Alan E. Rubin

Original structures, and fragmentation and reassembly histories of asteroids - Evidence from meteorites

Abstract If chondritic meteorites were internally heated after accretion had ended, then the hottest material would have been buried the deepest and should have cooled the slowest. If this is correct, there ought to be a correlation between cooling rate and petrographic type, a measure of the extent to which chondrites were metamorphosed (i.e., heated). Published and new cooling rates derived from the compositions of metallic iron-nickel grains do not display this correlation, implying either that chondrite parent asteroids never had onion-shell structures or that bodies with onion-shell structures were broken up and reassembled prior to cooling to below 500°C, the temperature at which cooling-rate information is recorded in metallic iron-nickel. Chondritic regolith breccias formed from materials that resided on the surfaces of their parent asteroids. Metallic iron-nickel grains in H- and L-chondrite regolith breccias indicate that the breccia constituents cooled at rates ranging from 1 to > 1000°K/myr. Based on thermal calculations, these cooling rates suggest that the materials spread out on the surfaces of H- and L-chondrite parent asteroids originated at depths ranging from about one kilometer to several tens of kilometers. Craters deep enough to excavate tens of kilometers cannot form on typical asteroidal bodies only 100 to 300 km in diameter without disrupting them. Therefore, it appears that at least some asteroids, namely, the parent bodies of H and L chondrites, were disrupted after cooling to below 300°C, and then reassembled to create surfaces containing rocks that originated at a wide range of depths. These results support theoretical calculations suggesting that many asteroids were broken up and subsequently reassembled into gravitationally bound rubble piles.

Lithification of gas-rich chondrite regolith breccias by grain boundary and localized shock melting

We studied the fine-grained matrices (< 150 μm) of 14 gas-rich ordinary chondrite regolith breccias in an attempt to decipher the nature of the lithification process that converted loose regolith material into consolidated breccias. We find that there is a continuous gradation in matrix textures from nearly completely clastic (class A) to highly cemented (class C) breccias in which the remaining clasts are completely surrounded by interstitial, shock-melted material. We conclude that this interstitial material formed by shock melting in the porous regolith. In general, the abundances of solar-wind-implanted 4He and 20Ne are inversely correlated with the abundance of interstitial, shock-melted, feldspathic material. Chondrites with the highest abundance of interstitial, melted material (class C) experienced the highest shock pressures and temperatures and suffered the most extensive degassing. It is this interstitial, feldspathic melt that lithifies and cements the breccias together; those breccias with very little interstitial melt (class A) are the most porous and least consolidated.

Matrix material in type 3 chondrites—occurrence, heterogeneity and relationship with chondrules

Abstract Matrix material in type 3 chondrites forms rims on chondrules, metal-sulfide aggregates, Ca,Al-rich inclusions and chondritic clasts; it also forms lumps up to a millimeter in size, which may contain coarser silicates. Chondrules of all types were found with internal matrix lumps that appear to have entered the chondrules before the latter had crystallized. Mean concentrations of Mg, Na, Al and Ca in matrix occurrences show up to fivefold variations in a single chondrite. Variations between mean matrix compositions of individual type 3 ordinary chondrites are almost as large and partly reflect systematic differences between H, L and LL matrices. Such variations are probably a result of nebular separation of feldspathic material and ferromagnesian silicates. Compositions of chondrules and their matrix rims are normally unrelated, although rim compositions are correlated with those of matrix lumps inside chondrules. A single chondrule was found with a composition nearly identical to that of its internal matrix lump, suggesting that some chondrules may have formed from matrix material. Matrix lumps are as heterogeneous as chondrules, but mean chondrule and matrix compositions differ, even allowing for possible loss of metallic Fe,Ni during chondrule formation. Since bulk compositions of matrix lumps and rims have probably not changed significantly since their formation except for Fe-Mg exchange, our matrix samples cannot represent typical chondrule precursor materials.

Mineralogy and petrology of the Abee enstatite chondrite breccia and its dark inclusions

The Abee E4 enstatite chondrite breccia consists of clasts (many rimmed by metallic Fe, Ni), dark inclusions and matrix. The clasts and matrix were well equilibrated by thermal metamorphism, as evidenced by uniform mineral compositions, recrystallized chondrules, low MnO content of enstatite and high abundance of orthoenstatite. The clasts acquired their metal-rich rims prior to this metamorphic episode. The occurrence in Abee of relatively unmetamorphosed dark inclusions, clasts with nearly random magnetic orientations and a matrix with a uniform magnetic orientation [18,19] indicates that clast and matrix metamorphism occurred prior to the agglomeration of the breccia. The dark inclusions are an unusual kind of enstatite chondritic material, distinguished from the clasts and matrix by their relative enrichments in REE [21–23], low relative abundances of kamacite, total metallic Fe, Ni and silica, lower niningerite/(total sulfide) ratios, high relative abundances of oldhamite and martensite, smaller euhedral enstatite, more heterogeneous enstatite and metallic Fe, Ni, more calcic enstatite and more nickeliferous schreibersite. We propose the following model for the petrogenesis of the Abee breccia: The maximum metamorphic temperature of breccia parent material was⩾- 840°C (the minimum temperature of formation of Abee niningerite) and perhaps near 950–1000°C (the Fe-Ni-S eutectic temperature). Euhedral enstatite crystals in metallic Fe, Ni- and sulfide-rich areas grew at these metamorphic temperatures into pliable metal and sulfide. Breccia parent material was impact-excavated from depth, admixed with dark inclusions and rapidly cooled (700 to 200°C in about 2 hours) [15]. During this cooling, clast and matrix material acquired thermal remanent magnetization. Random conglomeration of clasts and unconsolidated matrix materials caused the clasts to have random magnetic orientations and the matrix areas to have net magnetic intensities of zero (due to the cancellation of numerous randomly oriented magnetic vectors of equal intensity in the matrix). A subsequent ambient magnetic field imparted a uniform net magnetic orientation to the matrix and caused the magnetic orientations of the clasts to be somewhat less random. The Abee breccia was later consolidated, possibly by shock or by shallow burial and very long-period/low-temperature (< 215°C) metamorphism.

