Record in metamorphic tourmalines of subduction-zone devolatilization and boron cycling

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Abstract

Boron concentrations and isotope compositions of fluids and lavas from subduction-zone settings show great potential for elucidating mass flux at Earth's modern convergent margins. However, the fluid-mineral-melt behavior of B and its two stable isotopes remains relatively poorly understood. Boron isotope analyses of tourmaline in metasedimentary rocks subducted to 15-90 km depths (1) demonstrate the ability of this mineral to retain information regarding prograde devolatilization history in even highly retrograded rocks and (2) indicate the importance of tourmaline in affecting whole-rock B loss and B isotope evolution during subduction-zone metamorphism. The B lost from micas during metamorphism of subducting sedimentary rocks and altered oceanic crust is isotopically more enriched in 11B than the B retained in the micas. Beneath forearcs and volcanic arcs, the B from micas is either removed from the subduction-zone rocks via metamorphic fluids or sequestered by growing tourmaline, in which the B can be entrained to even greater depths. Here we demonstrate that these metamorphic fluids could contribute to the relatively high δ11B signatures observed in most arc lavas and the across-arc trends of decreasing δ11B observed in several arcs.

Original languageEnglish
Pages (from-to)407-410
Number of pages4
JournalGeology
Volume31
Issue number5
DOIs
Publication statusPublished - May 2003

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Keywords

  • B/B
  • Metamorphism
  • Secondary ion mass spectrometry
  • Subduction zones
  • Tourmaline

ASJC Scopus subject areas

  • Geology

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