40Ar/39Ar impact ages and time-temperature argon diffusion history of the Bunburra Rockhole anomalous basaltic achondrite

The Bunburra Rockhole meteorite is a brecciated anomalous basaltic achondrite containing coarse-, medium- and fine-grained lithologies. Petrographic observations constrain the limited shock pressure to between ca. 10 GPa and 20 GPa. In this study, we carried out nine 40Ar/39Ar step-heating experiments on distinct single-grain fragments extracted from the coarse and fine lithologies. We obtained six plateau ages and three mini-plateau ages. These ages fall into two internally concordant populations with mean ages of 3640 ± 21 Ma (n=7; P=0.53) and 3544 ± 26 Ma (n=2; P=0.54), respectively. Based on these results, additional 40Ar/39Ar data of fusion crust fragments, argon diffusion modeling, and petrographic observations, we conclude that the principal components of the Bunburra Rockhole basaltic achondrite are from a melt rock formed at ~3.64 Ga by a medium to large impact event. The data imply this impact generated high enough energy to completely melt the basaltic target rock and reset the Ar systematics, but only partially reset the Pb-Pb age. We also conclude that a complete 40Ar* resetting of pyroxene and plagioclase at this time could not have been achieved at solid-state conditions. Comparison with a terrestrial analogue (Lonar crater) shows that the time-temperature conditions required to melt basaltic target rocks upon impact are relatively easy to achieve. Ar data also suggest that a second medium-size impact event occurred on a neighboring part of the same target rock at ~3.54 Ga. Concordant low-temperature step ages of the nine aliquots suggest that, at ~3.42 Ga, a third smaller impact excavated parts of the ~3.64 Ga and ~3.54 Ga melt rocks and brought the fragments together. The lack of significant impact activity after 3.5 Ga, as recorded by the Bunburra Rockhole suggest that (1) either the meteorite was ejected in a small secondary parent body where it resided untouched by large impacts, or (2) it was covered by a porous heat-absorbing regolith blanket which, when combined with the diminishing frequency of large impacts in the solar system, protected Bunburra from subsequent major heating events. Finally we note that the total (K/Ar) resetting impact event history recorded by some of the brecciated eucrites (peak at 3.8-3.5 Ga) is similar to the large impact history recorded by the Bunburra Rockhole parent body (ca. 3.64-3.54 Ga; this study) and could indicate a similar position in the asteroid belt at that time

A geochemical study of the winonaites: Evidence for limited partial melting and constraints on the precursor composition

The winonaites are primitive achondrites which are associated with the IAB iron meteorites. Textural evidence implies heating to at least the Fe, Ni–FeS cotectic, but previous geochemical studies are ambiguous about the extent of silicate melting in these samples. Oxygen isotope evidence indicates that the precursor material may be related to the carbonaceous chondrites. Here we analysed a suite of winonaites for modal mineralogy and bulk major- and trace-element chemistry in order to assess the extent of thermal processing as well as constrain the precursor composition of the winonaite-IAB parent asteroid. Modal mineralogy and geochemical data are presented for eight winonaites. Textural analysis reveals that, for our sub-set of samples, all except the most primitive winonaite (Northwest Africa 1463) reached the Fe, Ni–FeS cotectic. However, only one (Tierra Blanca) shows geochemical evidence for silicate melting processes. Tierra Blanca is interpreted as a residue of small-degree silicate melting. Our sample of Winona shows geochemical evidence for extensive terrestrial weathering. All other winonaites studied here (Fortuna, Queen Alexander Range 94535, Hammadah al Hamra 193, Pontlyfni and NWA 1463) have chondritic major-element ratios and flat CI-normalised bulk rare-earth element patterns, suggesting that most of the winonaites did not reach the silicate melting temperature. The majority of winonaites were therefore heated to a narrow temperature range of between ∼1220 (the Fe, Ni–FeS cotectic temperature) and ∼1370 K (the basaltic partial melting temperature). Silicate inclusions in the IAB irons demonstrate partial melting did occur in some parts of the parent body (Ruzicka and Hutson, 2010), thereby implying heterogeneous heat distribution within this asteroid. Together, this indicates that melting was the result of internal heating by short-lived radionuclides. The brecciated nature of the winonaites suggests that the parent body was later disrupted by a catastrophic impact, which allowed the preservation of the largely unmelted winonaites. Despite major-element similarities to both ordinary and enstatite chondrites, trace-element analysis suggests the winonaite parent body had a carbonaceous chondrite-like precursor composition. The parent body of the winonaites was volatile-depleted relative to CI, but enriched compared to the other carbonaceous classes. The closest match are the CM chondrites; however, the specific precursor is not sampled in current meteorite collections

