Petrology and origin of amoeboid olivine aggregates in CR chondrites

Abstract

Abstract-Amoeboid olivine aggregates (AOAs) are irregularly shaped, fine-grained aggregates of olivine and Ca, Al-rich minerals and are important primitive components of CR chondrites. The AOAs in CR chondrites contain FeNi metal, and some AOAs contain Mn-rich forsterite with up to 0.7 MnO and Mn:Fe ratios greater than one. Additionally, AOAs in the CR chondrites do not contain secondary phases (nepheline and fayalitic olivine) that are found in AOAs in some CV chondrites. The AOAs in CR chondrites record a complex petrogenetic history that included nebular gas-solid condensation, reaction of minerals with the nebular gas, small degrees of melting, and sintering of the assemblage. A condensation origin for the Mn-rich forsterite is proposed. The Mn-rich forsterite found in IDPs, unequilibrated ordinary chondrite matrix, and AOAs in CR chondrites may have had a similar origin.A type A calcium, aluminum-rich inclusion (CAI) with an AOA attached to its Wark-Lovering rim is also described. This discovery reveals a temporal relationship between AOAs and type A inclusions. Additionally, a thin layer of forsterite is present as part of the Wark-Lovering rim, revealing the crystallization of olivine at the end stages of Wark-Lovering rim formation. The Ca, Alrich nodules in the AOAs may be petrogenetically related to the Ca, Al-rich minerals in WarkLovering rims on type A CAIs. AOAs are chondrite components that condensed during the final stage of Wark-Lovering rim formation but, in general, were temporally, spatially, or kinetically isolated from reacting with the nebula vapor during condensation of the lower temperature minerals that were commonly present as chondrule precursors.

Chondrites and Their Components

“…The mineralogy, petrology, 16 O-rich compositions, depletion in moderately volatile elements, such as manganese, chromium, and sodium, and the general absence of low-calcium pyroxenes associated with AOAs strongly suggest that AOAs are aggregates of grains of forsterite, aluminum-diopside, spinel, anorthite, and Fe,Ni metal that condensed in the nebula from an 16 Orich gaseous reservoir and aggregated with CAIs that formed earlier (Krot et al, 2004a(Krot et al, ,b, 2005dRuzicka et al, 2012a;Sugiura et al, 2009;Weisberg et al, 2004). Mineralogical and chemical similarities between AOAs and forsterite-rich accretionary rims suggest that AOAs formed contemporaneously with these rims.…”

Laboratory condensation of refractory dust in protosolar and circumstellar conditions

“…However, studies have shown that some, such as type B and C CAIs (Stolper, 1982;Wark and Lovering, 1982;Beckett and Grossman, 1988), or the compact type A CAIs (e.g., Simon et al, 1999) have probably crystallized from molten objects and could have also underwent a complex series of events including evaporation (Grossman et al, , 2002Richter et al, 2002) or partial melting (MacPherson and Davis, 1993). The complex history of other CAIs, such as fluffly type A inclusions (e.g., MacPherson and Grossman, 1984), spinel rich objects (MacPherson et al, 1983;Fegley and Post, 1985;Kornacki and Wood, 1985;McGuire and Hashimoto, 1989;MacPherson and Davis, 1994;Lin and Kimura, 2003;Krot et al, 2004a) or amoeboid olivine aggregates (Grossman and Steele, 1976;Komatsu et al, 2001;Krot et al, 2004b;Weisberg et al, 2004) is still not well understood (MacPherson et al, 1988;MacPherson, 2004). Their texture (e.g., fluffy aggregates of concentric objects for the spinel-rich inclusions) and irregular shape (e.g., amoeboid olivine aggregates) argue against crystallization from a liquid, with some inclusions such as most fine-grained spinel rich objects, showing a group II rare earth element pattern, interpreted as a condensation signature (Boynton, 1975;Kornacki and Wood, 1985;Sylvester et al, 1993).…”

Comparing Wild 2 particles to chondrites and IDPs

“…About one-fourth of these Mn-and Cr-rich olivines contain <1% FeO. Olivines with enrichments in these elements have been reported in carbonaceous chondrites, micrometeorites, and chondritic IDPs, though they are rare (Klöck et al 1989;Rietmeijer 1998;Gounelle et al 2002;Simon and Grossman 2003;Weisberg et al 2004). The wide Mg-Fe composition range of Wild 2 olivine is similar to that for anhydrous chondritic IDPs.…”