VARIATIONS IN WATER CONTENTS AND HYDROGEN ISOTOPIC COMPOSITIONS OF IGNEOUS AND METAMORPHIC AMPHIBOLES

Amphiboles are hydrous rock-forming minerals in igneous and metamorphic rocks commonly used to infer petrogenetic processes. Ever since the advent of electron microprobe which led to the abatement of wet chemical techniques, water contents in amphiboles are not regularly analyzed along with major and minor element compositions but assumed to be 2 wt. %.

In order to investigate variations in amphibole water contents and hydrogen isotopic compositions as a function of mineral compositions and rock types, we analyzed various amphibole end-members sampled from metamorphic, volcanic, intrusive, and mantle xenoliths. We applied both an in-situ (Secondary-Ion Mass Spectrometry, SIMS) and a bulk method (High Temperature Conversation Elemental Analyzer, TC-EA) to test the robustness of the methods. This new data set was complemented with an extensive literature research of published amphibole water contents and hydrogen isotopic compositions to detect systematic differences between amphibole compositions and rock types.

Analyzed calcic to sodic amphiboles have water contents between 0.14 – 2 wt. % and hydrogen isotopic compositions of -160 to -72‰ (in permille relative to SMOW; SIMS data). Literature data show a wide range in water contents from 0.04 to 4.9 wt. % ( metamorphic 0.1 – 4.9 wt. %, volcanic 0.04 – 3.03 wt. %, intrusive 0.35 – 3.15 wt. %, and mantle amphiboles 0.6 – 2.76 wt. %) and hydrogen isotope compositions from -232 to +50‰ (metamorphic -163 – -9‰, volcanic -191 – +3‰, intrusive -113 – -26.8‰, and mantle amphiboles -232 – +50‰).This compilation clearly shows that metamorphic amphiboles have the greatest range of water contents and mantle amphiboles have the smallest range. Mantle amphiboles have the greatest range of hydrogen isotopic compositions and intrusive amphiboles the smallest. However, the reported extremely high water contents are ascribed to mixed analyses of amphibole and hydrous mineral inclusions (e.g. biotite) applying bulk techniques. Volcanic and mantle amphiboles tend to experience various degrees of dehydration as evident in a positive correlation of water contents and hydrogen isotopic compositions. Intrusive amphiboles did not experiences significant dehydration indicating that their hydrogen isotopic compositions most likely reflect magmatic conditions.