In-situ Sr-Pb isotope geochemistry of lawsonite: A new method to investigate slab-fluids

Tomomi Hara, Tatsuki Tsujimori, Qing Chang, Jun Ichi Kimura

Research output: Contribution to journalArticlepeer-review

4 Citations (Scopus)

Abstract

Lawsonite is a hydrous Ca-Al silicate mineral which is broadly stable under modern style subduction conditions. In order to understand the elemental fractionation and the sources of subducted materials, lawsonite crystals in lawsonite-eclogite facies basaltic and sedimentary rocks from South Motagua Mélange of Guatemala were investigated using LA-ICPMS and LA-MC-ICPMS. Mass balance calculation based on in-situ trace element and modal compositions in an eclogite with a low-variance mineral assemblage confirmed that lawsonite hosts most of the light rare earth element (LREE), Sr, Pb, Th and U in the bulk. In-situ Sr-Pb isotope analyses of eclogite-facies lawsonite crystals revealed isotopic variations reflecting their protoliths. Isotopic zoning was also detected in some crystals. Lawsonite crystals in a phengite-rich eclogite have relatively low 87Sr/86Sr = 0.70335–0.70355 with variations in 207Pb/206Pb = 0.8401–0.8512 and 208Pb/206Pb = 2.0757–2.0911. Lawsonite crystals from another eclogite are remarkably zoned. The cores have 87Sr/86Sr = 0.70558–0.70601 and the rims have relatively elevated 87Sr/86Sr = 0.70636–0.70662, exhibiting MORB-like 207Pb/206Pb = 0.843–0.844 at the cores and more enriched 207Pb/206Pb = 0.839–0.841 at the rims. The radiogenic Sr isotope composition would have been derived from seafloor alteration before subduction. In fact, lawsonite crystals in a metachert have higher 87Sr/86Sr = 0.70697–0.70757 with 208Pb/206Pb = ∼2.07. However, the isotopically zoned metabasaltic lawsonite crystals with MORB-like core compositions suggest that metasomatism by an external fluid from sediment protolith occurred during overgrowth of their rims. Our study indicates that lawsonite crystals record both the isotopic compositions of the protoliths and metasomatic fluids from different protoliths. The in-situ Sr-Pb isotope analysis has a potential to reveal records of the complicated metamorphic processes.

Original languageEnglish
Pages (from-to)93-104
Number of pages12
JournalLithos
Volume320-321
DOIs
Publication statusPublished - 2018 Nov

Keywords

  • Lawsonite
  • South Motagua Mélange
  • Sr-Pb isotope
  • Subduction zone fluid
  • Trace element

ASJC Scopus subject areas

  • Geology
  • Geochemistry and Petrology

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