Tracing the earliest stages of hydrothermal alteration on the CM chondrite parent body

Abstract

The CM carbonaceous chondrites are an important resource in our efforts to understand the role of volatiles in the formation of planetary systems. We report the bulk mineralogy, water abundance, and infrared (IR) reflectance spectra of the CM chondrites LaPaz Icefield (LAP) 04514, LAP 04796, LAP 04565, and LAP 02333. They contain abundant Fe‐ and Mg‐rich serpentines (~70–80 vol%), and based on their phyllosilicate fractions, we classify LAP 04514, LAP 04796, and LAP 04565 as petrologic subtype 1.6 and LAP 02333 as 1.4. This is consistent with estimated water abundances of 9.9 (±1.1) wt% for LAP 04796, 10.4 (±0.1) wt% for LAP 04565, and 11.5 (±0.5) wt% for LAP 02333. However, LAP 04514 contains less water (8.8 ± 0.3 wt%), has a shallower 3 µm band depth, and lacks tochilinite having experienced posthydration temperatures of ~300–400 °C. We conclude that LAP 04514, LAP 04796, and LAP 04565 are among the least altered CM chondrites, which retain primitive features from the initial building blocks of the CM parent body. Finally, we use the IR spectral features of LAP 04514, LAP 04796, and LAP 04565 to identify C‐complex asteroid surfaces that record mild levels of hydration.

Constraints on the ice composition of carbonaceous chondrites from their magnetic mineralogy

“…Bulk magnetic measurements were conducted on 22 CM, two CI, and two CO chondrite powders. The same powders were previously characterised using infrared (IR) spectrocopic analysis by Bates et al (2020), and X-ray diffraction (XRD) by King et al (2015King et al ( , 2017King et al ( , 2019King et al ( , 2021a who assigned each meteorite a petrologic type based on its phyllosilicate abundance (e.g. Howard et al (2015)).…”

“…b) Magnetite fraction of the CM chondrites calculated via magnetic measurements plotted against phyllosilicate fraction measured using XRD. Phyllosilicate and magnetite XRD data from Alexander et al (2018a); Howard et al (2015); King et al (2015King et al ( , 2017King et al ( , 2019King et al ( , 2021a and Lee et al (2019). Data for the CM1.7 chondrites Paris, DOM 03182 and GRA 06172 are unpublished and are represented by unfilled points.…”

“…3a). One lobe extends along the positive B u -axis, one spreads along the B c -axis, and the third spreads diagonally 2015); King et al (2015King et al ( , 2017King et al ( , 2019King et al ( , 2021a and Lee et al (2019). Data for the CM1.7 chondrites Paris, DOM 03182 and GRA 06172 are unpublished and are represented by unfilled points.…”

Large Carbonaceous Chondrite Parent Bodies Favored by Abundance–Volatility Modeling: A Possible Chemical Signature of Pebble Accretion

“…For all three CC groups studied here, the second fractionation events are modeled to have occurred at temperatures similar to the highest temperatures indicated by petrographic or geochemical thermometry of the chondrites themselves, though this agreement is abetted by the large range of temperatures reported for each of the different CCs: for the CM chondrites, peak metamorphic temperatures range from <600 to over 1000 K (King et al 2021;Velbel & Zolensky 2021). Temperature estimates for the CV chondrites also vary considerably: various authors (e.g., Brearley & Krot 2013;Bonal et al 2020) have noted the wide range of temperatures (330°C-700°C; 603-973 K) for the CV Allende alone.…”

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

“…Increasing thermal metamorphism first causes a decrease in measured H 2 O abundance as phyllosilicates are dehydrated into a "disordered phyllosilicate phase," before being recrystallized into secondary silicates, resulting in a drop in phyllosilicate abundance (e.g., . CM data are from Bates et al (2020), King et al (2020), and King, Mason, et al (2021); CI1 data are from and (King, Solomon, et al, 2015). Thermally altered CMs from stage I to stage IV data are from , , and Lee et al (2016).…”