Constraints on protosolar planetary disk formation from the potassium-isotope compositions of the chondritic meteorites

Abstract

Potassium isotopes unveil some of the early geological activities and processes in the early solar system. Due to its relative volatile and mobile characteristics, the chemical and isotopic composition of K in chondrites, especially, provides valuable insights in figuring out the nebular thermal history in the protosolar planetary disk as well as potential effects on K from the secondary processes on their parent bodies.

The principal tool in this thesis is analyzing K-isotope ratios (41K/39K) in meteorite samples. The measurement of K isotopic compositions using a Multi-Collector Inductively Coupled Plasma Mass Spectrometer (MC-ICPMS) has been significantly improved over the past five years, and the high precision analyses of K isotope were possible as a result of various innovations on MC-ICPMS, especially the use of collision cell, a technique that removes the interference of species of mass 41 (41K). Additionally, the higher detection sensitivity of the modern MC-ICPMS enables precise isotope measurements to be conducted on meteorites, many of which have traced abundances of K.

This thesis presents new constraints on major stages of the formation of our protosolar planetary disk through high-precision K isotope analyses of the bulk chondrites and their various components. The main idea which connects these studies is the high-temperature nebular processes that influence K content and isotope compositions in these oldest solar system materials. The first chapter of this thesis is the examination of the astronomical environment of the protosolar molecular cloud via analyzing the K isotopic compositions of a total of 55 meteorite samples. This study argues that the isotopic anomalies in meteorites are inherited from a heterogeneous protosolar molecular cloud, which is likely associated with type II supernova injection before the solar system formed. Such a result implies that the survival of K isotopic heterogeneity in both undifferentiated and differentiated meteorites involves limited mixing among formation regions of the various planetary and chondrite parent bodies.

The second chapter of the thesis is estimating the formation timescale of our solar system that was associated with the collapse of protosolar molecular cloud via analyzing the K isotopic compositions of Ca-Al-rich inclusions (CAIs), the oldest materials that dates the age of solar system. The initial abundance of 41Ca in the solar system can be inferred from the presence of its decay product 41K in CAIs, and it places important constraints on the timing of the solar system’s formation. This chapter reports a new initial 41Ca abundance that is about five times higher than the previously accepted value. This new value likely requires an injection of new stellar materials from a Wolf-Rayet or AGB star into the protosolar molecular cloud about 0.6 - 1.0 million years before the formation of the solar system.

The final chapter of this thesis is evaluating the formation environment of planetary building blocks, including small planetary bodies, in the protosolar planetary disk via analyzing the isotopic composition of K of chondrules and matrix from both carbonaceous and ordinary chondrites. This study found that a wider variation of K isotope composition in chondrules and matrix, compared to the bulk, likely reflects sample heterogeneity from the nebular environment instead of secondary processes. In addition, the lack of open-system behavior for the K isotopes suggests a near-equilibrium condition for the chondrules’ formation, possibly accompanied by a high ambient gas pressure.