Howardite-eucrite-diogenite Research Articles

Early solar system timescales according to 53Mn- 53Cr systematics

We present results of a study of the 53Mn- 53Cr systematics in various solar system objects: angrites, eucrites, chondrites, diogenites, pallasites, the Earth and the Moon, and SNC meteorites. The primary goal of this study was to explore the capabilities of the 53Mn- 53Cr isotope system as a chronometer and as a tracer for events in the early solar system, to obtain chronological information on different classes of meteorites, and to investigate the indigenous distribution of 53Mn in the late nebula. These studies have shown that all meteorite groups investigated so far have excess 53Cr relative to the terrestrial value. A lunar sample exhibits 53Cr/ 52Cr ratios which are the same as the terrestrial normal. The angrites, several eucrites, and the pallasites show clear evidence for the existence of life 53Mn during their formation while other meteorites were isotopically equilibrated after essentially all 53Mn had decayed. A well defined whole-rock 53Mn- 53Cr isochron for the HED (Howardite-Eucrite-Diogenite) parent body was obtained. The isochron indicates that this planetesimal was essentially totally molten and differentiated ∼7 Ma before the angrites crystallized. Using the absolute age of the angrites as a time marker this event has occurred 4565 Ma ago, within present uncertainties at the same time when high temperature meteorite inclusions (CAI) were formed in the nebula. The first basalts were deposited onto its surface within less than 3 Ma. The bulk Mn/Cr ratios of the HED parent body (presumably Vesta), the angrites, and the pallasites are consistent with a chondritic Mn/Cr ratio. The results from the SCN meteorites show that their 53Cr excesses are less than half of those found in the other meteorites. Thus, the characteristic 53Cr/ 52Cr ratio of Mars (assuming SNCs originate from this planet) are intermediate between that of the earth-moon system and those of the other meteorites. When these 53Cr excesses are plotted as a function of the heliocentric distance of the place of origin of the samples then a linear relationship is indicated. Provided that this variation is due to the decay of 53Mn then a radial heterogeneous distribution of 53Mn must have existed in at least the inner early solar system. It is argued that radial fractionation within the nebula, based on the slightly higher volatility of Mn as compared to that of Cr, is an unlikely cause for this distribution. Thus, it must be an intrinsic feature of the late solar nebula. Stochastic mixing processes at the planetary embryo stage did obviously not eradicate this heterogeneity. Based on the 53Mn- 53Cr systematics in HED meteorites and in chondrites rather narrow limits are inferred for the age of the solar system or, more accurately, for the start of the decay of 53Mn within the solar nebula; the range of possible values is 4568–4571 Ma. The lower limit is consistent with the Pb-Pb ages of CAI’s.

The Kapoeta howardite: Implications for the regolith evolution of the howardite‐eucrite‐diogenite parent body

Abstract— A large hand sample and numerous polished thin sections, made from the hand sample, of the Kapoeta howardite and its many diverse lithic clasts were studied in detail by optical microscopy and electron microprobe techniques in an attempt to understand the surface processes that operated on the howardite‐eucrite‐diogenite (HED) parent body (most likely the asteroid 4 Vesta). Four unique, unusually large clasts, designated A (mafic breccia), B (granoblastic eucrite), D (howardite) and H (melt‐coated breccia), were selected for detailed study (modal analysis, mineral microprobe analysis, and noble gas measurements). Petrographic studies reveal that Kapoeta consists of a fine‐grained matrix made mostly of minute pyroxene and plagioclase fragments, into which are embedded numerous different lithic and mineral clasts of highly variable sizes. The lithic clasts include pyroxene‐plagioclase (eucrite), orthopyroxenite (diogenite), howardite, impact‐melt, metal‐sulfide‐rich, and carbonaceous chondrite clasts. The howardite clasts include examples of lithic clasts that constitute breccias‐within‐breccias, suggesting that at least two regolith generations are represented in the Kapoeta sample we studied. The clast assemblage suggests that repeated shock lithification was an important process during regolith evolution. Noble gas analyses of clast samples fall into two populations: (a) solar‐gas‐rich clasts H (rim only) and D and (b) clasts A and B, which are essentially free of solar gases. The concentrations of solar noble gases in the two matrix samples differ by a factor of ∼40. It appears that clast D is a true regolith breccia within the Kapoeta howardite (breccia‐within‐breccia), while clast H is a regolith breccia that has been significantly impact reworked. Our data indicate that the Kapoeta howardite is an extraordinarily heterogeneous rock in modal mineral and lithic clast abundances, grain size distributions, solar‐wind noble gas concentrations and cosmic‐ray exposure ages. These results illustrate the repetitive nature of impact comminution and lithification in the regolith of the HED parent body.

