0 K, the equilibrium distribution of cations is as indicated by the structural formula. At higher temperatures, however, the cations can exchange between tetrahedral and octahedral sites to give an intermediate, imperfectly ordered distribution.

Figure 5. Pumice and ash specimens from MSH were put into a disordered state by rapid cooling from 600°C. Each specimen was then annealed at 350°C and 400°C for times ranging from ~10 min to 1000 hr. Following each anneal period, Curie tem-perature (T

C) was measured on heating and cooling

to 600°C. These TC measurements also served to put

the sample back in a disordered state prior to the next anneal period. (A) Results for MSH-002G show that T

C measured on warming increases progres-

sively with anneal time as the initially disordered state evolves towards the equilibrium degree of order. (B) First derivative emphasizes that while T

C

warming increases with anneal time, TC cooling is

constant. TC

cooling represents the relatively disor-dered equilibrium state achieved at high tempera-ture and frozen in by rapid cooling. (C) Summary of results from 4 MSH samples. (D) Difference between T

C measured on warming and cooling is related to

the degree of cation reordering that occurs during each anneal.

Samples from Novarupta display the same behavior.

What is cation distribution?

Flow surface

Ganguly, J. (1982). Mg-Fe order-disorder in ferromagnesian silicates: II. Thermodynamics, kinetics and geological applications, in Ad-vances in physical geochemistry 2, S. Saxena (ed.), 58-99.

Ghiorso, MS and BW Evans (2008). Thermodynamics of rhombohe-dral oxide solid solutions and a revision of the Fe-Ti two-oxide geothermometer and oxygen-barometer, Am. J. Sci., 308, 957-1039.

Harrison, RJ and A Putnis, (1999). The magnetic properties and crys-tal chemistry of oxide spinel solid solutions, Surv. Geophys., 19, 461-520.