Silica‐Rich Vein Formation in an Evolving Stress Field, Atlantis Bank Oceanic Core Complex

Abstract

Drilling 809‐m Hole U1473A in the gabbro batholith at the Atlantis Bank Oceanic Core Complex (OCC) found two felsic vein generations: late magmatic fractionates, rich in deuteric water, hosted by oxide gabbros, and anatectic veins associated with dike intrusion and introduction of seawater‐derived volatiles. Microtextures show a change from compressional to tensional stress during vein formation. Temperatures and oxidation state were obtained from amphibole‐plagioclase and oxide pairs in the adjacent gabbros. Type I veins generally have reverse shear‐sense, with restricted ΔFMQ, high Mt/Ilm ratios, and low‐amphibole Cl/F indicating deuteric fluids. They formed during percolation and fractionation of Fe‐Ti‐rich melts into the primary olivine gabbro. Type II veins are usually hosted by olivine gabbro, occur at dike contacts and the margins of normal‐sense shear zones. They are undeformed or weakly deformed, with highly variable ΔFMQ, low Mt/Ilm ratios, and high‐amphibole Cl/F, indicating seawater‐derived fluids. The detachment fault on which the gabbro massif was emplaced rooted near the base of the dike‐gabbro transition beneath the rift valley. The ingress of seawater volatiles began at >800°C and penetrated at least ~590 m into the lower crust during extensional faulting in the rift valley and adjacent rift mountains. The sequence of the felsic vein formation likely reflects asymmetric diapiric flow, with a reversal of the stress regime, and a transition from juvenile to seawater‐derived volatiles. This, in turn, is consistent with fault capture leading to the large asymmetries in spreading rates during OCC formations and heat flow beneath the rift mountains.

High‐Temperature Strain Localization and the Nucleation of Oceanic Core Complexes (16.5°N, Mid‐Atlantic Ridge)

“…The first hypothesis implies syn‐tectonic hybridization from a melt rich in H 2 O and incompatible elements. These late‐stage melts are not rare in the oceanic lithosphere, where they crystallize felsic lithologies characterized by amphibole and albitic plagioclase intruded within more primitive gabbros (Basch et al., 2020; Dick et al., 2019; Ma et al., 2020; MacLeod et al., 2017). For instance, Gardner et al.…”

“…For instance, Gardner et al (2020) recently showed that the injection of these late-stage melts within a deforming olivine-gabbro in the Atlantis Bank promoted strain localization necessary for nucleation of the oceanic detachment. The origin of melts forming felsic lithologies is debated and include extreme fractional crystallization starting from a basaltic melt (e.g., Nguyen et al, 2018;Basch et al, 2020) or partial melting of preexisting hydrothermally altered gabbros (France et al, 2013;Koepke et al, 2007), or both (Ma et al, 2020). The anomalous composition of the ultra-mylonite could be thus interpreted as resulting from hybridization of a felsic melt localized in the shear zone by crystal plastic deformation.…”

“…The first hypothesis implies syn-tectonic hybridization from a melt rich in H 2 O and incompatible elements. These late-stage melts are not rare in the oceanic lithosphere, where they crystallize felsic lithologies characterized by amphibole and albitic plagioclase intruded within more primitive gabbros (Basch et al, 2020;Dick et al, 2019;Ma et al, 2020;MacLeod et al, 2017). For instance, Gardner et al (2020) recently showed that the injection of these late-stage melts within a deforming olivine-gabbro in the Atlantis Bank promoted strain localization necessary for nucleation of the oceanic detachment.…”

“…Inception of strain localization is mainly attributed to GBS‐mediated switch from early diffusion creep at high temperature to localized semi‐brittle deformation at sub‐solidus conditions (Mehl & Hirth, 2008; Schroeder & John, 2004). The role of low‐temperature fluids and/or high‐temperature melts in promoting this process is well described in the Atlantis Bank (Southwest Indian Ridge) and related to (a) infiltration of seawater‐derived fluids, (b) occurrence of intergranular melts, and (c) formation of late‐stage magmatic veins (e.g., Gardner et al., 2020; Ma et al., 2020; Taufner et al., 2021).…”

Brown Amphibole as Tracer of Tectono-Magmatic Evolution of the Atlantis Bank Oceanic Core Complex (IODP Hole U1473A)

“…The origin of the different generations of felsic veins appears to be correlated with lower crustal exhumation during emplacement of the OCC. Felsic veins with generally diffuse boundaries, high magnetite/ilmenite ratios and low amphibole Cl/F ratio were interpreted to have formed by fractional crystallization under high-temperature conditions (Nguyen et al, 2018;Ma et al, 2020). Conversely, felsic veins with sharp boundaries and high amphibole Cl/F ratio formed by hydrous melting (anatexis) as a consequence of melt intrusion in hydrothermally altered gabbros (Ma et al, 2020).…”