Mathematics – Logic
Scientific paper
Sep 1995
adsabs.harvard.edu/cgi-bin/nph-data_query?bibcode=1995metic..30q.550m&link_type=abstract
Meteoritics, vol. 30, no. 5, page 550
Mathematics
Logic
Howardites, Mesosiderites, Parent Bodies, Eucrite Crust, Mesosiderite Crust, Meteorite
Scientific paper
The silicates in mesosiderites and howardites are polymict breccias of diverse crustal lithologies. To the extent that brecciation and impact mixing were relatively thorough on asteroidal-sized bodies [1,2], the compositions of these polymict breccias may reveal information on the bulk crusts of their parent bodies. I have performed INAA on a large number of mesosiderite and howardite bulk samples, concentrating on many of the previously poorly studied mesosiderites, in order to investigate the nature of their parent bodies' crusts. Mesosiderite silicates are petrologically very similar to howardites, although more highly metamorphosed, and relict primary igneous clasts in mesosiderites are indistinguishable from basaltic eucrites and diogenites [3]. Therefore, the crusts of their parent bodies ought to be compositionally similar. Howardites are often thought to be fairly representative of the crust of the eucrite parent body (EPB) [4] and the mesosiderites may be similarly representative of their parent body (MPB) [5]. Comparison of the bulk silicates may then serve to illuminate differences between the EPB and the MPB, and/or shortcomings in our ideas of regolith processes on asteroids. The major lithophile element distinctions between mesosiderite silicates and howardites are in their REE compositions. On average, mesosiderite silicates have lower Sm/Lu and higher Eu/Sm than do howardites. The average mesosiderite has a CI normalized Eu/Sm ratio of ~1.44 vs. 0.88 for howardites. The slight negative Eu anomaly for average howardite might be consistent with the "classic" EPB crustal structure [6]; the deeper cumulate gabbros (Eu-rich) are slightly undersampled relative to their residual liquids (Sm-rich), which would occur stratigraphically higher. However, it is difficult to understand then why orthopyroxenites, which ought to dominate in the deepest crustal layer, are so abundant in howardites. It is very difficult to envision how the bulk mesosiderite composition could have arisen on this type of crust. The positive Eu anomaly would require oversampling of the deep cumulate gabbro layer, and undersampling of the surficial basaltic layer. This might be accomplished on the MPB if early impact erosion of the surface of the parent body occurred such that the basaltic surface was preferentially stripped by ejection during impact. Because this didn't also occur on the EPB, the implication is that the MPB was either significantly smaller, or suffered more intense impact activity. A basic assumption of the above discussion is that polymict breccias are fairly representative of their bulk crusts. Astronomical study of 4 Vesta has shown that it contains extensive regions rich in orthopyroxenite (possibly recent), presumably exhumed from deep in the crust [7]. Therefore, an alternate analysis may be that the bulk silicates of the two polymict breccia types are not true representatives of their parent body crusts, but rather are compositions highly dependent on local geology. In this case, the Eu-enriched trace element signature of mesosiderite silicates would occur in a region of the MPB crust where one or more impacts penetrated the gabbroic layer, locally enriching the regolith in cumulate gabbro. However, study of a large number of clasts in mesosiderites indicates that a large fraction of them were remelted early in solar system history, and many were then slowly cooled such that they formed a new generation of cumulate gabbros [3,8]. This is more likely to have occurred on a large, perhaps parent body wide, scale, and therefore, I believe the Eu-enriched nature of bulk silicate mesosiderites is probably characteristic of much of the MPB crust. References: [1] Cintala et al. (1978) Proc. LPSC 9th, 3803. [2] Housen et al. (1979) Icarus, 39, 317. [3] Rubin and Mittlefehldt (1992) GCA, 56, 827. [4] Warren (1985) GCA, 49, 577. [5] Rubin and Mittlefehldt (1993) Icarus, 101, 201. [6] Takeda (1979) Icarus, 40, 455. [7] Gaffey (1995) Icarus, submitted. [8] Mittlefehldt (1990) GCA, 54, 1165.
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