Chlorine-rich amphibole in deep layered gabbros as evidence for brine/rock interaction in the lower oceanic crust: A case study from the Wadi Wariyah, Samail Ophiolite, Sultanate of Oman

Abstract

Hydrothermal veins and dykelets that cross-cut layered olivine gabbros deep in the plutonic section of the Samail Ophiolite, Sultanate of Oman, point towards the occurrence of hydrothermal circulation in the deep oceanic crust, and these features record interactions between rock and high temperature seawater-derived fluids or brines. Deep penetration of seawater-derived fluids down to 100 m above the Moho transition zone and the consequent interactions with the host rock lead to hydrothermal alteration from granulite facies grading down to greenschist facies conditions. Here we present a study of veins and dykelets formed by hydrothermal interaction cutting layered gabbro in the Wadi Wariyah, using petrographic, microanalytical, isotopic, and structural methods. We focus on amphiboles, which show a conspicuous compositional variation from high-Ti magnesiohastingsite and pargasite via magnesiohornblende and edenite, to Cl-rich ferropargasite and hastingsite (up to 1.5 a.p.f.u. Cl) and actinolite. These minerals record a wide range of formation conditions from magmatic to hydrothermal, and reveal a complex history of interactions between rock and hydrothermal fluid or brine in a lower oceanic crustal setting. Large variations in Cl content and cation configurations in amphibole suggest formation in equilibrium with fluids of different salinities at variable fluid/rock ratios. The presence of subsolidus amphibole extremely enriched in chlorine implies phase separation and brine/rock interactions. 87 Sr /86 Sr values of 0.7031 to 0.7039 and stable δ 18 O isotopic compositions of 4.1 to 5.6 ‰ of the different amphibole types suggest a rock-dominated environment, i.e. with low fluid/rock ratios. However, the slight departure from mean Oman isotope values may indicate there was some influence of seawater in the aforementioned fluid-rock interactions. Our study provides new petrological data for the subsolidus evolution of gabbro-hosted amphibole-rich veins in the presence of a seawater-derived fluid.

Formation of Igneous Layering in the Lower Oceanic Crust From the Samail Ophiolite, Sultanate of Oman

“…Hydrothermal circulation at high temperatures in Wadi Wariyah is supported by observations in a well‐studied outcrop ~ 300 m south of the locality where our fine‐layered sample was taken: This outcrop of layered gabbros is characterized by a complex system of hydrothermal veins containing brown pargasitic amphibole and green magnesio‐hornblende which have formed under high to very high temperatures (850°C and 1030°C; Bosch, 2004; Wolff, 2013). Very high Cl contents in hastingsites and pargasites within these veins (up to 5 wt%, Currin et al, 2018) provide evidence for the presence of highly saline, seawater‐derived fluids at depth. Evidence for deep hydrothermal activity at high and very high temperatures in layered gabbros from fast‐spreading systems has been reported in several studies (Bosch, 2004; Garrido et al, 2001; Koepke et al, 2014; Nozaka et al, 2016).…”

“…Journal of Geophysical Research: Solid Earth 5 wt%, Currin et al, 2018) provide evidence for the presence of highly saline, seawater-derived fluids at depth. Evidence for deep hydrothermal activity at high and very high temperatures in layered gabbros from fast-spreading systems has been reported in several studies (Bosch, 2004;Garrido et al, 2001;Koepke et al, 2014;Nozaka et al, 2016).…”

“…The layering occurs as millimeter-or centimeter-thick layers with significantly differing olivine contents ( Figure 2d). The locality where we collected the sample is located 300 m north to a well-studied outcrop of layered gabbro that preserves magmatic and hydrothermal veins ranging in formation temperature from very high (magmatic regime) to low temperatures which cut perpendicularly across the layering (Figure 2c; Bosch, 2004;Currin et al, 2018;Nicolas et al, 2003;Wolff, 2013). Foliation in plagioclase and olivine is here parallel to the igneous layering.…”

Melt hybridization and metasomatism triggered by syn-magmatic faults within the Oman ophiolite: A clue to understand the genesis of the dunitic mantle-crust transition zone

“…Mid-Ocean Ridge vs. arc-related basin) (e.g. Bosch et al, 2004;Currin et al, 2018;France et al, 2009France et al, , 2013Gilgen et al, 2016;Python et al, 2007;Zihlmann et al, 2018).…”

“…In the Oman ophiolite, petrological and isotopic studies have shown that the crust was affected by hydrothermal circulations ranging from greenschist to upper amphibolite/granulite facies (e.g. Bosch et al, 2004;Gregory and Taylor, 1981;Koepke et al, 2005;Nehlig and Juteau, 1988;Currin et al, 2018). The formation of diopsidite and rodingite dikes likewise involves high temperature hydrothermal fluids circulating down to the Moho and even below (Akizawa et al, 2011;Python et al, 2007).…”