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Documenting Mantle and Crustal Contributions to Flood Basalt Magmatism via Computational

Modeling of the Steens Basalt, Southeast Oregon

Sylvana Bendaña

Purpose and Introduction

Flood basalts are large terrestrial, volcanic events that occur within a geologically short period of

time (a few million years) and profoundly affect climate, topography, and flora and fauna

(Hofmann et al. 1997; Self et al. 2006). Due to the large extrusive and intrusive magmatic

volumes, an important question emerges: what is the mass balance between crustal (assimilation)

and mantle (mixing/magma recharge) input and does this change systematically with time? At

the Steens Mountain in SE Oregon, a major basin-range fault scarp provides almost 2000m of

relief over the Alvord Desert, exposing one of the world’s thickest single exposure of Tertiary

basalt (Johnson et al., 1998). Two distinct sets of Steens Basalt, collectively comprised of over

200 fresh lava flows, have been identified: Lower Steens is composed of more primitive flows

that contrast with more compositionally evolved Upper Steens flows higher in the stratigraphic

column. These compositional distinctions lead to the hypothesis that the Lower Steens magmatic

system was dominated by magma recharge whereas Upper Steens was dominated by crustal

assimilation. I propose to test this hypothesis via collection of whole rock elemental and isotopic

data, and computational modeling using Energy-Constrained Recharge Assimilation-Fractional

Crystallization (EC-RAFC; Bohrson & Spera, 2001).

Background

Geological and Petrologic Background

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The Steens Basalt is the oldest unit of the Columbia River Flood Basalts (Camp et al. 2013).

Starting at around 16.7 Ma, about 31,800 km3 of basalt erupted from a dike swarm that strikes

NNE and is located west of the Sr isotope 0.7060 line on the Oregon Plateau. Flows erupted

during a 300 kyr time window and encompass a magnetic polarity reversal from R0 to N0 (Fig.1;

Jarboe et al. 2008). The time of eruption of Upper Steens was concurrent with the eruption of the

Imnaha flows on the Columbia Plateau (Fig.1B). The Lower Steens flows are reversely polarized

tholeiitic lavas, whereas Upper Steens flows are normally polarized mildly alkaline to

trachyandesitic lavas (Fig. 2). The more primitive Lower Steens composition is shown by lower

concentrations of incompatible trace elements (Fig. 2, 3 and 5) and less radiogenic 87Sr/86Sr,

which is characteristic of recharge-dominated magmatic system. More evolved Upper Steens is

characterized by higher concentrations of rare-earth elements, lower MgO and Ni concentrations,

and higher 87Sr/86Sr, typical of a magmatic system dominated by crustal assimilation (Fig. 2, 3

and 5; Johnson et al. 1998; Camp et. al. 2013; Wolff et al. 2013).

The Steens Basalt flows range from phyric to aphyric, with ≤15% olivine and pyroxene

phenocrysts and up to 40% plagioclase, with some of the phenocrysts reaching up to 4 mm in

size (Wolff and Ramos, 2013). Radiogenic isotope studies suggest Steens magmas were

influenced by magma recharge/mixing of two mantle components: plume and Pacific MORB

asthenosphere (Fig. 3B; Wolff et al. 2008; Ramos et al. 2013) and crustal assimilation through

partial melting of Mesozoic accreted terrane (e.g., Camp et al. 2013; Wolff et al. 2008; Chesley

et al. 1998). On an individual crystal scale, anorthite content and increase of plagioclase 87Sr/86Sr

from core to rim are consistent with magmatic water content of 0-2 wt. % and differentiation at

crustal pressures of <1 GPa (Ramos et al., 2005, 2013). Consistent with this, preliminary

MELTS models define a crystallization zone at ~2 kb (N. Moore, pers. comm.). In-situ

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plagioclase and groundmass 87Sr/86Sr implicate shallow open system processes (e.g. magma

recharge and crustal assimilation) with short residence times (Ramos et al. 2005).

Background on Mass and Thermal Modeling Tool: EC-RAFC

EC-RAFC constrains masses, compositions (trace element and isotopic ratios), and thermal

characteristics for open-system magma bodies undergoing RAFC (e.g., Spera & Bohrson 2001).

Model input includes initial magma composition (whole rock isotopic, trace element), thermal

parameters (e.g. initial temperature of each recharge event and of crust), and initial mass ratios of

magma, crust and recharge magma. Output includes trace element and isotope characteristics of

derivative magmas, as well as relative masses of crystals fractionated and crust assimilated. An

example of EC-RAFC output can be seen in Fig. 4 which shows the best fit trajectory that

follows the trend of the compositional data implicating shoaling of the magmatic system at Skye

(Fowler et. al, 2004).

