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The effects of oxygen fugacity and sulfur on the pressure of vapor-saturation of magma

Ery C. Hughes, Philippa Liggins, Lee Saper and Edward M. Stolper
The effects of oxygen fugacity and sulfur on the pressure of vapor-saturation of magma
American Mineralogist (March 2024) 109 (3): 422-438


Geobarometers are commonly used to determine the pressure (and hence depth) of magmatic bodies. For instance, at equilibrium, the concentration of dissolved volatiles in a vapor-saturated melt can be used as a barometer: this is the pressure of vapor-saturation (P (sub sat) (super upsilon ) ). Most determinations of P (sub sat) (super upsilon ) assume that melt and vapor contain only oxidized C-O-H species. However, sulfur is the third most abundant volatile element in magmas, and oxygen fugacity (f (sub O2) ) exerts a strong influence on the speciation of the melt and vapor. To explore how S and f (sub O2) affect calculations of P (sub sat) (super upsilon ) model a Hawaiian tholeiite that contains both reduced and oxidized C-O-H-S species in the melt and vapor. We find that excluding reduced C-O-H species in the system can result in significant underestimations of P (sub sat) (super upsilon ) under reducing conditions (Delta FMQl<0). The effect of S on P (sub sat) (super upsilon ) is small except in the vicinity of the "sulfur solubility minimum" (SS (super min) ; 0<Delta FMQ<+2), where excluding S-bearing species can result in underestimates of P (sub sat) (super upsilon ) . The implications of these results depend on the volatile concentration of the system being investigated, its f (sub O2) , and the melt composition and temperature. Our results suggest there will be little impact on P (sub sat) (super upsilon ) calculated for mid-ocean ridge basalts because their f (sub O2) is above where reduced C-O-H species become important in the melt and vapor and yet below the SS (super min) . However, the f (sub O2) of ocean island and arc basalts are close enough to the SS (super min) and their S concentrations high enough to influence P (sub sat) (super upsilon ) . However, high-CO (sub 2) and high-H (sub 2) O concentrations are predicted to reduce the effect of the SS (super min) . Hence, P (sub sat) (super upsilon ) calculated for shallowly trapped melt inclusions and matrix glass are more affected by the SS (super min) than deeply trapped melt inclusions. Lunar and martian magmas are typically more reduced than terrestrial magmas, and therefore accurate P (sub sat) (super upsilon ) calculations for them require the inclusion of reduced C-O-H species.

ISSN: 0003-004X
EISSN: 1945-3027
Serial Title: American Mineralogist
Serial Volume: 109
Serial Issue: 3
Title: The effects of oxygen fugacity and sulfur on the pressure of vapor-saturation of magma
Affiliation: Te Pu Ao-GNS Science, National Isotope Centre, Lower Hutt, New Zealand
Pages: 422-438
Published: 202403
Text Language: English
Publisher: Mineralogical Society of America, Washington, DC, United States
References: 190
Accession Number: 2024-023624
Categories: Igneous and metamorphic petrology
Document Type: Serial
Bibliographic Level: Analytic
Illustration Description: illus. incl. 2 tables
Secondary Affiliation: University of Cambridge, GBR, United KingdomCalifornia Institute of Technology, USA, United States
Country of Publication: United States
Secondary Affiliation: GeoRef, Copyright 2024, American Geosciences Institute. Abstract, copyright, Mineralogical Society of America. Reference includes data from GeoScienceWorld, Alexandria, VA, United States
Update Code: 202414
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