Project VoiLA: Volatile Recycling in the Lesser Antilles.

Deep water cycle studies have largely focused on subduction of lithosphere formed at fast spreading ridges. However, oceanic plates are more likely to become hydrated as spreading rate decreases.


Deep water cycle studies have largely focussed on subduction of lithosphere formed at fast spreading ridges. However, oceanic plates are more likely to become hydrated as spreading rate decreases.
The Quill Volcano on St. Eustatius Island. Credit: George Cooper. At mid-ocean ridges, such as the one that runs along the full length of and 3 kilometers below the surface of the Atlantic Ocean, new oceanic plate material forms as the plates on either side are pulled apart and hot underlying mantle rises to fill the space. Material spreading from the ridge cools into an increasingly thick plate, topped by a crust of solidified mantle melts. Below the Pacific, spreading can reach speeds of over 15 centimeters per year. However, the plates pull apart slower below the Atlantic, at rates of only a few centimeters per year. This leads to cooler plates with less melting and more fracturing, which provides additional pathways for water to enter the plate. Upon reaching a subduction zone, where plate convergence forces one plate to dive below the other, the sinking plate will heat up and get squeezed, resulting in release of some or all of its water into the deeper Earth. Scientists have studied this subduction-water cycle the fast-spreading Pacific. Now, our Volatile Recycling in the Lesser Antilles (VoiLA) project targets one of only two zones that currently subduct lithosphere formed by slow spreading in the Atlantic.
surface into the mantle as fluids trapped in pores found in sediments and crust, as well as in volatile-bearing minerals in the deeper plate. As plates sink and heat up, released fluids can trigger seismicity and induce melting within the subducted plate and in the overlying mantle wedge. This type of melting selectively concentrates volatiles and transports them into the overlying plate. If an eruption occurs, the cycle then returns the volatiles into the oceans and atmosphere (Fig. 1). Fig. 1. Schematic diagram of the Antilles subduction zone showing the input of volatiles at fracture zones, oceanic core complexes (where mantle rather than crustal rocks are exposed at the seafloor) and bending faults (light green) and partial release due to breakdown of volatile-bearing phases below the forearc and arc (light blue), affecting seismicity (red stars). Volatiles (like H 2 O and CO 2 ) percolating into the mantle wedge will react and cause melting. Melts will ascend via porous flow/diapirs to interact with the crust and possibly erupt. Part of the volatile budget will remain trapped in the subducting plate to be carried to great depths.

Credit: Saskia Goes
Exactly where and how volatiles are released and how they modify the host rock remains an area of intense research. Much of a plate's hydration is acquired at midocean ridges. In the slowly spreading Atlantic lithosphere, the plate is hydrated more pervasively and heterogeneously than the fast spreading Pacific lithosphere (Fig. 2). Therefore, expressions of volatiles are likely to be more pronounced during the subduction of lithosphere that is formed by slow spreading. Today, subduction is predominantly consuming lithosphere formed at relatively fast spreading ridges. However, in the past, when the Tethys Ocean was being subducted, slow spreading was common.
We investigate how the volatile cycle for the little-studied subduction in the Atlantic differs from that in subduction zones in the Pacific (which has been extensively studied by efforts like GeoPRISMS). We do so by targeting the Lesser Antilles, located in the Caribbean Sea.

Hydration of Slow-vs. Fast-spreading Oceanic Lithosphere
Mid-ocean ridge research has revealed that the character of oceanic lithosphere changes strongly with spreading rate (Fig. 2). Classical magmatic accretion dominates at fast-spreading ridges. In contrast, at slow-spreading ridges much of the extension is taken up by faulting. The nature of this faulting varies along strike depending on the melt supply. When supply is low, large-scale detachment faults form that bring mantle rocks up to the seabed. Both processes tend to occur at seafloor-spreading segment ends and substantially increase the hydration potential of the oceanic lithosphere, as hydrous, or serpentinised, mantle can hold 2-3 times as much water as the equivalent volume of basaltic crust. Additional hydration occurs along transforms, where the cool thermal state of slow-spread lithosphere provides conditions for deep hydration, augmenting along-strike variations in its volatile budget.

