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Base-metal sulfides (BMSs) are minerals that host platinum-group elements (PGE) in mantle peridotites and significantly control the bulk PGE content. They have been investigated in detail down to the sub-micrometer scale to elucidate PGE behavior in the Earth's interior. Base-metal sulfides are supposedly subjected to supergene and seawater weathering, leading to the redistribution of PGEs at low temperatures. Careful and thorough measurements of BMSs are thus required to elucidate PGE behavior in the Earth's interior. In the present study, a sub-micrometer-sized PGE-bearing sulfide inclusion in a clinopyroxene crystal in a harzburgite xenolith from Tahiti (Society Islands, French Polynesia) was investigated in detail (down to the sub-micrometer scale) using transmission electron microscopy with energy-dispersive X-ray spectroscopy (TEM-EDS). The sulfide inclusion is of carbonatitic metasomatic origin, as it is enveloped by carbonaceous glass, and forms a planar inclusion array with other PGE-bearing sulfide inclusions. The following sulfide phases were identified using TEM-EDS: Fe- and Ni-rich monosulfide solid solutions (MSSs), Fe- and Ni-rich pentlandite, sugakiite, heazlewoodite, chalcopyrite, and Cu-Ir-Pt-Rh-thiospinel (cuproiridsite–malanite–cuprorhodsite). We established the formation process of the metasomatic PGE-bearing sulfide inclusion by considering morphological and mineral characteristics in addition to the chemical composition. A primary MSS first crystallized from metasomatic sulfide melt at ca. 1000 °C, followed by the crystallization of an intermediate solid solution (ISS) below 900 °C. A high-form (high-temperature origin) Fe-rich pentlandite simultaneously crystallized with the primary MSS below ca. 850 °C and recrystallized into a low-form (low-temperature origin) Fe-rich pentlandite below ca. 600 °C. The primary MSS decomposed to Fe- and Ni-rich MSSs, low-form Ni-rich pentlandite, sugakiite, and heazlewoodite. The ISS decomposed to chalcopyrite below ca. 600 °C. Meanwhile, a Cu-Ir-Pt-Rh-thiospinel crystallized directly from the evolved Cu-rich sulfide melt below ca. 760 °C. Thus, Ir, Pt, and Rh preferentially partitioned into the melt phase during the crystallization process of the metasomatic sulfide melt. Metasomatic sulfide melts could be a significant medium for the transport and condensation of Pt together with Ir and Rh during the fractionation process in the Earth's interior. We hypothesize that the compositional variability of PGEs in carbonatites is due to the separation of sulfide melt leading to the loss of PGEs in the carbonatitic melts.

Platinum-group elements (PGEs) are one of the key tracers to reveal early differentiation processes of the Earth due to their preferential distribution into the metallic core. Meanwhile, informative evidence for early differentiation has been greatly disturbed through metasomatic PGE disturbance, which has been demonstrated through a number of PGE data for natural mantle peridotites as well as base-metal sulfide and platinum group mineral grains therein. The mechanism and process of metasomatic PGE mobilization should be investigated in detail for an appropriate estimation of PGE abundance in the primitive upper mantle. However, this has not yet been achieved, because sub-micrometer-scale ( i.e. scale of less than a micrometer) descriptions for metasomatic effects imprinted in the mantle peridotites have not been sufficiently recorded. Here, we report a sub-micrometer-sized sulfide–glass inclusion array in a Tahitian harzburgite xenolith. The textural and chemical characteristics were disclosed with employing synchrotron X-ray and transmission electron microscope analyses. The results demonstrate that the sulfide and glass contain appreciable amounts of PGE (9.7 at.% Ir, 4.3 at.% Rh and 5.8 at.% Pt) and carbon (21.2 at.% C), respectively. The sulfide–glass inclusion array is hosted in sodium-enriched clinopyroxene (up to 1.8 wt% Na 2 O) that shows vein-like distribution and partly replaces orthopyroxene. Primitive mantle-normalized trace-element patterns of the clinopyroxene show a general increase from heavy rare-earth elements (REEs) to light REEs with negative anomalies in Pb and high-field-strength elements such as Zr, Hf and Ti, which indicate equilibration with Mg-rich carbonatitic melt. These results suggest that Na-bearing Mg-rich carbonatitic melts were involved in the harzburgite formation and that Ir, Rh and Pt were mobilized through carbonatitic metasomatism and eventually distributed in the sulfides.

The stabilization of coesite is a diagnostic indicator of ultrahigh-pressure metamorphism and in many cases it implies that a rock has been subducted to a minimum depth of 80 km. Coesite typically occurs as rare relicts in rigid host minerals, but most commonly transforms into α-quartz pseudomorphs during exhumation. The abundance of coesite-bearing rocks in orogens worldwide is a contentious issue in the petrological community, despite evidence from numerical modeling that suggests that coesite formation should be a common geological process during ultrahigh-pressure metamorphism. This knowledge gap must be addressed to improve the understanding of the geological aspects of subduction-zone geodynamics. Here we report that minuscule coesites (