干旱区生态环境知识资源中心

delta O-18-delta D measurements were made on sepiolite and associated saponite and kerolite-stevensite mixed-layers from Miocene sediments of the Madrid Basin in Spain to evaluate the relative impact of the isotopic composition of the interacting fluids and the crystallization temperature on these minerals in specific continental crystallization environments such as alluvial, palustrine and mudflat areas. Preliminary experiments of progressive heating of pure sepiolite and saponite under vacuum allowed to set a pretreatment at 200 degrees C to remove the non-bonded water of both sepiolite and smectite before analysis of the structural oxygen and hydrogen. In combining crystallization temperatures of the different Mg-clays with potential isotopic compositions of the interacting fluids, it can be stated that the studied clay minerals precipitated during sedimentation in connection with various fluids of meteoric origin for saponite and stevensite, and with an evaporated, more saline water for sepiolite. The calculated crystallization temperatures ranged from as low as 7 +/- 5 to 35 +/- 5 degrees C for sepiolite and from about 22 +/- 5 to 48 +/- 5 degrees C for saponite. In the case of the kerolite-stevensite mixed-layer, it is likely that the stevensite domains crystallized first at a calculated temperature of about 10 +/- 5 degrees C being progressively replaced by kerolite domains at a significantly higher temperature towards 55 +/- 5 degrees C.

The calculated crystallization temperature of sepiolite appears also to increase from palustrine, mudflat, mudflat/palustrine, to alluvial environments. The increasing trend goes from palustrine sepiolite formed at as low as 7 +/- 5 degrees C from a parent water that interacted also with saponite, to sepiolite crystallized from evaporated waters at higher temperatures in mudflat/palustrine and alluvial facies. If such mineral formation occurred quasi-continuously rather than episodically, sepiolite (and saponite) formed at any given locality over a range of temperatures from waters with varied isotopic compositions. Any given Mg-mineral might then result from a mixture of crystals with different isotopic compositions consisting of end-members that interacted with the initial parental water of the smectite (delta O-18 = -7.25 and delta D = -48 parts per thousand) at low temperature of about 10 +/- 5 degrees C, and evaporated water (delta O-18 = -4.00 and delta D = -32 parts per thousand) at temperatures up to 40 +/- 5 degrees C. (C) 2012 Elsevier Ltd. All rights reserved.