The geochemistry of a chert breccia (Y. Kolodny, A. Katz,
M. Chaussidon)
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 (18O, D, 11B) composition. 11B was analyzed by ionprobe (at Nancy, France) 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 18O (29 to 35‰ vs. 21 to 33‰). D is a less efficient discriminator, though
compatible with fresher water diagenesis of the matrix: -115‰ to –76‰ for hydrogen in
the chert of the fragments, compared to –141 to -85‰ for the matrix. 11B in the matrix
shows some of the lowest values recorded in sediments (11B= -33‰), 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.
Profiles of 11B (c0 and B concentration from matrix into a fragment in sample
MK302. The profile is marked on both a polished section photograph (a) and a boron
map (b). Note the increase of B concentration at the border of the fragment compared to
the very uniform 11B value in the fragment. The boron map was prepared by placing a
cellulose nitrate film in contact with a polished rock section, irradiating with thermal
neutrons and recording the -tracks [10B(n,)7Li] . The film was then used in a photoenlarger (see Kolodny et al. 1980) as a “negative”.