Fluid-mediated mass transfer from a paleosubduction channel to its mantle wedge: Evidence from jadeitite in the Guatemala Suture Zone
Abstract
Jadeitites in serpentinite mélanges typically crystallized from aqueous fluids that transferred components from a subduction channel into overlying mantle rocks [1]. Mélange bodies adjacent to the Motagua fault system (MFS) of the Guatemala Suture Zone contain evidence of this process. A set of 93 whole rock major and trace element analyses track aspects of the fluid transfer from the channel to the wedge at pressures from 1 to >2 GPa. Published data indicate transfer of alkali silicate and fluid-mobilized large-ion lithophile elements (LILE) such as Ba, Sr, and NH4 (and Li), [2,3,4,5] from both altered oceanic crust and overlying sediments in a paleosubduction setting. PUM-normalized REE patterns for jadeitites vary geographically along the 200 km long exhumation trace in the MFS. The grouped patterns are mostly enriched compared to PUM, variable with respect to N-MORB, and generally flat with a trend toward a concave shape. Both positive and negative Eu anomalies occur. These are probably controlled in part by pyroxene crystal chemistry, in which elements such as Li, Sr, Cr and Ca are variably compatible depending on jadeite content. Relative LREE enrichment in some samples may point to fluid contribution from either OIB or a sediment source. All samples show relative enrichments in the HFSE Ta, Nb, Hf, Zr, U, Th, and the LILE Ba and Cs, contrasted by depletions in K and sometimes Pb or Sr. Most samples are also depleted in the highly compatible elements Cr, Sc and Ni. Our samples show a strong similarity with GLOSS (globally subducted oceanic sediment) and other sediments in terms of their trace-element patterns, but are offset to lower abundances. Jadeitites thus incorporate a strong signature derived from sediments mixed with that from fluid derived from altered oceanic crust. Although many samples show little contribution from infiltrated mantle protolith, some are enriched in Cr and Ni. Enrichment in the HFSE argues for mobility of these elements in aqueous fluids at high P/T conditions in the subduction channel. Jadeite crystallization, while modifying fluids derived from oceanic crustal sources, does provide a tangible record. Variations in this record may reflect either process variation or geographic variation in sediment sampled laterally along the subduction zone. All Guatemalan jadeitites and omphacitites are interpreted as fluid precipitates; some are depleted in LREE, while others are enriched. Thus, LREE enrichment is not an indicator of the formation process of jadeitites (fluid precipitated vs. metasomatic; [6]). Instead, LREE enrichment may be an indicator of sediments in the source of the fluids that precipitated jadeitite. References: [1] Tsujimori & Harlow 2012, Eur. J. Mineral. 24, 371-390; [2] Sorensen et al. 2006; Am. Mineral. 91, 979-996; [3] Sorensen et al. 2010, Int. Geo. Rev. 52(9), 899-940; [4] Simons et al. 2010, Geochim. Cosmochim. Acta 74, 3621-3641; [5] Harlow et al. 2007, Abstracts w/ Programs, GSA An. Meeting 39(6), 393; [6] Shigeno et al. 2012, Eur. J. Mineral. 24, 247-262.
- Publication:
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AGU Fall Meeting Abstracts
- Pub Date:
- December 2012
- Bibcode:
- 2012AGUFM.V41D..01H
- Keywords:
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- 1031 GEOCHEMISTRY / Subduction zone processes;
- 1065 GEOCHEMISTRY / Major and trace element geochemistry;
- 3613 MINERALOGY AND PETROLOGY / Subduction zone processes