Winchcombe: An example of rapid terrestrial alteration of a CM chondrite

Self Cite

The Winchcombe meteorite fell on February 28, 2021 and was the first recovered meteorite fall in the UK for 30 years, and the first UK carbonaceous chondrite. The meteorite was widely observed by meteor camera networks, doorbell cameras, and eyewitnesses, and 213.5 g (around 35% of the final recovered mass) was collected quickly—within 12 h—of its fall. It, therefore, represents an opportunity to study very pristine extra‐terrestrial material and requires appropriate careful curation. The meteorite fell in a narrow (600 m across) strewn field ~8.5 km long and oriented approximately east–west, with the largest single fragment at the farthest (east) end in the town of Winchcombe, Gloucestershire. Of the total known mass of 602 g, around 525 g is curated at the Natural History Museum, London. A sample analysis plan was devised within a month of the fall to enable scientists in the UK and beyond to quickly access and analyze fresh material. The sample is stored long term in a nitrogen atmosphere glove box. Preliminary macroscopic and electron microscopic examinations show it to be a CM2 chondrite, and despite an early search, no fragile minerals, such as halite, sulfur, etc., were observed.

Section: Terrestrial Alterationmentioning

confidence: 99%

Section: Curationmentioning

confidence: 99%

See 1 more Smart Citation

Recovery and curation of the Winchcombe (CM2) meteorite

Russell

King

Bates

et al. 2023

Self Cite

“…Its relatively low abundance may be due to partial dehydration while the samples were held under vacuum prior to the pyrolysis (the three Winchcombe samples were under vacuum for between 2 and 6 days; Table S2). The source of the weakly bound-2 fraction is also likely to be adsorbed molecular water, although hydrous sulfates may have made a minor contribution because very small quantities of terrestrially formed gypsum and bassanite (CaSO 4 Á0.5H 2 O) were described by Jenkins et al (2023) from a different piece of Winchcombe that was recovered a week after its fall. Our conclusions from the stepwise pyrolysis data are supported by thermogravimetric analysis (TGA) results from a sample of Winchcombe that was analyzed 5 days after it fell showing a mass loss between 20 and 200°C (inferred to be mainly due to water) of 3.0 wt% (King et al, 2022).…”

Section: Rate Of Terrestrial Contamination Of CM Fallsmentioning

confidence: 99%

The water content of CM carbonaceous chondrite falls and finds, and their susceptibility to terrestrial contamination

Lee,

Hallis,

Daly

et al. 2023

Self Cite

CM carbonaceous chondrites can be used to constrain the abundance and H isotopic composition of water and OH in C‐complex asteroids. Previous measurements of the water/OH content of the CMs are at the higher end of the compositional range of asteroids as determined by remote sensing. One possible explanation is that the indigenous water/OH content of meteorites has been overestimated due to contamination during their time on Earth. Here we have sought to better understand the magnitude and rate of terrestrial contamination through quantifying the concentration and H isotopic composition of telluric and indigenous water in CM falls by stepwise pyrolysis. These measurements have been integrated with published pyrolysis data from CM falls and finds. Once exposed to Earth's atmosphere CM falls are contaminated rapidly, with some acquiring weight percent concentrations of water within days. The amount of water added does not progressively increase with time because CM falls have a similar range of adsorbed water contents to finds. Instead, the petrologic types of CMs strongly influence the amount of terrestrial water that they can acquire. This relationship is probably controlled by mineralogical and/or petrophysical properties of the meteorites that affect their hygroscopicity. Irrespective of the quantity of water that a sample adsorbs or its terrestrial age, there is minimal exchange of H in indigenous phyllosilicates with the terrestrial environment. The falls and finds discussed here contain 1.9–10.5 wt% indigenous water (average 7.0 wt%) that is consistent with recent measurements of C‐complex asteroids including Bennu.

“…The record of variable aqueous alteration in CM chondrites can indicate the conditions on the CM parent body/bodies; such as differences in water/rock ratios, temperatures, and duration of alteration (Fujia, 2018; Guo & Eiler, 2007; Howard et al., 2015; King, Mason, et al., 2021; Palguta et al., 2010; Vacher et al., 2019; Verdier‐Paoletti et al., 2017, 2019). Winchcombe in particular gives a unique opportunity to investigate aqueous alteration due to its status as one of the quickest recovered meteoritic falls, with the first samples being collected within 12 h of the fireball, therefore minimizing the influence of terrestrial alteration (Jenkins et al., 2022; Russell et al., 2022).…”

Section: Introductionmentioning

confidence: 99%

The bulk mineralogy, elemental composition, and water content of the Winchcombe CM chondrite fall

Bates

King

Shirley

et al. 2023

On the microscale, the Winchcombe CM carbonaceous chondrite contains a number of lithological units with a variety of degrees of aqueous alteration. However, an understanding of the average hydration state is useful when comparing to other meteorites and remote observations of airless bodies. We report correlated bulk analyses on multiple subsamples of the Winchcombe meteorite, determining an average phyllosilicate fraction petrologic type of 1.2 and an average water content of 11.9 wt%. We show the elemental composition and distribution of iron and iron oxidation state are consistent with measurements from other CM chondrites; however, Winchcombe shows a low Hg concentration of 58.1 ± 0.5 ng g−1. We demonstrate that infrared reflectance spectra of Winchcombe are consistent with its bulk modal mineralogy, and comparable to other CM chondrites with similar average petrologic types. Finally, we also evaluate whether spectral parameters can estimate H/Si ratios and water abundances, finding generally spectral parameters underestimate water abundance compared to measured values.