Mineralogical records of early planetary processes on the howardite, eucrite, diogenite parent body with reference to Vesta

Abstract— Mineralogical information recovered from the howardite, eucrite, diogenite (HED) meteorites was employed to reconstruct the history of the parent body and relate it to 4 Vesta. These interpreted crustal evolution processes were then compared to the expected geological features on the surface of a likely proto‐planet, 4 Vesta. The original crustal materials of the HED parent body were preserved as mineral grains and lithic clasts, but in many eucrites, Fe/Mg ratios in pyroxenes were homogenized by diffusion after crystallization. The crystallization trend of the protocrust has been deciphered by (1) examining monomict and crystalline samples and using their mineralogical and chemical information to formulate a sequence of crystallization and cooling trends; and by (2) reconstructing the original crust prior to cratering events from lithic clasts and mineral fragments in polymict breccias such as howardites and polymict eucrites. Mineral components are identical, both in the individual HED and in polymict breccias, and no remnants of primitive materials were preserved in the polymict breccias. A layered crust model reconstructed from such breccias consists of an upper crust with extrusive lava‐like eucrites that have been brecciated and metamorphosed, diogenite mantle, and cumulate eucrites of varying thickness between them. This model can be used to explain the surface geological features of Vesta observed from the Hubble space telescope. A large crater with diogenitic orthopyroxene at the crater floor is consistent with the deepest diogenitic layer of the layered crust model; and an underlying olivine layer is expected from early crystallized olivine in the crystal fractionation model. The old terrain of eucritic surface materials of Vesta can be howardites, polymict eucrites, or regolith‐like eucrites produced from eucrites extruded and impacted on the surface. Partial melting models of eucrites seem to be favored by the rare‐earth element (REE) chemistry and experimental studies. Unfortunately, partial melting models have not demonstrated how the HED parent body is converted to a layered crust without producing any metamorphosed primitive material in the layered crust. The origin of cumulate eucrites with systematic variation of textures and chemistries of pyroxene can be explained by the layered crust model with excavation and mixing of trapped liquid. Discovery of basaltic materials with Na‐rich plagioclase and augite in iron meteorites, which are the products of partial melting, suggests that eucrites may be unique to a body that underwent large‐scale differentiation and metamorphism.

The fate of pyroclasts produced in explosive eruptions on the asteroid 4 Vesta

Abstract— We evaluate the consequences of explosive activity having taken place during volcanic eruptions on the differentiated asteroid 4 Vesta, which is the likely parent body of the howardite‐eucrite‐diogenite (HED) meteorites. For a wide range of magma volatile contents, we calculate the eruption speeds and subsequent trajectories of the pyroclastic magma droplets produced. By considering the size distribution and eruption speeds of the droplets, and the mass fluxes at which they are ejected, we show that, under all realistic circumstances, the droplets will have formed lava fountains that were extremely optically dense. As a result, virtually all of the droplets will have reached the surface having suffered a negligible amount of radiative cooling and will have coalesced into lava ponds feeding lava flows. Typically, <1% of the pyroclasts will have undergone enough cooling to allow them to accumulate into a recognizable fall deposit consisting of unwelded or partly welded volcanic glass beads. This result is consistent with the apparent absence of identifiable pyroclastic material in the HED (and other types of) differentiated meteorites.

A magma ocean on Vesta: Core formation and petrogenesis of eucrites and diogenites

Abstract—Available evidence strongly suggests that the HED (howardite, eucrite, diogenite) meteorites are samples of asteroid 4 Vesta. Abundances of the moderately siderophile elements (Ni, Co, Mo, W and P) in the HED mantle indicate that the parent body may have been completely molten during its early history. During cooling of a chondritic composition magma ocean, equilibrium crystallization is fostered by the suspension of crystals in a convecting magma ocean until the crystal fraction reaches a critical value near 0.80, when the convective system freezes and melts segregate from crystals by gravitational forces. The extruded liquids are similar in composition to Main Group and Stannern trend eucrites, and the last pyroxenes to precipitate out of this ocean (before convective lockup) span the compositional range of the diogenites. Subsequent fractional crystallization of a Main Group eucrite liquid, which has been isolated as a body of magma, produces the Nuevo Laredo trend and the cumulate eucrites. The predicted cumulate mineral compositions are in close agreement with phase compositions analyzed in the cumulate eucrites. Thus, eucrites and diogenites are shown to have formed as part of a simple and continuous crystallization sequence starting with a magma ocean environment on an asteroidal size parent body that is consistent with Vesta.