Defining the mechanism for compaction of the CV chondrite parent body

The Allende meteorite, a relatively unaltered member of the CV carbonaceous chondrite group, contains primitive crystallographic textures that can inform our understanding of early Solar System planetary compaction. To test between models of porosity reduction on the CV parent body, complex microstructures within ~0.5-mm-diameter chondrules and ~10-µm-long matrix olivine grains were analyzed by electron backscatter diffraction (EBSD) techniques. The large area map presented is one of the most extensive EBSD maps to have been collected in application to extraterrestrial materials. Chondrule margins preferentially exhibit limited intragrain crystallographic misorientation due to localized crystal-plastic deformation. Crystallographic preferred orientations (CPOs) preserved by matrix olivine grains are strongly coupled to grain shape, most pronounced in shortest dimension < a >, yet are locally variable in orientation and strength. Lithostatic pressure within plausible chondritic model asteroids is not sufficient to drive compaction or create the observed microstructures if the aggregate was cold. Significant local variability in the orientation and intensity of compaction is also inconsistent with a global process. Detailed microstructures indicative of crystal-plastic deformation are consistent with brief heating events that were small in magnitude. When combined with a lack of sintered grains and the spatially heterogeneous CPO, ubiquitous hot isostatic pressing is unlikely to be responsible. Furthermore, Allende is the most metamorphosed CV chondrite, so if sintering occurred at all on the CV parent body it would be evident here. We conclude that the crystallographic textures observed reflect impact compaction and indicate shock-wave directionality. We therefore present some of the first significant evidence for shock compaction of the CV parent body

Successful Recovery of an Observed Meteorite Fall Using Drones and Machine Learning

We report the first-time recovery of a fresh meteorite fall using a drone and a machine-learning algorithm. The fireball was observed on 2021 April 1 over Western Australia by the Desert Fireball Network, for which a fall area was calculated for the predicted surviving mass. A search team arrived on-site and surveyed 5.1 km2 area over a 4 day period. A convolutional neural network, trained on previously recovered meteorites with fusion crusts, processed the images on our field computer after each flight. Meteorite candidates identified by the algorithm were sorted by team members using two user interfaces to eliminate false positives. Surviving candidates were revisited with a smaller drone, and imaged in higher resolution, before being eliminated or finally being visited in person. The 70 g meteorite was recovered within 50 m of the calculated fall line, demonstrating the effectiveness of this methodology, which will facilitate the efficient collection of many more observed meteorite falls

Early crustal processes revealed by the ejection site of the oldest martian meteorite

The formation and differentiation of the crust of Mars in the first tens of millions of years after its accretion can only be deciphered from incredibly limited records. The martian breccia NWA 7034 and its paired stones is one of them. This meteorite contains the oldest martian igneous material ever dated: ~4.5 Ga old. However, its source and geological context have so far remained unknown. Here, we show that the meteorite was ejected 5–10 Ma ago from the north-east of the Terra Cimmeria—Sirenum province, in the southern hemisphere of Mars. More specifically, the breccia belongs to the ejecta deposits of the Khujirt crater formed 1.5 Ga ago, and it was ejected as a result of the formation of the Karratha crater 5–10 Ma ago. Our findings demonstrate that the Terra Cimmeria—Sirenum province is a relic of the differentiated primordial martian crust, formed shortly after the accretion of the planet, and that it constitutes a unique record of early crustal processes. This province is an ideal landing site for future missions aiming to unravel the first tens of millions of years of the history of Mars and, by extension, of all terrestrial planets, including the Earth

Metamorphism and partial melting of ordinary chondrites: Calculated phase equilibria

