Element mobility in paragneisses and metabasites of the Continental Deep Drilling Project (KTB/Germany)

 Leaching experiments (pH=3, T=70 °C) have been performed on paragneisses and metabasites from the KTB drill cores. The experiments yielded the fraction of total element content that is easily available to acidic aqueous fluids. The intensity of mineralogic alteration in the studied samples is expressed in terms of easily leachable fractions of K or Ba. Its reliability is shown by correlation with a petrographically determined alteration index. For the paragneisses, the easily available fractions of Fe, Mg, Mn, Ca, Na, K, Ba, Co, Cr, Ni, Sr, V, and Y together with the variables whole-rock content and depth of samples were subjected to a factor analysis, the results of which are presented as a five-factor model. Correlation plots of easily leachable element fraction and whole-rock content vs an alteration index (percentage of leachable K) reveal the changes in leachability as a function of intensity of alteration. In a cataclastic boundary between paragneisses and metabasites at 1610 m depth, the changes in whole-rock content and leachability of elements are studied in detail and compared with the general trends in alteration. The results point to considerable intercrystalline element redistribution during retrograde metamorphism, with only insignificant element transport on a meter scale. Although considerable fractions of many elements were easily available to acidic fluids (3<pH<4) in the experiments, the whole-rock element content was maintained during retrogression, suggesting that water/rock ratio and fluid flow were low. During alteration K, Ba, Fe, Co, Ni, Cr, and V are immobilized in situ, whereas Sr, Ca, and Y become more leachable. Across the studied, strongly altered, lithologic contact, element transport up to 7 m normal to foliation can be traced. Here, Ca, Sr, Mg, Co, Ni, and Cr are slightly enriched, whereas K, Ba, Fe, and Mn are slightly depleted.