Methods and logistics

I will test the hypothesis that the Lower Steens was dominated by magma recharge/mixing and

fractional crystallization and the Upper Steens was dominated assimilation and fractional

crystallization through petrographic analysis, whole rock trace element data, Sr isotopic data, O

isotope data on minerals, and EC-RAFC modeling. This Steens project will be in collaboration

with Professor Anita Grunder, her PhD student Nicole Moore at Oregon State University and

with a CWU undergraduate researcher that I will mentor. Thus compositional data and numerical

model results will be shared amongst each other to help with evaluation of the hypothesis. There

is a set of 35 hand-samples and associated whole-rock data (XRF- major and some trace

elements, Johnson et al. 1998) that I will use as the starting point of my research. Petrographic

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analysis will allow for choice of fresh samples for additional whole rock ICP-MS analysis of

critical trace elements (such as rare earth elements) at Washington State University. Based on

these data, a subset of ~18-20 existing samples will be chosen for whole-rock Sr isotope analysis

(New Mexico State University with Dr. Ramos) and for O isotope data collection on select

minerals (U. of Oregon with Dr. Bindeman). Combining new and existing data, comprehensive

documentation of major and trace element and O and Sr isotope changes during the course of the

Steens eruptive event will emerge, allowing flows to be broken into packages that are

petrologically related (Fig.5; Camp et al. 2013). In particular, trace elements, and Sr and O

isotopes will identify trends that are dominated by magma recharge or crustal assimilation. For

example, magma recharge events will be distinguished by increases in Ni and MgO and crustal

assimilation and fractional crystallization events by increases in incompatible trace elements and

Sr isotopes (Wolff et al. 2013).

For each petrologic package, iterative EC-RAFC modeling will define the mass of recharge

magma, assimilant and crystals fractionated. For each package's parental and recharge magmas,

trace element and isotopic compositions will be input, and crustal component input will be

estimated from the literature (e.g., Ramos et al. 2005; Johnson et al. 1998; Wolff et al. 2013).

Thermal data for parent magma will be taken from Spera and Bohrson (2001). A simulation

starts at the magma liquidus temperature, accommodates multiple recharge events and is

completed when magma and crust are in thermal equilibrium. Systematic variations in mass and

thermal parameters (e.g., mass of each recharge event, crust; initial temperature of crust, etc.)

will yield model compositional trends that will be compared with observed or measured

compositional trends. I anticipate that 30-50 simulations will be required to define the best-fit

model for each petrologic package.

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Anticipated Results

Best-fit result(s) will bracket trace element and isotopic trends and yield relative mass

contributions of crystallization, recharge and assimilation of each package. Evaluation of how

recharge (i.e., contribution from mantle components) and crustal masses change up-section will

allow me to assess if the composition of the Upper Steens is crustal assimilation-dominated

whereas the Lower Steens is magma recharge-dominated.

Significance of Research

The spectacular exposure at Steens Mountains affords an opportunity to evaluate time-

transgressive changes in magma composition. Application of EC-RAFC permits quantitative

documentation of the roles that recharge and assimilation play as a flood basalt event progresses.

The hypothesis that the Lower Steens is recharge-dominated suggests that early flood basalt

magmatism may have been dominated by mantle flux, whereas assimilation-dominated Upper

Steens indicates that conditions developed that were conducive to crustal melting. Verification of

this hypothesis would suggest fundamental changes in mantle vs. crustal involvement with time,

and highlight the need for quantitative modeling studies at other flood basalt provinces.

References

Bohrson WA, Spera FJ, 2001. “Energy-Constrained Open-System Magmatic Processes II: Application of Energy-Constrained Assimilation–Fractional Crystallization (EC-AFC) Model to Magmatic Systems.” Journal of Petrology 42 (5): 101941.doi:10.1093/petrology/42.5.1019. Camp VE, Ross ME, Duncan RA, Jarboe NA, Coe RS, Hanan BB, Johnson JA, in press 2013, The Steens Basalt: earliest lavas of the Columbia River Basalt Group: Geological Society of America Special Paper.  Chesley JT, Ruiz J. 1998. Crust–mantle interaction in large igneous provinces: implications from

the Re–Os isotope systematics of the Columbia River flood basalts: Earth and Planetary Science Letters, v. 154, no. 1, p. 1-11.

