Helium isotope evidence for mixing of mantle-derived fluids and deeply penetrating surface waters in an obducted peridotite massif
- Creators
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Swindle, Carl
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Clark, Doug
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Farley, Kenneth A.
Abstract
We measured He and Ar in fresh peridotites from the Twin Sisters massif, Washington USA. ³He/⁴He ratios measured on >35 samples are highly variable (∼0.8 to ∼6 times the atmospheric ratio, RA). Step-heating of a subset of these samples in every case reveals a low ³He/⁴He component (∼1 R_A) released at <1000 °C and a high ³He/⁴He component (>3 R_A) above that temperature, but these components are not effectively isolated by crushing and powder fusion analysis. He-Ar systematics indicate an intimate association of two fluid-inclusion hosted components in the peridotites. The first is a ∼6 R_A mantle component that is released at higher temperatures during step heating and is more abundant in dunite bands than surrounding harzburgites. The second component, released at low temperatures, has a ³He/⁴He ratio of 1.0 ± 0.5 R_A, atmospheric ⁴⁰Ar/³⁶Ar, and ⁴He/⁴⁰Ar far above atmospheric. It appears to be a mixture of mantle and radiogenic He sources introduced during obduction-related serpentinization, sometimes invisible, by surface-derived waters enriched with deeply-sourced helium. These data indicate that mantle noble gas signatures can be retained in lithospheric peridotites against both diffusive loss and radiogenic ingrowth over at least 10⁸ year timescales, likely due to concentration and immobilization of He in fluid inclusions. However, the mantle signature can be greatly modified by pervasive and potentially cryptic fluid alteration during emplacement.
Additional Information
© 2023 Elsevier. We are grateful to Sarah Francis and Basil Tikoff for providing samples and Jonathan Treffkorn and Frank Pavia for help with analytical work. We acknowledge the Water and Environment Lab at Caltech for providing the analytical facilities to measure U and Th. We thank George Rossman for guidance associated with Raman spectroscopy. We thank Jeffery Catalano, Junju Yamamoto and the anonymous reviewers for the constructive feedback. We appreciate Chi Ma's assistance with the scanning electron microscope, and Claire Bucholz for time on the petrographic microscope. We recognize Lillian Adriana Piña Paez from California Institute of Technology for assistance with ArcGIS software. This work was partially supported by National Science Foundation Graduate Research Fellowship Grant DGE‐1745301. The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.Attached Files
Supplemental Material - 1-s2.0-S0016703723002351-mmc1.pdf
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Additional details
- Eprint ID
- 122193
- Resolver ID
- CaltechAUTHORS:20230710-599244800.3
- NSF Graduate Research Fellowship
- DGE‐1745301
- Created
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2023-07-13Created from EPrint's datestamp field
- Updated
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2023-07-13Created from EPrint's last_modified field
- Caltech groups
- Division of Geological and Planetary Sciences