Origin of volatile-enriched mantle domains in Iceland

Volatile elements such as H, He, B, C, N, F, S, Cl and Ar build up the biosphere, hydrosphere and atmosphere, but are also minor constituents of the solid Earth: crust, mantle and core. These large entities (possibly even the core) are linked through global volatile cycles that operate on geologic timescales of hundreds of millions of years. In short, volcanism delivers mantle volatiles to the surface, subducting plates transport them back to the mantle. The mantle is not a chemically monotonous body of rock it was once thought to be. The presence of recycled and primordial components in the deep mantle is demonstrated in a striking fashion by chemical signatures of ocean island basalts that are thought to be fed by deep-reaching mantle plumes. Recycled components represent oceanic and continental crust, lithosphere and sediments that have entered the mantle via the subduction conveyor belt, while primordial material has remained in the deep mantle since the Early Earth without taking part in mantle convection.

Stable isotope ratios of volatile elements in ocean island basalts (OIBs) are as close to mantle DNA as one gets. They can be used to learn more about the origin different mantle components and the evolution of the mantle. Stable isotopes can also preserve information about the accretion history of the Earth.

In this project we aim to solve a specific problem of the Iceland mantle puzzle posed by the Kverkfjöll volcanic system. The Kverkfjöll basalts sample a mantle enriched in Cl and H₂O with a Pb isotopic signature that is different from the mantle sampled by most Icelandic volcanoes. Where does the volatile-rich mantle source of Kverkfjöll come from, how old is it, and why is it located beneath central-east Iceland?

To find answers, we analyzed concentrations and multiple volatile stable isotope systems (H, B, Cl and S) in the Kverkfjöll magma suite.

How to study volatiles in the mantle

The starting point of mantle volatile studies is to find good material to analyze. As the mantle can only rarely be directly sampled, our best bet is to find primitive basalts (partial melts of the mantle). The hard part is to find samples that haven't lost volatiles by degassing, and have not interacted with the crust, sea or atmosphere. Interactions like this could easily overprint the mantle signatures that we are after.

Subglacial glasses

In Iceland, we are spoiled with abundant and accessible outcrops of pillow basalts, which were erupted below Pleistocene ice sheets. The pressure below > 1 km thick ice is high enough to prevent heavy degassing of most volatile elements. The rims (typically about 1 cm thick) of the pillows are made out of rapidly quenched glass. These subglacial glasses are the raw material that I use for stable isotope analysis of H, B, Cl and S.

Melt inclusions

Another type of material that we use are glassy melt inclusions (MIs) in igneous crystals such as olivine. These capsules of melt were trapped by crystals growing in magma reservoirs deep in the crust (in Iceland, typically at 5-15 km depth) and are our best chance of directly sampling undegassed primitive melts. Commonly, MIs from a single eruption showcase a greater chemical and isotopic range than matrix glasses. This is interpreted as evidence that deep magma reservoirs receive melt injections from multiple sources, and subsequently mix and become homogenised to an average composition represented by the bulk lavas that erupt on the surface. There is a downside to working with MIs compared to subglacial glasses: sample preparation takes a long time, and the available analytical methods are limited by the small sample size (on the order of 30-200 μm). There are also added complexities to consider, such as post-entrapment crystallization and diffusion of volatiles in or out of the MI.