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Eagar KC., Fouch, MJ., James DE.,Carlson RW.. 2011. “Crustal Structure beneath the High Lava Plains of Eastern Oregon and Surrounding Regions from Receiver Function Analysis.” Journal of Geophysical Research: Solid Earth 116 (B2). Fowler, S. J., Bohrson, W. A., & Spera, F. J. (2004). Magmatic evolution of the Skye igneous centre, western Scotland: modelling of assimilation, recharge and fractional crystallization. Journal of Petrology, 45(12), 2481-2505. Hofmann A. 1997, Mantle geochemistry: the message from oceanic volcanism: Nature, v. 385,

no. 6613, p. 219-229. Jarboe NA, Coe RS, Renne PR, Glen JMG, Mankinen, EA. 2008, Quickly erupted volcanic

sections of the Steens Basalt, Columbia River Basalt Group: Secular variation, tectonic rotation, and the Steens Mountain reversal: Geochemistry Geophysics Geosystems, v. 9, no. 11.

Johnson JA, Hawkesworth CJ, Hooper PR, Binger GB. 1998, Major and trace element analyses of Steens Basalt, southeastern Oregon: U.S. Geological Survey Open-File Report 98482, 26 p.2 Johnson, J., Nielsen, R., and Fisk, M., 1996, Plagioclase-hosted melt inclusions in the Steens

basalt, southeastern Oregon: Petrology, v. 4, p. 247-254. Ramos FC, Wolff JA, Tollstrup DL. 2005, Sr isotope disequilibrium in Columbia River flood

basalts: Evidence for rapid shallow-level open-system processes: Geology, v. 33, no. 6, p. 457-460.

Ramos FC, Wolff JA, Starkel W, Eckberg A, Tollstrup DL, Scott S. 2013. The Changing Nature of Sources Associated with Columbia River Flood Basalts: Evidence from Strontium Isotope Ratio Variations in Plagioclase Phenocrysts. Geological Society of America Special Papers 497: 231–57.

Spera FJ, Bohrson WA. 2001, Energy-constrained open-system magmatic processes I: General model and energy-constrained assimilation and fractional crystallization (EC-AFC) formulation: Journal of Petrology, v. 42, no. 5, p. 999-1018.

Wolff JA, Ramos FC. In press 2013, Source materials for the main phase of the Columbia River Basalt Group: geochemical evidence and implications for magma storage and transport Geological Society of America Special Paper. Wolff JA, Ramos FC, Hart GL, Patterson JD, Brandon AD. 2008, Columbia River flood basalts

from a centralized crustal magmatic system: Nature Geoscience, v. 1, no. 3, p. 177-180.  

Schedule

-Selection of rocks for thin and thick sections (May and June, 2014)

-Petrographic analysis during summer 2014 (July)

-Trace element data collection summer at WSU 2014 (August)

-Collection of Sr, O isotopes Summer and Fall 2014 at NMSU and University of Oregon

-EC-RAFC modeling late Fall 2014, Winter and Spring 2015

-Write up of thesis Winter and Spring of 2015

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Budget

Budget Item Total Amount

Budgeted

Sr Isotope analyses at

NMSU

$2160

Travel to NMSU

(airfare)

$340

Per diem NMSU (no

lodging required)

$400

ICP-MS analytical

work at WSU

$2400

O Isotope analyses at

University of Oregon

$840

Mileage, per diem,

lodging WSU

$200

Mileage, per diem,

lodging, University

of Oregon

$365

Total $6705

Budget Justification

Whole rock trace element ICP-MS data for an estimated 20 samples required to augment existing

data and be used as input for the computational modeling. Whole-rock Sr isotope data will be

collected on 18 samples at New Mexico State University. Dr. Frank Ramos establishes the

charge at $120/sample. Airfare is estimated to be ~$340 and I plan to work at NMSU for 20 days

in order to complete column chemistry and mass spectrometry runs. Sr isotopes are a critical

data type because existing data demonstrate a range that has been interpreted to indicate

involvement of crustal contaminants and two distinct mantle sources (Figure 1B, e.g., Wolff et

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al. 2008). Oxygen isotope data will be collected on 20 samples (multiple minerals, for a total of

40 analyses) at $21 each (cost based on email from Dr. Bindeman). Mileage, per diem and hotel

for University of Oregon trip is thus also included in the budget. O isotopes are also a critical

data type because δ18O values along with 87Sr/86Sr data can provide important compositional

characterization that serve as input into the magmatic system (e.g. primitive basaltic magmas

derived from the upper mantle will have different δ18O values than those found in accreted

terrane crust). Thus, O and Sr isotopic data are essential for evaluating the roles of recharge and

assimilation and will be used as input into computational models. Data will also be used to

evaluate model output and define the best-fit models.

