Isotopic tracers of the origin of Ca in a carbonate crust from the Middle Atlas, Morocco

Carbonate crusts developed on basalt were studied at Tazouta in the Morocca n Middle Atlas. The chosen representative weathering profile is 2.8 m thick and is composed from base to top of a fresh basalt overlain by a coherent saprolite, a friable saprolite and a surface soil. Carbonate crust genesis occurs by replacement of primary and secondary clay minerals by low Mg calc ite. The C-14 dates of the calcite range from 15 to 14 ka in the coherent s aprolite, and from 12 to 8.5 ka in the friable saprolite. Smectitic weathering and replacement of Si-bearing minerals by carbonate ar e coupled processes leading to a strong calcium net gain (300 kg/m(3)) with in the friable saprolite. The low weathering rate of basalt compared to the calcrete genesis rate, and the high mobility of calcium in supergene weath ering mantles, limit the basaltic Ca contribution to calcrete genesis. The contribution of the Triassic sediment is also limited by the mineralogical unavailability of Ca. The Sr-87/Sr-86 ratios confirm the low contribution o f basalt and indicate an external origin of Ca. The Sr isotope ratios of ca lcrete are different from those of the adjacent Liassic carbonates and are close to rainfall values. The carbon isotope compositions of carbonates (-8 parts per thousand to -9 parts per thousand vs. PDB) imply a significant contribution of C3-dominate d organic carbon during carbonate precipitation. Oxygen isotopic compositio ns of carbonates (average - 5.24 parts per thousand vs. PDB), show that car bonates are O-18-enriched compared to meteoric waters (-4 parts per thousan d to - 3.5 parts per thousand vs. SMOW). (C) 2001 Elsevier Science B.V. All rights reserved.