Phase equilibrium constraints on angrite petrogenesis

Parameterizations of liquidus boundaries and solid solution in the CMAS + F e system (Shi, 1992) have been employed to depict the liquidus equilibria r elevant to the petrogenesis of angrites, Angrites are basaltic achondrites characterized by highly aluminous augite (fassaite), intermediate Mg-Fe oli vine, and late-stage CaFe-olivine (kirschsteinite). Two important features of the equilibria on the olivine liquidus surface relevant to angrite petro genesis are: 1) the presence of a thermal divide on the of + aug + plag + l iq boundary curve, which separates the compositions of source materials tha t produce low-silica angritic melts that crystallize highly aluminous augit e from those that produce higher silica melts with tholeiitic to eucritic c rystallization patterns; and 2) the change in the pseudo-invariant point on the low-silica side of the thermal divide from a plagioclase-peritectic in volving spinel (ol + aug + plag + sp + liq) at high to intermediate Mg' (Mg /[Mg + Fe]) to two pseudo-eutectics involving kirschsteinite (ol + aug + pl ag + kir + liq and ol + kir + plag + sp + liq) at low Mg'. The fassaitic (aluminous augite) pyroxene composition in Angra Dos Reis (AD OR), the presence of minor green spinel, and the absence of primary kirschs teinite (Prinz et al., 1977) indicate that crystallization of the ADOR pare ntal liquid was governed by the intermediate-Mg' set of equilibria such tha t, following crystallization of ol + aug + plag, the plagioclase reacted co mpletely at the plagioclase-peritectic with the interstitial liquid, which subsequently crystallized beyond the plagioclase-peritectic onto the ol + a ug + sp liquidus boundary curve. The ADOR bulk, composition is consistent w ith trapping similar to 10% of the parental liquid in a cumulate with cotec tic proportions of fassaite and olivine. Lewis Cliff (LEW)86010 crystallize d from a liquid with Mg' similar to that of ADOR, but on the ol + plag cote ctic closer to the thermal divide such that the first pyroxene to crystalli ze had much lower Al content than that of ADOR. In the late stages of cryst allization the 86010 residual liquid land that of LEW87051) encountered the low Mg' set of equilibria involving kirschsteinite. These relationships re quire either a higher degree of melting for the 86010 parent magma or sourc e region different than ADOR's. These relationships are also consistent wit h compositionally dependent REE partition coefficients between fassaite and the ADOR liquid being as much as 1.5-2 times higher than those for the 860 10 liquid at the onset of pyroxene crystallization. The combination of a tr apped liquid component, higher partition coefficients, and smaller degrees of melting help to explain the observation that ADOR, an apparent cumulate, has REE concentrations twice as high as those in 86010 (Mittlefehdlt and L indstrom, 1990), an apparent chilled liquid. The absence of a strong negati ve Eu-anomaly in the ADOR parent liquid. however, requires relatively high degrees of partial melting to eliminate plagioclase in the source region (r esorption of plagioclase at the peritectic eliminates the Eu-anomaly that d evelops during crystallization), so ultimately different source regions are required. Progressive iron loss from devolatilized primitive chondrites (Allende, Mur chison) produces source regions capable of producing a wide range of melt c ompositions with angritic to eucritic crystallization behavior. The composi tions of carbonaceous and ordinary chondrite provide a similar range of pot ential source region compositions. However, primitive chondrite(+/-Fe) sour ce regions that produce angrite-like melts have Mg' that is too low, wherea s chondrite(+/-Fe) sources that have Mg' sufficiently high to yield the Mg' in angrite minerals have too much silica (or orthopyroxene) component to y ield angrite-like liquids. No single group of meteorites + Fe simultaneousl y satisfies the constraints of Mg' and silica component. However, mixtures of Fe-depleted chondrite plus a low-silica component similar to Ca-Al-rich inclusions (CAIs) can satisfy the constraints. The absence in angrites of C a-48 and Ti-50 anomalies, typical of CAIs (Lugmair and Galer, 1992), sugges ts that the low-silica component was not simply an enrichment of CAIs, but was the result of direct accretion of high-temperature condensate (Grossman , 1972) into sizable, thermally shielded planetesimals. Thus angrites crypt ically record mixing of planetesimal-sized heterogeneities in the early sol ar system. Copyright (C) 1999 Elsevier Science Ltd.