Chondrite, ungrouped, type 3.5–3.8, low-FeO
(L3.8-an in MetBull 53)
standby for moorabie photo
Found before 1965
30° 6′ S., 141° 4′ E. One mass of 31 pounds was found by Mr. L. Russell while prospecting about 130 miles north of Broken Hill close to Boolka, and ~10 miles south of the Moorabie Bore in New South Wales, Australia. Chondrules representing a large variety of types are closely spaced within a sparse white matrix. These chondrules exhibit foliation and preferential alignment, probably produced from an impact-shock event; Moorabie is shocked to stage S4–5. Olivine grains show undulatory extinction and mosaicism. Shock-heating also produced in situ local melting of some FeNi-metal and troilite grains at temperatures of ~950°C (from a pre-shock accretion-related temperature of ~400°C). These grains coalesced into larger clasts, one of which has been found to enclose chondrules identical to those in the host (Fujita and Kitamura, 1992). In addition, silicate–metal–troilite melt pockets and clear maskelynite are present in the matrix of Moorabie. Slow cooling followed the shock event.

Although Moorabie has been grouped with L-group chondrites, it forms a group of reduced chondrites (Fa1013) of mostly H-group association which might represent one or more unique objects distinct from the H, L, and LL ordinary chondrite parent bodies (Fa1620). Besides Moorabie, other low-FeO chondrites include Beni Semguine [H5-an], Burnwell [H4-an], Cerro los Calvos [H4-an], Suwahib (Buwah) [H3.8-an], Willaroy [H3.8-an], Wray (a) [H4-an], EET 96031 [H4-like], LAP 04757/73 [H-like], MIL 07273 [H5-an], and QUE 94570 [L-like]. All of these reduced chondrites have mineral compositions outside the established range for the known ordinary chondrite groups. Furthermore, their variable metal contents, low concentrations of Co in kamacite, and high troilite contents support a derivation from one or more unique parent bodies.

Notably, the O-isotopic compositions of EET 96031, LAP 04757/73, and MIL 07273 all plot within a region delineated by the H chondrites. The equilibrated chondrite Burnwell is a fall that has similar low-FeO properties and which initially was shown to plot on an extension of the H–L–LL trend towards more reducing compositions (Russell et al., 1998); however, additional O-isotopic analyses conducted on Burnwell (Rumble III et al., 2007) gave a compositional value that clearly plots within the H chondrite field.

Troiano et al. (2010, 2011) and Friedrich et al. (2011) studied a number of low-FeO chondrites with isotopic, mineralogical, and trace element values in the range of H chondrites, including Burnwell, EET 96031, LAP 04757/73, and MIL 07273. Their studies provide evidence for the origin of the low-FeO chondrites on the H chondrite parent body. It is theorized that these low-FeO chondrites experienced extreme redox processes in which an oxidizing agent such as ice reacted with material containing a higher amount of metal than that present in typical H chondrites.

The low-FeO contents within this group of chondrites, as well as the variable abundances of metal and troilite, are considered by some to be primary features that were established early within the solar nebula (McCoy et al., 1994). These properties were likely a consequence of the heterogeneous incorporation of nebular components within the parent body, rather than resulting from later parent body metamorphism. However, Mössbauer spectroscopy investigations indicate that reduction occurred in unequilibrated ordinary chondrites as metamorphic grade increased, perhaps from progressive dehydration of phyllosilicates. This scenario would attribute the reduced nature of these ordinary chondrites to parent body processes rather than to the accretion of primary low-FeO components on one or more unique parent bodies. However, no reducing agent such as C-rich material has been observed in these chondrites thus far.

Despite compositional and isotopic similarities with the low-FeO chondritic clasts in IIE irons such as Netshaëvo, the low petrologic type of Moorabie and other reduced unequilibrated ordinary chondrites are inconsistent with the highly metamorphosed character of the silicates in these irons. Therefore, the IIE parent body cannot be the parent body of the reduced ordinary chondrites. By the same token, no low-FeO clasts have been identified within H chondrite breccias, indicating that this is not the parent body either. These observations would suggest that this low-FeO group samples one or more unrecognized ordinary chondrite parent bodies. Resolution of the true origin of the reduced unequilibrated ordinary chondrites such as Moorabie will require further investigation.

Trapped noble gas studies were conducted by Matsuda et al. (2010) on both a dark inclusion and a matrix sample from Moorabie. The trapped noble gas ratios in the dark inclusion were found to be similar to Q-type gases (‘Q’ for ‘quintessence’, including He, Ne, Ar, Kr, and Xe), while those from the matrix had a much lower abundance and a different composition. It was suggested that thermal metamorphism may be the cause of the lower abundance and different composition of Q noble gases in the matrix.

It has been found that a positive correlation exists between petrologic type and shock stage (Rubin, 2004). However, Moorabie is not typical of most other unequilibrated chondrites in that it is significantly shocked to stage S4–5. This is contrary to the expectation that a porous, volatile-rich, petrologic type-3 chondrite would most likely be destroyed in an impact that resulted in a shock stage as high as S4–5. The Moorabie specimen shown above is a 49.4 g partial slice.

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