Experiments show that diamond floats in a primitive mantle melt at around 20 gigapasca and 2360°C and in a melt formed by partial melting of the transition zone at about 10 gigapascals and 2270°C. These observations constrain magma densities at high pressure. Diamond precipitated or trapped in a silicate melt at the base of the transition zone or the lower mantle floats and has been accumulating in the transition zone since early in Earth's history. Thus, the transition zone could be a reservoir of diamond.
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