Other
Scientific paper
Jul 1992
adsabs.harvard.edu/cgi-bin/nph-data_query?bibcode=1992metic..27..240j&link_type=abstract
Meteoritics, vol. 27, no. 3, volume 27, page 240
Other
Scientific paper
We reported earlier (e.g., Jurewicz et al., 1991) that the compositions of experimental partial melts from the Allende (CV) chondrite depend strongly on oxygen fugacity (f(sub)o2). At low f(sub)o2 (IW-1) partial melts resemble eucrites, whereas at high f(sub)o2 (IW+2) they resemble angrites. In the past we have concentrated our discussions on the petrogenesis of angritic magmas. Here we explore the implications of our experiments for the origin of eucrites. Powders of Allende and Murchison were equilibrated at an f(sub)o2 of IW-1 and at temperatures ranging from 1140 degrees C (below the solidus) to 1325 degrees C. Melt was observed at >=1150 degrees C for Allende and >= 1160 degrees C for Murchison. Allende partial melts, while broadly eucritic, do not match most eucrites in detail. Thus, most of our discussion here will be devoted to partial melts of Murchison. The major element chemistry of our 1180-1170 degrees C melts of Murchison are nearly identical to the composition of Sioux County, and the experimental melts plot near the peritectic point of the OL-PL-SI pseudoternary (Stolper, 1977). Our olivine and pigeonite compositions and KD(Fe/Mg)+s from the Murchison 1170 degrees C experiment are nearly identical to the 1170 degrees C experiment of Stolper (1977) on Sioux County. Thus, the major element compositions of phases produced by partially melting anhydrous Murchison agree with the eucrite experiments of Stolper. Additionally, our partial melting experiments reproduce the Stannern-Bouvante trend, which has been inferred to represent partial melting. These agreements appear to favor a partial melting origin for primitive eucrites, as opposed to fractional crystallization of diogenitic pyroxene (e.g., Hewins and Newsom, 1988). However, MnO contents of Allende and Murchison are subchondritic and, consequently, our partial melts have high FeO/MnO ratios. As has been pointed out by others, if the eucrites were produced by simple partial melting, the MnO content of the eucrite parent body must be rather high compared to either Allende or Murchison (e.g., Dreibus and Wanke, 1978). Chromium contents of eucrites and of our spinel-saturated Murchison and Allende experiments are extremely similar (Cr2O3 = ~0.31 +- 0.02 wt%), in agreement with Stolper's (1977) suggestion that eucrites are spinel saturated. This observation also favors a partial melting origin for eucrites such as Sioux County. Because Cr is compatible in pyroxene, extensive fractionation of pyroxene should deplete Cr in eucritic liquids and move them away from the chromite saturation surface. However, this model should be tested by crystallization experiments on high-temperature partial melts. Not all eucrites are as simple as Sioux County. Most of the main group eucrites have slightly lower Mg#s, slightly higher CaO/Al2O3 ratios, and slightly higher incompatible element concentrations, suggesting that, relative to Sioux County, most of the main group eucrites either have experienced small amounts of fractional crystallization or were produced by slightly smaller amounts of partial melting. We are unable to distinguish between these two possibilities. However, for this redox state and bulk composition, neither mechanism will produce signficant amounts of diogenites, as little pyroxene is left in the residue after plagioclase is exhausted. We note two other interesting inferences based on our experiments. (i) Because of their high Cr contents (Cr2O3 >= 0.4 wt%), it is possible that two eucrites do not represent melt compositions, ALHA81001 and Pomozdino. The strange composition of Pomozdino has already been noted (Warren and Jerde, 1987). (ii) Ibitira appears to differ signifcantly from the other eucrites and partial melts of Murchison. Instead, Ibitira seems most similar to the partial melts of Allende. If further analyses of Ibitira sustain this similarity, there must either be multiple eucrite parent bodies or the EPB must be heterogeneous. References: Dreibus G. and Wanke H. (1978) Z. Naturforsch. 35a, 204-216. Hewins R.H. and Newsom H.E. (1988) In Meteorites and the Early Solar System (Kerridge and Matthews, eds.), U. Arizona Press. pp. 73-101. Jurewicz A.J.G., Mittlefehldt D.W. and Jones J.H. (1991) Science 252, 695-698. Stolper E.M. (1977) Geochim. Cosmochim. Acta 41, 587-611. Warren P.H. and Jerde E.A. (1987) Geochim. Cosmochim. Acta 51, 713-725.
Houston Jones Jane
Jurewicz Amy J. G.
Mittlefehldt Dave W.
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