Magma Dynamics of Axial Melt Lens at Fast‐Spreading Mid‐Ocean Ridges

Abstract

AbstractMultichannel seismic studies performed at fast‐spreading mid‐ocean ridges revealed the presence of a thin (tens to hundreds of meters high), narrow (< 1–2 km wide) axial melt lens (AML) in the mid‐crust, which is underlain by crystal/melt mush that is in turn laterally surrounded by a transition zone of mostly solidified material. In order to shed light on the complexity of magmatic and metamorphic processes ongoing within and at the roof of axial melt lenses, we have focused on the petrological and geochemical record provided by fossilized AMLs. Of particular significance is Hole 1256D in the equatorial Pacific drilled by the International Ocean Discovery Program (IODP), where for the first time, the transition between sheeted dikes and gabbros in intact fast‐spreading crust was penetrated, providing a drill core with a more or less continuous record of the upper part of an AML (Teagle et al., 2006; Koepke et al., 2008). This can be regarded as rosetta stone to answer long‐standing questions on the complex magmatic evolution within an AML, as well as on metamorphic and anatectic processes ongoing at the roof of a dynamic AML, rising upward in the mid‐crust as a consequence of a replenishment event. The plutonic rocks drilled from Hole 1256D consist of quartz‐bearing gabbros, diorites and tonalites, which might represent the upper part of a fossilized AML. The gabbros and diorites are consistent with modeled products of MORB fractional crystallization, composed of mixed melt and cumulate in varying ratios. Modeled trace elements support a model in which the tonalites originated from low‐degree partial melting of the sheeted dikes overlying the AML, rather than extreme fractional crystallization (Erdmann et al., 2015; Zhang et al., 2017a). Therefore, the upper part of AML, largely composed of low density and high‐viscosity felsic magmas, may serve as a barrier to eruptible MORB melts in the lower part of AML. Zoning of apatites from three different lithologies, tonalites, diorites, and gabbros, is common and shows a consistent evolution trend with depletion in Cl and REEs from core to rim. The cores are usually homogenous in composition and interpreted as magmatic origin, whereas zones with lower Cl and REEs are disseminated with heterogeneous concentrations, indicating exchanges with hydrothermal fluids. The high‐Cl apatite core indicates assimilation of high‐Cl brines at a magmatic stage, which is interpreted as immiscibility product from cycling seawater‐derived fluids at a high temperature (Zhang et al., 2017b). The variation of F/Cl and Br/Cl ratios of bull rocks may reflect the mixing between MORB magmas and seawater‐derived fluids, crystallization of apatite and amphibole, and/or extraction of magmatic fluids (Zhang et al., 2017c).