HISLA-DR

Hydrogen Incorporation in Subducting Lithosphere after Dehydration Reactions

 Partecipanti

Mappa

 Word cloud

Esplora la "nuvola delle parole (Word Cloud) per avere un'idea di massima del progetto.

 Obiettivo del progetto (Objective)

'Subduction zones play a fundamental role in the deep water cycle making the Earth unique among other terrestrial planets. During subduction dehydration of hydrous minerals produces a fluid phase. A part of this fluid phase will be recycled water back to the Earth’s surface through hydrothermal aqueous fluids or through hydrous arc magmas, whereas another part of the water will be transported to the deep mantle by Nominally Anhydrous Minerals (NAMs) such as olivine, pyroxene and garnet. The partitioning of water between these two processes is crucial for the understanding the deep water cycle which has profound implications for the rheology of the upper and lower mantle. However, the transition from hydrous minerals to NAMs has been very little explored and only rough estimations are available.

The main thrusts of the project are (1) to undertake a series of novel experiments with hydrous mantle compositions using high-pressure experimental facilities to constrain the actual water contents that are recycled in NAMs in shallow subduction zones (1.5-5.5 GPa and <900ºC) and (2) compare the products with natural assemblages that underwent extensive dehydration during subduction (through antigorite and chlorite high-pressure breakdown reactions). Additionally this project will investigate the link between mechanisms of deformation and mechanism of hydrogen incorporation in NAMs and how they may affect the rheology of the incoming slab and uppermost mantle.

This project will be arrange by assuming two years at Australian National University (outgoing host institution), during which experiments will be performed and natural rocks will be analyzed for trace elements and water contents, and one year at Géosciences Montpellier (Return host institution), where microstructural investigations of natural samples will be carried out.'

Introduzione (Teaser)

Although the Earth is a rocky planet, many aqueous dynamic processes take place beneath its surface making it unique among terrestrial planets. New insight into the role of fluids on properties of materials is directly relevant to earthquakes and volcanic eruptions.

Descrizione progetto (Article)

Subduction zones are geological regions in which two tectonic plates (pieces of the Earth's lithosphere) violently collide, causing one of them to sink into the Earth's mantle under the other. In general, movements of the oceanic lithosphere (crust and upper mantle from the oceans) facilitate incorporation of water into hydrous minerals. During subduction, some of these hydrous minerals are partially dehydrated, creating a fluid phase.

Some of the water goes back up to the Earth's surface and part is transported to the deep mantle by nominally anhydrous minerals (NAMs). The EU-funded project 'Hydrogen incorporation in subducting lithosphere after dehydration reactions' (HISLA-DR) set out to explore the poorly understood mechanisms of water transfer to NAMs.

Scientists first collected new data on hydrogen diffusion into NAMs, particularly olivine. Lab experiments exploiting well-constrained hydrous defect chemistry were followed by collection of rock samples from high-pressure metaperidotite (e.g. from the Eastern Alps). Analysis of hydrogen content in NAMs from the field samples constitute the most complete data set on water content in natural peridotites from subduction zones.

Experimental evidence showed that diffusion of hydrogen in the olivine lattice could be either the fastest of any other species or orders of magnitude slower. Scientists have linked the rate to defects in which hydrogen is bound. The hydrogen associated with the most common defects in olivine in the upper mantle diffuses up to 1 000 times slower than previously assumed.

Samples obtained from the Alps and data from laboratory experiments demonstrate preservation of water for long time periods and correlation of hydrogen content to temperature and pressure. Both of these support the observed slow hydrogen diffusion coefficients. Combined field and lab data enable for the first time a quantitative evaluation of maximum water capacity in NAMs in subduction zones, suggesting that the mantle wedge acts as a water reservoir.

Subduction zones are the most geologically active regions on Earth, playing a critical role in seismic events and volcanic eruptions. HISLA-DR has provided profound insight into dynamic aqueous events in these regions, filling an important gap in knowledge and providing insight relevant to hazard prevention.