STABLE-ISOTOPE CHEMISTRY OF GAYLUSSITE AND NAHCOLITE FROM THE DEPOSITS OF THE CRATER LAKE AT MALHA, NORTHERN SUDAN

Gaylussite (Na2CO3. CaCO3. 5H(2)O) and nahcolite (NaHCO3)( )in deposit s of the crater lake at Malha are predictably characterized by a stron g O-18 enrichment that is typical of evaporitic environments, Both min erals also yield high delta(13)C values, which, in the case of gayluss ite, are covariant with the delta(18)O values. The fact that the covar iant trend, which is a feature that is commonly recognized for the dep osits of closed basins, pertains both to the synsedimentary and pedoge nic gaylussite deposits of the Malha basin demonstrates that C-13 enri chment was mainly controlled by inorganic processes. An important biol ogical control on the carbon isotope composition is also contradicted by the occurrence of a covariance for synsedimentary gaylussite that m ainly formed by bottom growth in a well-mixed water mass. Degassing of carbon dioxide is the most likely cause of C-13 enrichment, although this implies that CO2 loss is a process that occurs concurrently with the evaporation of water, without, as could be expected, being driven by changes in the relative abundance of compounds within the CO2-H2O s ystem. The lack of a covariance of delta(18)O and delta(13)C for nahco lite seems to be partly related to bacterial activity, which did play an important role during lake stages with nahcolite formation, and par tly to the lack of a further increase of delta(18)O with continued eva poration at advanced stages of the evaporitic concentration of brines. (C) 1998 Elsevier Science B.V. All rights reserved.