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High-pressure phase transitions in MgCr (sub 2) O (sub 4) .Mg (sub 2) SiO (sub 4) composition; reactions between olivine and chromite with implications for ultrahigh-pressure chromitites

Masaki Akaogi, Airi Kawahara, Hiroshi Kojitani, Kazuaki Yoshida, Yuki Anegawa and Takayuki Ishii
High-pressure phase transitions in MgCr (sub 2) O (sub 4) .Mg (sub 2) SiO (sub 4) composition; reactions between olivine and chromite with implications for ultrahigh-pressure chromitites
American Mineralogist (January 2018) 103 (1): 161-170

Abstract

Phase relations in the Mg (sub 2) SiO (sub 4) -MgCr (sub 2) O (sub 4) system were investigated in the pressure range of 9.5-27 GPa at 1600 degrees C to examine the possible deep mantle origin of ultrahigh-pressure (UHP) chromitites in ophiolites. The experimental results indicate that MgCr (sub 2) O (sub 4) -rich chromite (Ch) coexists with Mg (sub 2) SiO (sub 4) -rich olivine (Ol) below approximately 13.5 GPa in the equimolar Mg (sub 2) SiO (sub 4) .MgCr (sub 2) O (sub 4) composition. Above approximately 13.5 GPa, they react to form a three-phase assemblage: garnet (Gt) solid solution in the Mg (sub 4) Si (sub 4) O (sub 12) -Mg (sub 3) Cr (sub 2) Si (sub 3) O (sub 12) system, modified ludwigite (mLd)-type Mg (sub 2) Cr (sub 2) O (sub 5) phase and Mg (sub 14) Si (sub 5) O (sub 24) -rich anhydrous phase B (Anh-B). At approximately 19.5 GPa, Anh-B is replaced by Mg (sub 2) SiO (sub 4) -rich wadsleyite (Wd). At 22 GPa, MgCr (sub 2) O (sub 4) -rich calcium titanate (CT) phase coexists with Mg (sub 2) SiO (sub 4) -rich ringwoodite (Rw). The assemblage of CT+Rw changes to CT + MgSiO (sub 3) -rich bridgmanite (Brg) + MgO periclase at 23 GPa. These sequential phase changes indicate that Ch+Ol do not directly transform to CT+Rw but to the three-phase assemblage, Gt+mLd+Anh-B (or Wd), that becomes stable at pressures corresponding to the upper and middle parts of the mantle transition zone. Our results suggest that the UHP chromitites that have been studied so far did not reach transition zone depths during mantle recycling processes of the chromitites, because there is no evidence of the presence of the reaction products of Ol and Ch. If the reaction products, in particular mLd and Anh-B, are found in the UHP chromitites, they are good indicators to estimate the subduction depth of the chromitites.


ISSN: 0003-004X
EISSN: 1945-3027
Coden: AMMIAY
Serial Title: American Mineralogist
Serial Volume: 103
Serial Issue: 1
Title: High-pressure phase transitions in MgCr (sub 2) O (sub 4) .Mg (sub 2) SiO (sub 4) composition; reactions between olivine and chromite with implications for ultrahigh-pressure chromitites
Affiliation: Gakushuin University, Department of Chemistry, Tokyo, Japan
Pages: 161-170
Published: 201801
Text Language: English
Publisher: Mineralogical Society of America, Washington, DC, United States
References: 50
Accession Number: 2018-035524
Categories: Geophysics of minerals and rocks
Document Type: Serial
Bibliographic Level: Analytic
Illustration Description: illus. incl. 3 tables
Country of Publication: United States
Secondary Affiliation: GeoRef, Copyright 2018, American Geosciences Institute. Abstract, copyright, Mineralogical Society of America. Reference includes data from GeoScienceWorld, Alexandria, VA, United States
Update Code: 2018
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