Ni-rich olivine in minettes from Two Buttes, Colorado: A connection between potassic melts from the mantle and low Ni partition coefficients

Details Ni-rich olivine in minettes from Two Buttes, Colorado: A connection between potassic melts from the mantle and low Ni partition coefficients Ni-rich olivine in minettes from Two Buttes, Colorado: A connection between potassic melts from the mantle and low Ni partition coefficients

Jan 1993

adsabs.harvard.edu/cgi-bin/nph-data_query?bibcode=1993gecoa..57..123d&link_type=abstract

Geochimica et Cosmochimica Acta, vol. 57, Issue 1, pp.123-129

Computer Science

Scientific paper

Olivine minettes from Two Buttes, Colorado, contain diverse populations of olivine, some of which is unusually rich in Ni. Olivine compositions range from Fo 83 to Fo 94 with 0.13 to 0.64 wt% NiO. Host minettes have 15.5 to 17.9 wt% MgO and Ni (ppm)/MgO (wt%) of 25 to 32. Ni-rich olivine grains have large homogeneous cores ( 1300-2900 m diameter, 0.55-0.64 wt% NiO) with thin rims (50-150 m) that are dramatically poorer in Ni and Mg and richer in Ca, Mn, and Fe. The olivine cores are too rich in Ni to be xenocrysts from normal peridotite. K D olivine - bulk rock Fe / Mg values are 0.35 and 0.29, consistent with crystallization of Ni-rich olivine from the host minettes. The association confirms a genetic relationship between Ni-rich olivine and potassic, mafic magmas. High potassium and volatiles may have inhibited polymerization of the melts and caused K D olivine - melt (Ni) to be low during partial melting, resulting in high Ni contents of the magma as suggested for Spanish lamproites by et al. (1984). Low oxygen fugacities may also have been important. Following crystallization of phlogopite and loss of volatiles, the Ni-rich olivine may have crystallized. Numerical simulations of Ni diffusion in olivine constrain times for magma evolution and for formation of intricate oscillatory zoning in diopside phenocrysts that coexist with the Ni-rich olivine. The steep gradients at olivine rims can be simulated in less than 1 year at 1250°C, and so the homogeneous high-Ni cores could have existed in the more evolved melt for only a short time. The Ni-rich olivines are evidence that the K- and Mg-rich bulk rock compositions at Two Buttes represent near-primary magmas.

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