UC Davis

The mantle is Earth’s largest reservoir as it constitutes 80% of its volume, and the chemical composition and structure of the mantle can provide constraints on the formation and evolution processes of the Earth. The geochemistry of rocks derived from the mantle are extensively utilized to elucidate the nature of the mantle. For example, the lithophile element geochemistry of oceanic basalts portray a heterogeneous, geochemically layered mantle that is extensively altered by crust formation and subduction processes. However, the story as told by the lithophile elements provide only a partial history of the Earth as the volatile elements behave distinctly from the refractory elements. The noble gases are excellent tools for constraining mantle evolution processes due to their inert nature, low abundance, and ubiquitous presence in all mantle and surface components. Furthermore, as the solar system volatile reservoirs contains uniquely identifying “fingerprints” of noble gas isotopic ratios, the precise measurements of the noble gases in the mantle can provide clues as to which building blocks contributed to the formation of the Earth. However, despite the utility of the noble gases, there are several challenges that hinder the precise determination of the mantle noble gas isotopic ratios, such as atmospheric contamination, low sample abundance, and difficulties in cryogenic separation of the noble gases for mass spectrometric measurements. The chapters in this dissertation report on the successful measurement of the noble gas data of mantle derived mid-ocean ridge basalts (MORBs) collected from the Mid-Atlantic Ridge (MAR) and provides insight into the origin, evolution, and heterogeneity of the volatiles in the mantle.Chapter 1 presents He, Ne, Ar, and Xe data determined from the depleted MORBs (D-MORBs) collected along the MAR between the Kane and Atlantis fracture zones from 24-30 °N. iii D-MORBs are likely representative of the depleted mantle, more so than E-MORBs, which may have incorporated components of plume or recycled material. The isotopic ratios of the light noble gases He and Ne from this ridge segment are MORB-like and show little variations, with the exception of a highly localized plume-like signature at 29 °N. However, the isotopic ratios of the heavy noble gases Ar and Xe show extreme heterogeneity that span ~30% and ~50%, respectively, of the isotopic range globally observed in oceanic basalts. The 40Ar/36Ar and 129Xe/130Xe generally shows a decrease in ratios with increasing latitude, which we interpret as being evidence for the preferential subduction of the heavy noble gases. These observations provide evidence for complex distributions of heterogeneities within the MORB source that are not influenced by plumes, and which are not captured by the lithophile element geochemistry. Chapter 2 presents the first ever determination of all six krypton isotopes from MORB samples that are well-resolved from air, and the primordial isotopes of Xe. Krypton is an ideal tracer of the provenance of terrestrial volatiles as the krypton isotopes of solar system volatile reservoirs (e.g. solar, chondritic, phase Q) are well resolved. The results show that Phase Q, a carbonaceous phase that is often the only carrier of heavy noble gases in achondrites, and ordinary and enstatite chondrites, cannot be the source of mantle Kr and Xe. Rather, carbonaceous chondrites provide a better match for the mantle composition. As Phase Q was recently suggested to be a major source for atmospheric gases, the Phase Q component could not have been sourced from mantle outgassing. Instead, this Q component must have been delivered after the last equilibration event that effectively mixed the mantle gases with the atmosphere, the moon forming giant impact. However, no meteorite group in our current collection satisfies the requirements of having sufficiently high volatile contents while having only Phase Q noble gases to satisfy the atmospheric volatile budget. A volatile-rich material, not yet sampled, may be a iv major source of atmospheric noble gases. Additionally, we show that the initial MORB source must have been depleted of Xe relative to Kr by several orders of magnitude with respect to the average carbonaceous chondrites. Three possible explanations for this Xe depletion are preferential partitioning of Xe into the core, equilibrium magma ocean outgassing, and low Xe/Kr in the accreting volatile source. Chapter 3 presents the components of xenon in the Kane-Atlantis MORB (KA-MORB) source as determined from the 130Xe and 132Xe normalized xenon ratios. The xenon in MORB can be modeled as a mixture of 5 components: air, primordial Xe (in this case AVCC), 244Pu-derived xenon, 238U-derived xenon, and 129I-derived xenon. As each of these components have unique xenon isotopic ratios, the proportions of each component in the MORB mixture can be deconvoluted, given sufficient xenon isotopic measurements. Traditional methods of deconvolution utilized only five xenon ratios, but this chapter shows that utilizing all eight xenon isotopes produces deconvolution results with lowest uncertainty. The high precision determination of the components results in 129Xe*/136XePu and 136XePu/(136XePu+136XeU) that are distinct from that determined for 2ΠD43, a MORB sample often used to represent the heavy noble gas composition of the MORB mantle. As 244Pu is extinct by about 500 Myrs, variations in the 129I-derived/244Pu-derived xenon ratios between the two MORB sources must have been established before 4 Ga. Furthermore, the 129Xe*/136XePu and 136XePu/(136XePu+136XeU) ratios between the two MORB sources display seemingly contradictory degassing histories. To reconcile this observation three possible scenarios are proposed: 1) the I/Pu ratio in the MORB sources were not the same in the aftermath of accretion, 2) the two reservoirs experienced unique multi-stage degassing histories, or 3) differing amounts of late veneer material was added to the two reservoirs. Lastly, the I-Pu-Xe system has traditionally be used to calculate the Xe-Xe v mantle closure age, which marks the point in time when a reservoir transitioned from a open-system to a closed-system that retained most of the produced xenon isotopes. We calculate a Xe-Xe closure age of 88.3 ± 7.7 Ma, which may constrain the timing of the moon-forming giant impact.