NDLI: Amphibole stability in primitive arc magmas: effects of temperature, H2O content, and oxygen fugacity

Content Provider	Springer Nature Link
Author	Krawczynski, Michael J. Grove, Timothy L. Behrens, Harald
Copyright Year	2012
Abstract	The water-saturated phase relations have been determined for a primitive magnesian andesite (57 wt% SiO2, 9 wt% MgO) from the Mt. Shasta, CA region over the pressure range 200–800 MPa, temperature range of 915–1,070 °C, and oxygen fugacities varying from the nickel–nickel oxide (NNO) buffer to three log units above NNO (NNO+3). The phase diagram of a primitive basaltic andesite (52 wt% SiO2, 10.5 wt% MgO) also from the Mt. Shasta region (Grove et al. in Contrib Miner Petrol 145:515–533; 2003) has been supplemented with additional experimental data at 500 MPa. Hydrous phase relations for these compositions allow a comparison of the dramatic effects of dissolved H2O on the crystallization sequence. Liquidus mineral phase stability and appearance temperatures vary sensitively in response to variation in pressure and H2O content, and this information is used to calibrate magmatic barometers-hygrometers for primitive arc magmas. H2O-saturated experiments on both compositions reveal the strong dependence of amphibole stability on the partial pressure of H2O. A narrow stability field is identified where olivine and amphibole are coexisting phases in the primitive andesite composition above 500 MPa and at least until 800 MPa, between 975–1,025 °C. With increasing H2O pressure ( $${P}_{\text {H}_2{\rm O}}$$ ), the temperature difference between the liquidus and amphibole appearance decreases, causing a change in chemical composition of the first amphibole to crystallize. An empirical calibration is proposed for an amphibole first appearance barometer-hygrometer that uses Mg# of the amphibole and $$f_{\text {O}_2}$$ : $$ P_{\text{H}_{2}{\rm O}}({\rm MPa})=\left[{\frac{{\rm Mg\#}}{52.7}}-0.014 * Δ {\rm NNO}\right]^{15.12} $$ This barometer gives a minimum $${P}_{\text{H}_{2}{\rm O}}$$ recorded by the first appearance of amphibole in primitive arc basaltic andesite and andesite. We apply this barometer to amphibole antecrysts erupted in mixed andesite and dacite lavas from the Mt. Shasta, CA stratocone. Both high H2O pressures (500–900 MPa) and high pre-eruptive magmatic H2O contents (10–14 wt% H2O) are indicated for the primitive end members of magma mixing that are preserved in the Shasta lavas. We also use these new experimental data to explore and evaluate the empirical hornblende barometer of Larocque and Canil (2010).
Starting Page	317
Ending Page	339
Page Count	23
File Format	PDF
ISSN	00107999
Journal	Contributions to Mineralogy and Petrology
Volume Number	164
Issue Number	2
e-ISSN	14320967
Language	English
Publisher	Springer-Verlag
Publisher Date	2012-04-07
Publisher Place	Berlin/Heidelberg
Access Restriction	One Nation One Subscription (ONOS)
Subject Keyword	Amphibole Experimental petrology Shasta Water in magma Subduction zone volcanism Mineralogy Geology Mineral Resources
Content Type	Text
Resource Type	Article
Subject	Geochemistry and Petrology Geophysics

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