Chemical mass balance of calcrete genesis on the Toledo granite (Spain)

The chemical mass balance of calcrete genesis is studied on a typical seque nce developed in granite, in the Toledo mountains, Central Spain. Field evidence and petrographic observations indicate that the texture and the bulk volume of the parent rock are strictly preserved all along the stu died calcrete profile. Microscopic observations indicate that the calcitization process starts wit hin the saprolite, superimposed on the usual mechanisms of granite weatheri ng: the fresh rock is first weathered to secondary clays, mainly smectites, which are then pseudomorphically replaced by calcite. Based on this eviden ce, chemical mass transfers are calculated, assuming iso-volume transformat ion from the parent rock to the calcrete. The mass balance results show the increasing loss of matter due to weatheri ng of the primary phases, from the saprolite towards the calcrete layers hi gher in the sequence. Zr, Ti or Th, which are classically considered as imm obile during weathering, are also depleted along the profile, especially in the calcrete layer. This results from the prevailing highly alkaline condi tions, which could account for the simultaneous precipitation of CaCO3 and silicate dissolution. The calculated budget suggests that the elements exported from the weatheri ng profile are provided dominantly by the weathering of plagioclase and bio tite. We calculate that 8-42% of the original Ca remains in granitic relies , while only 15% of the authigenic Ca released by weathering is reincorpora ted in the calcite. This suggests that 373 kg/m(2) of calcium (i.e., three times the original amount) is imported into the calcrete from allochtonous sources, probably due to aeolian transport from distant limestone formation s. (C) 2000 Elsevier Science B.V, All rights reserved.