Atmospheric input vs in situ weathering in the genesis of calcretes: An Srisotope study at Galvez (Central Spain)

Strontium isotopes have been analyzed in a typical calcrete profile develop ed on granite in the Toledo mountains, Central Spain. The pedogenic carbona tes show clear petrographic evidence of pseudomorphic replacement of the we athered parent granite. Calcretes display Sr-87/Sr-86 ratios between 0.70961 and 0.71059 in sharp c ontrast to the granite whole rock (0.72856) and minerals (0.71359 to 0.9135 1). This difference shows that the contribution of Sr from the granite to t he calcretes is at most 33% and may be as low as 3%. Direct measurements in rains and aerosols show that the allochtonuus source of Ca and Sr is clear ly related to the atmospheric input, mainly as dry deposit. A slight decrease of Sr concentration is observed from the upper horizon co mposed of continuous calcrete to the deeper calcrete veins in the saprolite . This may be due to a kinetic control of the Sr/Ca fractionation, and diff erent crystallization rates of the carbonates in the different units of the profile. Finally, local groundwaters have Sr isotopic compositions similar to the ca lcretes and the atmospheric input, very different from waters running on th e granite. Copyright (C) 1999 Elsevier Science Ltd.