Evolutionary History of the Mesosiderite Asteroid: A Chronologic and Petrologic Synthesis

Recent petrologic, chemical, and isotopic analyses of mesosiderite clasts and whole rocks allow reconstruction of the evolutionary history of the mesosiderite parent body (MPB). Chronologic measurements indicate that initial melting of the MPB occurred shortly after accretion ∼4.56 Ga ago, at about the same time that the eucrites and angrites formed. Significant crustal remelting ≥4.47 Ga ago, probably associated with metal-silicate mixing, is responsible for forming abundant gabbroic clasts with extreme positive Eu anomalies. Localized impact melting 4.5-3.9 Ga ago produced quench-textured clasts and matrices and may be responsible for causing mesosiderite metamorphism as well as the extensive melting of type-4 mesosiderites. Collisional disruption and gravitational reassembly ∼3.9 Ga ago is indicated by thorough Ar degassing at this time. Metallographic cooling rates of ∼1°C/Ma were likely initiated by this event; the history of higher temperature events measured by thermoluminescence was annealed out. Impact excavation and ejection of deeply buried breccias occurred ⪡3.9 Ga ago; the ∼10-160 Ma cosmic-ray-exposure ages of mesosiderites represent the epoch when individual mesosiderites were excavated from the MPB or from a ≥10-m-size fragment. It is unlikely that mesosiderites and HED (howardite-eucrite-diogenite) samples come from the same parent body. There is no evidence for metal-silicate mixing (with attendant redox effects) or major crustal remelting on the HED body. Bulk howardites have flat chondrite-normalized rare-earth-element (REE) patterns, whereas many bulk mesosiderites have REE patterns with positive Eu anomalies and relative enrichments in light REE. The nature of the heating mechanism which caused the initial differentiation of mesosiderites and howardites is problematic.

Olivine diogenites: The mantle of the eucrite parent body

Two olivine-rich Antarctic diogenites (ALH A77256 and ALH 84001) of the howardite-eucritediogenite (HED) meteorite association have olivine/pyroxene ratios similar to normative ratios in devolatilized ordinary chondrites. Based on chemical data and petrological analysis, these meteorites represent the residuum of partial melting of the mantle in the eucrite parent body (EPB). Mineral assemblages in these olivine-rich diogenites record a continuum in thermal histories from initial partial melting (1150–1200°C) to subsolidus re-equilibration ( 795 ± 55°C). The small number of olivine-rich diogenites known hints that only the outer portion of the EPB has been sampled.

Age of a eucrite clast from the Bholghati howardite

39Ar- 40Ar, Rb-Sr, and Sm-Nd investigations of the “large” eucrite clast (,1) studied by the Bholghati Consortium show that it crystallized from a melt ~ 4.51 Ga ago. The crystallization age is given most precisely by a Sm-Nd internal isochron of 4.510 ± 0.034 Ga (2σ error limits) for λ( 147Sm) = 0.00654 (Ga) −1. 142Nd 144Nd measurements show that the clast contained live 146Sm at the time of crystallization. A solar system initial 146 Sm 144Sm 0 = 0.00431 +0.0039 −0.0026 at 4.555 Ga ago can be calculated from a 146Sm- 142Nd isochron and the age of the clast. Alternatively, using ( 146Sm 144Sm ) 0 = 0.0045 as previously reported in the literature, a 146 Sm 142 Nd formation interval of 0.05 +0.12 −0.08 Ga can be calculated, which is concordant with the 147Sm- 143Nd isochron age and in agreement with Pu-U-Xe and Pu-Nd-Xe formation intervals reported in a companion Consortium paper. The initial 143Nd 144 Nd ( I Nd ) of the clast was I Nd = 0.505958 ± 50 (2σ error limits), normalized to 146Nd/ 144Nd = 0.724140 and is consistent with the CHUR reference value for evolution in a source having chondritic relative REE abundances prior to magma generation. However, when interlaboratory bias is considered, the data are also permissive of a LREE depleted source as suggested in a companion Consortium paper. A primary Rb-Sr isochron yields an age of 4.54 ± 0.06 Ga (2σ error limits) for λ( 87Rb) = 0.01402 (Ga) −1, in agreement with the more precise Sm-Nd age. The initial 87Sr 86Sr (I Sr) of the clast was I Sr = 0.699006 ± 21 and was the same within error limits as I Sr previously determined for a pristine eucrite clast from polymict eucrite Y75011; compatible with a common parent body. The 39Ar- 40Ar data and some of the Rb-Sr data show that Bholghati was affected by a major thermal event ~ 2–3 Ga ago and also provide weak evidence for a minor thermal event ≤ 1 Ga ago. The age and isotopic systematics of the clast are consistent with those of other eucrites and support the widely held assumption of a common parent body for howardites and eucrites. The isotopic investigations of polymict eucrites and howardites point to a prolonged cratering record on the howardite-eucrite-diogenite (HED) parent body and suggest that it survived the “cataclysm” affecting the Moon ~ 3.8-4.0 Ga ago.

