T3 Advances in Nontraditional Stable Isotope Measurements and Utility as Proxies in Modern and Paleo-Settings

Sunday, 22 September 2024
8:00 AM - 12:00 PM
207A

[T32] FROM SOURCE ROCK TO CINNABAR – HOW THE GIANT MERCURY DEPOSITS IN EARTH’S CRUST FORMED

Paper number: T3-11
Program: Technical Sessions
Time: 10:45 AM – 11:00 AM

Authors

Abstract

The largest concentrations of Hg on Earth exist as giant deposits of cinnabar (HgS). How such enrichments of Hg formed, based on its known crustal abundance, has never been fully resolved nor has the source(s) of Hg been unequivocally established. Hg isotopes were used to elucidate crustal processes leading to the formation of these giant cinnabar deposits.

Hg isotopes provide compelling evidence for a genetic association of Hg in cinnabar deposits being sourced from sediments deposited during global LIP events. HgS deposits in the Terlingua mining district of SW Texas, and the New Idria Hg mine in the California Coast Range are examples of Hg recycling from organic-rich marls and tuffaceous black shales of Cenomanian-Turonian age, having a Hg isotopic signature confirming a genetic relationship to the OAE-2 LIP event.

Mass dependent fractionation (MDF) of Hg isotopes show remarkable enrichment of δ202Hg in cinnabar relative to its upper mantle source. Two mechanisms contribute to this enrichment: one is the low temperature, early diagenetic loss of volatile 198Hg(g) to an extant gas phase; the other is oxidation during cinnabar deposition. Loss of 198Hg(g) results in δ202Hg enrichment of residual Hg in source sediments. Evidence for this significant loss of 198Hg(g) is observed as large depletions in δ202Hg in proximal gas condensate liquids in high pressure – high temperature (HP/HT) reservoirs in the central North Sea (CNS).

Supporting evidence is also provided by the ratios of Δ199Hg / Δ201Hg on mass independent fractionation (MIF) isotope plots, where cinnabar from Huancavelica (Peru) has a slope of 1.97 (± 0.033; 1SE), indistinguishable from measured slopes observed in Hg0(g) vaporization experiments (2 ± 0.3; 1SE). The large changes in MDF of Hg isotopes appears to be independent of MIF of Hg isotopes, which generally preserve their primary mantle Hg isotopic signature.

Hydrocarbon migration from Hg-enriched sediments transports reduced Hg0(org, aq) to the site of cinnabar deposition, where oxidation of Hg0(org, aq) and H2S further enhances enrichment of δ202Hg in cinnabar.

Geological Society of America Abstracts with Programs. Vol. 56, No. 5

doi: 10.1130/abs/2024AM-401957

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