The role of aqueous fluids in the slab-to-mantle transfer of boron, beryllium, and lithium during subduction: Experiments and models
- Univ. of Toronto, Ontario (Canada). Dept. of Geology
- Lawrence Livermore National Lab., CA (United States)
The low atomic mass elements B, Be, and Li are viewed as sensitive tracers of the involvement of subducted materials in the genesis of island arc magmas. In order to better assess the role of dense aqueous fluids in the slab-to-mantle transfer of these elements during subduction, measurements have been made of partition coefficients for B, Be, and Li between aqueous fluid and minerals likely to be present in the basaltic portion of the downgoing slab, namely clinopyroxene and garnet. Experiments at 900 C and 2.0 GPa reveal that the average clinopyroxene-fluid partition coefficient for Be ({approximately}2) exceeds that for either Li ({approximately}0.2) or B ({approximately}0.02) and values are 100{times} (B,Li) to 1,000{times} (Be) larger than partition coefficients for garnet. Clinopyroxene-fluid partition coefficients were found to vary with the alumina content of run-product clinopyroxenes, but this variation is interpreted to reflect the specific exchange reaction that governs the incorporation of these elements into the pyroxene structure, and not mineral-fluid disequilibrium. Calculations indicate that by the time the slab reaches a depth of 200 km, B/Be and B/Nb in the dehydration residue has been reduced to {approximately}5--12% of initial values. Thus, the preferential loss of B during dehydration is viewed as a viable mechanism to prevent the excess B acquired during near-surface alteration of oceanic crust from being cycled into the mantle, thereby maintaining the distinction in B/Be and B/Nb for mantle and crustal reservoirs.
- Sponsoring Organization:
- USDOE, Washington, DC (United States); Natural Sciences and Engineering Research Council of Canada, Ottawa, ON (Canada)
- DOE Contract Number:
- W-7405-ENG-48
- OSTI ID:
- 338615
- Journal Information:
- Geochimica et Cosmochimica Acta, Vol. 62, Issue 19-20; Other Information: PBD: Oct 1998
- Country of Publication:
- United States
- Language:
- English
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