The high-pressure synthesis of lawsonite in the MORB+H₂O system

Abstract

Lawsonite is an important water reservoir in subducting oceanic crust below the amphibole dehydration depth ϳ70 km. To determine the maximum pressure stability of lawsonite in the MORBϩH 2 O system, experiments were carried out using a 1000 ton uniaxial multi-anvil apparatus (SPI-1000). Mixtures of synthetic gelϩ2wt% H 2 O were used for the starting materials with the average MORB composition. Experimental P-T conditions were T ϭ 700-900 ЊC and P ϭ 5.5-13.5 GPa. Run durations were 12 and 24 h.Lawsonite was synthesized stably up to 10 GPa and T Ͻ 700 ЊC in the stishovite stability field, and Ͻ900 ЊC at 8 GPa and 750 ЊC at 5.5 GPa in the coesite stability field, with a steep positive slope for the lawsonite-out reaction. The lawsonite-out reaction in the coesite stability field changes to have a gentle negative slope in the stishovite stability field. The reaction leading to the disappearance of lawsonite is a continuous reaction due to the compositional enlargement of garnet toward the grossular end-member with increasing T and P. Lawsonite disappears when the tie line connecting grossular-rich garnet with omphacitic clinopyroxene reaches the bulk composition on the conventional AC(FM) ternary diagram.

Morphological stability of hydrous liquid droplets at grain boundaries of eclogite minerals in the deep upper mantle

“…1). A considerable number of these studies were conducted on basalt + H 2 O systems because the basaltic crust is one of the main carriers of water in the upper parts of subducting slabs (e.g., Schmidt and Poli, 1998;Okamoto and Maruyama, 1999). In this type of system, amphibole is the most abundant hydrous mineral in the uppermost mantle, although its stability field is limited to temperatures (T ) below 700°C and pressures (P) below~2 GPa (Schmidt and Poli, 1998).…”

“…In this type of system, amphibole is the most abundant hydrous mineral in the uppermost mantle, although its stability field is limited to temperatures (T ) below 700°C and pressures (P) below~2 GPa (Schmidt and Poli, 1998). At depths beneath the amphibole stability field, lawsonite, which is stable up to 900°C at 8 GPa and 700°C at 10 GPa (Okamoto and Maruyama, 1999), represents the main reservoir of water. When a slab is cold, lawsonite may carry small amounts of water (< 0.5 wt%) to the middle depths of the upper mantle, i.e., to ~300 km (P~10 GPa) (e.g., Okamoto and Maruyama, 1999).…”

“…At depths beneath the amphibole stability field, lawsonite, which is stable up to 900°C at 8 GPa and 700°C at 10 GPa (Okamoto and Maruyama, 1999), represents the main reservoir of water. When a slab is cold, lawsonite may carry small amounts of water (< 0.5 wt%) to the middle depths of the upper mantle, i.e., to ~300 km (P~10 GPa) (e.g., Okamoto and Maruyama, 1999). The hydrous minerals in the basaltic part of the slab release all water at pressures of~2-4 GPa if the slab is hot (e.g., Syracuse et al, 2010).…”

“…The liquid always forms an interconnected network along the grain boundaries and segregates easily from the host rock. Blue areas represent the stability field of hydrous minerals from Okamoto and Maruyama (1999). The dark-blue and pale-blue areas are the fields of H 2 O > 1 wt% and H 2 O ≤ 1 wt%, respectively.…”

Is quartz a potential indicator of ultrahigh-pressure metamorphism? Laser Raman spectroscopy of quartz inclusions in ultrahigh-pressure garnets

“…10. BSE images of garnet porphyroblasts and P-T paths for coesite-(a-d) and diamond-grade (e-f) metamorphic rocks from the Kokchetav massif plotted on metamorphic facies diagram (after Liou et al, 1998;Okamoto & Maruyama, 1999). The P-T stages indicated along the path of the coesite-and diamond-grade metamorphic rocks are based on petrologic data of Parkinson (2000), Theunissen et al (2007) and Hermann et al (2001), Korsakov et al (2007), respectively.…”