Xonotlite and pectolite in rodingites from the Samail ophiolite: Markers of reducing conditions and element transfer during peridotite-gabbro interaction

Abstract

The Oman Drilling Project cored Holes BA1B, BA3A and BA4A into serpentinized peridotites in Wadi Lawayni of the Samail ophiolite, Oman. These mantle peridotites are crosscut by numerous gabbro, olivine gabbro and clinopyroxenite dikes that are altered to calc-silicate assemblages (hydrogarnet, diopside, xonotlite, vesuvianite and pectolite), chlorite, and phlogopite. These mineralogical assemblages are typical of rodingites. Xonotlite [Ca₆Si₆O₁₇(OH)₂] and pectolite [NaCa₂Si₃O₈(OH)] forming as fibrous or acicular/radial crystals in milk-white vein-like are common in these holes. In contrast, only a few occurrences of these minerals have been reported worldwide. Bulk rock analyses indicate two types of altered dikes: chlorite-bearing dikes displaying enrichments in Mg, Fe, Mn, Ti, Cr, Ni and volatiles as well as loss of Na compared to lower crustal gabbros; and rodingitized dikes with enrichments in Ca and Na during alteration. Mass gains in chlorite-bearing dikes are consistent with intense mass transfer from peridotite to gabbroic dikes. These results are consistent with thermodynamic modelling showing a strong control of the peridotite-gabbro ratio on the mineralogy of the altered gabbro: wollastonite, pectolite, and vesuvianite present here are predicted to form in peridotite-dominated systems, whereas prehnite, epidote and pumpellyite not reported in the studied boreholes are predicted to form in gabbro-dominated systems. Fluid inclusions containing H₂ and CH₄ are observed in both primary orthopyroxene and pectolite. They indicate the presence of a carbon-rich reduced fluid during rodingitization. The xonotlite stability field calculated with thermodynamic modelling is indeed limited by carbonate formation as oxygen fugacity increases. The rodingites from Oman display numerous markers of strong fluid/rock interactions, including enrichment in mobile elements such as Na, K and Li, typically associated with seawater interaction and hydrothermal fluid transport. The presence of xonotlite, pectolite and phlogopite is consistent with the addition of external elements during rodingitization since the extent of the calculated stability fields of these minerals increases when adding Ca, Na and K to the system. High δ^44/40Ca values in chlorite-bearing dikes (1.65 ± 0.09 ‰ to 1.91 ± 0.09 ‰), rodingitized dikes (0.28 ± 0.09 ‰ to 1.25 ± 0.02 ‰) and serpentinized harzburgites (1.04 ± 0.05 ‰ to 1.64 ± 0.06 ‰) reveals mineral-fluid fractionation of Ca isotopes and/or interaction of seawater with gabbro-peridotite samples.