Evaluation of dehydration mechanism during heating of hydrous asteroids based on mineralogical and chemical analysis of naturally and experimentally heated CM chondrites

Abstract

Based on the evidence derived from spectroscopic observation and meteorite analysis, some hydrous asteroids were heated and dehydrated for a certain period of time after aqueous alteration. In order to reproduce the dehydration processes, we experimentally heated Murchison CM chondrite at 600°C for 1 h (600°C/1 h), 600°C/96 h, 900°C/1 h, and 900°C/96 h under controlled oxygen partial pressures. The experimental products were compared with Belgica (B-)7904 CM chondrite, a meteorite from a dehydrated asteroid in terms of characteristic mineralogical and compositional properties. B-7904 shows properties intermediate between the two experimental products heated at 900°C/1 h and 900°C/96 h. In addition, the presence or the absence of some temperature-sensitive minerals in B-7904 suggests that it experienced heating at a temperature higher than 700°C but lower than 890°C. The duration of heating, based on the diffusion time needed to achieve the Fe-Mg zoning profile in olivine in B-7904, was estimated to be between 10 and 103 days at 700°C and between 1 to 102 h at 890°C. The obtained durations are much shorter than those expected from the internal heating model which requires prolonged heating over million years. Therefore, it is unlikely that the short-lived radionuclide of 26Al is a heat source for the dehydration of B-7904. Instead, short-duration local heating, such as that from impacts or solar radiation, is a more promising heat source.

Activity in Geminid Parent (3200) Phaethon

“…The surface is not hot enough for rock itself to significantly sublimate, but the perihelion temperatures exceed those needed to thermally decompose some rocks. For example, experiments with the hydrated mineral serpentine show structural changes on laboratory timescales (hours and days) beginning at T ∼ 600 K (Akai 1992;Nozaki et al 2006;Nakato et al 2008). These changes become extensive by 900 K, comparable to the peak temperatures anticipated on Phaethon.…”

Modal mineralogy of CI and CI-like chondrites by X-ray diffraction

“…A significant fraction of C-type asteroids appear to have dehydrated surfaces with spectral features similar to those of the thermally metamorphosed CI-like chondrites (Hiroi et al, 1993(Hiroi et al, , 1996Osawa et al, 2005;Cloutis et al, 2012). The timing and mechanism of the thermal metamorphism remains poorly constrained, although studies of mineral textures (Nakato et al, 2008) and organics (Yabuta et al, 2010;Orthous-Daunay et al, 2013;Quirico et al, 2014) in the heated CM chondrites indicate that the process was short-lived and probably took place on the order of hours to several years.…”

An oxygen isotope dichotomy in CM2 chondritic carbonates—A SIMS approach

“…CM chondrites are the most common carbonaceous chondrite type and show a wide range of aqueous alteration extent-classified from intermediate (2) to complete (1) (Zolensky et al, 1997;Weisberg et al, 2006). At least 12 CM chondrites are identified as being thermally metamorphosed after hydration, possibly through impact heating (Nakamura, 2005;Nakato et al, 2008). CM chondrites preserve mineralogical, chemical, textural, and isotopic evidence of aqueous alteration of anhydrous minerals.…”