Low-Ca pyroxenes from LL group chondritic meteorites: crystal structural studies and implications for their thermal histories

Chondrites and Their Components

“…The cooling rates determined by the metallographic technique, which are mostly 1-10 3 C Ma -1 in the temperature range of 400-500 C, are compatible with cooling rates in the range 100-500 C determined by the fission track technique. A third technique based on Fe-Mg ordering in orthopyroxene gives cooling rates in the range 340-480 C that are systematically higher by several orders of magnitude (Folco et al, 1997), probably because metamorphism was insufficient to reequilibrate the ordering imposed during chondrule formation (Artioli and Davoli, 1994). Cooling rates can also be estimated from radiometric age data for mineral grains that close isotopically at different temperatures Bogard, 1995Bogard, , 2011Ganguly and Tirone, 2001).…”

Linking meteorites to their asteroid parent bodies: The capabilities of dust analyzer instruments during asteroid flybys

“…Since OCs are essentially composed of olivine and LCP (two‐thirds to three‐fours), as well as HCP, plagioclase, FeNi‐metal, and sulfide, four different types of spectra are expected to arise from a low‐speed flyby: While the impact of olivine and LCP grains will produce spectra showing only a distinct Mg peak, HCP grain impacts will result in Ca and Mg‐dominated spectra (+ potentially some Na and Al from augite in type 3 and 4 OCs; Brearley & Jones, 1998), whereas plagioclase grains exhibit simultaneous Ca, Na, and Al (without Mg), and the spectra from sulfide and FeNi‐metal grain impacts will not show a clear peak signature. Although LCP may contain some Ca, the risk of misidentification is nevertheless estimated to be low, as the wollastonite content (of LCP) in all three groups does typically not exceed 2 mole% (e.g., Artioli & Davoli, 1994; Slater‐Reynolds & McSween Jr., 2005), therefore producing a spectrum strongly dominated by Mg. Consequently, a classification as H, L, or LL type may be possible based on the number of Mg‐dominated spectra, deriving from olivine and LCP impacts, compared to the number of undistinctive spectra from FeNi‐metal and sulfide grain impacts (Figure 4). It should be noted, however, that distinguishing between H and LL chondrites in a flyby of an H‐analogue asteroid using the proportion of Mg‐dominated spectra would require the detection of 585 grains for a 1‐sigma identification, a number that is above the baseline assumption of this work.…”