Effect of diagenetic phase transformation on the silicon isotope composition of opaline sinter deposits of Geysir, Iceland

Geilert, Sonja, Vroon, Pieter Z. and van Bergen, Manfred J. (2016) Effect of diagenetic phase transformation on the silicon isotope composition of opaline sinter deposits of Geysir, Iceland Chemical Geology, 433 . pp. 57-67. DOI 10.1016/j.chemgeo.2016.04.008.

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Detectible δ30Si variations in present-day chemical silica deposits have stimulated the application of silicon isotopes to infer environmental conditions from ancient equivalents. Interpretations of δ30Si signatures remain problematic in view of potential post-depositional changes, of which magnitudes and underlying mechanisms are largely unknown. A critical issue in the interpretation of isotope data from cherts concerns the extent to which early-diagenetic processes modify original δ30Si signatures. Here, we report δ30Si variations in opal-A, opal-A/CT and opal-CT from fossil sinter deposits in an active discharge apron in the Geysir geothermal field, Iceland. Opal-A samples show an average δ30Si of − 0.7 ± 0.2‰, while opal-CT samples are isotopically lighter with an average δ30Si of − 2.0 ± 0.4‰, implying a sizable shift of approximately 1.3‰ between the different phases. This shift can be explained by repetitive dissolution/re-precipitation processes, diffusive transport or temperature differences during phase transitions. On average, the fossil opal-A tends to be less negative in δ30Si than amorphous silica that recently precipitated from the hydrothermal water. The difference can be attributed to primary variability in isotopic fractionation that accompanies precipitation out of spring water at the surface, or to a post-depositional release of surface 28Si at the onset of diagenetic formation. Our results corroborate the perception that original silicon isotope signatures of silica, acquired during chemical precipitation from a saturated solution, may not be preserved in the geological record, and that post-depositional changes must be taken into account when interpreting data from ancient chert deposits.

Document Type: Article
Additional Information: WOS:000375417900006
Keywords: Geysir, Early-diagenesis of silica sinter, Fossil opals, Silicon isotopes, Dissolution/re-precipitation
Research affiliation: OceanRep > GEOMAR > FB2 Marine Biogeochemistry > FB2-MG Marine Geosystems
Refereed: Yes
DOI etc.: 10.1016/j.chemgeo.2016.04.008
ISSN: 0009-2541
Date Deposited: 27 May 2016 10:25
Last Modified: 03 May 2017 11:41
URI: http://eprints.uni-kiel.de/id/eprint/32939

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