Boron isotope variations of Franciscan serpentinites, northern California

Chinatsu Yamada, Tatsuki Tsujimori, Qing Chang, Jun Ichi Kimura

Research output: Contribution to journalArticlepeer-review

6 Citations (Scopus)

Abstract

Serpentinites and serpentinized mantle peridotites with various tectonic origins occur in the Franciscan Complex of the Northern California Cordillera, USA. Boron isotopes of serpentinites differentiate with fluid-mediated processes, and have great potential for key geologic markers in convergent margins. To understand boron isotope behavior within the Franciscan subduction zone system, we apply a newly developed ablation volume correction (AVC) method for in-situ isotope/elemental analyses using a laser-ablation multiple collector inductively-coupled-plasma mass spectrometry (LA-MC-ICPMS) on seventeen different Franciscan serpentinites (sensu lato) collected from eight separate areas. Boron abundances and isotope compositions of the studied serpentinites show large variations B = 1.6–239 μg·g −1 , δ 11 B = −12.0 to +24.4‰, which allow to discriminate the serpentinites into two groups: (1) a lighter δ 11 B of −12.0 to +8.8‰ with a lesser B < ~56 μg·g −1 and (2) a heavier δ 11 B of +7.2 to +24.4% with a greater B ~34–239 μg·g −1 . These groups lithologically correspond to the presence or absence of associated blueschist-facies metamorphic rocks, respectively. The blueschist-bearing and/or blueschist-associated serpentinites might have been affected by a deep forearc slab fluids in the depth of > ~2 GPa. Preferential partitioning of 11 B into fluids released from the subducted slab at shallow leaves lighter δ 11 B in the slab resulting in lighter δ 11 B in the deep slab fluids. In contrast, the blueschist-absent serpentinites with heavier δ 11 B may have formed at a shallow environment where shallow slab or hydrothermal fluids with heavier 11 B were present. Lesser versus greater amounts of B in the deep versus shallow serpentinites are also consistent with the dehydration profile of B from a slab. Our results show the versatility of boron isotopes and composition for identification of the origin of serpentinite in Pacific-type orogenic belts.

Original languageEnglish
Pages (from-to)180-189
Number of pages10
JournalLithos
Volume334-335
DOIs
Publication statusPublished - 2019 Jun 1

Keywords

  • Boron isotope
  • California Coast Ranges
  • Franciscan complex
  • Serpentinites

ASJC Scopus subject areas

  • Geology
  • Geochemistry and Petrology

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