Other
Scientific paper
Jul 1992
adsabs.harvard.edu/cgi-bin/nph-data_query?bibcode=1992metic..27r.261m&link_type=abstract
Meteoritics, vol. 27, no. 3, volume 27, page 261
Other
Scientific paper
Two groups of basaltic eucrites may be primary partial melts; the Stannern group (Stannern, Bouvante, Pomozdino, and ALHA81001) and the Juvinas group (all others except Nuevo Laredo, Lakangaon, and possibly Vetluga). The S group is widely believed to represent small degrees of partial melting of the eucrite parent body (EPB), but there is still controversy regarding the origin of the J group. One model holds that the J group eucrites are residual liquids, while a second considers them to be primary partial melts. This study attempts to understand the possible relationships between the J and S groups through trace element modeling of partial melting processes. Method. Different model sources were assumed to have Mg/Si and Cr/Si ratios similar to major chondritic groups (e.g. CI, H, EH, etc.). The Fe/Mg and Fe/Mn ratios were set to plausible eucrite source region values (molar MgO/(MgO+FeO) ~0.65, FeO/MnO ~40; Stolper, 1977). The refractory elements (Ca, Al, REE, Ti, Sc = CARTS) were added in chondritic relative proportions and adjusted to match trace element contents of model melts with natural eucrites. The Na/Si ratio was set to yield eucritic Na/La ratios in model melts. Normative mineralogies of source regions were defined as functions of the amount of CARTS present, which determine normative plagioclase (from Al) and calcic pyroxene (from remaining Ca). The proportions of pyroxene and plagioclase melting were set to match eucrite mineralogy. Partition coefficients for REE and Sc were taken from Colson et al. (1988), McKay et al. (1986), Paslick et al. (1990), and Weill and McKay (1975). Modeling was limited to La (indicator of the degree of melting), Sc (indicator of the pyroxene content of the source region), and Eu (indicator of the plagioclase content of the source region). Results. Considering Sc and La only, acceptable source regions for the J group eucrites require an inverse relationship between Mg/Si and CARTS/Si ratios: carbonaceous chondrites with Mg/Si ~1x CI need ~1x CI CARTS/Si, while EH chondrites with Mg/Si ~0.69x CI need ~1.5x CI CARTS/Si. Including Eu in the modeling limits the range of acceptable source regions to carbonaceous chondrite sources with ~1x CI CARTS/Si. If partition coefficients are constant, then the J group and S group eucrites require different source regions. S group sources require lower CARTS/Si ratios for a given Mg/Si ratio than do J group sources: a CI source region with 1x CI CARTS/Si can yield J group eucrites, while only 0.95x CI CARTS/Si are needed to the produce S group. Discussion. The modeling indicates that acceptable source regions for eucritic basalts have Mg/Si and CARTS/Si ratios close to those of CI chondrites. Further, only two possible scenarios appear acceptable for the relationship between the J group and the S group. Either the J and S groups represent a range of primary partial melts from slightly different source regions(J group ~12-17%; S group ~6-9%), or the J group represents residual liquids from ~14-18% partial melts of the S group source region. About 5-15% fractional crystallization of cumulate eucrites is required to generate primitive J group eucrites from a plausible primary melt. At the stage of melting when plagioclase is exhausted from either J or S group source regions, very little pyroxene remains. Hence, it is difficult to produce diogenites from the same source region that produces J or S group eucrites. The results of this modeling are highly compatible with melting experiments on carbonaceous chondrites (Jones et al., 1992). References: Colson, R.O., McKay G.A., and Taylor L.A. (1988) Geochim. Cosmochim. Acta 52, 539. Jones J.H., Jurewicz A.J.G., and Mittlefehldt D.W. (1992) Meteoritics. (abstract), this issue. Paslick C.R., Jones J.H., and McKay G.A. (1990) Lunar Planet. Sci. (abstract) 18, 936. McKay G.A., Wagstaff J., and Yang S.-R. (1986) Geochim. Cosmochim. Acta 50, 927. Stolper E. (1977) Geochim. Cosmochim. Acta 41, 587. Weill D.F. and McKay G.A. (1975) Proc. Lunar Sci. Conf. 6th, 1143.
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