Lower oceanic crust formed by in situ melt crystallisation revealed by seismic layering

Oceanic crust forms at mid-ocean spreading centres through a combination of magmatic and tectonic processes, with the magmatic processes creating two distinct layers: the upper and the lower crust. While the upper crust is known to form from lava flows and basaltic dikes based on geophysical and drilling results, the formation of the gabbroic lower crust is still debated. Here we perform a full waveform inversion of wide-angle seismic data from relatively young (7-12-million-year-old) crust formed at the slow spreading Mid-Atlantic Ridge. The seismic velocity model reveals alternating, 400-500 m thick, high and low velocity layers with ±200 m/s velocity variations, below ~2 km from the oceanic basement. The uppermost low-velocity layer is consistent with hydrothermal alteration, defining the base of extensive hydrothermal circulation near the ridge axis. The underlying layering supports that the lower crust is formed through the intrusion of melt as sills at different depths, that cool and crystallise in situ. The layering extends up to 5-15 km distance along the seismic profile, covering 300,000-800,000 years, suggesting that this form of lower crustal accretion is a stable process.

Three-dimensional magnetic stripes require slow cooling in fast-spread lower ocean crust

Earth's magnetic field is recorded as oceanic crust cools, generating lineated magnetic anomalies that provide the pattern of polarity reversals for the past 160 million years¹. In the lower (gabbroic) crust, polarity interval boundaries are proxies for isotherms that constrain cooling and hence crustal accretion. Seismic observations^2-4, geospeedometry^5-7 and thermal modelling^8-10 of fast-spread crust yield conflicting interpretations of where and how heat is lost near the ridge, a sensitive indicator of processes of melt transport and crystallization within the crust. Here we show that the magnetic structure of magmatically robust fast-spread crust requires that crustal temperatures near the dike-gabbro transition remain at approximately 500 degrees Celsius for 0.1 million years. Near-bottom magnetization solutions over two areas, separated by approximately 8 kilometres, highlight subhorizontal polarity boundaries within 200 metres of the dike-gabbro transition that extend 7-8 kilometres off-axis. Oriented samples with multiple polarity components provide direct confirmation of a corresponding horizontal polarity boundary across an area approximately one kilometre wide, and indicate slow cooling over three polarity intervals. Our results are incompatible with deep hydrothermal cooling within a few kilometres of the axis^2,7 and instead suggest a broad, hot axial zone that extends roughly 8 kilometres off-axis in magmatically robust fast-spread ocean crust.

Consequences of a crystal mush-dominated magma plumbing system: a mid-ocean ridge perspective

Crystal mush is rapidly emerging as a new paradigm for the evolution of igneous systems. Mid-ocean ridges provide a unique opportunity to study mush processes: geophysical data indicate that, even at the most magmatically robust fast-spreading ridges, the magma plumbing system typically comprises crystal mush. In this paper, we describe some of the consequences of crystal mush for the evolution of the mid-ocean ridge magmatic system. One of these is that melt migration by porous flow plays an important role, in addition to rapid, channelized flow. Facilitated by both buoyancy and (deformation-enhanced) compaction, porous flow leads to reactions between the mush and migrating melts. Reactions between melt and the surrounding crystal framework are also likely to occur upon emplacement of primitive melts into the mush. Furthermore, replenishment facilitates mixing between the replenishing melt and interstitial melts of the mush. Hence, crystal mushes facilitate reaction and mixing, which leads to significant homogenization, and which may account for the geochemical systematics of mid-ocean ridge basalt (MORB). A second consequence is cryptic fractionation. At mid-ocean ridges, a plagioclase framework may already have formed when clinopyroxene saturates. As a result, clinopyroxene phenocrysts are rare, despite the fact that the vast majority of MORB records clinopyroxene fractionation. Hence, melts extracted from crystal mush may show a cryptic fractionation signature. Another consequence of a mush-dominated plumbing system is that channelized flow of melts through the crystal mush leads to the occurrence of vertical magmatic fabrics in oceanic gabbros, as well as the entrainment of diverse populations of phenocrysts. Overall, we conclude that the occurrence of crystal mush has a number of fundamental implications for the behaviour and evolution of magmatic systems, and that mid-ocean ridges can serve as a useful template for trans-crustal mush columns elsewhere. This article is part of the Theo Murphy meeting issue 'Magma reservoir architecture and dynamics'.

Mafic tiers and transient mushes: evidence from Iceland

It is well established that magmatism is trans-crustal, with melt storage and processing occurring over a range of depths. Development of this conceptual model was based on observations of the products of magmatism at spreading ridges, including Iceland. Petrological barometry and tracking of the solidification process has been used to show that the Icelandic crust is built by crystallization over a range of depths. The available petrological evidence indicates that most of the active rift zones are not underlain by extensive and pervasive crystal mush. Instead, the microanalytical observations from Iceland are consistent with a model where magmatic processing in the lower crust occurs in sills of decimetric vertical thickness. This stacked sills mode of crustal accretion corresponds to that proposed for the oceanic crust on the basis of ophiolite studies. A key feature of these models is that the country rock for the sills is hot but subsolidus. This condition can be met if the porosity in thin crystal mushes at the margins of the sills is occluded by primitive phases, a contention that is consistent with observations from cumulate nodules in Icelandic basalts. The conditions required for the stabilization of trans-crustal mushes may not be present in magmatic systems at spreading ridges. This article is part of the Theo Murphy meeting issue 'Magma reservoir architecture and dynamics'.

