Author ORCID Identifier
https://orcid.org/0000-0002-1761-3917
Date of Award
Spring 5-28-2025
Document Type
Thesis (Ph.D.)
Department or Program
Earth Sciences
First Advisor
Prof. Mukul Sharma
Abstract
This dissertation introduces a novel progressive digestion technique for osmium (Os) isotope analysis, capable of sequentially isolating Os from surface contaminants, bulk silicate matrix, and refractory phases within a single whole-rock powder. This approach overcomes challenges in distinguishing pristine mantle signatures from contamination in young volcanic rocks. Applied to basaltic reference materials BHVO-2 and EN026 10D-3, the method revealed intrasample Os-isotope heterogeneity and labile, radiogenic contamination in BHVO-2, likely introduced during sample preparation. For EN026 10D-3, while Step II Os-isotope ratios align with primitive upper mantle, radiogenic labile Os underscores the vulnerability of low-Os MORBs to disturbance—emphasizing the need for high-fidelity analytical protocol. This progressive digestion technique for Os isotope analysis was applied to studies of lavas from Hawaii and Laacher See (Germany) tephra
Mauna Loa lavas (1852–2022), exhibit a nearly constant Os-isotope range (187Os/188Os = 0.134–0.136) across ~200 years, despite subtle surficial contamination capable of shifting ratios by up to ~2%. Intrasample heterogeneity in older flows likely reflects magma mixing. The striking invariance of Os isotopes—unlike variable lithophile systems—suggests isotopic similarity to the “shared” mantle source with Kīlauea or, alternatively, low Os contribution due to residual sulfides. Mauna Loa’s more radiogenic 187Os/188Os ratios (average 0.1357 ± 0.0013) compared to that of Kīlauea (0.1285 ± 0.0005) supports a recycled component with high Re/Os in its source.
Os and Pb isotope data from the Laacher See tephra are used to track open-system magmatic processes. Os-isotopic heterogeneity in the Rothenberg basanite records variable contamination, while increasing Os and Pb ratios through differentiation reflect sulfide fractionation and subtle assimilation. A downward Os-isotope trend in evolved units suggests convective redistribution of sulfide phases. Isotopic overlap with Eifel lithospheric mantle xenoliths indicates a parental melt from enriched subcontinental lithosphere, modified by crustal interaction.
This work demonstrates the exceptional sensitivity of Os isotopes to magmatic sources and processes and establishes a robust framework for decoding mantle and crustal signatures in young volcanic systems especially those prone to disturbance due to their low Os content.
Original Citation
Ch.2. published as:
Bharadwaj, S., Sharma, M., 2024. Intrasample osmium isotope disequilibrium in young volcanic rocks: Insights from a new progressive digestion procedure. Geochimica et Cosmochimica Acta 384, 14–26.
Recommended Citation
Bharadwaj, Siddhartha, "Intrasample Os-isotope disequilibrium in young volcanic rocks" (2025). Dartmouth College Ph.D Dissertations. 370.
https://digitalcommons.dartmouth.edu/dissertations/370
