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This study illustrates the field relationships of jadeitite-bearing block-in-matrix sequences on Syros and Tinos, Cycladic Blueschist Unit, and adds additional U–Pb zircon ages for jadeitites to the geochronological database. The results confirm the importance of Cretaceous (c. 80 Ma) and Eocene (c. 50 Ma) processes in their geological evolution. Interpretations suggesting that the jadeitites were formed by complete metasomatic replacement of a pre-existing rock are not fully supported by field observations. In at least some cases, the formation of jadeitite is likely due to precipitation from Na-Al-Si-rich aqueous fluids, which also caused variable metasomatic alteration of the host rock. Unambiguous age constraints for formation of the Syros and Tinos jadeitites are not available. A relationship to Eocene blueschist facies metamorphism recorded in the associated metamafic rocks seems plausible. However, since high-pressure overprinting of pre-Eocene jadeitite is also conceivable, there is a much larger time window for jadeitite formation, framed by Cretaceous (c. 80–76 Ma) protolith ages of various mélange blocks and the waning stages of blueschist facies metamorphism (c. 40 Ma). Field observations are consistent with the interpretation that the mélange-like occurrences on Syros and Tinos record, to varying extent, multi-stage processes that include detachment of mafic rocks from the subducting plate, local infiltration of Na-Al-Si-rich aqueous fluids, exhumation via a serpentinitic matrix in a subduction channel and reworking of the primary block-in-matrix fabric by sedimentary or tectonic processes during accretionary wedge formation.

Distinctly different groundmass mineralogy characterise the hypabyssal facies, Mesoproterozoic diamondiferous P3 and P4 intrusions from the Wajrakarur Kimberlite Field, southern India. P3 is an archetypal kimberlite with macrocrysts of olivine and phlogopite set in a groundmass dominated by phlogopite and monticellite with subordinate amounts of serpentine, spinel, perovskite, apatite, calcite and rare baddeleyite. P4 contains mega- and macrocrysts of olivine set in a groundmass dominated by clinopyroxene and phlogopite with subordinate amounts of serpentine, spinel, perovskite, apatite, and occasional gittinsite, and is mineralogically interpreted as an olivine lamproite. Three distinct populations of olivine, phlogopite and clinopyroxene are recognized based on their microtextural and compositional characteristics. The first population includes glimmerite and phlogopite–clinopyroxene nodules, and Mg-rich olivine macrocrysts (Fo 90–93) which are interpreted to be derived from disaggregated mantle xenoliths. The second population comprises macrocrysts of phlogopite and Fe-rich olivine (Fo 81–89) from P3, megacrysts and macrocrysts of Fe-rich olivine (Fo 85–87) from P4 and a rare olivine–clinopyroxene nodule from P4 which are suggested to have a genetic link with the precursor melt of the respective intrusions. The third population represents clearly magmatic minerals such as euhedral phenocrysts of Fe-rich olivine (Fo 85–90) crystallised at mantle depths, and olivine overgrowth rims formed contemporaneously with groundmass minerals at crustal levels. Close spatial association and contemporaneous emplacement of P3 kimberlite and P4 lamproite is explained by a unifying petrogenetic model which involves the interaction of a silica-poor carbonatite melt with differently metasomatised wall rocks in the lithospheric mantle. It is proposed that the metasomatised wall rock for lamproite contained abundant MARID-type and phlogopite-rich metasomatic veins, while that for kimberlite was relatively refractory in nature.

