Geochemistry of a chert breccia

摘要

A complex history of diagenetic interactions between a siliceous sediment, seawater and fresh water is revealed by intraformational chert breccias. Chert breccias were formed in the Campanian Mishash Formation in Israel, by "practically contemporaneous" fracturing of lithified cherty layers followed by silicification and lithification of the matrix. Pairs of fragments and matrix were compared with respect to their chemical (Ca, Sr, Na, K, Mg, Li, B, SO4, Ba) and isotopic (delta(18)O, deltaD, delta(11)B) composition. delta(11)B was analyzed by ion-probe and includes a profile across a fragment-matrix contact. The epicontinental cherts of the Mishash Fm. are enriched by a factor of 10 to 50 in all elements other than O and Si in comparison with Deep-Sea cherts. All results are compatible with the proposition that the lithification of the matrix occurred in contact with fresh-water, as opposed to seawater in which the fragments, as well as most of the Mishash sediments were formed. The strongest evidence for this difference is in the higher concentration of B in the fragments (27-70 ppm vs. 11-21ppm in the matrix) and higher delta(18)O (29 to 35parts per thousand vs. 21 to 33parts per thousand). deltaD is a less efficient discriminator, though compatible with fresher water diagenesis of the matrix: - 115parts per thousand to -76parts per thousand for hydrogen in the chert of the fragments, compared to -141 to -85parts per thousand for the matrix. delta(11)B in the matrix shows some of the lowest values recorded in sediments (delta(11)B = -33parts per thousand), but varies strongly, suggesting that the source of boron in the matrix is a mixture of a freshwater and a marine component. Both seawater and the freshwater that has equilibrated with the cherts underwent varying degrees of evaporation. Ca, Sr and SO4 are carried by apatite, trapped as detritus in the matrix. The concentration of lithium in the matrix is high (11-16 ppm), whereas in the adjacent fragments it is mostly only within 1-2 ppm. Li probably enters the matrix from the interstitial solution, during the opal --> quartz transformation. The second, prolonged, transformation takes place in a (freshwater) flow-through, open system. This allows a much larger mass of Li to be scavenged by the transforming silica despite its low concentration in freshwater. Copyright (C) 2005 Elsevier Ltd.