High-Temperature Events in the Protoplanetary Disk: Formation Conditions of Silica in Chondrules

Abstract

This thesis looks at the occurrence of silica in two specific settings: silica in silica-rich igneous rims (SIRs) in carbonaceous Renazzo-like (CR) chondrites and silica-bearing chondrules in enstatite chondrites (ECs). We combine observations of natural samples and experimental analogs to constrain formation conditions of both SIRs and silica-bearing chondrules in ECs. Silica-rich igneous rims (SIRs) are a unique feature of the CR chondrites which typically surround type I porphyritic chondrules. We studied SIRs surrounding 27 chondrules from 5 CR chondrites and noted that different textural types of SIRs exist (common, Si-glass, and Si-edge). SIRs typically contain silica, low-Ca pyroxene, Ca-rich pyroxene ± glass/mesostasis ± plagioclase ± rare olivine. Using, Raman spectroscopy, we determined that cristobalite is the only silica polymorph, with the exception of one tridymite SIR. The stability field of cristobalite lies between 1470-1713 degrees Celsius suggesting that SIRs were heated to high temperatures. Since there is little evidence at the edge of chondrules surrounded by SIRs that the host chondrule was re-melted, we suggest that the temperature of formation of SIRs is likely at the lower end of the temperature range and that the duration of heating was relatively short. Accretion and condensation models have been proposed for SIR formation, but it is difficult to determine which of these occurred as both are poorly constrained. However, based on our observations of sharp interfaces between SIRs and host chondrules, lack of melting along this interface, and distinct compositional boundaries observed in X-ray maps between some host chondrules and SIRs, we support the accretion model of SIR formation. To better constrain the formation conditions of SIRs, we conducted a series of experiments to reproduce natural SIRs. Experiments conducted at peak temperatures of 1310-1507 degrees Celsius with linear cooling rates between 30-90 degrees Celsius /hr successfully reproduce textures and mineral compositions of natural SIRs. For SIRs which contain fine-grained mesostasis, slower cooling rates at lower temperatures are required (e.g., 6 degrees Celsius/hr from 1200 degrees Celsius). Additionally, we have shown experimentally that cristobalite can form in experiments that had peak temperatures less than 1470 degrees Celsius and therefore we demonstrate that silica polymorphs are not robust temperature indicators and the presence of cristobalite in natural SIRs does not necessarily indicate high peak temperatures. Overall, our experiments demonstrate that SIRs plausibly formed by accretion of silica-rich dust to the surface of a solid chondrule, followed by a short heating event comparable to the chondrule-forming heating process. Silica-bearing chondrules are ubiquitous in ECs. We studied both EH and EL chondrites to characterise the texture, silica composition, and silica polymorphs of silica-bearing chondrules. We studied a range of chondrule textural types including porphyritic pyroxene, radial pyroxene, and barred pyroxene. Silica appears to be the last phase to crystallise in these systems. Silica is associated with low-Ca pyroxene ± Ca-rich pyroxene ± glass ± plagioclase ± Fe-sulfides. Using Raman spectroscopy, we determined that cristobalite is the dominant polymorph with the exception of one chondrule which contained both quartz and cristobalite. Despite the common occurrence of silica-bearing chondrules in ECs, no previous study has specifically aimed to reproduce them in order to understand their formation conditions. We conducted dynamic crystallisation experiments to investigate the formation conditions of silica-bearing EC chondrules. Experiments conducted at 1575 degrees Celsius with linear cooling rates of 50 and 30 degrees Celsius/hr contain silica grains whereas experiments cooled rapidly (90 degrees Celsius/hr) at high temperatures do not. Our experiments reproduced some of the textures observed for ma

Date of Award	1 Aug 2022
Original language	English
Awarding Institution	The University of Manchester
Supervisor	James Gilmour (Supervisor) & Rhian Jones (Supervisor)