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Effect of alkalis on the reaction of clinopyroxene with Mg-carbonate at 6 GPa; implications for partial melting of carbonated lherzolite

Anton Shatskiy, Ivan V. Podborodnikov, Anton V. Arefiev, Konstantin D. Litasov, Artem D. Chanyshev, Igor S. Sharygin, Nikolai S. Karmanov and Eiji Ohtani
Effect of alkalis on the reaction of clinopyroxene with Mg-carbonate at 6 GPa; implications for partial melting of carbonated lherzolite
American Mineralogist (September 2017) 102 (9): 1934-1946

Abstract

The reaction between clinopyroxene and Mg-carbonate is supposed to define the solidus of carbonated lherzolite at pressures exceeding 5 GPa. To investigate the effect of alkalis on this reaction, subsolidus and melting phase relations in the following systems have been examined at 6 GPa: CaMgSi (sub 2) O (sub 6) +2MgCO (sub 3) (Di+2Mgs); CaMgSi (sub 2) O (sub 6) +NaAlSi (sub 2) O (sub 6) +2MgCO (sub 3) (Di+Jd+2Mgs); CaMgSi (sub 2) O (sub 6) +Na (sub 2) Mg(CO (sub 3) ) (sub 2) (Di+Na (sub 2) Mg); and CaMgSi (sub 2) O (sub 6) +K (sub 2) Mg(CO (sub 3) ) (sub 2) (Di+K (sub 2) Mg). The Di+2Mgs system begins to melt at 1400 degrees C via the approximate reaction CaMgSi (sub 2) O (sub 6) (clinopyroxene) + 2MgCO (sub 3) (magnesite) = CaMg(CO (sub 3) ) (sub 2) (liquid) + Mg (sub 2) Si (sub 2) O (sub 6) (orthopyroxene), which leads to an essentially carbonate liquid (L) with composition Ca (sub 0.56) Mg (sub 0.44) CO (sub 3) + 3.5 mol% SiO (sub 2) The initial melting of the Di+Jd+2Mgs system occurs at 1350 degrees C via the reaction 2CaMgSi (sub 2) O (sub 6) (clinopyroxene) + 2NaAlSi (sub 2) O (sub 6) (clinopyroxene) + 8MgCO (sub 3) (magnesite) = Mg (sub 3) Al (sub 2) Si (sub 3) O (sub 12) (garnet) + 5MgSiO (sub 3) (clinopyroxene) + 2CaMg(CO (sub 3) ) (sub 2) (liquid) + Na (sub 2) CO (sub 3) (liquid) + 3CO (sub 2) (liquid and/or fluid), which yields the carbonate liquid with approximate composition of 10Na (sub 2) CO (sub 3) .90Ca (sub 0.5) Mg (sub 0.5) CO (sub 3) + 2 mol% SiO (sub 2) . The systems Di+Na (sub 2) Mg and Di+K (sub 2) Mg start to melt at 1100 and 1050 degrees C, respectively, via the reaction CaMgSi (sub 2) O (sub 6) (clinopyroxene) + 2(Na or K) (sub 2) Mg(CO (sub 3) ) (sub 2) (solid) = Mg (sub 2) Si (sub 2) O (sub 6) (orthopyroxene) + (Na or K) (sub 4) CaMg(CO (sub 3) ) (sub 4) (liquid). The resulting melts have the alkali-rich carbonate compositions Na (sub 2) Ca (sub 0.4) Mg (sub 0.6) (CO (sub 3) ) (sub 2) + 0.4 mol% SiO (sub 2) and 43 K (sub 2) CO (sub 3) .57Ca (sub 0.4) Mg (sub 0.6) CO (sub 3) + 0.6 mol% SiO (sub 2) . These melts do not undergo significant changes as temperature rises to 1400 degrees C, retaining their calcium number and a high Na (sub 2) O, K (sub 2) O, and low SiO (sub 2) . We suggest that the clinopyroxene-Mg-carbonate reaction controlling the solidus of carbonated lherzolite is very sensitive to the carbonate composition and shifts from 1400 to 1050 degrees C at 6 GPa, which yields K-rich carbonate melt if the subsolidus assemblage contains the K (sub 2) Mg(CO (sub 3) ) (sub 2) compound. Such a decrease in solidus temperature has been previously observed in the K-rich carbonated lherzolite system. Although a presence of eitelite, Na (sub 2) Mg(CO (sub 3) ) (sub 2) , has a similar effect, this mineral cannot be considered as a potential host of Na in carbonated lherzolite, because the whole Na added into the system dissolves as jadeite component in clinopyroxene if bulk Al/Na > or = 1. The presence of jadeite component in clinopyroxene has little impact on the temperature of the solidus reaction decreasing it to 1350 degrees C at 6 GPa.


ISSN: 0003-004X
EISSN: 1945-3027
Coden: AMMIAY
Serial Title: American Mineralogist
Serial Volume: 102
Serial Issue: 9
Title: Effect of alkalis on the reaction of clinopyroxene with Mg-carbonate at 6 GPa; implications for partial melting of carbonated lherzolite
Affiliation: Russian Academy of Science, V.S. Sobolev Institute of Geology and Mineralogy, Novosibirsk, Russian Federation
Pages: 1934-1946
Published: 201709
Text Language: English
Publisher: Mineralogical Society of America, Washington, DC, United States
References: 43
Accession Number: 2017-082717
Categories: Geophysics of minerals and rocks
Document Type: Serial
Bibliographic Level: Analytic
Illustration Description: illus.
Secondary Affiliation: Novosibirsk State University, RUS, Russian FederationTohoku University, JPN, Japan
Country of Publication: United States
Secondary Affiliation: GeoRef, Copyright 2017, American Geosciences Institute. Abstract, copyright, Mineralogical Society of America. Reference includes data from GeoScienceWorld, Alexandria, VA, United States
Update Code: 201743
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