EES Virtual Seminar: Halogen cycle in the mantle seen through melt inclusions

Dr. Estelle Rose-Koga
Laboratoire Magma et Volcans, Université de Clermont-Auvergne, France

Abstract: Halogen elements are the critical geochemical tracers marking (i) the fractionation at subduction zone and (ii) the recycling into the deep mantle. While these elements form volatile compounds in magmatic systems, they are well-preserved in melt inclusions hosted in minerals.

Melt inclusions are small droplets of magma trapped in minerals during their growth. When hosted by primitive olivine, their compositions therefore reflect that of the magma prior to extensive degassing or magma-crust interaction. Furthermore, melt inclusions give us access to the volatile composition (H2O, CO2, F, Cl and S) of the magma sources in general, and particularly, the most faithful messages are given by the halogen (F and Cl).

In this presentation, I will outline several key observations with melt inclusion data demonstrating the fractionation of F and Cl in subduction zones. Through these, it is possible to show where halogen data gave new information on how the elements were transferred from the slab to the magma source; an example of a single arc volcano in a young and hot subduction setting. At Mt. Shasta, the fractionation of halogen correlates with the different magma types that are considered as originated from distinct magma genesis processes. An example of a cold subduction setting, at Mt. Iwate showed that F was withheld in the slab, and resulted in comparable F abundances to MORB, while Cl was elevated as expected for H2O-rich arc basalts. Halogen data can be applied to a complex arc system such as Aeolian arc. Combining F, REE and Pb isotopes data from melt inclusions, one can identify the distinct sources in the Italian subduction zone, and identifies a potential lower mantle component. Whatever fractionation took place at the subduction zone, the remaining slab descends into mantle and mixes into the convective mantle. In fact, recycled slab material can be detected in OIB, for example in the ocean island of Azores. Here, it is possible to identify the recycled material by combining halogen over trace element ratio versus Pb isotopes.

The community has learned a lot about the halogen cycle in the mantle over the years. However, there is still remaining questions. In the talk, I will present “an enigma of the recycling of F depleted signature”. An answer to this mystery might come from ongoing direction of research, such as the investigation of metamorphic rocks to better constrain the halogen carrier phases in the mantle, and their expected composition (e.g. Debret et al., 2013, 2016). We also know that amphibole is an important reservoir of fluorine (and Cl) and that recycle of F will depend on amphibole stability in the slab (e.g. Rose-Koga et al., 2014). Another potentially powerful direction of research is the use of stable isotopes of chlorine as tracer of Cl carrier-phase.



Online via Zoom
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Peteris Rozenbaks