Arc magmas commonly are mixtures of newly arriving primitive melts, stored magmas at shallow levels, and xenolithic material added on ascent. Almost every eruption has a unique assembly of these components, which may record magmatic processes occurring in the plumbing system prior to an eruption. In this study, we focus on complexly zoned olivines (crustal xenocrysts) to obtain a better understanding of the magmatic processes and the assembly of the 1963-65 erupted magmas of Irazu volcano, one of the most voluminous active volcanoes in Costa Rica. We performed high-precision in situ Fe-Mg isotope analyses by femtosecond-LA-MC-ICP-MS on these olivines, to unravel the origin of their complex chemical zoning (growth, diffusion, or a combination of both processes). This information was used to establish a refined diffusion model to explore magma mixing-to-eruption timescales. Furthermore, trace element analyses using LA-ICP-MS were performed. Chromium displays a chemical zoning in the investigated olivine, which coincides spatially as well as in terms of length scale and geometry with Fe-Mg zoning and that was used to constrain Cr diffusivity in natural olivine. Our findings show that Fe-Mg zoning in Irazu olivine mainly results from Fe-Mg inter-diffusion after two crystal growth episodes as indicated by strongly coupled chemical and isotopic zoning. Simulations of this diffusive process indicate that mixing of these crystals into ascending primitive melts occurred <600 days before their eruption, consistent with a previously reported diffusion study based on Ni zonation in Mg-rich olivines. Trace element characteristics of olivine suggest that the complex-zoned olivine crystals originate from a crystal mush/cumulate in the middle or lower crust and deeper than the shallow magma chamber and were mobilized by mantle-derived magma bearing Mg-rich olivines. Finally, modeling of the observed Cr zoning in the Irazu olivines indicates that the diffusion coefficient for Cr in olivine (DCr) is smaller than DFe-Mg by a factor of 4.9 ± 2.9 at the conditions experienced by these crystals consistent with Cr diffusion experiments at high silica activity in the melt. Our results show that by combining elemental and isotope zoning studies in individual minerals we can refine the timing/assembly of magmatic eruptions and provide independent constraints on element diffusivities. Last, it confirms that primitive arc magmas at Irazu are not aphyric during ascent, but carry primitive phenocrysts from lower crust or Moho depth to the surface.

A wide number of genetic models have been proposed for volcanically transported ruby and sapphire deposits around the world. In this contribution we compare the trace element chemistry, mineral and melt inclusions, and oxygen isotope ratios in blue to reddish-violet sapphires from Yogo Gulch, Montana, U.S.A., with rubies from the Chantaburi-Trat region of Thailand and the Pailin region of Cambodia. The similarities between Thai/Cambodian rubies and Yogo sapphires suggest a common origin for gem corundum from both deposits. Specifically, we advance a model whereby sapphires and rubies formed through a peritectic melting reaction when the lamprophyre or basalts that transported the gem corundum to the surface partially melted Al-rich lower crustal rocks. Furthermore, we suggest the protolith of the rubies and sapphires was an anorthosite or, in the case of Thai/Cambodian rubies, an anorthosite subjected to higher pressures and converted into a garnet-clinopyroxenite. In this model the rubies and sapphires are rightfully considered to be xenocrysts in their host basalts or lamprophyre; however, in this scenario they are not \"accidental\" xenocrysts but their formation is intimately and directly linked to the magmas that transported them to the surface. The similarities in these gem corundum deposits suggests that the partial melting, non-accidental xenocryst model may be more wide-reaching and globally important than previously realized. Importantly, in both cases the gem corundum has an ostensibly \"metamorphic\" trace element signature, whereas the presence of silicate melt (or magma) inclusions shows they ought to be considered to be \"magmatic\" rubies and sapphires. This discrepancy suggests that existing trace element discriminant diagrams intended to separate \"metamorphic\" from \"magmatic\" gem corundum ought to be used with caution.

Zircon serves as a robust tracer for crustal recycling processes owing to its wide stability under diverse geological conditions. Its cryptic occurrence within ophiolites offers valuable insights into regional paleotectonic evolution. In this study, we identify a few zircon xenocrysts in both peridotite and basalt units from the Neoproterozoic South Anhui Ophiolite (SAO) in the southeastern Yangtze Block, South China. Zircon xenocrysts within the peridotite yield U-Pb ages ranging from ca. 2.7 to 1.0 Ga (n = 21), with three peaks of 2.8–2.5 Ga, 2.2–1.8 Ga, and 1.2–1.0 Ga. Comparative analysis of age spectra suggests these xenocrysts likely originated from recycled subducted continental materials within the Yangtze Block. In the basaltic rocks, zircon xenocrysts exhibit ages of ca. 2.1–0.9 Ga (n = 27), with peaks of 1.1–0.9 Ga, 1.5–1.4 Ga, and 2.1–1.7 Ga. These zircons are interpreted to have been inherited from wall rocks through crustal contamination during magma ascent, as their age spectra closely resemble those of the surrounding basement strata. Collectively, these findings support that the SAO possibly formed in a back-arc basin setting, characterized by significant crust–mantle interactions.

