ISOTOPE GEOCHEMISTRY OF METASOMATIC MINERALS IN SUBCONTINENTAL MANTLE Intellectual Merit Boron is a fluid-transported element that undergoes significant isotope fractionation at the low temperatures of Earths surface. It is therefore a potentially powerful tracer of crust-mantle interactions, particularly those involving a hydrous fluid phase. As such, it may also be a convenient proxy tracer of the global H cycle. Although significant advances in understanding the global deep B cycle have been made in recent years by the analysis of B in oceanic basalts, this approach is sometimes hampered by the apparent tendency of these magmas to assimilate B from seawater-altered oceanic crust. We propose, instead, to study the partitioning of B and its isotopes among mantle reservoirs by the analysis of B in minerals from xenoliths derived from the subcontinental lithospheric mantle (SCLM). These samples offer advantages: (1) their B is not notably affected by crustal processes and (2) their isotopic variations (including nonboron elements) are often much greater than those in oceanic basalts, thus offering more precise estimation of the compositions of end-member components, (3) the isotopic effects of specific mantle processes can be traced in cogenetic samples, (4) metasomatic assemblages deriving from both plume and subduction sources occur (5) SCLM archives the signatures of metasomatic fluids and melts over time, thus potentially recording secular changes. We propose to use SIMS to measure the B content and isotope composition of phlogopite and associated metasomatic minerals, in order to determine 11B/10B of old, recycled LIL-depleted oceanic crust (HIMU component), the enriched mantle components (EMI, EMII), the deep mantle source of kimberlites, and of magma sources putatively related to the African Superplume. These measurements will permit us to test for secular variations in the isotopic composition of the SCLM and to test the hypotheses that: (1) the HIMU recycled component in the mantle has been depleted in B (and dehydrated), and (2) recycled SCLM can account for 11B/10B in EM-type oceanic basalts. The initial focus will be on well-documented xenoliths from the Kaapvaal Craton of South Africa where three generations of metasomatism dominated by HIMU, EMI and EMII type components occur. Isotopic compositions of Pb in mica and Sr in coexisting clinopyroxene will be determined by LA-MC-ICP-MS in collaboration with P. Janney and P. le Roux at the University of Cape Town. In collaboration with Dr. J. Wartho at ASU, ages of micas will be determined or constrained by laser Ar-Ar profiles across large grains that retain age information in their cores, permitting us to seek evidence for secular evolution of B isotope composition of the SCLM that may reflect changing recycling conditions over 3 Ga. The study will be extended to other SCLM samples and to comparative material from suboceanic mantle. The results will provide new insights into the contribution of recycled materials to mantle reservoirs with particular reference to their histories of hydration and dehydration.
|Effective start/end date||7/1/13 → 6/30/19|
- National Science Foundation (NSF): $249,961.00