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Experimental constraints on rutile saturation during partial melting of metabasalt at the amphibolite to eclogite transition, with applications to TTG genesis

Xiong Xiaolin, Hans Keppler, Andreas Audetat, Gudmundur Gudfinnsson, Sun Weidong, Song Maoshuang, Xiao Wansheng and Yuan Li
Experimental constraints on rutile saturation during partial melting of metabasalt at the amphibolite to eclogite transition, with applications to TTG genesis
American Mineralogist (August 2009) 94 (8-9): 1175-1186

Abstract

TiO (sub 2) solubility in rutile-saturated felsic melts and coexisting minerals was determined at 1.5-3.5 GPa, 750-1250 degrees C, and 5-30 wt% H (sub 2) O. TiO (sub 2) solubility in the melt primarily increases with temperature and melt basicity; it increases slightly with water content in the melt, and it decreases with pressure. A general TiO (sub 2) solubility model was obtained and is expressed as: ln(TiO (sub 2) ) (sub melt) = ln(TiO (sub 2) ) (sub rutile) +1.701-(9041/T)-0.173P+0.348FM+0.016H (sub 2) O, where TiO (sub 2) and H (sub 2) O are in wt%, T is in Kelvin, P in GPa, and FM is the melt composition parameter given by FM=(1/Si).[Na+K+2(Ca+Fe+Mn+Mg)]/Al, in which the chemical symbols represent cation fractions. TiO (sub 2) solubility in amphibole, garnet, and clinopyroxene also increases with temperature and empirical equations describing this temperature dependence were derived. These data were used to assess the protolith TiO (sub 2) content required for rutile saturation during partial melting of hydrous metabasalt at the amphibolite to eclogite transition. The results show that only 0.8-1.0 wt% TiO (sub 2) is required for rutile saturation during low-degree (<20%) melting. Rutile is stable up to approximately 1150 degrees C with 1.6 wt% TiO (sub 2) in the protolith and 30-40% melting for dehydration melting and up to approximately 1050 degrees C and 50-60% melting for fluid-present melting. The data also show that 0.7-0.8 wt% TiO (sub 2) in the protolith is needed for rutile saturation during subsolidus dehydration. Therefore, nearly all basaltic protoliths in deep-crustal settings and subduction zones will be saturated with rutile during subsolidus dehydration and low-degree melting at hydrous conditions. Archean tonalites-trondhjemites-granites (TTG) are widely accepted to be the products of low-degree melting of metabasalts at the amphibolite to eclogite transition, with rutile being present in the residue. Comparison of natural TTG compositions with our experimental rutile solubility data indicates that the dominant TTG magmas were produced at temperatures of 750-950 degrees C, which requires that the partial melting occurred at hydrous conditions. Models involving melting at the base of oceanic plateaus are inadequate to explain TTG genesis because the plateau root zones are likely dominated by anhydrous cumulates. A slab-melting model satisfies the requirement of a hydrous metabasalt, which during subduction would melt to produce voluminous TTG melts under high Archean geothermal gradients. The geothermal gradients responsible are estimated to be between 10 and 19 degrees C/km based on a pressure range of 1.5-2.5 GPa for the amphibolite to eclogite transition.


ISSN: 0003-004X
EISSN: 1945-3027
Coden: AMMIAY
Serial Title: American Mineralogist
Serial Volume: 94
Serial Issue: 8-9
Title: Experimental constraints on rutile saturation during partial melting of metabasalt at the amphibolite to eclogite transition, with applications to TTG genesis
Affiliation: Chinese Academy of Sciences, Guangzhou Institute of Geochemistry, Guangzhou, China
Pages: 1175-1186
Published: 200908
Text Language: English
Publisher: Mineralogical Society of America, Washington, DC, United States
References: 67
Accession Number: 2009-085394
Categories: Igneous and metamorphic petrology
Document Type: Serial
Bibliographic Level: Analytic
Illustration Description: illus. incl. 4 tables
Secondary Affiliation: Universitaet Bayreuth, DEU, Federal Republic of Germany
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: 200946
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