The effect of metamorphism on the trace element composition of subducted oceanic crust and sediment

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dc.contributor.advisorLeeman, William P.en_US
dc.creatorMoran, Ann Elizabethen_US
dc.date.accessioned2009-06-04T00:37:51Zen_US
dc.date.available2009-06-04T00:37:51Zen_US
dc.date.issued1993en_US
dc.description.abstractMetasedimentary and metabasaltic rocks of the Pelona schist of Sierra Pelona, southern California, preserve a relatively high P/T inverted metamorphic gradient (Graham and Powell, 1984) and provide an opportunity to study possible compositional changes in an oceanic slab progressively metamorphosed under P-T conditions similar to those in shallow parts of some subduction zones. Progressive metamorphism resulted in continuous major element compositional change in plagioclase, white mica, amphibole, and epidote. Variations in whole-rock compositions do not correlate with increasing metamorphic grade and largely appear to reflect protolith heterogeneity. Loss of H$\sb2$O-rich fluid during chlorite breakdown reactions largely accounts for the decrease in weight percent LOI and H content with increasing metamorphic grade. Comparison with unmetamorphosed equivalents and lower temperature, high-pressure metamorphic rocks (including metabasaltic samples from the Shuksan schist and the Franciscan Complex) suggest that As and Sb may be lost from metasedimentary rocks and that B may be lost from metabasaltic rocks at temperatures less than 450$\sp\circ$C. Trace element analyses obtained by ion microprobe for minerals in the Pelona, Catalina, and Shuksan schists document the mineral residencies of trace elements and the redistribution of trace elements among minerals as modal abundances vary. Boron, Ba, Li, Rb, and Cs are strongly concentrated in micas. In samples with coexisting white mica and biotite, higher concentrations of B, Ba, and Sr are observed in white mica, relative to Li, Rb, and Cs, which preferentially substitute into biotite. Cesium and Rb partition in a relatively constant ratio between white mica and biotite. Strontium is strongly concentrated in epidote, but is increasingly incorporated into white mica and plagioclase as the modal abundance of epidote decreases and as plagioclase compositions become more calcic. Similarly, Li appears to be repartitioned into amphibole as chlorite decreases in modal abundance. These observations support a model of gradual release of fluid-mobile trace elements during progressive metamorphism (as from subducted slabs). Prograde continuous reactions involving mica may particularly control the mobility of alkali and alkaline earth elements enriched in arc magmas and provide a mechanism for fractionating incompatible element ratios (e.g. B/Cs) during subduction zone metamorphism.en_US
dc.format.extent378 p.en_US
dc.format.mimetypeapplication/pdfen_US
dc.identifier.callnoThesis Geol. 1993 Moranen_US
dc.identifier.citationMoran, Ann Elizabeth. "The effect of metamorphism on the trace element composition of subducted oceanic crust and sediment." (1993) Diss., Rice University. <a href="https://hdl.handle.net/1911/16653">https://hdl.handle.net/1911/16653</a>.en_US
dc.identifier.urihttps://hdl.handle.net/1911/16653en_US
dc.language.isoengen_US
dc.rightsCopyright is held by the author, unless otherwise indicated. Permission to reuse, publish, or reproduce the work beyond the bounds of fair use or other exemptions to copyright law must be obtained from the copyright holder.en_US
dc.subjectGeochemistryen_US
dc.subjectMineralogyen_US
dc.subjectGeologyen_US
dc.titleThe effect of metamorphism on the trace element composition of subducted oceanic crust and sedimenten_US
dc.typeThesisen_US
dc.type.materialTexten_US
thesis.degree.departmentEarth Scienceen_US
thesis.degree.disciplineNatural Sciencesen_US
thesis.degree.grantorRice Universityen_US
thesis.degree.levelDoctoralen_US
thesis.degree.nameDoctor of Philosophyen_US
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