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 language | English |
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Pages (from-to) | 407-410 |
Number of pages | 4 |
Journal | Geology |
Volume | 31 |
Issue number | 5 |
DOIs | |
Publication status | Published - May 1 2003 |
Keywords
- B/B
- Metamorphism
- Secondary ion mass spectrometry
- Subduction zones
- Tourmaline
ASJC Scopus subject areas
- Geology