Gpochimica et Cosmochimica Ada Vol. 57, pp. 123-129 Copyright 0 1993 zyxwvutsrqponmlkjihgfedcbaZYXWVUTSRQPONMLKJIHGFEDCBA Pergamon Press Ltd. Printed in U.S.A. 0016-7037/93/55.00 + .OO zyxwvuts Ni-rich olivine in minettes from Two Buttes, Colorado: A connection between potassic melts from the mantle and low Ni partition coefficients LINDA L. DAVIS and DOUGLAS SMITH Department of Geological Sciences, The University of Texas at Austin, Austin, TX 787 12, USA zyxwvutsrqponmlkjihg (Received November 14, 1991; accepted in revisedform June 30, 1992) Abstract-Olivine minettes from Two Buttes, Colorado, contain diverse populations of olivine, some of which is unusually rich in Ni. Olivine compositions range from Foss to Fog4 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 pm diameter, 0.55-0.64 wt% NiO) with thin rims (50-150 pm) that are dramatically poorer in Ni and Mg and ,ficher in Ca, Mn, and Fe. The olivine cores are too rich in Ni to be xenocrysts from normal peridotite. K$“‘“*b”‘krock 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 Kpemdt (Ni) to be low during partial melting, resulting in high Ni contents of the magma as suggested for Spanish lamproites by VENTURELLIet 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 125O”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. INTRODUCTION CORESOF SOMEANHEDRAL OliVine gI%nS in mafiC minettes from Two Buttes, Colorado, have NiO contents up to 0.64 wt%. Such Ni contents are higher than any reported for olivine in kimberlite (typically about 0.38 wt% NiO; MOORE, 1988) and komatiite (up to about 0.55 wt% NiO; GREEN et al., 1975; ARNDT et al., 1977). They also exceed the range of 0.3 to 0.5 wt% NiO that characterizes almost all olivine in mantle xenoliths (SIMKIN and SMITH, 1970; SATO, 1977; BVSP, 198 I ). Origins of Mg- and Ni-rich primary melts have been widely discussed (e.g., O’HARA et al., 1975; SATO, 1977; HART and DAVIS, 1978; BVSP, 198 zyxwvutsrqponmlkjihgfedcbaZYXWVUTSRQPONMLKJIHGFEDCBA 1 ), and uncertainties in Ni partitioning between olivine and melts have led to various interpretations of Ni-MgO relationships for mantle-derived mafic rocks. The Mg-Ni exchange coefficient has been used to place limits on the Ni and MgO content of primary melts, but determination of melt compositions is often difficult, be- cause olivine fractionation and accumulation can modify the primary magma composition. The distinction of xenocrysts from phenocrysts is also important in interpreting the nature and evolution of Mg- and Ni-rich melts. Olivine and host minette at Two Buttes have been analyzed in detail to gain insights into the processes responsible for the unusual Ni- rich compositions and into the genesis and evolution of po- tassium-rich mafic magmas. Hypotheses regarding the genesis of potassic rocks continue to be controversial, both in general (BERGMAN, 1987; FOLEY, 1990; CARMICHAEL,199 1; MITCHELL and BERGMAN, 1990) and in the western USA (e.g., DUDAS, 1991; O’BRIEN et al., 199 1; TINGEY et al., 199 1) . Understanding the genesis of Ni- rich olivine is likely to contribute to understanding the origins of such melts, because almost all similarly Ni-rich magmatic olivine is also in potassic mafic rocks, one exception being that in high-MgO tholeiites from Hawaii (CLAGUE et al., I99 1). Other potassic rocks hosting Ni-rich olivines are lam- proites (BERGMAN, 1987) from Spain, Antarctica, and Aus- tralia. Lamproites (Ni ppm/MgO wt% = 48 + 5) from southeastern Spain have anhedral, kinked grains of olivine that contain 0.47 to 0.70 wt% NiO and undeformed phe- nocrysts with 0.31 to 0.34 wt% NiO ( VENTURELLI et al., 1984): the host rock is jumillite that BERGMAN(1987) de- scribes as the “most lamproite-like of all the Murcia-Almeria rocks.” Lamproites from Gaussberg, Antarctica, contain mi- crophenocrysts of olivine with 0.16 to 0.62 wt% NiO ( SHER- ATON and CUNDARI, 1980). In the West Kimberley lam- proites in Australia, olivine microphenocrysts contain up to 0.64 wt% NiO (JAQUES et al., 1986). The occurrences of Ni-rich olivine may provide insights into the polymerization of the parent melts, the oxygen fu- gacities of melts and sources, or both. The melts that crys- tallized these olivines may have been. Ni-rich because they were partial melts of peridotite under conditions with a low olivine/melt Ni partition coefficient. Olivine/melt Ni par- tition coefficients are significantly influenced by potassium content (BURNS and FIFE, 1964; IRVINE, 1974;KUSHIRO, 1975;IRVINE and KUSHIRO, 1976;HART and DAVIS, 1978; MYSEN and VIRGO, 1980), and VENTURELLIet al. (1984) suggested that Ni-rich olivine in potassic rocks in Spain may have formed because parent melts had low polymerization with high contents of alkaline elements and water. The par- tition coefficients also may be influenced by oxygen fugacities of the melts, but evidence is mixed ( COLSON, 1990; MORSE etal., 1991; STEELE~~~~., 1991; SNYDER ~~~CARMICHAEL, 123