Isotopic Variability as an Indicator for Earth and Solar System Processes



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Isotope analyses of natural materials are used by geochemists and cosmochemists to investigate Earth and solar system processes. Resolvable differences in the measured isotopic compositions for natural materials are caused by numerous natural and laboratory-induced processes, including mass-dependent fractionation, radioactive decay, nucleosynthetic anomalies, and mass-independent fractionation. Earth-surface processes were investigated using variations in 40Ar/39Ar ages in muscovites in the Central Nepalese Himalayas. Age variations in these samples are due to the radioactive decay of 40K to 40Ar, coupled with temperature-dependent diffusion of Ar. The isotopic variability in these samples, coupled with geomorphologic and structural data, provided information about uplift and erosion. We used this variability to interpret a zone of increased erosion that is centered above recently proposed locations of mid-crustal ramps in the Main Himalayan Thrust. This zone of increased erosion was consistent with a duplex growing along the ramp back‐tilting and uplifting older duplex faults. This zone also coincides with a dynamically supported bulge in the High Himalaya where a reservoir of elastic strain appears to be maintained over numerous seismic cycles. Solar system processes, including accretion in the protoplanetary disk, were investigated using nucleosynthetic Nd isotope variations and mass-independent fractionation in enstatite and carbonaceous chondrite samples. Our Nd isotopic analyses show that Earth and enstatite chondrites have resolvable differences in their 142Nd/144Nd ratios, even after correcting for radiogenic 142Nd. This likely indicates that the materials in the solar nebula that accreted to from Earth and enstatite chondrites had different starting Sm/Nd ratios. This resolvable difference can be explained by mineral sorting in the solar nebula, via mineral evaporation fronts in the protoplanetary disk that could preferentially enrich Earth with material that has a higher Sm/Nd ratio. The Nd isotopic compositions of CK (Karoonda-like) chondrites are not resolvable from CV (Vigarano-like) chondrites. Numerous chondrite samples analyzed for their Nd isotopic compositions have 150Nd/144Nd anomalies that are inconsistent with nucleosynthetic models. The Nd isotopic compositions of these samples are not consistent with laboratory-induced mass-independent fractionation. Rather, we propose that they are consistent with a combination of nucleosynthetic anomalies and nuclear field shift effects produced by natural processes.



Isotope, Erosion, Nucleosynthesis, Chondrites


Portions of this document appear in: Johnston, S. N., J. M. Cannon, and Peter Copeland. "Post‐Miocene erosion in Central Nepal controlled by midcrustal ramp position, duplex growth, and dynamically maintained elastic strain." Tectonics 39, no. 12 (2020): e2020TC006291.