A linear model and topology for the host-inclusion mineral system involving diamond

A linear model of internal pressure for the host-inclusion system has been developed for diamond and a test selection of thirty minerals (excluding sulfides). Central to this model, for each diamond-inclusion pair, is the isovolume locus in P-T space, along which the relative volumes of the two phases change identically. The key ratio 0, equal to slope of the isovolume locus divided by the slope of the graphite-diamond transition, permits unconditional assignment of a mineral to one of four universal topological groups of inclusions, namely 1) a heritage-T group, 2) a heritage-P group, 3) a group with mixed response (complete decompression inside high temperature diamond), and 4) a group that decompresses completely. About half of the test minerals may be used to determine the P-T conditions of formation through measurements of internal pressures on inclusions in natural diamond. All minerals that are typically used to age-date diamond belong to the mixed group: high-temperature inclusions in diamond will tend to reset to the emplacement age. Diamond from Argyle, Australia, only fits this model if diamond forms and adjusts through prolonged secular cooling (500 degrees C over 500 M.y.). Most inclusions in superdeep diamond should fracture the host diamond, with published examples indicating some resealing near the conditions of formation of cratonic diamond (? temporary storage). The model topology for the converse setting (diamond as the inclusion) is critically different. The converse setting is mostly hostile to survival of diamond during delivery to the Earth's surface. Only seven of the tested host minerals (including spinel) are predicted to protect included diamond against conversion to graphite, but most minerals actually reported to carry microcrystals of diamond are excluded. However, the model predicts that the copresence of supercritical fluids (H(2)0-CO2) in the inclusion chamber would protect diamond included in almost any host mineral (glass also protects, but at a critically lower level). Minerals in graphite-bearing and coesite-bearing eclogites should be examined as potential hosts for microcrystals of diamond protected by this mechanism.