Constraining the metamorphic pressures (P) and temperatures (T) recorded by meteorites is key to understanding the size and thermal history of their asteroid parent bodies. New thermodynamic models calibrated to very low P for minerals and melt in terrestrial mantle peridotite permit quantitative investigation of high-T metamorphism in ordinary chondrites using phase equilibria modelling. Isochemical P–T phase diagrams based on the average composition of H, L and LL chondrite falls and contoured for the composition and abundance of olivine, ortho- and clinopyroxene, plagioclase and chromite provide a good match with values measured in so-called equilibrated (petrologic type 4–6) samples. Some compositional variables, in particular Al in orthopyroxene and Na in clinopyroxene, exhibit a strong pressure dependence when considered over a range of several kilobars, providing a means of recognising meteorites derived from the cores of asteroids with radii of several hundred kilometres, if such bodies existed at that time. At the low pressures (<1 kbar) that typify thermal metamorphism, several compositional variables are good thermometers. Although those based on Fe–Mg exchange are likely to have been reset during slow cooling, those based on coupled substitution, in particular Ca and Al in orthopyroxene and Na in clinopyroxene, are less susceptible to retrograde diffusion and are potentially more faithful recorders of peak conditions. The intersection of isopleths of these variables may allow pressures to be quantified, even at low P, permitting constraints on the minimum size of parent asteroid bodies. The phase diagrams predict the onset of partial melting at 1050–1100 °C by incongruent reactions consuming plagioclase, clinopyroxene and orthopyroxene, whose compositions change abruptly as melting proceeds. These predictions match natural observations well and support the view that type 7 chondrites represent a suprasolidus continuation of the established petrologic types at the extremes of thermal metamorphism. The results suggest phase equilibria modelling has potential as a powerful quantitative tool in investigating, for example, progressive oxidation during metamorphism, the degree of melting and melt loss or accumulation required to produce the spectrum of differentiated meteorites, and whether the onion shell or rubble pile model best explains the metamorphic evolution of asteroid parent bodies in the early solar system

Modal mineralogy of CM chondrites by X-ray diffraction (PSD-XRD): Part 2. Degree, nature and settings of aqueous alteration

Within 5 million years after formation of calcium aluminium rich inclusions (CAI), high temperature anhydrous phases were transformed to hydrous phyllosilicates, mostly serpentines, which dominate the matrices of the most primitive carbonaceous chondrites. CMs are the largest group of meteorites to provide samples of this material. To understand the nature of the availability, and role of H2O in the early solar system – as well as the settings of aqueous alteration – defining CM petrogenesis is critical. By Position Sensitive Detector X-ray Diffraction (PSD-XRD), we determine the modal abundance of crystalline phases present in volumes >1% for a suite of CMs – extending Part 1 of this work that dealt only with CM2 falls (Howard et al., 2009) to now include CM2 and CM1 finds. CM2 samples contain 13–31% Fe,Mg silicates (olivine + pyroxene) and from 67% to 82% total phyllosilicate (mean 75% ± 1.3 2σ). CM1 samples contain 6–10% olivine + pyroxene and 86–88% total phyllosilicate. Magnetite (0.6–5.2%), sulphide (0.6–3.9%), calcite (0–1.9%) and gypsum (0–0.8%) are minor phases across all samples. Since phyllosilicate forms from hydration of anhydrous Fe,Mg silicates (olivine + pyroxene), the ratio of total phyllosilicate to total anhydrous Fe,Mg silicate defines the degree of hydration and the following sequence results (in order of increasing hydration): QUE 97990 < Y 791198 < Murchison < Murray < Mighei < ALHA 81002 < Nogoya ≤ Cold Bokkeveld ≤ Essebi < QUE 93005 < ALH 83100 < MET 01070 < SCO 06043. High activities of Al (mostly from reactive mesostasis) and Si help to explain the composition and structure of CM serpentines that are distinct from terrestrial standards. Our data allows inference as to CM mineralogy at the point of accretion and challenges the conceptual validity of progressive alteration sequences. Modal mineralogy also provides new insights into CM petrogenesis and hints at a component of aqueous alteration occurring in the nebula, in addition to on the CM parent body(ies)

Crystallography of an Early Amazonian Meteorite: Implications for Conditions at Crystallization

No abstract available