Thermal History of Matrix Forsterite Grains from Murchison Based on High-resolution Tomography

Giulia Perotti,Dario Ferreira Sanchez,Elishevah M M E Van Kooten,Henning O Sørensen,Esther H R Tsai,Kim N Dalby,Anja C Andersen,Henning Haack,Daniel Grolimund,Mirko Holler,Tue Hassenkam

doi:10.3847/1538-4357/ac26bc

Abstract

Abstract Protoplanetary disks are dust- and gas-rich structures surrounding protostars. Depending on the distance from the protostar, this dust is thermally processed to different degrees and accreted to form bodies of varying chemical compositions. The primordial accretion processes occurring in the early protoplanetary disk such as chondrule formation and metal segregation are not well understood. One way to constrain them is to study the morphology and composition of forsteritic grains from the matrix of carbonaceous chondrites. Here, we present high-resolution ptychographic X-ray nanotomography and multimodal chemical microtomography (X-ray diffraction and X-ray fluorescence) to reveal the early history of forsteritic grains extracted from the matrix of the Murchison CM2.5 chondrite. The 3D electron density maps revealed, at unprecedented resolution (64 nm), spherical inclusions containing Fe–Ni, very little silica-rich glass and void caps (i.e., volumes where the electron density is consistent with conditions close to vacuum) trapped in forsterite. The presence of the voids along with the overall composition, petrological textures, and shrinkage calculations is consistent with the grains experiencing one or more heating events with peak temperatures close to the melting point of forsterite (∼2100 K), and subsequently cooled and contracted, in agreement with chondrule-forming conditions.

Highlights

Carbonaceous chondrites contain materials that provide key insights onto the physical and chemical properties of solar nebula dust grains that later agglomerated into the building blocks of the planets in the Solar System (e.g., Scott 2007)
The primordial accretion processes occurring in the early protoplanetary disk such as chondrule formation and metal segregation are not well understood
The energy dispersive X-ray spectroscopy (EDXS) only investigates composition within a few hundred nm or micrometers of the surface, but since the tomography of the forsteritic matrix is homogeneous throughout the sample, it is reasonable to assume that the EDXS reflects the composition of the bulk of the particle

Summary

Introduction

Carbonaceous chondrites contain materials that provide key insights onto the physical and chemical properties of solar nebula dust grains that later agglomerated into the building blocks of the planets in the Solar System (e.g., Scott 2007). They consist of a fine-grained matrix which harbours spherical chondrules (once molten silicate droplets that formed during transient heating events in the solar nebula) and refractory inclusions (i.e., the first formed solids; Connelly et al 2012). We used a novel powerful method, Ptychographic X-ray Computed nano-Tomography (PXCT) which, to our knowledge, is here applied to planetary science for the first time. Apart from adopting PXCT, we employed other non-destructive techniques such as micro X-ray diffraction (μ-XRD) tomography, micro X-ray fluorescence (μXRF) tomography, micro X-ray Absorption Near-edge Spectroscopy (μ-XANES) and energy dispersive X-ray spectroscopy (EDXS)

Methods

Discussion

Conclusion