Bunburra Rockhole: Exploring the geology of a new differentiated asteroid

Bunburra Rockhole is the first recovered meteorite of the Desert Fireball Network. We expanded a bulk chemical study of the Bunburra Rockhole meteorite to include major, minor and trace element analyses, as well as oxygen and chromium isotopes, in several different pieces of the meteorite. This was to determine the extent of chemical heterogeneity and constrain the origin of the meteorite. Minor and trace element analyses in all pieces are exactly on the basaltic eucrite trend. Major element analyses show a slight deviation from basaltic eucrite compositions, but not in any systematic pattern. New oxygen isotope analyses on 23 pieces of Bunburra Rockhole shows large variation in both δ17O and δ18O, and both are well outside the HED parent body fractionation line. We present the first Cr isotope results of this rock, which are also distinct from HEDs. Detailed computed tomographic scanning and back-scattered electron mapping do not indicate the presence of any other meteoritic contaminant (contamination is also unlikely based on trace element chemistry). We therefore conclude that Bunburra Rockhole represents a sample of a new differentiated asteroid, one that may have more variable oxygen isotopic compositions than 4 Vesta. The fact that Bunburra Rockhole chemistry falls on the eucrite trend perhaps suggests that multiple objects with basaltic crusts accreted in a similar region of the Solar System

Multiple water isotope proxy reconstruction of extremely low last glacial temperatures in Eastern Beringia (Western Arctic)

Precipitation isotopes are commonly used for paleothermometry in high latitude regions. Here we present multiple water isotope proxies from the same sedimentary context – perennially frozen loess deposits in the Klondike Goldfields in central Yukon, Canada, representing parts of Marine Isotope Stages (MIS) 4, 3 and 2 – allowing us to uniquely corroborate fractionations and temperature conversions during these Late Pleistocene cold stages. We include new and existing proxy data from: relict wedge ice, a direct archive for snowmelt; relict pore ice, an archive for bulk soil water integrating year-round precipitation; and hydrated volcanic glass shards and fossil plant waxes, which are also thought to integrate year-round precipitation but are subject to large fractionations. In some cases, our temperature estimates based on existing proxy data are much cooler than previously estimated due to our use of source water corrections for the glacial ocean, new transfer functions calibrated specifically for northern North America (dDprecip = 3.1‰·°C-1 × T – 155‰; and d18Oprecip = 0.41‰·°C-1 × T – 20.2‰), and novel insights on the apparent net fractionation correction for Eastern Beringian steppe-tundra plant waxes (ewax/precip = -59 ± 10‰). The snowmelt origin of wedge ice ensures a relatively constrained winter-spring seasonality of contributing precipitation, as supported by the consistency between water isotope measurements from Late Holocene wedge ice and modern winter-spring precipitation. Wedge ice dating to the transitional MIS 3/2 is isotopically depleted relative to modern spring-winter precipitation by an amount that indicates a temperature depression of ~14 ± 5 °C below modern. The soil water origin of pore ice, and other proxies integrating year-round precipitation from soil water, allows for a more variable precipitation seasonality. The isotopic composition of modern pore ice is consistent with mean annual precipitation. However, the isotopic composition of pore ice during MIS 3/2 converges on wedge ice values, signalling an increase in the ratio of cold-to-warm-season precipitation integrated by pore ice during glacial times, possibly due to drier summers as supported by the fossil record and climate model simulations. In the study region, water isotope proxies integrating year-round precipitation may overestimate annual temperature differences between today and recent cold stages due to transient precipitation seasonality, as detected here, and thus are best interpreted as upper bound estimates. Based on these proxies, we estimate that annual temperatures during MIS 4, 3/2 and 2 were depressed below the modern climate to a maximum of ~18 °C, 16 °C and 21 °C ± 4–5 °C, respectively. Our study highlights the value of multiple water isotope proxies towards understanding changes in precipitation seasonality and developing robust reconstructions of past climate, and may be particularly important for studies of the major climate transformations over glacial-interglacial timescales

Hamburg: A Pristine H4 Chondrite Fall

Here we present the initial results from a consortium study that was formed to thoroughly characterize the meteorite Hamburg, an H4 OC that fell in January 2018

The CM carbonaceous chondrite regolith Diepenveen.

A carbonaceous chondrite was recovered immediately after the fall near the village of Diepenveen in the Netherlands on October 27, 1873, but came to light only in 2012. Analysis of sodium and poly-aromatic hydrocarbon content suggests little contamination from handling. Diepenveen is a regolith breccia with an overall petrology consistent with a CM classification. Unlike most other CM chondrites, the bulk oxygen isotopes are extremely O-16 rich, apparently dominated by the signature of anhydrous minerals, distributed on a steep slope pointing to the domain of intrinsic CM water. A small subset plots closer to the normal CM regime, on a parallel line 2 parts per thousand lower in delta O-17. Different lithologies in Diepenveen experienced varying levels of aqueous alteration processing, being less aqueously altered at places rather than more heated. The presence of an agglutinate grain and the properties of methanol-soluble organic compounds point to active impact processing of some of the clasts. Diepenveen belongs to a CM clan with similar to 5Ma CRE age, longer than most other CM chondrites, and has a relatively young K-Ar resetting age of similar to 1.5Ga. As a CM chondrite, Diepenveen may be representative of samples soon to be returned from the surface of asteroid (162173) Ryugu by the Hayabusa2 spacecraft

The fall, recovery, classification, and initial characterization of the Hamburg, Michigan H4 chondrite.

The Hamburg meteorite fell on January 16, 2018, near Hamburg, Michigan, after a fireball event widely observed in the U.S. Midwest and in Ontario, Canada. Several fragments fell onto frozen surfaces of lakes and, thanks to weather radar data, were recovered days after the fall. The studied rock fragments show no or little signs of terrestrial weathering. Here, we present the initial results from an international consortium study to describe the fall, characterize the meteorite, and probe the collision history of Hamburg. About 1 kg of recovered meteorites was initially reported. Petrology, mineral chemistry, trace element and organic chemistry, and O and Cr isotopic compositions are characteristic of H4 chondrites. Cosmic ray exposure ages based on cosmogenic He-3, Ne-21, and Ar-38 are similar to 12 Ma, and roughly agree with each other. Noble gas data as well as the cosmogenic Be-10 concentration point to a small 40-60 cm diameter meteoroid. An Ar-40-Ar-39 age of 4532 +/- 24 Ma indicates no major impact event occurring later in its evolutionary history, consistent with data of other H4 chondrites. Microanalyses of phosphates with LA-ICPMS give an average Pb-Pb age of 4549 +/- 36 Ma. This is in good agreement with the average SIMS Pb-Pb phosphate age of 4535.3 +/- 9.5 Ma and U-Pb Concordia age of 4535 +/- 10 Ma. The weighted average age of 4541.6 +/- 9.5 Ma reflects the metamorphic phosphate crystallization age after parent body formation in the early solar system