Thallium as a tracer of fluid–rock interaction in the shallow Mariana forearc

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2015-09Author
Nielsen, Sune G.
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Klein, Frieder
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Kading, Tristan
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Blusztajn, Jerzy S.
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Wickham, Katie
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https://hdl.handle.net/1912/7628As published
https://doi.org/10.1016/j.epsl.2015.09.001Keyword
Serpentinization; Thallium isotopes; Mariana forearc; Hydrothermal alteration; Rodingitization; SubductionAbstract
Fluids driven off the subducting Pacific plate infiltrate the shallow Mariana 26 forearc and lead to
extensive serpentinization of mantle peridotite. However, the sources, pathways, and chemical
modifications of ascending, slab-derived fluids remain poorly constrained and controversial. In this study,
we use thallium (Tl) concentrations and isotopic ratios of serpentinized peridotite and rodingitized
diabase from the South Chamorro and Conical Seamounts to discriminate between potential fluid sources
with distinct Tl isotope compositions. Serpentinite samples from the Mariana forearc all display ε205Tl > -
0.5 (where ε205Tl = 10,000 x (205Tl/203Tlsample-205Tl/203TlSRM 997)/(205Tl/203TlSRM 997)), which is significantly
enriched in 205Tl compared to the normal mantle (ε205Tl = -2). Given that high temperature hydrothermal
processes do not impart significant Tl isotope fractionation, the isotope compositions of the serpentinites
must reflect that of the serpentinizing fluid. Pelagic sediments are the only known slab component that
consistently display ε205Tl > -0.5 and, therefore, we interpret the heavy Tl isotope signatures as signifying
that the serpentinizing fluids were derived from subducting pelagic sediments. A rodingitized diabase
from Conical Seamount was found to have an ε205Tl of 0.8, suggesting that sediment-sourced
serpentinization fluids could also affect diabase and other mafic lithologies in the shallow Mariana
forearc. Forearc rodingitization of diabase led to a strong depletion in Tl content and a virtually complete
loss of K, Na and Rb. The chemical composition of hybrid fluids resulting from serpentinization of
harzburgite with concomitant rodingitization of diabase can be highly alkaline, depleted in Si, yet
enriched in Ca, Na, K, and Rb, which is consistent with the composition of fluids emanating from mud
volcanoes in the Mariana forearc. Our study suggests that fluid-rock interactions between sedimentary,
mafic, and ultramafic lithologies are strongly interconnected even in the shallowest parts of subduction
zones. We conclude that transfer of fluids and dissolved elements at temperatures and pressures below
400°C and 1GPa, respectively, must be taken into account when elemental budgets and mass transfer
between the subducting plate, the forearc, the deep mantle and the ocean are evaluated.
Description
Author Posting. © The Author(s), 2015. This is the author's version of the work. It is posted here by permission of Elsevier for personal use, not for redistribution. The definitive version was published in Earth and Planetary Science Letters 430 (2015): 416-426, doi:10.1016/j.epsl.2015.09.001.
Suggested Citation
Preprint: Nielsen, Sune G., Klein, Frieder, Kading, Tristan, Blusztajn, Jerzy S., Wickham, Katie, "Thallium as a tracer of fluid–rock interaction in the shallow Mariana forearc", 2015-09, https://doi.org/10.1016/j.epsl.2015.09.001, https://hdl.handle.net/1912/7628Related items
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