OBSERVED AND PREDICTED CRYSTALLIZATION SEQUENCE IN BARBERTON KOMATIITE

The 3.3 Ga Weltevreden Formation of the Barberton Greenstone Belt includes massive komatiite flows that are exceptionally fresh, preserving original igneous textures, minerals, and glass. Fresh minerals include olivine(OL), orthopyroxene(OPX), pigeonite(PIG), augite(AUG) and chromite(CHR). Microprobe analyses yield Mg# as magnesian as 95.6 ± 0.1 for OL cores and 91.3 for OL adjacent to melt inclusions. These are consistent with the observed OL control of bulk rock compositions within flows which suggest OL Mg# 96. Such OL is in predicted equilibrium with the random spinifex layers of these flows, which have MgO 35.1 wt%, and require low pressure temperatures of 1700C. High temperatures for these komatiites and OL are also consistent with the minor element concentrations observed. OL has Cr₂O₃ up to 0.32 wt% and NiO up to 0.56 wt%. The high observed Cr contents in these OL, and lack of Al substituting for Si, require Cr²⁺, and further support extremely high temperatures. Maximum Mg# in CHR is 72.3 for cores and decreases to values as low as 35.9 at rims. This large variation in Mg# is likely due to partial subsolidus equilibration and/or serpentinization. Cr# is 82.3 for cores and decreases only to about 72.0 in most crystals. OPX cores have Mg# 91.3 and Wo 0.02, but are strongly zoned to compositions that are more appropriately regarded as PIG (though crystals are optically continuous and appear to be orthorhombic) with Mg# 87.0 and Wo 0.13. AUG occurs as optically distinct overgrowths on PIG within the spinifex layers, and as discrete crystals within massive portions of the flows. AUG ranges in composition from Mg# 87.0 and Wo 0.31 to Mg# 75.0 and Wo 0.43. The remainder of the rock is composed of a fine chloritic groundmass that represents altered quenched melt. No plagioclase is observed.

MELTS was used to model fractional crystallization of this system at 1 atm and fO₂ of Ni-NiO. OL crystallization begins at 1660C with a Mg # 96.5 and ends at 1360C with Mg# 90.1. CHR begins crystallization at 1580C with Cr# significantly less than observed (70.0 vs. 82.0). OPX crystallization begins at 1360C, when OL crystallization ceases, at Mg# 89.6, and ends at 1240C with onset of PIG and AUG crystallization, which Mg# 78.5 and 79.9, respectively. Plagioclase begins crystallization at 1220C.