Noble Gas, Halogen and Water Recycling into the Terrestrial Mantle

Lead Research Organisation: University of Manchester
Department Name: Earth Atmospheric and Env Sciences


Incompatible and volatile elements like noble gases and the halogens are continually brought from the mantle to the Earth's surface at mid ocean ridges and other regions of volcanic activity. It has always been assumed that this is a one-way process for these species. This assumption is made when using these tracers to investigate planetary volatile origin, accretionary processes, mantle dynamics and atmosphere evolution in the case of the noble gases, or tracking the halogen budget of the oceans and assessing the role of ocean salinity in the evolution and sustenance of life in the oceans. Recent Manchester work suggests that surface noble gases are being subducted into the deep mantle and that this source dominates the mantle heavy noble gas budget (Ballentine et al., Nature 2005; Holland and Ballentine, Nature 2006). What is surprising is that the mantle shows an Ar/Kr/Xe signature identical to that of seawater. This is an elemental/isotopic composition unique in the solar system and we can rule this out as an original accretionary mantle component. Because of the volatile nature of noble gases we might expect this seawater ratio to be perturbed during the process of subduction. What then is the mechanism that preserves the seawater signature? Where in the oceanic crust are these gases found? How and why are they preserved in the subducting slab? We present new pilot data in our proposal showing that we can identify the same seawater signature in fluids released from dewatering slabs that have been taken to at least 100km depth in the mantle. With a technique pioneered at Manchester, we also show that the halogens in this fluid, which are equally susceptible to fractionation, are identical to values found in marine pore fluids. It would appear that noble gas and halogen subduction are linked. We propose the first complete and systematic analytical survey of the oceanic crust and associated sediment to identify the location and elemental character of the phases or hosts that dominate the noble gas and halogen budget of subducting material. For little extra effort we will also obtain the abundance and hydrogen isotopic composition of the associated water. We also propose to extend the pilot study results to two more terrains, with contrasting thermal regimes, where we expect to be able to sample and identify the noble gas and halogen composition of deep (~100km) subducted fluids. This combined data set will be the first to link noble gases, halogens and related water in a systematic way from subducting to subducted fluid composition in differernt thermal settings. This will enable us to identify the major carrier/ers of noble gas into the mantle and use our understanding of noble gas concentrations and convection behaviour in the mantle to start to model and identify the associated subducted halogen and water impact on the respective total mantle budget and the evolution of these tracers in the mantle system - systems that underpin our understanding of the Earth.
Description From the outset our main goal has been to characterise the noble gas and halogen content of the oceanic crust from an observationally very low starting point. Such an assay is a critical start in characterizing their concentrations and distribution in the oceanic crust and essential for investigating the potential of the ocean crust to subduct these elements into the deep mantle; a very topical subject of wide interest. We have achieved this and have also explored the control on and relationships between noble gases and halogens by the mineralogy, petrology and altered ocean crust textures. We acknowledge that there is much more that can be done as the petrology of such samples is complex at a microscopic scale and samples limited. Our work nevertheless significantly advances our understanding of the controls on halogen and noble gas distribution in the oceanic crust and provides an updated, and significantly changed, estimate of their flux into subducting systems. Importantly we identify a distinct composition control on the halogen and noble gas concentration and relative elemental abundance between the upper oceanic crust and the underlying sheeted dykes below the transition zone as ca8-900m sub basement. While beyond the scope of this work, this is important information for models considering the detail of dehydration and loss of volatiles during subduction (e.g. work by Van Keken and colleagues).

This work is in-review for publication by Geochimica Cosmohimica Acta.

A closely related paper resulting from this funding is in-press but not available on the system yet to enter in the publications:

Sedimentary halogens and noble gases within Western Antarctic xenoliths: Implications of extensive volatile recycling to the sub continental lithospheric mantle

GEOCHIMICA ET COSMOCHIMICA ACTA Volume: 176 Pages: 139-156 Published: MAR 1 2016

In this paper we present a combined noble gas and halogen study on mantle xenoliths from the Western Antarctic Rift System (WARS) to better understand the flux of subducted volatiles to the sub continental lithospheric mantle (SCLM) and assess the impact this has on mantle chemistry. Noble gases and halogens within these xenoliths have recorded past episodes of volatile interaction within the SCLM and can be used to reconstruct a tectonic history of the WARS. Marine halogen and noble gas signatures within the SCLM xenoliths provide evidence for the introduction and retention of recycled volatiles within the SCLM by subduction related metasomatism, signifying that not all volatiles that survive subduction are mixed efficiently through the convecting mantle. The global SCLM therefore represents a potentially important reservoir for the long term residence of subducted volatiles.
Exploitation Route These results are an essential starting point for understanding how volatile elements are recycled into the mantle and how they impact mantle convection and ultimately influence planetary habitability.
Sectors Environment