Participation of fresh water in chert diagenesis: evidence from oxygen isotopes and boron α-track mapping

Sedimentary chert breccias are the product of two stages of silicification: one which produced ‘fragments’, the other, during which the ‘matrix’ crystallized. Fragments and matrix of twenty-five breccias from the Senonian Mishash Formation in Southern Israel were separated and analysed for δ¹⁸O in quartz. The matrix is significantly depleted in ¹⁸O in comparison with fragments (δ¹⁸O of fragments ranges between +29.4 and +34.7‰, for matrix the range is +21.1–32.6‰). Boron has been mapped in the same rocks using the ¹⁰B(n, α)⁷Li reaction and α-track etching. Fragments are significantly enriched in boron in comparison to the matrix. The correspondence between boron maps and rock texture is perfect. These results when combined with Steinitz's (1977) report of evaporites and evaporite ghosts in the same rocks are interpreted as indicating two different environments of chert diagenesis: a first, normal marine to hypersaline stage during which the microquartz in fragments crystallized, and a second stage of matrix chertification which in most cases occurred when fresh water was interacting with the crystallizing sediment. These conclusions are corroborated by isotopic analysis of carbonates which coexist with the cherts, as well as stratigraphical and palaeontological analysis.