Carbon in olivine: Results from nuclear reaction analysis

The solubility behavior of C in olivine has been investigated using the 12C(d, p)13C nuclear reaction analysis technique. The technique provides an in situ microanalysis of concentration as a function of depth below the surface. In the case of olivine the depth profile extends to approximately 2.6 μn. Microbeam capability is achieved by focusing the deuteron beam with a superconducting solenoid. However, thermal degradation of the sample imposes a practical limit on the lateral spatial resolution of approximately 150 μm. The detection limit for C in forsteritic olivine is 65 ppm (atomic) and limited mainly by important background contributions from both Mg and Si. Olivines of similar compositions (Fo90–92) but from diverse geologic environments were studied, including (l) olivine crystallized in the presence of saline hydrothermal fluid in the shallow crust, (2) upper mantle megacrysts and crystals separated from spinel Iherzolite xenoliths from alkali basalts, (3) natural crystals equilibrated with graphite and magnesite at 30 kbar and 1400°C, and (4) olivine from a diamond-bearing garnet Iherzolite xenolith from a kimberlite. Contrary to expectation based on previous experiments, none of the samples were found to contain C in concentrations greater than the detection limit. Concentration profiles generated by the use of combined sputtering and Auger techniques demonstrate that the adsorbed C-rich layer on polished surfaces is typically restricted to the upper 100 A of the surface.

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