Uranium-thorium chronometry of weathering rinds: Rock alteration rate and paleo-isotopic record of weathering fluids

E. Pelt, F. Chabaux, C. Innocent, A. K. Navarre-Sitchler, Peter Benjamin Sak, Susan Louise Brantley

Research output: Contribution to journalArticle

85 Citations (Scopus)

Abstract

The potential of 238U-234U-230Th chronometry for constraining the formation rate of weathering rinds developed on fresh rocks is assessed by analyzing a weathering rind on a basaltic clast from a 125 kyr old Costa Rican alluvial terrace. Eighteen subsamples were collected from one section of the clast by drilling cores (4 mm in diameter and 5 mm depth) along two transects straddling the core-rind boundary. Variations of major and trace element concentrations along the two transects point out (a) intense loss of alkaline and alkaline-earth elements, (b) conservative behaviour of elements such as Zr, Hf and Th, and (c) external input of U into the rind without any evidence of U loss during basalt weathering. In addition, variations in U concentrations along the transects show that the main U-Th fractionation process associated with the weathering of the basaltic clast is an external input of U (without addition nor loss of Th) in the basalt rind transition zone, and that, once deposited U is immobile in the weathering rind. In the frame of this interpretation scheme, a weathering rate of 0.5 ± 0.2 mm/kyr can be calculated for the studied clast, which is consistent with geological and isotopic evidence constraining the depositional ages of the terraces. In addition, the variations in the (234U/238U) ratio along the analysed transects as well as the increase in Sr isotopic ratios within the weathering rind are best explained by temporal variation of the U activity ratios and Sr isotope ratios of the soil solutions brought into the rind. This work highlights how well detailed U-Th chronological studies of weathering rinds can (1) constrain the formation rates of weathering systems, and (2) record the time variation of isotopic composition of weathering fluids.

Original languageEnglish (US)
Pages (from-to)98-105
Number of pages8
JournalEarth and Planetary Science Letters
Volume276
Issue number1-2
DOIs
StatePublished - Nov 30 2008

Fingerprint

Thorium
Uranium
thorium
weathering
Weathering
uranium
Rocks
rocks
Fluids
fluid
fluids
rock
clast
transect
terrace
basalt
Core drilling
rate
weathering rate
isotope ratios

All Science Journal Classification (ASJC) codes

  • Geophysics
  • Geochemistry and Petrology
  • Earth and Planetary Sciences (miscellaneous)
  • Space and Planetary Science

Cite this

Pelt, E. ; Chabaux, F. ; Innocent, C. ; Navarre-Sitchler, A. K. ; Sak, Peter Benjamin ; Brantley, Susan Louise. / Uranium-thorium chronometry of weathering rinds : Rock alteration rate and paleo-isotopic record of weathering fluids. In: Earth and Planetary Science Letters. 2008 ; Vol. 276, No. 1-2. pp. 98-105.
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abstract = "The potential of 238U-234U-230Th chronometry for constraining the formation rate of weathering rinds developed on fresh rocks is assessed by analyzing a weathering rind on a basaltic clast from a 125 kyr old Costa Rican alluvial terrace. Eighteen subsamples were collected from one section of the clast by drilling cores (4 mm in diameter and 5 mm depth) along two transects straddling the core-rind boundary. Variations of major and trace element concentrations along the two transects point out (a) intense loss of alkaline and alkaline-earth elements, (b) conservative behaviour of elements such as Zr, Hf and Th, and (c) external input of U into the rind without any evidence of U loss during basalt weathering. In addition, variations in U concentrations along the transects show that the main U-Th fractionation process associated with the weathering of the basaltic clast is an external input of U (without addition nor loss of Th) in the basalt rind transition zone, and that, once deposited U is immobile in the weathering rind. In the frame of this interpretation scheme, a weathering rate of 0.5 ± 0.2 mm/kyr can be calculated for the studied clast, which is consistent with geological and isotopic evidence constraining the depositional ages of the terraces. In addition, the variations in the (234U/238U) ratio along the analysed transects as well as the increase in Sr isotopic ratios within the weathering rind are best explained by temporal variation of the U activity ratios and Sr isotope ratios of the soil solutions brought into the rind. This work highlights how well detailed U-Th chronological studies of weathering rinds can (1) constrain the formation rates of weathering systems, and (2) record the time variation of isotopic composition of weathering fluids.",
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Uranium-thorium chronometry of weathering rinds : Rock alteration rate and paleo-isotopic record of weathering fluids. / Pelt, E.; Chabaux, F.; Innocent, C.; Navarre-Sitchler, A. K.; Sak, Peter Benjamin; Brantley, Susan Louise.

In: Earth and Planetary Science Letters, Vol. 276, No. 1-2, 30.11.2008, p. 98-105.

Research output: Contribution to journalArticle

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T1 - Uranium-thorium chronometry of weathering rinds

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AU - Pelt, E.

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