An experimental study of serpentine decomposition at high pressure (4.5 GPa) was carried out to elucidate if water can be preserved in the system in the form other than structural admixtures in minerals. This problem is of interest because it is water that plays a leading role in the melting in a subducted slab and a mantle wedge. To estimate the possible content of an aqueous fluid in deep-seated rocks, a BARS pressless split-sphere apparatus was used in complex with thermobarogeochemistry and gas chromatography. It has been established that the serpentine decomposition is accompanied by the release of water, which concentrates in inclusions in the produced minerals (olivine and orthopyroxene) and their interstices. Chromatographic analysis with a stepwise heating of samples showed that most of the released water is localized in the interstices, and the rest is conserved in fluid inclusions in the minerals. The produced solid phases conserve 0.13 to 2.43 wt.% fluids as inclusions, with water amounting to 0.1–2.06 wt.%. The content of inclusions determined by microscopic examination falls in this region. Since the mobility of the fluid conserved as inclusions in the olivine and orthopyroxene is significantly lower than that in the interstices, this fluid might be better preserved in olivine-containing rocks subsided to depth.
Abstract. The origin and evolution of metal melts in the Earth's mantle and
their role in the formation of diamond are the subject of active discussion.
It is widely accepted that portions of metal melts in the form of
pockets can be a suitable medium for diamond growth. This raises
questions about the role of silicate minerals that form the walls of these
pockets and are present in the volume of the metal melt during the
growth of diamonds. The aim of the present work was to study the
crystallization of diamond in a complex heterogeneous system: metal-melt–basalt–carbon. The experiments were performed using a multianvil
high-pressure apparatus of split-sphere type (BARS) at a pressure of 5.5 GPa and a temperature of 1500 ∘C. The results demonstrated
crystallization of diamond in metal melt together with garnet and
clinopyroxene, whose chemical compositions are similar to those of eclogitic
inclusions in natural diamond. We show that the presence of silicates in the
crystallization medium does not reduce the chemical ability of metal melts
to catalyze the conversion of graphite into diamond, and, morphologically,
diamond crystallizes mainly in the form of a cuboctahedron. When the content
of the silicate material in the system exceeds 5 wt %, diamond forms
parallel-growth aggregates, but 15 wt % of silicate phases block the
crystallization chamber, preventing the penetration of metallic melt into
them, thus interrupting the growth of diamond. We infer that the studied
mechanism of diamond crystallization can occur at lower-mantle conditions
but could also have taken place in the ancient continental mantle of the
Earth, under reducing conditions that allowed the stability of Fe–Ni melts.
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