Low-FeO Ordinary Chondrites: A Nebular Origin and New Chondrite Parent Body

We have studied five meteorites [Wray (a), Cerro 10s Calvos, Willaroy, Moorabie and Suwahib (Buwah)] which differ from other chondritic meteorites and appear to form a new group. They are closely related to ordinary chondrites, but have lower FeO in the mafic silicates than H chondrites. Furthermore, their oxygen isotopic compositions are intermediate between E and H chondrites; they have highly variable metal contents, low Co concentrations in kamacite, and high troilite contents. The differences between low-FeO and H, L and LL chondrites appear to have been established in the solar nebula, not through parent body processes. Low-FeO chondrites did not originate on the H chondrite or IIE iron parent bodies, but probably represent a new ordinary chondrite parent body. PROPERTIES The low-FeO contents of Cerro 10s Calvos, Willaroy, Moorabie and Suwahib (Buwah) have been noted by several authors [e.g.,1,2] and these meteorites have been examined in detail [2]. We recently recognized the low-FeO nature of Wray (a) [3] and here report on our comparative studies. Classification Low-FeO chondrites are weakly metamorphosed and exhibit wide ranges of shock and weathering. Classification of petrologic types, shock stages [4] and weathering [5] are as follows: Wray (a): 4, S4, W1; Cerro 10s Calvos: 4, S2, W3; Willaroy: 3.8, S2, W4; Moorabie: 3.9, S4, Wl ; and Suwahib (Buwah): 3.8, S5, W1. MaJic Silicate Compositions Fa concentrations in low-FeO chondrites (Fa12.8-15.3) are below those of H chondrites (Fa16.9-20.4). Low-Ca pyroxene compositions are intermediate between H and E chondrites. Metal and Troilite Abundances Fe,Ni metal abundances in low-FeO chondrites are highly variable (1 1.2-19.4 wt.%, weathering corrected) and overlap with H and L chondrites. Troilite contents in the mildly weathered Moorabie (9.1 wt.%) and Suwahib (Buwah) (8.6 wt.%) are higher than found in any ordinary chondrite [6], although analyses of these meteorites by [7] suggest normal OC abundances of troilite. Sampling heterogeneity may be responsible. Cobalt in Kamacite The Co concentrations in kamacite of low-FeO chondrites (0.30-0.45 wt.% [2, this work]) are near or lower than those in H chondrites (0.44-0.51 wt.% [8]). Chondrule Sizes Low-FeO chondrites have mean chondrule diameters around 400 pm w r a y (a), Cerro 10s Calvos, Willaroy] and 600 pm [Moorabie, Suwahib (Buwah)]. These values are close to H (-300 pm) and L (-600-800 pm) chondrites [9]. The poorly constrained ranges of chondrule sizes in H, L and LL chondrites and large standard deviations for chondrule sizes in low-FeO chondrites suggest that this parameter is unreliable for establishing group membership. Oxygen Isotopes The A 1 7 0 values for low-FeO chondrites (0.32-0.87%0 [10,11]) are less than or equal to H chondrites (0.73B.09%0,[10]). Moorabie and Suwahib (Buwah), which have slightly larger chondrules, also have higher A17O. Figure 1 plots the compositions of lowFeO chondrites, along with the fields for H, L, LL and E chondrites. WASSON et al. [2] suggested that Willaroy & Cerro 10s Calvos and Suwahib (Buwah) & Moorabie are H and L chondrites, respectively, that were incorporated into, and partially equilibrated with, aubritic material, primarily because their study of Cumberland Falls chondritic inclusions indicated formation by this mechanism. Low-FeO chondrites also lack the high siderophile abundances of Netschaevo, which [12] called HH chondritic material based on extrapolation of H-L-LL trends. Because of the lack of definitive evidence for reduction (e.g., Fs>Fa) or a reducing host, high troilite contents, and short diffusion distances for oxygen in type 5 ordinary chondrites [lo], we suggest nebular processes produced the low-FeO concentrations. O Lunar and Planetary Institute Provided by the NASA Astrophysics Data System 866 LPSCXYV LOW-FeO CHONDRITES