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

Abstract

Carbonaceous chondrites experienced varying degrees of aqueous alteration on their parent asteroids, which influenced their mineralogies, textures, and bulk chemical and isotopic compositions. Although this alteration was a crucial event in the history of these meteorites, their various alteration pathways are not well understood. One phase that formed during this alteration was magnetite, and its morphology and abundance vary between and within chondrite groups, providing a means of investigating chondrite aqueous alteration. We measured bulk magnetic properties and first-order reversal curve (FORC) diagrams of CM, CI, CO, and ungrouped C2 chondrites to identify the morphology and size range of magnetite present in these meteorites. We identify two predominant pathways of aqueous alteration among these meteorites that can be distinguished by the resultant morphology of magnetite. In WIS 91600, Tagish Lake, and CI chondrites, magnetite forms predominantly from Fe-sulfides as framboids and stacked plaquettes. In CM and CO chondrites, <0.1 μm single-domain (SD) magnetite and 0.1–5 μm vortex (V) state magnetite formed predominantly via the direct replacement of metal and Fe-sulfides. After ruling out differences in temperature, water:rock ratios, terrestrial weathering effects, and starting mineralogy, we hypothesise that the primary factor controlling the pathway of aqueous alteration was the composition of the ice accreted into each chondrite group’s parent body. Nebula condensation sequences predict that the most feasible method of appreciably evolving ice concentrations was the condensation of ammonia, which will have formed a more alkaline hydrous fluid upon melting, leading to fundamentally different conditions that may have caused the formation of different magnetite morphologies. As such, we suggest that WIS 91600, Tagish Lake, and the CI chondrites accreted past the ammonia ice line, supporting a more distal or younger accretion of their parent asteroids.

Lifetime of the Outer Solar System Nebula From Carbonaceous Chondrites

“…Below we discuss why magnetite is likely to dominate the magnetic record of these samples. In a recent study, first‐order reversal curves of DOM 08006 show a tri‐lobate geometry, which is consistent with the magnetic carriers being predominately in vortex state (Sridhar et al., 2021). Because magnetite is the most abundant magnetic recorder and has the highest susceptibility (see Section 2.1.2), we conclude that the magnetite grains are dominantly in the vortex state, such that they are robust magnetic recorders with relaxation times greater than the age of the solar system (Nagy et al., 2019).…”

“…Below we discuss why magnetite is likely to dominate the magnetic record of these samples. Recent FORC analysis of DOM 08006 shows a tri-lobate geometry which is consistent with the magnetic carriers being predominately in vortex states (Sridhar et al, 2021). Because magnetite is the most abundant magnetic recorder and with the highest susceptibility (see below), we conclude that the magnetite grains are dominantly in the vortex state, such that they are robust magnetic recorders with relaxation times greater than the age of the solar system (Nagy et al, 2019) Because magnetite formed during parent body alteration (Davidson et al, 2019) and was not subsequently heated above 300 o C, the magnetic record of these samples should be predominately in the form of crystallization remanent magnetization (CRM).…”

“…First, they formed magnetite during aqueous alteration on their parent body, so they can potentially provide constraints on the intensity of the local ambient magnetic field present during this process (Doyle et al., 2014). Second, they have ferromagnetic inclusions (e.g., magnetite; see below) with high‐fidelity paleomagnetic properties (Alexander et al., 2018; Davidson et al., 2019; Sridhar et al., 2021). Third, they experienced very minor heating after parent body aqueous alteration, thereby better preserving a magnetic record dating to the time of this early alteration rather than slightly later during subsequent metamorphic heating and cooling (Davidson et al., 2019).…”

“…Unfortunately, very little is known about the mechanism of CRM acquisition during parent body alteration and how it can affect the recovered paleointensity. A recent study suggests that magnetite can form through different pathways in chondrites (Sridhar et al., 2021). Magnetite grains in most CM and CO chondrites likely formed through pseudomorphic replacement of metal, while magnetite in framboids and plaquettes, which are found in comparatively high abundance in several chondrites, including the CM Paris, are thought to have formed through the dissolution of sulfide and precipitation of magnetite in these exotic morphologies.…”

The Winchcombe meteorite, a unique and pristine witness from the outer solar system

“…Measurements were made in high-resolution mode with m/∆m > 8000, yielding a sensitivity of 25 V/ppm of 52 Cr. Beams of 49 Ti, 50 Cr, 51 V, 52 Cr, 53 Cr, 54 Cr, and 56 Fe were simultaneously collected using cups L4, L2, L1, C, H1, H2, and H4. Each was connected to amplifiers with 10 11 -ohm feedback resistors, except the cup collecting 52 Cr with a 10 10 -ohm feedback resistor and L1 collecting 51 V with a 10 12 -ohm feedback resistor.…”