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Thallium concentration and thallium isotope composition of lateritic terrains

Thallium concentration and thallium isotope composition of lateritic terrains
Thallium concentration and thallium isotope composition of lateritic terrains

Continental weathering plays a key role in modifying the geochemical budget of terrestrial reservoirs. Laterites are the products of extreme sub-aerial continental weathering. This study presents the first investigation of thallium (Tl) abundances and stable isotopic compositions of lateritic terrains. Two laterite profiles from India of differing protolith and age are studied. Thallium concentrations range between 7 and 244 ng/g for a basalt-based lateritic profile and 37–652 ng/g within a greywacke lateritic profile. The average Tl stable isotope composition of the two profiles is similar to many typical igneous materials, however, the intense tropical weathering causes a small but resolvable fractionation of Tl stable isotopes towards heavy values in the residual soils. The profiles are dominated by significant positive isotope excursions (reported as ε205Tl relative to standard NBS997) of +3.5 ± 0.5 2sd and +6.2 ± 0.5 2sd at the inferred palaeowater tables within both laterite profiles. These signatures likely reflect combined changes in redox state and mineralogy. Extensive mineral dissolution under through-flowing fluids alongside the formation of new phases such as phyllosilicates and Mn- and Fe-oxides and hydroxides likely account for some of the Tl mobilisation, sorption and coprecipitation. In the case of laterites, the formation of the new phases and role of surface sorption likely contribute to stable Tl isotope fractionation. The identification of strong isotope excursions at inferred palaeowater tables encourages future research to determine specific mineral phases that may drive significant fractionation of Tl stable isotopes. This study showcases the magnitude of natural variation possible in terrestrial soils. Such information is key to the nascent application of Tl isotope compositions as tracers of anthropogenic pollution.

Birnessite, Ferromanganese minerals, Laterites, Thallium isotopes
0016-7037
446-462
Howarth, S.
dd6533a2-a4bb-4cdc-b8b2-64a4303ee819
Prytulak, J.
acc25269-e2d2-4724-a319-9eda76e7551e
Little, S.H.
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Hammond, S.J.
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Widdowson, M.
cc798c19-592e-4a36-adbb-e5efc18e31c3
Howarth, S.
dd6533a2-a4bb-4cdc-b8b2-64a4303ee819
Prytulak, J.
acc25269-e2d2-4724-a319-9eda76e7551e
Little, S.H.
e85f87f2-ac3f-461c-bcc7-034a684b93b9
Hammond, S.J.
9c913ae2-4dd3-49ad-b943-7d20f1291bb0
Widdowson, M.
cc798c19-592e-4a36-adbb-e5efc18e31c3

Howarth, S., Prytulak, J., Little, S.H., Hammond, S.J. and Widdowson, M. (2018) Thallium concentration and thallium isotope composition of lateritic terrains. Geochimica et Cosmochimica Acta, 239, 446-462. (doi:10.1016/j.gca.2018.04.017).

Record type: Article

Abstract

Continental weathering plays a key role in modifying the geochemical budget of terrestrial reservoirs. Laterites are the products of extreme sub-aerial continental weathering. This study presents the first investigation of thallium (Tl) abundances and stable isotopic compositions of lateritic terrains. Two laterite profiles from India of differing protolith and age are studied. Thallium concentrations range between 7 and 244 ng/g for a basalt-based lateritic profile and 37–652 ng/g within a greywacke lateritic profile. The average Tl stable isotope composition of the two profiles is similar to many typical igneous materials, however, the intense tropical weathering causes a small but resolvable fractionation of Tl stable isotopes towards heavy values in the residual soils. The profiles are dominated by significant positive isotope excursions (reported as ε205Tl relative to standard NBS997) of +3.5 ± 0.5 2sd and +6.2 ± 0.5 2sd at the inferred palaeowater tables within both laterite profiles. These signatures likely reflect combined changes in redox state and mineralogy. Extensive mineral dissolution under through-flowing fluids alongside the formation of new phases such as phyllosilicates and Mn- and Fe-oxides and hydroxides likely account for some of the Tl mobilisation, sorption and coprecipitation. In the case of laterites, the formation of the new phases and role of surface sorption likely contribute to stable Tl isotope fractionation. The identification of strong isotope excursions at inferred palaeowater tables encourages future research to determine specific mineral phases that may drive significant fractionation of Tl stable isotopes. This study showcases the magnitude of natural variation possible in terrestrial soils. Such information is key to the nascent application of Tl isotope compositions as tracers of anthropogenic pollution.

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More information

Accepted/In Press date: 16 April 2018
e-pub ahead of print date: 25 April 2018
Published date: 15 October 2018
Keywords: Birnessite, Ferromanganese minerals, Laterites, Thallium isotopes

Identifiers

Local EPrints ID: 424433
URI: http://eprints.soton.ac.uk/id/eprint/424433
ISSN: 0016-7037
PURE UUID: 960e3eaf-65bd-4f29-b832-3cbfe8b17611
ORCID for S. Howarth: ORCID iD orcid.org/0000-0002-8017-8954

Catalogue record

Date deposited: 05 Oct 2018 11:37
Last modified: 17 Dec 2019 01:28

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