TY - JOUR

T1 - Quantification of the CO2 budget and H2O-CO2 systematics in subduction-zone magmas through the experimental hydration of melt inclusions in olivine at high H2O pressure

AU - Mironov, Nikita

AU - Portnyagin, Maxim

AU - Botcharnikov, Roman

AU - Gurenko, Andrey

AU - Hoernle, Kaj

AU - Holtz, François

N1 - Funding Information: We are grateful to S. Simakin and E. Potapov for SIMS trace element analyses in Yaroslavl; M. Thöner for assistance with electron microprobe in Kiel; S. Fanara, A. Husen, A. Stechern and F. Vetere for experimental assistance; R. Almeev, H. Behrens, V. Naumov, A. Sobolev and Y. Taran for helpful discussions. T. Plank, an anonymous reviewer and editor T. Mather are sincerely thanked for providing insightful comments and corrections to the earlier version of this manuscript. Experiments were performed under support of the DFG grant HO1337/29 . The Russian Science Foundation grant No. 14-17-00582 and RFBR grant No. 12-05-01131 provided support to N.M. and M.P. during manuscript preparation. The analytical costs were covered through the GEOMAR Helmholtz Center funding. Publisher Copyright: © 2015 Elsevier B.V. Copyright: Copyright 2019 Elsevier B.V., All rights reserved.

PY - 2015/9/1

Y1 - 2015/9/1

N2 - Reliable evaluation of CO2 contents in parental arc magmas, which can be preserved in melt inclusions in phenocrysts, is required to verify the proposed efficiency of CO2 recycling at convergent margins. Quantification of bulk CO2 concentration in melt inclusions requires their complete homogenization. Using samples from lavas from the Bulochka vent of Klyuchevskoy Volcano (Kamchatka), we applied a novel experimental approach to homogenize and re-equilibrate naturally dehydrated (<1 wt.% H2O) melt inclusions from high-Fo (85-91 mol.%) olivine. The experiments were performed at temperatures of 1150-1400 °C, pressures of up to 500 MPa, under dry to H2O-saturated conditions and with oxygen fugacity ranging from CCO to QFM+3.3. No homogenization was achieved at dry conditions. Complete dissolution of fluid bubbles (homogenization) in the melt inclusions was achieved at H2O pressures of 500 MPa and temperature of 1150 °C, when water content in the melt inclusions reached 4-5 wt.% H2O. The CO2 content in the homogenized inclusions is 3800±140ppm and CO2/Nb = 3000 ± 420, which are the highest values reported so far for the typical middle-K primitive arc melts and fall within the range of values inferred from the magmatic flux and volcanic gas data for primary arc magma compositions. About 83% of the CO2 in Klyuchevskoy magmas is likely to be derived from the subducting slab and can be attributed to flux melting with a fluid having a CO2/H2O ratio of ~0.06. The H2O and CO2 contents in the melt inclusions after hydrous experiments were found to correlate positively with each other and negatively with the volume of fluid bubble, reflecting increasing internal pressure in melt inclusions with increasing melt hydration. Therefore, similar trends observed in some natural sets of melt inclusions can be attributed to a partial dehydration of melts after entrapment, operating simultaneously with or following post-entrapment crystallization. Our study implies that the process of post-entrapment dehydration can be completely reversed under high pressure experimental conditions. If temperature, redox conditions and pressure of melt inclusion entrapment can be independently estimated, then our novel experimental approach (homogenization at high H2O pressure) can be used to reconstruct the initial CO2 content and also the entire composition of melt inclusions in olivine, including their initial H2O content, from any type of volcanic rock. With this approach volatiles in ancient lavas can also be determined, expanding our knowledge of volatile recycling further back in Earth history.

AB - Reliable evaluation of CO2 contents in parental arc magmas, which can be preserved in melt inclusions in phenocrysts, is required to verify the proposed efficiency of CO2 recycling at convergent margins. Quantification of bulk CO2 concentration in melt inclusions requires their complete homogenization. Using samples from lavas from the Bulochka vent of Klyuchevskoy Volcano (Kamchatka), we applied a novel experimental approach to homogenize and re-equilibrate naturally dehydrated (<1 wt.% H2O) melt inclusions from high-Fo (85-91 mol.%) olivine. The experiments were performed at temperatures of 1150-1400 °C, pressures of up to 500 MPa, under dry to H2O-saturated conditions and with oxygen fugacity ranging from CCO to QFM+3.3. No homogenization was achieved at dry conditions. Complete dissolution of fluid bubbles (homogenization) in the melt inclusions was achieved at H2O pressures of 500 MPa and temperature of 1150 °C, when water content in the melt inclusions reached 4-5 wt.% H2O. The CO2 content in the homogenized inclusions is 3800±140ppm and CO2/Nb = 3000 ± 420, which are the highest values reported so far for the typical middle-K primitive arc melts and fall within the range of values inferred from the magmatic flux and volcanic gas data for primary arc magma compositions. About 83% of the CO2 in Klyuchevskoy magmas is likely to be derived from the subducting slab and can be attributed to flux melting with a fluid having a CO2/H2O ratio of ~0.06. The H2O and CO2 contents in the melt inclusions after hydrous experiments were found to correlate positively with each other and negatively with the volume of fluid bubble, reflecting increasing internal pressure in melt inclusions with increasing melt hydration. Therefore, similar trends observed in some natural sets of melt inclusions can be attributed to a partial dehydration of melts after entrapment, operating simultaneously with or following post-entrapment crystallization. Our study implies that the process of post-entrapment dehydration can be completely reversed under high pressure experimental conditions. If temperature, redox conditions and pressure of melt inclusion entrapment can be independently estimated, then our novel experimental approach (homogenization at high H2O pressure) can be used to reconstruct the initial CO2 content and also the entire composition of melt inclusions in olivine, including their initial H2O content, from any type of volcanic rock. With this approach volatiles in ancient lavas can also be determined, expanding our knowledge of volatile recycling further back in Earth history.

KW - Experiment

KW - Kamchatka

KW - Melt inclusion

KW - Olivine

KW - Volatiles

UR - http://www.scopus.com/inward/record.url?scp=84930616079&partnerID=8YFLogxK

U2 - 10.1016/j.epsl.2015.05.043

DO - 10.1016/j.epsl.2015.05.043

M3 - Article

AN - SCOPUS:84930616079

VL - 425

SP - 1

EP - 11

JO - Earth and Planetary Science Letters

JF - Earth and Planetary Science Letters

SN - 0012-821X

ER -