Koh-i-Noor and other world-famous diamonds such as Hope, Orloff, Great Mogul, Nizam, and Pitt, well-known in the industry as ‘Golkonda Diamonds’ are very well recognised for their rare colours, large carat sizes, and because of the paucity of nitrogen atoms majority of them have been classified as Type IIa diamonds. These renowned Golkonda diamonds were recovered from placers mined on the banks of the Krishna River in southern India; however, their primary source rocks (either kimberlites or lamproites) remain questioned and untraced. Precise identification of the primary sources of such large-sized, dominantly Type IIa diamonds (i.e., CLIPPIRS) is crucial for understanding their deep mantle origin, nature and timing of magmatism carrying them and essential from economic and geological perspectives. We employed a multidisciplinary approach incorporating xenocrystic mineral composition and bulk-rock geochemistry, field geological and remote sensing (GIS) studies to locate the probable primary sources of these renowned diamonds, know the origin of Type IIa Golkonda diamonds in southern India, and to understand the mechanism and timing of diamond transport and dispersal as placers in the Krishna River basin. Our study rules out the possibility of various lamproite occurrences of the Eastern Dharwar Craton and Banganapalle conglomerates as being sources of Koh-i-Noor and other Golkonda diamonds. The absence of Type IIa diamonds in the highly diamondiferous Late Cretaceous kimberlites of Wajrakarur likewise excludes them as source of Golkonda diamonds. Among southern India's two significant kimberlite fields, i.e., Wajrakarur and Narayanpet, compositions of indicator minerals from the Wajrakarur Kimberlite Field (WKF) reveal their ultimate diamond preservation potential, presence of strong diamondiferous mantle roots and deeper source regions, hence recognising them to be the potential sources of Golkonda diamonds. GIS and remote sensing tools were used to calculate moisture content, vegetation indices, and to locate paleo-channel of the Penner River, which was primarily responsible for the transportation of diamonds from their Mesoproterozoic (ca. 1.1 Ga) source rocks at Wajrakarur to their final sites of recovery, i.e., Kolluru and other mines situated on the banks of the Krishna River. The occurrence of alluvial placer deposits in Krishna River drainage system is analogous to the Orange River drainage system in South Africa. Both areas have diamonds sourced from primary kimberlite pipes, transported by rivers, and deposited in specific areas. Similarities in the origin, mechanism of diamond transport, dispersal and deposition have played a crucial role in significant diamond production from alluvial deposits in Krishna and Orange Rivers.

The temperature-dependent exchange of Ni and Mg between garnet and olivine in mantle peridotite is an important geothermometer for determining temperature variations in the upper mantle and the diamond potential of kimberlites. Existing calibrations of the Ni-in-garnet geothermometer show considerable differences in estimated temperature above and below 1100 °C hindering its confident application. In this study, we present the results from new synthesis experiments conducted on a piston cylinder apparatus at 2.25–4.5 GPa and 1100–1325 °C. Our experimental approach was to equilibrate a Ni-free Cr-pyrope-rich garnet starting mixture made from sintered oxides with natural olivine capsules (Niolv ≅ 3000 ppm) to produce an experimental charge comprised entirely of peridotitic pyrope garnet with trace abundances of Ni (10–100 s of ppm). Experimental runs products were analysed by wave-length dispersive electron probe microanalysis (EPMA) and laser ablation inductively coupled plasma mass spectrometry (LA-ICP-MS). We use the partition coefficient for the distribution of Ni between our garnet experimental charge and the olivine capsule lnDgrt/olvNi;NigrtNiolv, the Ca mole fraction in garnet (XgrtCa; Ca/(Ca + Fe + Mg)), and the Cr mole fraction in garnet (XgrtCr; Cr/(Cr + Al)) to develop a new formulation of the Ni-in-garnet geothermometer that performs more reliably on experimental and natural datasets than existing calibrations. Our updated Ni-in-garnet geothermometer is defined here as:T∘C=-8254.568XgrtCa×3.023+XgrtCr×2.307+lnDgrtolvNi-2.639-273±55where Dgrt/olvNi=NigrtNiolv, Ni is in ppm, XgrtCa = Ca/(Ca + Fe + Mg) in garnet, and XgrtCr= Cr/(Cr + Al) in garnet. Our updated Ni-in-garnet geothermometer can be applied to garnet peridotite xenoliths or monomineralic garnet xenocrysts derived from disaggregation of a peridotite source. Our calibration can be used as a single grain geothermometer by assuming an average mantle olivine Ni concentration of 3000 ppm. To maximise the reliability of temperature estimates made from our Ni-in-garnet geothermometer, we provide users with a data quality protocol method which can be applied to all garnet EPMA and LA-ICP-MS analyses prior to Ni-in-garnet geothermometry. The temperature uncertainty of our updated calibration has been rigorously propagated by incorporating all analytical and experimental uncertainties. We have found that our Ni-in-garnet temperature estimates have a maximum associated uncertainty of ± 55 °C. The improved performance of our updated calibration is demonstrated through its application to previously published experimental datasets and on natural, well-characterised garnet peridotite xenoliths from a variety of published datasets, including the diamondiferous Diavik and Ekati kimberlite pipes from the Lac de Gras kimberlite field, Canada. Our new calibration better aligns temperature estimates using the Ni-in-garnet geothermometer with those estimated by the widely used (Nimis and Taylor, Contrib Mineral Petrol 139:541–554, 2000) enstatite-in-clinopyroxene geothermometer, and confirms an improvement in performance of the new calibration relative to existing versions of the Ni-in-garnet geothermometer.

