Date of Award
Doctor of Philosophy (PhD)
In Chapter 1, I review the geochemical and cosmochemical properties of potassium (K). Classified as a moderately volatile element, K serves as a robust proxy for understanding the key processes during nebular/planetary evaporation events. I review different mechanisms of volatile depletion among various planetary materials: mass-dependent variations in “nebular” processes; mass-independent variations among different nucleosynthetic sources; or planetary-scale volatile depletion processes. Furthermore, I provide an optimized K purification procedure via ion-exchange chromatography and a high-precision K isotope analysis technique using Neptune plus Multiple Collector Inductively Coupled Plasma Mass Spectrometer (MC-ICPMS) in both “cold plasma” and “dry plasma” methods. I also conduct an inter-laboratory comparison of 41K on various instrumentations on geo-reference materials.
In Chapter 2, I apply the high-precision K isotope measurements onto a suite of howardite–eucrite–diogenite (HED) meteorites. The focus on HED meteorites is driven by the evidence for extreme volatile depletion in K and other moderately volatile elements, which is inferred on the basis of its low K/U. The results show substantial heavy K isotope enrichments among HED meteorites relative to the bulk silicate Earth (+0.36 ± 0.16‰ versus 0.43 ± 0.17‰ for average HEDs and bulk silicate Earth, respectively). I propose that the average K isotopic composition obtained from HED meteorites is representative of bulk silicate 4-Vesta. The significant depletion of moderately volatile element K and its associated K heavy isotope enrichment can be interpreted as either (1) Precursor: 4-Vesta accreted from volatile-poor materials already depleted in volatiles, either through incomplete condensation from the proto-solar nebula or partial re-evaporation in the nebular environment; (2) Accretion: 4-Vesta acquired its volatile depletion and heavy K isotopic feature from the vapor loss during its accretionary growth; or (3) Magma ocean: 4-Vesta experienced volatile degassing during a global and/or in-situ magma ocean phase.
Previous K stable isotope studies largely focused on placing the collected data into the broad framework of lunar volatile depletion as well as the evolution of the Earth-Moon system. Nevertheless, the distribution of K isotope among different reservoirs in the lunar interior and K isotopic fractionation after the Moon-forming Giant Impact event remain loosely constrained. In Chapter 3, I conduct the high-precision K isotope measurements onto a new suite of lunar samples, covering a range in lunar geochemical and petrological diversity. These new data along with data from previous studies provide a comprehensive evaluation of K isotopic composition of the Moon and the processes of volatile redistribution between major reservoirs. I define the K isotopic composition of the bulk silicate Moon as 0.07 ± 0.09‰ (2SD). Furthermore, I interpret the heavy K isotope enrichment in the (or a portion of the) urKREEP reservoir as a natural consequence of extended degassing of K from the late-stage Lunar Magma Ocean (LMO) as the K chloride species. The complementary isotopically light K-bearing vapor phases condense back onto the lunar surface, causing substantial K isotope variations in lunar nonmare rocks. Nevertheless, the heterogeneous K isotope distribution should not change the K isotopic composition of the bulk silicate Moon, which is well represented by the lunar mantle.
Using spacecraft data and elemental abundances derived from martian meteorites, earlier studies set a paradigm of a volatile- and water-rich Mars relative to Earth. Nevertheless, inherent difficulty in determining the volatile budget of bulk silicate Mars makes it challenging to directly compare the extents of volatile depletions among differentiated bodies in the Solar System. Moreover, the debate about volatile depletion originating in the solar nebula or on planetary bodies has resurfaced quite strongly recently, calling for new isotope proxies. In Chapter 4, I provide an alternative for evaluating the nature of volatiles on Mars as well as mechanisms for planetary volatile retention using K isotopes. Combining available high-precision K isotope data to-date among differentiated planetary bodies, I reveal the role of fundamental physical properties in dictating the degree of volatile loss during planet formation, and thus the K isotopic variability observed. The new K isotopic composition of bulk silicate Mars and the strong correlation between 41K and planet mass suggest that the planetary body sizes fundamentally control their ability to retain volatiles. This could further shed light on water content and habitability of planets and assist with constraining unknown parent body sizes.
Chair and Committee
Bruce Fegley Jr.
Tian, Zhen, "Potassium Isotope Cosmochemistry" (2021). Arts & Sciences Electronic Theses and Dissertations. 2623.