The Antilles subduction end-member
Understanding volatile cycling in subduction zones requires scientists to synthesize observations from different settings with contrasting incoming plate age, spreading rate, degree of sedimentation, fracture zone density, and plate bending. Of the few zones presently subducting slow-spread lithosphere, none has been studied with a comprehensive multi-disciplinary effort.
Our VoiLA experiment, funded by the UK Natural Environment Research Council, targets the Antilles subduction zone, which consumes slow-spread lithosphere with a number of very pronounced fracture zones (Fig. 3). It also subducts old plate at an unusually slow rate. There are appreciable differences in sediment thickness and composition along the arc, owing to the large supply of terrestrial sediment in the south from the Orinoco River. Plate coupling and earthquake productivity are strongest in the north, often attributed to variable input of fluids and/or sediments. The volcanic islands vary significantly in size, evidence of variable long-term magmatic productivity, possibly linked to variable slab dehydration. A few previous studies [Schlaphorst et al., 2016;Paulatto et al., 2017] found along-arc variations in seismicity rates and seismic velocity structure in the slab and overlying mantle wedge that may correlate with fracture zone distribution. Our project (October 2015-March 2021) is comprised of two marine scientific cruises on the RRS James Cook (JC133 and JC149), temporary seismic deployments on the islands, geological fieldwork, geochemical and petrological analyses and numerical modelling (Fig. 3).
Between March 2016 and May 2017, we deployed 24 German DEPAS trillium compact and 10 Scripps trillium 240s broadband ocean bottom seismometers (BBOBS) on the Lesser Antilles arc, forearc, and back arc. Thirty-two of these instruments successfully recorded natural earthquake data, as well as the airgun shots from two along-arc lines (Fig. 3). All 8 stations deployed on the southern islands, from St. Lucia to Grenada, recorded earthquakes between January 2017 and January 2018 and 7 registered the arc-back-arc shots. Nine of ten stations deployed on Guadeloupe-Martinique from Oct 2016 until March 2017 recorded earthquake data for at least part of this time. The BBOBS data quality is excellent, with low noise levels comparable to many land stations, and the island stations exhibit noise levels only slightly higher than permanent island stations.
During JC149, we shot three active-source wide-angle refraction lines and supplementary multi-channel reflection lines on the incoming plate to survey structures across the fracture zones and to determine where the plate bends into the trench. We successfully recorded data with 133 instruments from the German DEPAS and UK Ocean-Bottom Instrumentation Facility. Here too, data quality is excellent and early travel-time models show a strong contrast in seismic velocity structure across the medium-offset fracture zones.
In addition, we collected gravity, magnetic, and bathymetry data throughout the two cruises and dredged a number of samples from one of the bathymetric ridges. We surveyed underwater volcano Kick-em Jenny using swath bathymetry on both cruises, bracketing its most recent eruption [Allen et al., 2018]. We conducted geological fieldwork on St. Martin, St. Kitts, Nevis, Guadeloupe, and La Désirade to collect mafic volcanic material to target melt inclusions and to complement the existing, extensive Bristol/Durham collections of volcanic and xenolith samples along the arc. Initial results from VoiLA surface wave imaging and Wadati-Benioff seismicity relocation indicate pronounced and variable fluid signatures along the arc.

Future work
In ongoing work, we are imaging the structure of the incoming plate to test the extent of hydration and serpentinisation near fracture zones and during plate bending. We will improve the characterisation of seismicity in and between this and the upper plate to aid insight into the region's poorly-characterised seismic hazard. A combination of active and passive-source seismic data will provide new constraints on the highly variable crustal structure along the arc and the enigmatic back-arc Grenada basin. We plan to use arc structure combined with petrologic and geochemical data to investigate the source and depth of melt generation, its transport and evolution. Bodywave, surface-wave and attenuation tomography and imaging of seismic anisotropy in the mantle wedge, together with geochemical analyses and thermal and flow models, will help constrain mantle flow and fluid pathways. We are analysing volcanic samples for trace elements including ore metals, boron isotopes and volatile contents, with a particular focus on how along-arc variations may be coupled to the structure and hydration state of the incoming plate.
Our passive seismic data set should be ready to be archived and made generally available through IRIS by June 2021. Underway geophysical data will be accessible via the British Oceanographic Data Centre. Geochemical data will be added to GeoROC. Our website will provide further updates and outcomes.