Boron isotopes in cherts; DSDP and on-land; fractionation and diagenesis

Author(s): Kolodny, Yehoshua; Chaussidon, Marc
Author Affiliation(s): Primary:
Hebrew University of Jerusalem, Institute of Earth Sciences, Jerusalem, Israel
Other:
CRPG-CNRS, France
Volume Title: Geological Society of America, 2002 annual meeting
Source: Abstracts with Programs - Geological Society of America, 34(6), p.152; Geological Society of America, 2002 annual meeting, Denver, CO, Oct. 27-30, 2002. Publisher: Geological Society of America (GSA), Boulder, CO, United States. ISSN: 0016-7592 CODEN: GAAPBC
Note: In English
Summary: 22 samples of cherts from the Central Pacific (Leg17, Hole 167) were analyzed for B content and δ11B by ion probe. We also analyzed 3 samples of chert breccias from the Campanian Mishash Fm., Israel. In all samples δ18O has been previously measured; for the DSDP samples 87Sr/86Sr, Ge was also determined. In the breccias B concentration was mapped by α-tracks. δ11B of DSDP cherts varies between -5.5 and +8ä, indicating a fractionation of 32 to 45ä with respect to present day seawater B. This fractionation is similar to the observed values in opal-A radiolarians (+2 to +4.5ä, Ishikawa and Nakamura, 1993). Assuming no secular variation in δ11BSW our results can be the reflection of a three-stage precipitation-dissolution process of DSDP cherts: the first leading to Opal-A and involving a large fractionation of B isotopes, the second and third resulting in quartz via opal-CT, involving high solid/liquid ratios and hence a small isotopic fractionation. Alternatively the δ11B-Age curve may be a reflection of a secular change of δ11BSW. In that case the curve would be shifted by some 40-50 m.y towards the present with respect to the curve proposed by Lemarchand et al. (2000). Such a shift was suggested for δ18O in DSDP cherts by Kolodny and Epstein (1976). The chert breccias show a sharp difference in B concentration between fragments (50-90 ppm) and matrix (3-20 ppm), confirming earlier α-track mapping and the interpretation that fragments were silicified in contact with seawater, whereas the matrix was transformed to quartz in contact with water of meteoric origin. Indeed δ11B in fragments is in a similar range as that the marine DSDP cherts, confirming their marine origin. δ11B in the matrix partly overlaps with the values in the fragments but reaches lower values (-33ä), suggesting that whereas the water was primarily meteoric, the salts were in part derived from seawater.
Year of Publication: 2002
Research Program: DSDP Deep Sea Drilling Project
Key Words: 02 Geochemistry; Alkaline earth metals; Asia; B-11/B-10; Boron; Campanian; Central Pacific; Chemically precipitated rocks; Chert; Cretaceous; DSDP Site 167; Deep Sea Drilling Project; Diagenesis; Ion probe data; Isotope fractionation; Isotope ratios; Isotopes; Israel; Leg 17; Mass spectra; Matrix; Mesozoic; Metals; Meteoric water; Middle East; Mishmash Formation; Pacific Ocean; Precipitation; Processes; Sea water; Secular variations; Sedimentary rocks; Senonian; Silicification; Solution; Spectra; Sr-87/Sr-86; Stable isotopes; Strontium; Upper Cretaceous
Coordinates: N070406 N070406 W1764930 W1764930
Record ID: 2004033170
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