Where melting begins in the water-fluxed peridotite of the mantle wedge above a subducting slab is governed by the water-saturated ("wet") solidus and by the second critical endpoint (SCEP), the pressure beyond which hydrous silicate melt and aqueous fluid become completely miscible as a single supercritical fluid and a discrete melting reaction ceases to exist. After five decades of work, published wet solidi still disagree by 200–600°C at a given pressure, and the SCEP of the peridotite–H2O system rests on a single in situ X-ray determination (Mibe et al., 2007). Wang et al. (2020) bracketed the wet solidus between 950 and 1000°C at 3 GPa and the SCEP between 3 and 4 GPa with multi-anvil quench experiments at 3–6 GPa, and proposed that a deep hydrous partial melting zone (DHPMZ) at 90–130 km depth beneath NE Japan feeds hydrous melts to the mantle wedge, controls the volcanic front, and could generate high-Mg magmas (boninite, picrite, komatiite). The problem asks for determination of the SCEP within the peridotite–H2O system itself, location of the true water-saturated solidus between 3 GPa and the SCEP, and tests of the DHPMZ model.

Background

In a subduction zone, water released from the downgoing slab enters the mantle wedge — the hot peridotite of the overriding plate that lies above the slab — and lowers the temperature at which that peridotite begins to melt. The curve describing the onset of melting when water is present in excess, so that an aqueous fluid coexists with the first silicate melt, is the water-saturated or "wet" solidus. Its position has been an outstanding issue for more than 50 years: published wet solidi disagree by 200–600°C at a given pressure, with quench experiments — high-pressure runs whose products are rapidly cooled (quenched) so that the equilibrium state is preserved for later examination — on hydrous peridotite by Grove et al. (2006), who found the water-saturated solidus of primitive peridotite falling from about 940°C at 1.2 GPa to about 800°C at 3.2 GPa, and by Till et al. (2012), who found H2O-saturated melting nearly isothermal at about 800–820°C between 3 and 6 GPa (the true wet solidus below the SCEP and a pseudo-solidus above it), far below older determinations; a related controversy concerns which hydrous minerals (for example chlorite) are stable on the solidus.

At sufficiently high pressure the wet solidus terminates. Hydrous silicate melt and aqueous fluid become completely miscible, forming a single supercritical fluid — a phase intermediate in composition between melt and water in which the distinction between melting and dissolution disappears. The termination point, where the hydrous solidus meets the melt–fluid critical curve, is the second critical endpoint (SCEP). Below the SCEP pressure, water-saturated melting yields coexisting melt and fluid; above it, only one fluid phase exists and heating produces a gradual transition rather than a discrete melting reaction, so the boundary measured above the SCEP is a pseudo-solidus rather than a true solidus. The SCEP pressure therefore sets the depth below which water cannot enter the mantle wedge as a separate aqueous phase.

Direct SCEP determinations in rock–H2O systems are scarce. Using in situ X-ray radiography at the SPring-8 synchrotron — watching the meniscus between fluid and melt disappear in a heated multi-anvil cell — Mibe et al. (2007) bracketed the peridotite–H2O SCEP between about 3.6 and 4.0 GPa. Related determinations place the basalt–H2O SCEP at about 3.4 GPa and 770°C (Mibe et al., 2011), the sediment–H2O and high-Mg-andesite–H2O endpoints at about 83 and 92 km depth (Kawamoto et al., 2012), and supercritical behavior across 4–6 GPa in water-saturated eclogite (Kessel et al., 2005).

Wang et al. (2020) re-examined the peridotite–H2O system with large-volume multi-anvil quench experiments (a multi-anvil press is a hydraulic apparatus that reaches mantle pressures on a cubic assembly of anvils) on a fertile peridotite — fertile meaning unmelted and close in composition to primitive mantle — at 3–6 GPa and 950–1200°C, using textures and compositions of quenched materials to identify melt and fluid. They bracketed the wet solidus between 950 and 1000°C at 3 GPa and the SCEP between 3 and 4 GPa. Above the SCEP they located the pseudo-solidus at 4 and 6 GPa. From these phase relations they proposed that the lithosphere–asthenosphere boundary — the transition from the rigid tectonic plate to the ductile, slowly flowing mantle beneath it — in subduction zones is defined by the wet solidus, and that a deep hydrous partial melting zone (DHPMZ) — a layer of partial melting along the wet solidus/pseudo-solidus at roughly 90–130 km depth, spatially coincident with a known low-seismic-velocity anomaly beneath NE Japan — supplies hydrous melts to the shallower primary melting zone of the wedge and may control the position of the volcanic front (the trench-parallel line of arc volcanoes) and the magma production rate. Their experimental high-pressure hydrous melts also match natural high-Mg magmas — boninite, picrite, and komatiite (ultramafic volcanic rocks highly magnesian in composition) — in major elements, suggesting a deep, water-rich origin for these magmas.

Problem Statement

Building on Wang et al. (2020), whose own contribution was to bracket rather than pin down these phase relations, the problem is to complete the water-saturated melting relations of the peridotite-H2O system at upper-mantle pressures and to establish whether the resulting deep hydrous melting regime governs magma genesis in the mantle wedge. Its objective is to determine, within the peridotite-H2O system itself, the phase relations between 3 and 6 GPa — the second critical endpoint (SCEP) that terminates the wet solidus, and the wet solidus itself below the SCEP — and to test the deep hydrous partial melting zone (DHPMZ) model that follows from them:

(1) A determination of the SCEP pressure within the peridotite-H2O system, pinning the endpoint down beyond the current 3-4 GPa bracket of Wang et al. (2020) and the debated range of reported endpoint values (<4 to >6 GPa).

(2) A location of the true water-saturated solidus between 3 GPa and the SCEP pressure — the segment where hydrous melt and aqueous fluid still coexist as separate phases — for which the published determinations disagree and which Wang et al. (2020) bracketed at only one pressure (950-1000°C at 3 GPa), while above the SCEP only the pseudo-solidus is located, at 4 and 6 GPa.

(3) A test of the DHPMZ model — whether deep hydrous partial melting along the wet solidus/pseudo-solidus at roughly 90-130 km depth beneath NE Japan accounts for the observed low-velocity anomaly there rather than other causes, supplies hydrous melts to the primary melting zone of the mantle wedge, controls the position of the volcanic front and the magma production rate, and can generate natural high-Mg magmas (boninite, picrite, komatiite) from deep, water-rich sources.

Items (1) and (2) are one phase-relation objective, since the wet solidus terminates at the SCEP; item (3) tests the DHPMZ model and each of its four enumerated consequences.

References

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