The stable isotopic composition of cryptocrystalline magnesite occurrences in Turkey and Austria and implications for their origin

Cryptocrystalline magnesite occurs predominantly in ultramafitic rocks of ophiolite sequences and associated sediments. Two types are recognized, the Kraubath type (KT), which occurs – tectonically controlled – in ultramafic rocks as veins, networks and zebra ore, and the Bela Stena type (BST), which occurs as nodules and layers in sediments.
   The two types not only differ by the nature of their host rock but also by their C and O isotopic composition. The KT has lower C isotope values (-18 to -6‰ VPDB) than the BST (-1 to +4‰ VPDB). δ18O values of both types overlap, whereby the KT shows a tendency to lower values (+22 to +29‰ VSMOW) than the BST (+26 to +36‰ VSMOW).
   This study is based on extensive fieldwork and a total of 320 samples from Austria (Kraubath) and Turkey (western and eastern Anatolia).
   Kraubath (Austria) contains the lowest C isotope values (-22.5 to -11.3‰ VPDB). Turkish KT magnesite contains higher C isotope values (-12.7 to -3.1‰ VPDB). Turkish BST magnesite contains mainly positive C isotope values (+1.5 to +6.9‰ VPDB) with the exceptions of Dutluca and Bahtyiar (Eskişehir/Western Anatolia).
   In the operated magnesite deposit of Dutluca (Eskişehir/Turkey) KT magnesite and zebra ore (-10.6 to -7.8‰ VPDB) are covered by sediments with BST magnesite, which occurs as nodules and layers (-6.5 to -5.8‰ VPDB).
   The deposit of Bathyiar (Eskişehir/Turkey) shows a transition from network to iron-rich zebra ore and BST. The network shows normal isotope values, but the δ¹⁸O values of the zebra ore are extraordinary high (+29 to +35‰ VSMOW).
   The δ¹⁸O values of KT magnesite suggest general formation temperatures between ca. 60 and 70°C. Exceptions are the deposits of Tavşanlı/Turkey, which formed at temperatures of ca. 80°C. Zebra ore of Bathyiar (Eskişehir/Turkey) formed at temperatures below ca. 30°C.
The range of the δ¹³C values (-22.5 to +6.1‰ VPDB) suggests that CO₂ was derived from several sources (biogenic – atmospheric, decarboxylated, mantle-derived or magmatogene) and was transported by meteoric waters. Supergene water with dissolved HCO₃^- invaded the serpentinite and leached Mg²⁺. The release of Mg²⁺ and OH^- into the solution raised the pH. Extension of strike-slip system provided pathways for migration and the formation of hydrothermal convection cells. At shallow levels the drops in pCO₂ due to outgassing caused supersaturation and formation of magnesite.