Systematics of Ni and Co partitioning between olivine and peridotitic liquids

Details Systematics of Ni and Co partitioning between olivine and peridotitic liquids Systematics of Ni and Co partitioning between olivine and peridotitic liquids

Dec 2006

adsabs.harvard.edu/cgi-bin/nph-data_query?bibcode=2006agufm.v24b..02l&link_type=abstract

American Geophysical Union, Fall Meeting 2006, abstract #V24B-02

Physics

1025 Composition Of The Mantle, 1037 Magma Genesis And Partial Melting (3619), 3610 Geochemical Modeling (1009, 8410), 3630 Experimental Mineralogy And Petrology, 3672 Planetary Mineralogy And Petrology (5410)

Scientific paper

We have measured olivine/liquid partition coefficients (D) for Ni and Co in runs from 1550 to 2015° C and 1.5 to 6 GPa. D values (weight units) range from 7.2 to 1.4 for Ni and 2.0 to 0.9 for Co. The Ds correlate weakly with 1/T and P/T, but well with D/*(MgO) in accordance with the Jones'95 model (D/*MO = aD/*MgO +b, where D/* is the partition coefficient expressed in cation mole fraction, M is a divalent cation, and a and b are constants). Our experiments have D/*MgO in the range of 2.4 to 1.4. The model predicts that Ni and Co will become incompatible (D>1) for values of D/*MgO < 1.39 and 1.76, respectively, which correspond to peridotitic and komatiitic compositions. Calculations of the fractional crystallization of a lunar magma ocean with bulk composition similar to a degassed terrestrial upper mantle and an initial bottom pressure of 3.0 GPa show that both Ni and Co are initially incompatible, but that Ni becomes compatible after 10/% crystallization and Co after about 40/%. Ni in olivine has a prominent peak at intermediate fractions of crystallization; whereas Co in olivine in creases monotonically until the very latest stages. The patterns of Ni and Co concentration calculated in magma ocean olivine mimic patterns in observed in lunar rocks: olivines for the Mg-suite have the highest Mg/#s, but relatively low Ni; mare basalts have less magnesian olivine but much higher Ni in olivine, and ferroan anorthosites have olivines with lower Mg/# still, but also low Ni. Co is also low in the most magnesian olivines, but olivines from ferroan anorthosites have Co concentrations similar to olivine in mare basalts, so the overall pattern is a monotonic increase of Co as Mg/# decreases. Within the mare basalt series the expected pattern of Ni and Co concentrations in olivine decreasing as Mg/# decreases is observed. So the apparently anomalous behavior of Ni and Co can only be explained by crystallization of a magma ocean in which accumulation of the earliest olivines contributed to the formation of the Mg-suite source region, accumulation of intermediate olivine cumulates led to the formation of the mare basalts, and in the later stages ferroan anorthosites formed either directly in the magma ocean or their source regions did.

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