Martin, Pierre-Etienne Michel Clément (2025) Calcium-aluminium-rich inclusion populations and abundances within CM chondritic lithologies. PhD thesis, University of Glasgow.
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Abstract
C-type asteroids are considered prime candidates for the delivery of water to Earth in the early stages of the Solar System, due to their high-water and volatile content. Carbonaceous Mighei-like (CM) chondrites are water-rich polymict breccias that are thought to originate from these planetesimals. These carbonaceous meteorites have experienced extensive parent body alteration processes and display a diverse range of degrees of aqueous alteration. Among their components, Calcium-Aluminium-rich Inclusions (CAIs) are the first solids to have formed within the Solar System and accreted early onto these asteroidal parent bodies after migrating from their near-solar forming region. Therefore, these inclusions constitute ideal candidates for recording the aqueous alteration events that affected the CM parent body(-ies).
To this end, CAIs from 21 samples from six CM chondrites spanning a wide range of degrees of aqueous alteration (CM2.0-2.9) were selected for investigation. Using a combination of backscatter electron images and energy dispersive X-ray spectroscopy, CAIs were identified, selected, and measured within Asuka 12236, DNG 06004, Paris, and Winchcombe, and CAIs from DNG 06004, LAP 02239, and Winchcombe were classified according to their mineralogical and morphological features. A singular large altered complex aggregate (CAI) from Winchcombe was further studied using electron probe micro-analysis, transmission electron microscopy, and transmission Kikuchi diffraction to determine the potential role of its unusual pre-accretionary mineralogy on secondary mineralisation processes.
Measurements of CAI modal abundances, combined with data from the literature, reveal a clear correlation: CM CAI modal abundance decreases relative to the increasing aqueous alteration of their host lithology. This is due to the progressive alteration and destruction of CAIs through fluid-mediated replacement reactions. Further petrological and geochemical investigations of the CAI populations within Winchcombe reveal that CAIs have been affected differentially by fluid-mediated replacement reactions. Further evidencing that the CM parent body(-ies) likely underwent heterogeneous aqueous alteration processes. This suggests that the initial distribution of CAIs within the CM parent body(-ies) was likely homogeneous, and heterogeneity in the CM CAI record was caused by subsequent fluid-driven alteration events.
During the large-scale investigation of CM CAIs, three rare CompoundChondrule-CAIs (CCCAIs) were identified within Aguas Zarcas and LAP 02239. These objects are described as CAIs enclosed within chondrules and have been previously reported in other major carbonaceous groups (CO, CV, and CH). One CCCAI was subjected to electron backscatter diffraction to test the potential for CAIs to act as nucleation sites for chondrule formation. Additionally, in order to trace the origin of the CCCAI components, oxygen isotopes were measured using secondary ion mass spectrometry. The targets included the CAI and chondrule components of CCCAIs, as well as other CAIs and chondrules from Aguas Zarcas, DNG 06004, LAP 02239, Paris, and Winchcombe for comparison.
Based on the oxygen isotope analysis, the components of the CCCAIs belong to the same CAI and chondrule populations found within the CM lithologies. This led to the proposition of a model for the formation of CCCAIs by the incorporation of CAIs within a precursor ferromagnesian melt during transient heating events in the chondrule forming region. This suggests an early outward migration of near-solar refractory material within 1 Ma after CAI formation (before the formation of a physical barrier, e.g. proto-Jupiter).
The CM CAI populations are diverse in their pre- and post-accretionary histories and can provide invaluable information by constraining the initial and early compositions of the CM parent body(-ies). The methods developed during this thesis could be applied to other carbonaceous groups in order to improve our understanding of early accretion and alteration of asteroidal parent bodies.
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