Trace element fractionation in deep subduction zones inferred from a lawsonite eclogite xenolith from the Colorado Plateau

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doi: 10.1016/j.chemgeo.2006.08.009
Authors:Usui, Tomohiro; Kobayashi, Katsura; Nakamura, Eizo; Helmstaedt, Hewart
Author Affiliations:Primary:
Okayama University at Misasa, Pleasant Memorial Laboratory, Tottori, Japan
Washington State University, United States
University of Bern, Switzerland
University of Tennessee, Knoxville, United States
Queen's University, Canada
Volume Title:Geochemical processes responsible for element mobility fom the slab to the surface
Volume Authors:Simon, Adam C., editor; King, Robert L.; Pettke, Thomas; Usui, Tomohiro
Source:Geochemical processes responsible for element mobility fom the slab to the surface, edited by Adam C. Simon, Robert L. King, Thomas Pettke and Tomohiro Usui. Chemical Geology, 239(3-4), p.336-351. Publisher: Elsevier, Amsterdam, Netherlands. ISSN: 0009-2541
Publication Date:2007
Note:In English. 42 refs.; illus., incl. 4 tables
Summary:A coesite-bearing, lawsonite-eclogite xenolith from the Colorado Plateau, interpreted as a fragment of the subducted Farallon plate, is used to characterize trace element behavior in subducted oceanic crust. The xenolith consists of almandine-rich garnet, omphacite, lawsonite, phengite, rutile, pyrite and zircon as the primary mineral assemblage. Garnet crystals are extremely zoned with respect to their Mn contents, with core to rim variation from ∼1.4 to ∼0.2 wt.%. The euhedral zoning feature of garnet crystals and its included mineral assemblages suggest that the garnet continued to grow in the coesite stability field during prograde lawsonite eclogite facies metamorphism. In the lawsonite-eclogite xenoliths, garnet dominates the heavy rare earth elements (HREE), and lawsonite dominates both light rare earth elements (LREE) and Sr inventories. Combining the mineralogical and petrographic observations with precise spatial resolution ion microprobe analyses (<15 µm) of zoned garnet as well as lawsonite inclusions in garnet, we investigated trace element fractionation in coesite stability field during lawsonite eclogite facies metamorphism. Garnet shows progressive HREE depletion from core to rim, suggesting that HREE, which once partitioned into garnet crystal, would not be involved in postdated metamorphic reactions due to the high partition coefficients of HREE into garnet. Lawsonite inclusions in garnet, which represent lower metamorphic condition relative to lawsonite in the matrix, have LREE concentrations ∼10 times lower than those of matrix lawsonite. On the contrary, the concentration of Sr in the included lawsonite is (<20 relative %) lower than that of the matrix lawsonite. Based on constraints from metamorphic history recorded in the prograde-zoned garnet and mass balance among all constituent minerals in the lawsonite-eclogite xenolith, this contrasting feature for Sr and LREE of lawsonite is most plausibly explained by the hypothesis that allanite coexisting with included lawsonite might have decomposed during prograde metamorphism. The LREE released from the decomposing allanite would have been incorporated into lawsonite crystals. Consequently, REE and Sr could be retained in subducting oceanic crust even in the coesite stability field, if the slab is sufficiently cold enough to pass though the lawsonite eclogite facies.
Sections:Geochemistry; Petrology
Subsections:Metamorphic rocks; Oceanic petrology
Subjects:Basalts; Chemical composition; Chemical fractionation; Coesite; Crust; Crystal chemistry; Eclogite; Farallon Plate; Framework silicates; Geochemistry; Igneous rocks; Inclusions; Ion probe data; Lawsonite; Major elements; Mass balance; Mass spectra; Metals; Metamorphic rocks; Metamorphism; Mid-ocean ridge basalts; Mineral assemblages; Mineral composition; Oceanic crust; Orthosilicates; Petrography; Plate tectonics; Prograde metamorphism; Rare earths; SEM data; Silica minerals; Silicates; Sorosilicates; Spectra; Subduction; Subduction zones; Textures; Trace elements; Volcanic rocks; Xenoliths; Colorado Plateau; United States
Abstract Numbers:07M/2011
Record ID:2008009835
Copyright Information:GeoRef, Copyright 2019 American Geosciences Institute. Reference includes data from CAPCAS, Elsevier Scientific Publishers, Amsterdam, Netherlands
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