The origin of type C inclusions from carbonaceous chondrites

Abstract

Type C inclusions are plagioclase-rich, Ca-, Al-rich inclusions found in carbonaceous chondrites. They formed as solid condensates which were later melted in an event that destroyed the original condensate grains. Neither the melting event nor secondary alteration had a significant effect on bulk composition. Two stages in the condensation history can be discerned on the basis of major element bulk compositions. As in type A inclusions, the condensate phase assemblage originally consisted of melilite + spinel + perovskite + hibonite. Type C's were, however, significantly enriched in spinel relative to unaltered portions of most type A's. In contrast to type A's, condensate grains of spinel and melilite in type C's reacted partially with a coexisting gas to produce anorthite + diopside. One-half to two-thirds of the silica now in type C inclusions was introduced by this process. The reactions involving melilite and spinel that are predicted by equilibrium condensation calculations for a cooling gas of solar composition did not occur, probably due to kinetic constraints. Type C's may be related to Al-rich chondrules in ordinary chondrites by the addition of olivine and albite. Bulk compositions of Al-rich chondrules in enstatite chondrites are consistent with the addition of orthopyroxene and albite to type C inclusions. Thus, types A and C inclusions and Al-rich chondrules could represent sequences of condensates removed from interaction with the primitive solar nebula at progressively lower temperatures. If Al-rich chondrules represent the high-temperature component in chondritic material, then type C inclusions rather than the more common subgroups of CAIs could be the true parents of chondrites.

Oxygen isotopic compositions of Allende Type C CAIs: Evidence for isotopic exchange during nebular melting and asteroidal metamorphism

“…Another group of "intermediate" objects are the Type C CAIs, which are rare relative to other CAI varieties and which have been little studied (Wark, 1987). Their bulk compositions are close to those of many Al-rich chondrules and, like the latter, deviate significantly from the predictions of equilibrium condensation of a hot solar gas (Beckett and Grossman, 1988;Yoneda and Grossman, 1995;see discussion by MacPherson and Huss, 2005).…”

Experimental studies of trace element partitioning in Ca,Al-rich compositions: Anorthite and perovskite

“…Our expectation was that pyroxene would be the liquidus phase, but large (2 100 p) euhedral crystals of anorthite appeared instead. The QPX composition was prepared as a fassaite glass standard for ion probe analysis, based on a Ti-rich fassaite composition from BECKETT ( 1986). Pe-1507 rovskite was observed to be the liquidus phase, presenting an opportunity for the measurement of perovskite D values.…”

“…However, if the alteration products, abundant in Allende Type B CAls and assigned to melilite in computing modes ( BECKETT, 1986), were instead formed primarily from anorthite, this would substantially increase anorthite modes and lead to lower predicted anotthite Mg contents for F close to Fp. This explanation is admittedly speculative and has not been proposed from any petrographic study of CAIs, but it probably cannot be ruled out.…”

“…Reversely zoned melilite is observed in TS23 and TS34, but not in TS33, althouah auantitative data are onlv available for TS23. For TS34, (Ak) 3 was scaled by the ratios of the average core Ak for TS34 and TS23 (BECKETT, 1986). For TS33, (Ak), was taken to be 0.6, but a separate model with 0.5 showed insignificant differences.…”

Fine‐grained, spinel‐rich inclusions from the reduced CV chondrites Efremovka and Leoville: I. Mineralogy, petrology, and bulk chemistry

“…Stolper (1982) discussed the significance of the predicted versus observed compositional trends in the context of type B CAIs, but in fact, the type C CAIs exhibit the greatest deviation from the predicted trend. Beckett and Grossman (1988) specifically considered the problem of type Cs and proposed a model in which the predicted equilibrium reactions (e.g., Grossman 1972) involving gehlenitic melilite + spinel to form åkermanitic melilite + pyroxene were kinetically inhibited relative to reactions in a silica-rich gas that produced pyroxene + anorthite instead. Beckett and Grossman (1988) predicted that the precursors to type C CAIs were spinel-rich relative to types A or B CAIs and consisted of primary aluminous melilite (avg.…”