Water and its influence on the lithosphere–asthenosphere boundary

Abstract

The Earth has distinctive convective behaviour, described by the plate tectonics model, in which lateral motion of the oceanic lithosphere of basaltic crust and peridotitic uppermost mantle is decoupled from the underlying mechanically weaker upper mantle (asthenosphere). The reason for differentiation at the lithosphere-asthenosphere boundary is currently being debated with relevant observations from geophysics (including seismology) and geochemistry (including experimental petrology). Water is thought to have an important effect on mantle rheology, either by weakening the crystal structure of olivine and pyroxenes by dilute solid solution, or by causing low-temperature partial melting. Here we present a novel experimental approach to clarify the role of water in the uppermost mantle at pressures up to 6 GPa, equivalent to a depth of 190 km. We found that for lherzolite in which a water-rich vapour is present, the temperature at which a silicate melt first appears (the vapour-saturated solidus) increases from a minimum of 970 °C at 1.5 GPa to 1,350 °C at 6 GPa. We have measured the water content in lherzolite to be approximately 180 parts per million, retained in nominally anhydrous minerals at 2.5 and 4 GPa at temperatures above and below the vapour-saturated solidus. The hydrous mineral pargasite is the main water-storage site in the uppermost mantle, and the instability of pargasite at pressures greater than 3 GPa (equivalent to more than about 90 km depth) causes a sharp drop in both the water-storage capacity and the solidus temperature of fertile upper-mantle lherzolite. The presence of interstitial melt in mantle with more than 180 parts per million of water at pressures greater than 3 GPa alters mantle rheology and defines the lithosphere-asthenosphere boundary. Modern asthenospheric mantle acting as the source for mid-oceanic ridge basalts has a water content of 50-200 parts per million (refs 3-5). We show that this matches the water content of residual nominally anhydrous minerals after incipient melting of lherzolite at the vapour-saturated solidus at high pressure.

The responses of the four main substitution mechanisms of H in olivine to H2O activity at 1050 °C and 3 GPa

“…This is an important consideration when laboratory observations are to be extrapolated to olivine in natural environments such as the Earth's mantle. In the laboratory, experiments are typically conducted at high a H 2 O to maximise H 2 O contents, which facilitates measurements, but in the Earth's mantle, a H 2 O is lowered by the presence of other components, and an upper limit to a H 2 O at a given T, P and composition is imposed by partial melting (Green et al 2010). Whenever amphibole is present, a H 2 O is constrained by amphibole-pyroxene-olivine equilibria and is considerably lower than if a free aqueous fluid phase were present (Lamb and Popp 2009).…”

Application of thermodynamic modelling to natural mantle xenoliths: examples of density variations and pressure–temperature evolution of the lithospheric mantle

“…Moreover, it is important to emphasize that our calculations assume no volatile-bearing phases and only subsolidus conditions (and hence do not consider the possible presence of melt at the LAB or the presence of hydrous phases; cf. Green et al 2010;Sakamaki et al 2013;Stern et al 2015). Figure 3 depicts these geothermal gradients together with two examples of predicted density distribution in P-T space.…”

Abiogenic Deep Origin of Hydrocarbons and Oil and Gas Deposits Formation

“…If hydrocarbon fluids could be generated in the mantle they could be generated in the asthenosphere zone only. In the paper [Green et al 2010] published in Nature the modern considerations about thermobaric conditions on the depth down to 200 km are shown (Fig. 2).…”

“…Experimental data published in Nature recently [Green et al 2010] shows that water-storage capacity in the uppermost mantle "is dominated by pargasite and has a maximum of about 0.6 wt% H2O (30% pargasite) at about 1.5 GPa, decreasing to about 0.2 wt% H2O (10% pargasite) at 2.5 GPa". Another possible source of hydrogen is hydroxyl group in some minerals (biotite, muscovite).…”

“…Another possible source of hydrogen is hydroxyl group in some minerals (biotite, muscovite). [Green et al 2010].…”