Prinya Promprated

The significance of mineral inclusions in large diamonds from Yakutia, Russia

Abstract Large diamonds (10-200 carats) from Udachnaya, Mir, and Aikhal kimberlite pipes of Yakutia contain variable mineral inclusions typical of diamonds of smaller size. Among these inclusions, extracted from outer portions of these diamonds, the majority are garnets of harzburgitic paragenesis, with a small number of inclusions belonging to lherzolitic and eclogitic groups, in addition to chromite diamond inclusions (DIs). The most striking feature of the garnet DIs is the sinusoidal rareearth elements (REE) patterns observed for all harzburgitic, and some lherzolitic garnets, whereas eclogitic garnets form normal LREE-depleted patterns. These sinusoidal REE patterns are indicative of a complex petrogenesis for the garnets, involving partial-melting, followed by metasomatic enrichment, particularly of the LREEs/MREEs. The metasomatic agents were probably not those of carbonatitic fluids, but rather C-O-H-N-S fluids, as indicated by the negative anomalies of Ba and Sr in the garnets. T he general axiom among investigators that most, if not all, DIs are syngenetic with their diamonds is seriously questioned. The petrogenetic history of the harzburgitic garnet DIs involved complex processing prior to their diamond encapsulation. That is, the garnet DIs retained a signature of their earlier, pre-diamond encapsulation history. This result is taken as proof that basically all harzburgitic garnet DIs are non-syngenetic (i.e., protogenetic) with their host diamonds.

Petrochemistry of Mafic Granulite Xenoliths from the Chantaburi Basaltic Field: Implications for the Nature of the Lower Crust beneath Thailand

Mafic granulite-facies xenoliths in alkali basalts from Chantaburi province represent the only known lower-crustal material in Thailand. Most xenoliths contain garnet (or its secondary product—kelyphite), plagioclase, and clinopyroxene, ± traces of corundum, and are grouped into garnet-rich and clinopyroxene-rich granulites (Groups 2 and 3, respectively). One sample is classified as olivine-garnet clinopyroxenite (Group 1), reflecting the presence of olivine and spinel and the absence of plagioclase. Whole-rock chemistry suggests that the protoliths of these granulite xenoliths have high normative olivine, plagioclase, and diopside, and, in most cases, low abundances and generally flat REE profiles with positive Eu anomalies. These characteristics indicate that the protoliths originated as basaltic cumulates, similar to troctolite and olivine-gabbro. On cooling, cumulate protoliths transformed to granulite-facies assemblages, mainly by the reaction: Pl + Ol = Gt + Cpx. Interestingly, some granulite xenoliths contain corundum, interpreted as a product of incongruent melting of plagioclase, that may have crystallized sometime prior to the complete transformation of protoliths to garnet granulites. P-T estimates indicate that the xenoliths last equilibrated at ~ 1100 to 1200°C and 15 to 18 kbar, consistent with depths around 50 to 60 km. The occurrence of mafic granulite xenoliths in Thailand may indicate prolonged periods of basalt underplating that generated new lower crust, and eventually culminated with Late Cenozoic alkali basalt volcanism.

Petrochemistry of the Mantle beneath Thailand: Evidence from Peridotite Xenoliths

Spinel-lherzolites are most abundant among ultramafic xenoliths in Late Cenozoic alkali basalts, which were erupted on Thailands two lithospheric fragments, namely the Shan-Thai and Indochina microcontinents. Major-element compositions of the Cr-diopside—group xenoliths (Type I) vary considerably—e.g., 0.92 to 5.80 wt% A12O3, 32.7 to 43.2 wt% MgO—covering a range from fertile to depleted compositions relative to primary mantle models. The extra-fertile mantle is evidenced in subchondritic Ca/Al ratios of whole-rocks and the occurrence of high modal orthopyroxene xenolith (websterite), suggesting metasomatism of mantle rocks. Interaction of the xenoliths with host basalts is observed as the formation of spongy rinds and symplectite on clinopyroxene and orthopyroxene, respectively. This results in a decrease of A12O3 and Na2O with increasing CaO and MgO in the primary clinopyroxene, and locally the formation of K-Na-rich, feldspathic glass within the spongy rinds. Subsolidus re-equilibration also produced ...