DETERMINATION OF THE MODAL MINERALOGY OF CM CARBONACEOUS CHONDRITES USING QEMSCAN®: CONSTRAINTS ON PROGRESSIVE ASTEROIDAL AQUEOUS ALTERATION

Introduction: Evidence for the interaction of water with early solar system materials is widespread among carbonaceous chondrite meteorites [1]. Among the most heavily hydrated chondrites are the CM carbonaceous chondrites. Several studies have suggested that the CM chondrites have undergone progressive aqueous alteration within an asteroidal parent body (e.g. 1-5) that has transformed primary anhydrous phases to secondary alteration products. In this study we have evaluated the modal mineralogy of 14 CM chondrites, ranging from petrologic type 2 to type 1 using using QEMSCAN^® (Quantitative Evaluation of Minerals Using Scanning Electron Microscopy).

Techniques. Polished thin sections were mapped in automated mode using a 5 x 5 µm beam stepping interval. X-ray spectra for each pixel were obtained by simultaneous acquisition from 4 Bruker SDD EDS X-ray detectors. Phase ID was carried out utilizing EDS X-ray spectral mineral libraries, calibrated for individual, boundary and intergrowth phases.

Results. Even the least altered CM chondrites contain high abundances of secondary phases – 15 vol% Mg-Fe serpentine and 32 vol% tochilinite-cronstedtite intergrowths (T/C), with only 20 vol% primary anhydrous silicates (olivine+low-Ca pyroxene). The abundance of anhydrous silicates decreases as the abundance of serpentine increases, but the correlation is not 1:1. The most significant correlation observed in the data is a strong linear decrease in the abundance of tochilinite-cronsteditite with increasing modal abundance of Mg-Fe serpentine.

Discussion: Our dataset shows that the process of aqueous alteration is complex, but involves two major processes. First, primary Mg-rich silicates, dominantly olivine and low-Ca pyroxene in chondrules undergo alteration forming Mg-Fe silicates and releasing Si and Mg into the aqueous fluid. This increase in the activity of Si and Mg results in the destabilization of tochilinite-cronstedtite, which undergoes reaction to form more Mg-Fe serpentine. Hence the decrease in the abundance of primary silicates and Mg-Fe serpentines is clearly coupled.

[1] Brearley, A.J. (2006) MESS II. pp. 587; [2] Browning, L.B. et al. (1996) GCA 60, 2621. [3] Rubin, A.E. et al. (2007) GCA 71, 2361. [5] Howard, K.T. et al. (2009) GCA 73, 4576-4589. [5] Howard, K.T. et al. (2011) GCA 75, 2735-2751.