Microchondrule-bearing clast in the Piancaldoli LL3 meteorite: a new kind of type 3 chondrite and its relevance to the history of chondrules

In the Piancaldoli LL3 chondrite, we found a mm-sized clast containing ~100 chondrules 0.2–64 μm in apparent diameter (much smaller than any previously reported) that are all of the same textural type (radial pyroxene; FS1–17). This clast, like other type 3 chondrites, has a fine-grained Ferich opaque silicate matrix, sharply defined chondrules, abundant low-Ca clinopyroxene and minor troilite and Si- and Cr-bearing metallic Fe,Ni. However, the very high modal matrix abundance (63 ± 8 vol. %), unique characteristics of the chondrules, and absence of microscopically-observable olivine indicate that the clast is a new kind of type 3 chondrite. Most chondrules have FeO-rich edges, and chondrule size is inversely correlated with chondrule-core FeO concentration (the first reported correlation of chondrule size and composition). Chondrules acquired Fe by diffusion from Fe-rich matrix material during mild metamorphism, possibly before final consolidation of the rock. Microchondrules (those chondrules ⩽ 100 μm in diameter) are also abundant in another new kind of type 3 chondrite clast in the Rio Negro L chondrite regolith breccia. In other type 3 chondrite groups, microchondrule abundance appears to be anticorrelated with mean chondrule size, viz. 0.02–0.04 vol. % in H and CO chondrites and ⩽0.006 vol. % in L, LL, and CV chondrites. Microchondrules probably formed by the same process that formed normal-sized droplet chondrules: melting of pre-existing dustballs. Because most compound chondrules in the clast and other type 3 chondrites formed by collisions between chondrules of the same textural type, we suggest that dust grains were mineralogically sorted in the nebula before aggregating into dustballs. The sizes of compound chondrules and chondrule craters, which resulted from collisions of similarly-sized chondrules while they were plastic, indicate that size-sorting (of dustballs) occurred before chondrule formation, probably by aerodynamic processes in the nebula. We predict that other kinds of type 3 chondrites exist which contain chondrule abundances, size-ranges and proportions of textural types different from known chondrite groups.

Derivation of a heterogeneous lithic fragment in the Bovedy L-group chondrite from impact-melted porphyritic chondrules

The Bovedy L-group chondrite contains a light-colored poikilitic lithic fragment with olivine, low-Ca pyroxene and kamacite compositions characteristic of porphyritic chondrules from unequilibrated ordinary chondrites. Its texture, compositional similarities to porphyritic chondrules, and low Na2O, K2O and P2O5 content indicate that the fragment represents a solidified, slightly fractionated impact melt formed from a source that was rich in porphyritic chondrules. The fragment is heterogeneous, with a progressive increase in the bulk MgOFeO ratio and in MgO content of olivines and low-Ca pyroxenes across its length. 39Ar40Ar analyses of the fragment and host indicate that the meteorite experienced extensive degassing due to reheating. The approximate age of 0.5–0.94 Byr dates the reheating event and not the formation of the lithic fragment or the Bovedy breccia. This reheating event renders the fragments and hosts metallographic cooling rate of ~ 5 C/Myr (through 500°C) imprecise. However, the absence of martensite and the presence of kamacite. zoned taenite and tetrataenite in the fragment and host are consistent with such slow cooling through 500°C. This cooling rate must have resulted from burial of the fragment-host assemblage beneath insulating material on the Bovedy parent body. If the thermal diffusivity (κ) of this overburden was approximately comparable to that of the lunar regolith (10−4cm2/sec), then the fragment was buried at a depth ≌ 6.5 km; if K = 10−2 cm2/sec (similar to chondritic material), then the fragment was buried at a depth ≅65 km.

New kind of type 3 chondrite with a graphite-magnetite matrix

Abstract We have discovered four clasts in three ordinary-chondrite regolith breccias which are a new kind of type 3 chondrite. Like ordinary and carbonaceous type 3 chondrites, they have distinct chondrules, some of which contain glass, highly heterogeneous olivines and pyroxenes, and predominantly monoclinic low-Ca pyroxenes. But instead of the usual fine-grained, Fe-rich silicate matrix, the clasts have a matrix composed largely of aggregates of micron- and submicron-sized graphite and magnetite. The bulk compositions of the clasts as well as the types of chondrules (largely porphyritic) are typical of type 3 ordinary chondrites, although chondrules in the clasts are somewhat smaller (0.1–0.5 mm). A close relationship with ordinary chondrites is also indicated by the presence of similar graphite-magnetite aggregates in seven type 3 ordinary chondrites. This new kind of chondrite is probably the source of the abundant graphite-magnetite inclusions in ordinary-chondrite regolith breccias, and may be more common than indicated by the absence of whole meteorites made of chondrules and graphite-magnetite.