The settings of aqueous alteration in the early solar system: A nanoscale STXM investigation of the Murchison CM2 chondrite

Authors: Paul Schofield (Natural History Museum, Diamond Light Source) , Ashley King (Natural History Museum) , Burkhard Kaulich , Majid Abyaneh (Diamond Light Source) , Tohru Araki (Diamond Light Source) , Sara Russell (Natural History Museum)
Co-authored by industrial partner: No

Type: Conference Paper
Conference: British Planetary Science Congress
Peer Reviewed: No

State: Published (Approved)
Published: December 2017
Diamond Proposal Number(s): 11426

Open Access Open Access

Abstract: Many studies have described petrographic evidence for in situ aqueous alteration on the asteroid parent body(ies) of CM chondrites [1]. However, the origin of finegrained rims (FGRs) of phyllosilicates that surround pristine anhydrous fragments in CM chondrites remains controversial. The textures of FGRs suggest that they formed through accretion onto their host objects, but it’s not clear whether hydration of the dust occurred in a nebula [2] or asteroid [3] environment. The settings of aqueous alteration in the early solar system may be inferred from crystal chemical variations within the sub-micron mineralogy of FGRs and matrix within the CM chondrites. Using synchrotron μXANES we observed systematic variations in Fe3+/ΣFe across FGRs but not within the matrix [4]. To identify the source of this variation we used STXM and nanoscale XRF to compare the crystal-chemical relationships of the matrix mineralogy with that of the FGRs. Changes in Fe L- and O K-edge spectra from matrix areas can be attributed to oxide and phyllosilicate minerals. For FGRs, small changes in the relative intensities of the Fe L3 edge main peaks suggest variations in the Fe3+/ΣFe ratio. More grain-scale variability is observed in the FGR than in the matrix, and Fe-sulphides are detected in the FGR but not in the matrix. Our data show that the composition of the FGR is mineralogically distinct from the matrix, showing it was surrounding the chondrules in their nebular sojourn prior to accretion and hence has sampled a different reservoir of dust to the matrix. A possible explanation for the data is the incorporation of ice and/or carbonaceous grains in the FGR which would cause local variability in redox states when these phases reacted on the asteroidal parent body.

Subject Areas: Earth Science

Instruments: I08-Scanning X-ray Microscopy beamline (SXM)