Interaction of Mantle Rocks with Crustal Fluids: Sagvandites of the Scandinavian Caledonides

• Published in: Journal of earth science (Wuhan, China) China), 2019-12, Vol.30 (6), p.1084-1094
• Main Authors: Bucher, Kurt, Stober, Ingrid
• Format: Article
• Language: English
• Subjects: Amphibolite facies; Amphibolites; Biogeosciences; Carbon dioxide; Chromite; Conversion; Crystal structure; Crystals; Dunite; Earth and Environmental Science; Earth mantle; Earth Sciences; Enstatite; Fluids; Forsterite; Geochemistry; Geology; Geotechnical Engineering & Applied Earth Sciences; Isotopes; Magnesite; Magnesium carbonate; Metamorphism and Orogenic Belts—Response from Micro- to Macro-Scale; Nappes; Olivine; Orogeny; Outcrops; Peridotite; Rocks; Silica; Silicon dioxide; Sodium chloride; Ultramafic materials; Ultramafic rocks; Upper mantle
• ISSN: 1674-487X; 1867-111X
• DOI: 10.1007/s12583-019-1257-2

Sagvandite is an enstatite+magnesite rock formed from dunite or harzburgite bodies occurring as tectonically emplaced fragments from the upper mantle in many orogenic belts by interaction with CO 2 -bearing crustal fluids at upper amphibolite facies P-T conditions. Sagvandite bodies occur widespread in distinct nappes in the Scandinavian Caledonides in Norway. Common to all of the many sagvandite outcrops is their general structure of radial bundles of very coarse cm-sized enstatite crystals and interstitial magnesite. Often some strongly resorbed primary olivine is preserved, in addition to minor accessory Cr-spinel and chromite. The dunite to sagvandite conversion is governed by three metasomatic reactions: (1) carbonatization of peridotite by CO 2 -bearing fluids; (2) interaction with external fluids containing dissolved silica; (3) loss of Mg by dissolution of forsterite in NaCl-rich deep fluids. Simultaneous progress ξ overall of all three reactions in proportions that conserve the volume of the original dunite can explain the observed structure and mode of sagvandite. The relationship among the progress ξ of the three reactions suggests that loss of Mg by the ultramafic rock is the dominating process in the iso-volume conversion of dunite to sagvandite.