Enstatite chondrites (EC) are a rare class of meteorites that record complex histories of accretion, thermal metamorphism, shock processing, and secondary alteration on their EH and EL parent bodies. They are highly reduced and are considered to be an important component during accretion of the Earth. Current EC petrologic type classification schemes, defining a sequence of thermal metamorphism, are qualitative only and suffer from the inadvertent inclusion of shock-melted samples that obscure metamorphic trends. Due to their unusual mineralogy, complex histories, and rarity, our understanding of the chronology of the EC is limited. In particular, the chronology of the EH3, which have apparent Ar-Ar and Rb-Sr dates of c. 2 Ga, are poorly understood. For our study, we assembled a diverse suite of EC from both EH and EL parent bodies. Based on the geochemistry of sulfides (high Cr in troilite, high Fe in the mixed Mg, Mn, Fe monosulfide) and metal (high Ni in kamacite) we developed quantitative criteria to identify quenched impact-melt rocks, allowing us to exclude these samples from our petrologic type study. Using this dataset of thermally metamorphosed EC, we constructed a quantitative classification scheme for petrologic types 3 to 6, for both EH and EL. Key indicators of progressive metamorphism include the absence of olivine in higher petrologic types, increasing abundance and grain size of feldspar, and changes in minor and trace element abundances in silicates, sulfides, and metals. We also refined the subtype classification within EH3 using Cr₂O₃ content in olivine, allowing us to distinguish the most primitive EH3. We also show that it is important to identify EH3 chondrites that contain the Qingzhen Reaction (QR), a fine-grained mineral assemblage associated with the K-bearing sulfide djerfisherite [K,Na)6(Fe,Cu,Ni)25S26Cl]. We analysed the texture, structure and geochemistry of the constituents of the QR. Our findings show that the QR is dominated by porous troilite with remnant djerfisherite and traces of halite and sphalerite. The QR is not correlated with shock stage or terrestrial weathering grade and is observed in nearly all djerfisherite mineral associations in QR-bearing EH3. We interpret the QR as an alteration product resulting from the interaction of djerfisherite with a limited volume of anhydrous fluid. The QR offers rare evidence of fluid-mediated alteration on the relatively un-altered EC parent bodies. To better understand the chronology of the EC, we conducted in situ and bulk Ar-Ar dating, and in situ Rb-Sr dating, on our EC sample suite. The Ar-Ar and Rb-Sr dates for EC samples are consistent, suggesting similar closure temperatures for both chronometers. Dates for the EH (excluding EH3-QRB) generally fall within a range from 4,400-4,550 Ma and for the EL 4,475-4,550 Ma. These dates either represent the age of parent body disruption, or the dates represent parent body cooling ages, it is not yet possible to resolve the two interpretations, given the similarity of the chronology data. The dates for both EH and EL impact-melt rocks are the same, at 4,543 ± 11 Ma, suggesting that major impacts affected both the EH and EL parent bodies around the same time. Systematics for the QR-bearing EH3 are disturbed. Some analyses retain evidence of their older parent body dates of c. 4.5 Ga, while most analyses yield a combined date of 1,620 ± 124 Ma, which we interpret as the formation age of the QR. The findings of this study, including revised quantitative classification schemes for the EC, detailed analyses of the QR alteration and its timing, and broader chronological insights into the EC, represent a meaningful advancement in our understanding of EC and by extension planetary science more broadly.
| Date of Award | 26 Aug 2025 |
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| Original language | English |
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| Awarding Institution | - The University of Manchester
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| Supervisor | Raymond Burgess (Co Supervisor), Rhian Jones (Main Supervisor) & Romain Tartese (Co Supervisor) |
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