An Approach to Genesis of Sepiolite and Palygorskite in Lacustrine Sediments of the Lower Pliocene Sakarya and Porsuk Formations in the Sivrihisar and Yunusemre-Biçer Regions (Eskişehir), Turkey

Abstract

The Lower Pliocene lacustrine sediments of the Sakarya and Porsuk Formations in the Sivrihisar and Yunusemre-Biçer regions consist of claystone, argillaceous carbonate, carbonate, and evaporites. No detailed studies of paleoclimatic conditions have been performed previously. The present study aimed to determine the depositional environment and paleoclimatic conditions for the formation of these economically important sepiolite/palygorskite/carbonate/evaporite deposits based on detailed mineralogical, geochemical, and isotopic studies. Samples from various lacustrine sediments were examined using polarized-light microscopy, X-ray diffraction, scanning electron microscopy, and chemical and isotopic analysis methods. Dolomites are predominantly of micrite, which is partly recrystallized to dolomicrosparite/dolosparite close to desiccation fractures. The presence of ostracods and dacycladecean algae in the carbonates reflects a restricted depositional environment. The formation of sepiolite and palygorskite fibers, either as cement between/enclosing dolomite and/or as calcite crystals, reflects occasional changes in physicochemical conditions provided by fluctuations in the lake-water level and influx of groundwater in relation to climatic changes during and after dolomite precipitation. The positive correlations of ΣREE with Al2O3, Nb, high-field-strength elements, and transition elements are due to alteration of feldspar and hornblende in the volcanic units. The high values of Ba and Sr relative to Cr, Co, Ni, and V also indicate that felsic rather than ophiolitic rocks were the parent material. The crossplot of whole-rock SiO2vs. Al2O3+K2O+Na2O and V/Cr ratio suggests deposition of carbonate-dolomitic sepiolite-sepiolitic dolomite under arid climate and oxic conditions, whereas the Ni/Co and V/(V+Ni) ratios of the sediments indicate deposition of organic-bearing sepiolite/palygorskite under anoxic-dysoxic conditions. An enrichment in δ13C and δ18O values of dolomite with respect to calcite is probably due to differences in mineral fractionations. The δ34S and δ18O values and 87Sr/86Sr isotope ratios for gypsum suggest an intensely evaporitic lacustrine environment fed by an older marine evaporitic source. The Si, Al, Mg, Ca, and enhanced TOT/C required for periodic precipitation of organic-rich brown sepiolite/palygorskite characterize deposition in a swampy environment, while dolomitic sepiolite and sepiolitic dolomite formed in ponds by partial drying of the main alkaline lake.

Dolomite dissolution: An alternative diagenetic pathway for the formation of palygorskite clay

“…One possibility is that all three parameters (pH, Mg and H 4 SiOH 4 ) did not evolve simultaneously, but were sequentially developed until the system was undersaturated with respect to dolomite and subsequently saturated with respect to palygorskite. Meteoric water is a likely candidate responsible for starting a series of reactions and has been interpreted as the source of silica or solution responsible for palygorskite formation in other localities (Singer & Norrish, 1974;Soong, 1992;Draidia et al, 2016;Kadir et al, 2016Kadir et al, , 2017. Such a fluid could contain elevated levels of H 4 SiO 4 (Knauth, 1979;Bennet & Siegel, 1987) as well as high pCO 2 (James & Choquette, 1990).…”

“…Palygorskite, with an ideal composition of (Mg, Al) 2 Si 4 O 10 (OH)Á4(H 2 O), is a magnesiumrich monoclinic and orthorhombic clay mineral with a characteristic fibrous habit (Weaver & Beck, 1977;Singer, 1979Singer, , 1984Singer, , 2002Galan, 1996;Galan & Carretero, 1999;Guggenheim & Krekeler, 2011;Murray et al, 2011). Palygorskite is observed in a wide variety of sedimentary deposits worldwide (Isphording, 1973;Callen, 1977Callen, , 1984Weaver & Beck, 1977;Torres-Ruiz et al, 1994;Akbulut & Kadir, 2003), with the largest deposits located within the Middle Tertiary sediments of China, Senegal, Spain, Turkey, Ukraine and the south-eastern United States (Krekeler, 2004;Garcia-Romero et al, 2007;Murray et al, 2011;Yeniyol, 2012Yeniyol, , 2014Kadir et al, 2016Kadir et al, , 2017. The mineral also serves as an important constituent in soils in arid to semi-arid environments (Singer & Norrish, 1974;Singer, 1984Singer, , 2002.…”

“…Numerous case studies have documented the co-occurrence of dolomite and palygorskite in the rock record (Weaver, 1975;Weaver & Beck, 1977;Shadfan et al, 1985a,b;C ß a gatay, 1988, 1990Ingles & Anadon, 1991;Verrecchia & Le Coustumer, 1996;Holail & Al-Hajari, 1997;Krekeler et al, 2007;Xie et al, 2013;Draidia et al, 2016;Kadir et al, 2017). Despite a lack of compelling petrological evidence, a number of models to explain the genesis of palygorskite in association with dolomite have been proposed.…”

Geodynamic seawater-sediment porewater evolution of the east central Atlantic Paleogene ocean margin revealed by U-Pb dating of sedimentary phosphates

“…The absence of widespread swelling clays and fibrous sepiolite-palygorskite growing on platy smectite suggests that the transformation of smectite to sepiolite-palygorskite was not the dominant mechanism in the Amizmiz phosphorites. Secondary partial dissolution of rhombohedric dolomitic crystals could have resulted from changes in physiochemical conditions of the environments, which would have favored subsequent sepiolite and palygorskite precipitation (Weaver and Beck, 1977;Akbulut and Kadir, 2003;Kadir et al, 2017). Considering the low amount of dolomite in the Amizmiz samples, it is unlikely that all sepiolite-palygorskite-hosted Mg derived from the dissolution of dolomite.…”

Transformation of Magnesite to Sepiolite and Stevensite: Characteristics and Genesis (Çayirbaği, Konya, Turkey)

“…Sepiolite and stevensite are authigenic Mg-rich clay minerals having different structural organizations. According to numerous published papers, these minerals form in a variety of geological environments: in Si-and Mg-rich alkaline lakes and shallow seas via direct precipitation from alkaline or saline water (Papke 1972;Tettenhorst and Moore 1978;Khoury et al 1982;Eberl et al 1982;Galan and Castillo 1984;Ece and Çoban 1994;Karakaş and Kadir 1998;Singer et al 1998;Pozo and Casas 1999;Buey et al 2000;Mayayo et al 2000;Kadir and Akbulut 2001;Kadir et al 2002;Akbulut and Kadir 2003;Karakaya et al 2004;Furquim et al 2008;Galan and Pozo 2011;Yeniyol 1992Yeniyol , 2007Yeniyol , 2012Yeniyol , 2014Kadir et al 2016Kadir et al , 2017; and many others), transformation from precursor Mg-bearing silicate phases (Stoessell and Hay 1978;Chahi et al 1997;Pozo and Casas 1999;Cuevas et al 2003;Hover and Ashley 2003;Yeniyol 1997Yeniyol , 2012Yeniyol , 2014, hydrothermal alteration of magmatic rocks (Faust and Murata 1953;Randal 1959;Imai et al 1970;Post 1984;April and Keller 1992;İrkeç and Ünlü 1993), and weathering of mafic and ultramafic rocks…”