A Portrait of Terrestrial Volatile Evolution From Mantle Noble Gases
Tucker, Jonathan M.
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AbstractVolatile elements and compounds provide important insights into large-scale planetary and tectonic processes. The chapters in this thesis exploit the wealth of information provided by noble gas measurements to address a variety of questions regarding the origins and distributions of volatiles in the Earth.
Chapter 1 presents new He-Ne-Ar-Xe data from basalts from the equatorial Mid-Atlantic Ridge, demonstrating a large degree of heavy noble gas heterogeneity in mid-ocean ridge basalts (MORBs). The He and Ne data can be explained primarily by a mixture of a depleted mantle component and a HIMU-like component, with the constraint that the HIMU component is a combination of recycled and primitive material. While most mantle-derived Xe is recycled atmospheric Xe, the compositions of depleted MORBs, HIMU-type MORBs, and the Iceland plume cannot be related solely by different amounts of recycled air. Rather, HIMU-type MORBs and the Iceland plume sample a less degassed reservoir. Furthermore, differences in the amount of 129I-derived 129Xe between the depleted and HIMU-type MORBs suggests that HIMU-type MORBs are sampling a reservoir that formed within the first 100 Myr of solar system history and has not extensively mixed with the depleted mantle since.
Chapter 2 explores the hypothesis that high 3He/22Ne ratios intrinsic to the depleted mantle were generated by multiple episodes of magma ocean outgassing and atmospheric loss during Earth's accretion. We argue that magma ocean outgassing in the aftermath of giant impacts during accretion can raise the mantle 3He/22Ne ratio, but multiple episodes of outgassing and atmospheric loss are required to achieve the ratio observed in the depleted mantle. The preservation of low 3He/22Ne ratios in primitive plumes suggests that later giant impacts such as the Moon-forming giant impact did not homogenize the whole mantle. The requirement for episodes of atmospheric loss to achieve high 3He/22Ne ratios during accretion may also provide an explanation for Earth's nonchondritic volatile element ratios such as N/H as N would be more susceptible to loss processes than H.
Chapter 3 uses the combined constraints of the radiogenic noble gases 4He*, 21Ne*, 40Ar*, and 136Xe*U to examine degassing processes at mid-ocean ridges. We show that ratios of radiogenic noble gases cannot be simultaneously explained by any equilibrium degassing model, questioning the use of such models to reconstruct pre-degassing magmatic contents and hence mantle fluxes of elements like C. We argue that kinetic fractionation prevents slowly diffusing volatiles from achieving their equilibrium partitioning between vesicles and melt and present a new simple model of disequilibrium degassing that self-consistently explains CO2-4He*-21Ne*-40Ar* compositions in MORBs. Application of this model suggests that the average MORB mantle C/3He ratio and C flux may be a factor of 2 higher than that inferred from equilibrium degassing-based estimates.
Chapter 4 presents new He data on depleted MORBs from the subtropical north Mid-Atlantic Ridge. Correlations between He and Pb isotopic compositions in this region as well as others globally suggest that, absent plume influence, He isotopes in MORBs can be explained by a mixture of an intrinsic, relatively unradiogenic depleted component and highly radiogenic recycled oceanic crust, a manifestation of the "marble cake" mantle. With a simple mixing model, we estimate that the mantle source of average MORBs has ~5% recycled oceanic crust.
Chapter 5 presents new He-Ne-Ar-Xe data on a subset of the depleted MORB samples described in Chapter 4. Ne isotopic compositions are less nucleogenic than average MORBs, indicating the influence of a relatively undegassed component, which may be especially strongly sampled near 29°N. Ar, and Xe isotopic compositions extend from moderately radiogenic values to highly unradiogenic values, demonstrating extreme variability absent significant variability in lithophile chemistry. Globally, Ar and Xe isotopic compositions correlate in oceanic basalts, interpreted to result from mixtures of degassed material having radiogenic Ar and Xe with undegassed material and recycled air having unradiogenic Ar and Xe.
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