Talc-dominated seafloor deposits reveal a new class of hydrothermal system

The Von Damm Vent Field (VDVF) is located on the flanks of the Mid-Cayman Spreading Centre, 13 km west of the axial rift, within a gabbro and peridotite basement. Unlike any other active vent field, hydrothermal precipitates at the VDVF comprise 85-90% by volume of the magnesium silicate mineral, talc. Hydrothermal fluids vent from a 3-m high, 1-m diameter chimney and other orifices at up to 215 °C with low metal concentrations, intermediate pH (5.8) and high concentrations (667 mmol kg(-1)) of chloride relative to seawater. Here we show that the VDVF vent fluid is generated by interaction of seawater with a mafic and ultramafic basement which precipitates talc on mixing with seawater. The heat flux at the VDVF is measured at 487±101 MW, comparable to the most powerful magma-driven hydrothermal systems known, and may represent a significant mode of off-axis oceanic crustal cooling not previously recognized or accounted for in global models.

Hybrid shallow on-axis and deep off-axis hydrothermal circulation at fast-spreading ridges

Hydrothermal flow at oceanic spreading centres accounts for about ten per cent of all heat flux in the oceans and controls the thermal structure of young oceanic plates. It also influences ocean and crustal chemistry, provides a basis for chemosynthetic ecosystems, and has formed massive sulphide ore deposits throughout Earth's history. Despite this, how and under what conditions heat is extracted, in particular from the lower crust, remains largely unclear. Here we present high-resolution, whole-crust, two- and three-dimensional simulations of hydrothermal flow beneath fast-spreading ridges that predict the existence of two interacting flow components, controlled by different physical mechanisms, that merge above the melt lens to feed ridge-centred vent sites. Shallow on-axis flow structures develop owing to the thermodynamic properties of water, whereas deeper off-axis flow is strongly shaped by crustal permeability, particularly the brittle-ductile transition. About 60 per cent of the discharging fluid mass is replenished on-axis by warm (up to 300 degrees Celsius) recharge flow surrounding the hot thermal plumes, and the remaining 40 per cent or so occurs as colder and broader recharge up to several kilometres away from the axis that feeds hot (500-700 degrees Celsius) deep-rooted off-axis flow towards the ridge. Despite its lower contribution to the total mass flux, this deep off-axis flow carries about 70 per cent of the thermal energy released at the ridge axis. This combination of two flow components explains the seismically determined thermal structure of the crust and reconciles previously incompatible models favouring either shallower on-axis or deeper off-axis hydrothermal circulation.

Primitive layered gabbros from fast-spreading lower oceanic crust

Three-quarters of the oceanic crust formed at fast-spreading ridges is composed of plutonic rocks whose mineral assemblages, textures and compositions record the history of melt transport and crystallization between the mantle and the sea floor. Despite the importance of these rocks, sampling them in situ is extremely challenging owing to the overlying dykes and lavas. This means that models for understanding the formation of the lower crust are based largely on geophysical studies and ancient analogues (ophiolites) that did not form at typical mid-ocean ridges. Here we describe cored intervals of primitive, modally layered gabbroic rocks from the lower plutonic crust formed at a fast-spreading ridge, sampled by the Integrated Ocean Drilling Program at the Hess Deep rift. Centimetre-scale, modally layered rocks, some of which have a strong layering-parallel foliation, confirm a long-held belief that such rocks are a key constituent of the lower oceanic crust formed at fast-spreading ridges. Geochemical analysis of these primitive lower plutonic rocks--in combination with previous geochemical data for shallow-level plutonic rocks, sheeted dykes and lavas--provides the most completely constrained estimate of the bulk composition of fast-spreading oceanic crust so far. Simple crystallization models using this bulk crustal composition as the parental melt accurately predict the bulk composition of both the lavas and the plutonic rocks. However, the recovered plutonic rocks show early crystallization of orthopyroxene, which is not predicted by current models of melt extraction from the mantle and mid-ocean-ridge basalt differentiation. The simplest explanation of this observation is that compositionally diverse melts are extracted from the mantle and partly crystallize before mixing to produce the more homogeneous magmas that erupt.

Drilling to Gabbro in Intact Ocean Crust

Sampling an intact sequence of oceanic crust through lavas, dikes, and gabbros is necessary to advance the understanding of the formation and evolution of crust formed at mid-ocean ridges, but it has been an elusive goal of scientific ocean drilling for decades. Recent drilling in the eastern Pacific Ocean in Hole 1256D reached gabbro within seismic layer 2, 1157 meters into crust formed at a superfast spreading rate. The gabbros are the crystallized melt lenses that formed beneath a mid-ocean ridge. The depth at which gabbro was reached confirms predictions extrapolated from seismic experiments at modern mid-ocean ridges: Melt lenses occur at shallower depths at faster spreading rates. The gabbros intrude metamorphosed sheeted dikes and have compositions similar to the overlying lavas, precluding formation of the cumulate lower oceanic crust from melt lenses so far penetrated by Hole 1256D.