Fractionation trends among IVA iron meteorites: Contrasts with IIIAB trends

A neutron-activation study of 48 group-IVA irons shows much lower negative slopes on Ir-Au and Ir-As diagrams than observed in the larger magmatic gro up IIIAB. This difference seams to reflect the tendency of D-Ir, D-Au, and D-As, to increase with increasing S content. Contents of S and other volati les are much lower in IVA irons than IIIAB irons. We show that both groups can be fit with D-X values that depend quadratically on S. with initial IVA S contents about 6X lower than those in IIIAB. The IVA scatter fields show a spread in Au or As at constant Ir that appears to reflect variations in the fraction of trapped melt between 0% and 30%. Copper shows an S-shaped t rend that may reflect moderate positive and negative changes in D-Cu as the magma evolved or, less likely. sampling variations in a broad band reflect ing fractionation and trapping of melt. Gibeon, the largess IVA iron with a mass > 30 tons, shows an appreciable range in compositions consistent eith er with differences in the degree of magma crystallization or with differen ces in the content of trapped melt. A striking difference between IVA and I IIAB is observed in the Ir/Au ratios in the most Ir-rich irons in the group s: that in IVA is 40% lower than the IIIAB ratio, and lower than those in o ther iron-meteorite groups. The IVA Ir/Au ratio is about half the ratios in the chondrite groups. We examined three possible explanations of this anom aly: (1) the high-lr irons contain large amounts of trapped melt; or (2) ha lf of the IVA core (i.e.. the first 50% to crystallize) is missing from the terrestrial set of IVA irons: or (3) the IVA magma formed by incomplete me lting of the metal in the chondritic precursor material, with the metal tha t remained in the mantle having high Ir and low Au contents. Plausibility a rguments favor the third possibility, The third scenario is the most plausi ble, but the second cannot be ruled out. We review recent evidence regardin g the cooling rates in group IVA. In contrast to recent interpretations, we note several lines of evidence that indicate constant cooling rates indepe ndent of composition, as expected if all IVA meteorites were in the same co re when cooling between 900 and 650 K occurred. Copyright (C) 2001 Elsevier Science Ltd.