Figures

 

 

 

 

 

 

 

 

 

 

 

 

 

 

A

N2

R1

N1

R1

N0

R0

Paleom

agne

ticStratig

raph

y

16.73 Ma (SR)

B

Figure 1-(A) Map of the extent of the Columbia Flood Basalts (CFB). Solid red line is approximate areal extent of Lower Steens, which likely extends north under Imnaha; dashed black line is Upper Steens. Modified from Camp & Ross (2004) and Camp & Hanan (2008). (B) - Schematic regional stratigraphy for the Columbia CFB. Paleomagnetic stratigraphy and Steens Reversal (SR) from Jarboe et al. (2008; 2010). Modified from Camp & Hanan (2008) and Wolff & Ramos (2013).  

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Figure 2- Compositional data of Lower and Upper Steens (Camp et al., 2013).

B 0

200

400

600

800

1000

1200

45 50 55

Height  in  Se

ction  [

m]

SiO2wt.%

Upper  SteensLower  Steens

0 100 200 300Ni  ppm

0 10 20 30 40La  ppm

0.703530.70384

0.70363

0.70333

0.703410.70351

0.70355

0.70369

0.703740.70398

2 4 6 8 10 12MgO  wt.%

0

200

400

600

800

1000

1200

45 50 55

Height  in  Se

ction  [

m]

SiO2wt.%

Upper  SteensLower  Steens

0 100 200 300Ni  ppm

0 10 20 30 40La  ppm

0.703530.70384

0.70363

0.70333

0.703410.70351

0.70355

0.70369

0.703740.70398

2 4 6 8 10 12MgO  wt.% A

Figure 3 –(A) Trace element composition of the Lower and Upper Steens as a function of stratigraphic position at Steens Mountain (data from Johnson et al. 1998; Wolff & Ramos 2013). (B) - Pb-Sr isotopic variations in the Columbia River Basalt lavas (Wolff & Ramos, 2013). Lower Steens Basalt (unfilled red boxes); Upper Steens Basalt (filled red boxes) (Bohrson & Spera 2001). Compositional data from other flows of the CRBs are in gray. Figure adapted from Grunder and Moore (pers. comm. 2013).

0

200

400

600

800

1000

1200

45 50 55

Height  in  Se

ction  [m

]

SiO2wt.%

Upper  SteensLower  Steens

0 100 200 300Ni  ppm

0 10 20 30 40La  ppm

0.703530.70384

0.70363

0.70333

0.703410.70351

0.70355

0.70369

0.703740.70398

2 4 6 8 10 12MgO  wt.%

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Figure 4- Plot of 206Pb/204Pb and Sr concentration vs. 87Sr/86Sr for the Skye magmatic system. Chemical data are shown in colored symbols and best-fit EC-RAFC trends are shown in the black and gray lines (Fowler et al, 2004).

 

 

 

 

 

 

 

 

 

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45# 50# 55#SiO2%wt.%%

0.7030#0.7035#0.7040#0.7045#87Sr/86Sr%

0# 10# 20# 30# 40#La%ppm%

0#

200#

400#

600#

800#

1000#

1200#

2# 4# 6# 8# 10# 12#

Height%in%Sec9o

n%[m

]#

MgO%wt.%%

Upper#Steens#Lower#Steens#

50# 60# 70# 80# 90#Olivine'Fo'Content'

###Phenocryst#(core,#rim)####Microphenocryst####Groundmass#

Figure 5- Silica and Sr isotope compositions from the Lower and Upper Steens Basalt as correlated with stratigraphic position at Steens Mountain (Johnson et al. 1998; Wolff & Ramos 2013; Ramos et al. 2013). SiO2 plot has red circles enclosing the 5 proposed petrologic packages, defined distinct SiO2 abundance. Sr isotopic compositions for whole rock are plotted as symbols, and the black lines (through or near the symbols) indicate corresponding mineral 87Sr/86Sr. Figure Modified from Grunder and Moore (pers. comm. 2013).

0

200

400

600

800

1000

1200

45 50 55

Height  in  Se

ction  [m

]

SiO2wt.%

Upper  SteensLower  Steens

0 100 200 300Ni  ppm

0 10 20 30 40La  ppm

0.703530.70384

0.70363

0.70333

0.703410.70351

0.70355

0.70369

0.703740.70398

2 4 6 8 10 12MgO  wt.%