Rb‐Sr and Sm‐Nd internal isochron ages of a subophitic basalt clast and a matrix sample from the Y75011 eucrite

The Yamato polymict eucrite Y75011 contains several coarse‐grained mesostasis‐rich basaltic clasts with subophitic texture. The preservation of extensive Mg‐Fe chemical zoning in pyroxenes and the presence of a dark mesostasis shows that these clasts are representative of pristine lava‐like basalts extruded on the surface of the Howardite‐Eucrite‐Diogenite (HED) parent body. Rubidium‐strontium analyses of a large clast, Y75011,84B, yielded a precise internal isochron age of 4.60 ± 0.05 b.y. for λ(87Rb) = 0.0139 (b.y.)−1 or 4.50 ± 0.05 b.y. for λ(87Rb) = 0.0142 (b.y.)−1. The Rb‐Sr systematics of a matrix sample are nearly identical to those of the pristine clast, showing that the matrix is dominantly comminuted mesostasis‐rich basalt. The Rb‐Sr internal isochron age of the matrix is 4.56 ± 0.06 b.y. for λ(87Rb) = 0.0139 (b.y.)−1 or 4.46 ± 0.06 b.y. for λ(87Rb) = 0.0142 (b.y.)−1. These ages are within error limits of the Rb‐Sr whole rock age of chondrites and are in agreement with 206Pb/207Pb and Sm‐Nd ages of ordinary monomict eucrites for λ(87Rb) = 0.01402 (b.y.)−1 based on a comparison of the Rb‐Sr and U‐Th‐Pb ages of chondrites (Minster et al., 1982). The good precision obtained for the Rb‐Sr ages of both clast and matrix is due to the presence of a mesostasis phase in which the Rb/Sr ratio is enriched 3–4 times above that of any eucrite phase previously analysed. The initial 87Sr/86Sr ratios of clast and matrix are 0.69894 ± 2 and 0.69896 ± 3, respectively, and are in satisfactory agreement with the BABI value (Papanastassiou and Wasserburg, 1969). There is evidence of isotopic disturbance in both the Rb‐Sr and Sm‐Nd systems of the most magnetic separate analysed. A Sm‐Nd internal isochron of 4.55 ± 0.14 b.y. for the combined data from both clast and matrix exclusive of the most magnetic separate is in agreement with the more precise Rb‐Sr age. The corresponding initial 143Nd/144Nd = 0.50587 ± 19, normalized to 148Nd/144Nd = 0.24308. The clast data alone, exclusive of the most magnetic separate, define an isochron age of 4.52 ± 0.16 b.y. and initial 143Nd/144Nd = 0.50593 ± 19. The isochron error limits quoted here for the Sm‐Nd data are 2σ from the York (1966) program and include the factor [S/(N−2)]½ used in that program to compensate for “geological error.” However, this factor is not appropriate for the Rb‐Sr isochrons and has been omitted from the quoted error limits. The initial 143Nd/144Nd of the pristine clast at 4.56 b.y. ago is 0.505877 ± 25, in agreement within error limits with the CHUR value (Jacobsen and Wasserburg, 1984). Clast Y75011,84 represents the oldest pristine, unambiguously basaltic lava as yet recovered from a solar system object. Rare earth element abundance measurements (Shimizu and Masuda, 1985) show it to be one of the most differentiated eucrites. Thus moderately evolved basaltic lavas were produced very early in the history of the HED parent body. Their preservation as clasts in the polymict eucrites suggests derivation from a different geologic setting on the parent body than that occupied by the more thermally metamorphosed monomict eucrites. We suggest that the preservation of pristine clasts in polymict eucrites may be due to their impact excavation to the surface of the parent body away from heat sources concentrated within impact craters. The monomict eucrites may be derived from the floors of craters where they were thermally metamorphosed by shock heating and by the presence of overlying impact melts and nearly molten breccias.