Abstract:
To reveal chemical fractionation during porcellanitization, we have examined chemical compositions of porcellanite clasts and their host sediments from DSDP sites 164 and 166. Porcellanite is an intermediate product during chert formation. Distinctly higher Si/Ti ratios are observed in porcellanite relative to the host sediment, indicating Si transfer from the host sediment. However, Ti-normalized values of Al, Fe, Mn, Ca, Na, K, P, rare earth elements, Sc, Rb, Y, Cs, Pb, Th, and Cr in porcellanite are similar to those of the host sediment, showing that these elements are affected by only very limited fractionation during porcellanitization and retain information about the sedimentary environment. The Si/Ti, P/Ti, and Al/Ti ratios in DSDP clayey samples fluctuate synchronously with their sedimentation ages. Because the Si/Ti and P/Ti ratios in clayey sediment reflect the contribution of biogenic siliceous and phosphatic debris, respectively, their high values in Cenozoic clayey sediments indicate vigorous biogenic productivity. Moreover, the observed similarity in age profiles of the Al/Ti ratio with the Si/Ti ratios and P/Ti ratio in clayey samples suggests that the Al/Ti ratio is affected by the supply of biogenic debris. The high Al/Ti ratios found in the Cenozoic samples can be ascribed to the contribution of adsorbed Al onto sinking biogenic debris. The small difference between the Al/Ti ratio of the Cenozoic porcellanite and the host sediment indicates that the adsorbed Al is also retained in porcellanite during Si enrichment. Therefore, the Al/Ti ratio in bedded cherts may be a potential indicator of ancient biogenic activity.