Zircons from nine Palaeoproterozoic granitoid intrusions within the southern part of the Fennoscandian Shield have been studied by laser ablation inductively coupled plasma source mass spectrometry to obtain U-Pb ages (in the range 1.88-1.68 Ga) and Hf isotope compositions. Six granitoids are from the 1.85-1.65 Ga Transscandinavian Igneous Belt; during that period more than 106 km3 of granitoid magma intruded the pre-existing crust. The large majority of magmatic zircons from the nine granitoids have a limited initial range, 176Hf/177Hf=0.2816-0.2818, and define an evolutionary trend given by an initial value of εHf(1.88Ga)≈+2±3 at an average 176Lu/177Hf=0.015. These data show that a geographically extensive, long-lived, relatively homogeneous, and dominant magma source resided within 2.1-1.86 Ga Svecofennian juvenile crust between 1.88 and 1.68 Ga. Zircon xenocrysts (1.91-1.98 Ga) with initial εHf=+0 to +2.5 from one of the intrusions provide additional evidence for such a long-lived crustal source of granitic magmas in central Fennoscandia. The granitoids were emplaced during a period of active mafic underplating that supplied heat to the anatectic zone in the lower-middle crust, but little or no mantle-derived Hf to the granitic magmas, in contrast to many mixed intermediate rocks.

This paper presents the reconstruction of the architecture of the lithospheric mantle, including its thermal state and thickness, as well as the scale and efficiency of its sampling by four kimberlites from the Arkhangelsk diamondiferous province: Arkhangelskaya, Lomonosovskaya, V. Grib, and TSNIGRI-Arkhangelskaya. These kimberlites differ in terms of their composition, diamond content, and location. Data presented include the major-element composition of clinopyroxene xenocrysts (>2000 grains), P–T calculations from compositionally filtered Cr-diopside grains, and the reconstruction of local paleogeotherms. Additionally, we used available data on Ni content in peridotitic garnet xenocrysts to calculate their T values and project them onto local Cr-diopside-derived geotherms to reconstruct the vertical distribution of mantle xenocrysts and assess the efficiency of lithospheric mantle sampling by different kimberlites. We identified the presence of a >200 km-thick lithospheric mantle beneath the region at the time of kimberlite emplacement. We also found that the diamond content of the studied pipes was, to some extent, dependent on the following set of factors: (1) the thermal state of the lithospheric mantle; (2) the width of the real “diamond window” marked by mantle xenocrysts, especially by diamond-associated garnets; and (3) the efficiency of lithospheric mantle sampling by kimberlite. The results of this study can be used to inform diamond exploration programs within the region.

Textures of Bushveld chromite from thin seams and accessory disseminations in the Platreef and the northernmost Waterberg Project area were compared with textures of xenocrystic chromite from mantle xenoliths found in Neogene basalt in the Kurile Island Arc. The sieve-textured to symplectic rims around the resorbed chromite in the Kurile samples resulted from the reaction between chromite and chromite-undersaturated basaltic melt, with the inclusions in chromite being entrapped during episodes of chromite primary growth, chemical dissolution, and reprecipitation or secondary growth. The relics of the lattice-oriented etch tunnels suggest that the dissolution preferentially developed along the crystallographic planes and defects. The Bushveld chromites exhibiting similar textures are interpreted as reaction-textured chromites, by analogy with the Kurile samples. The Bushveld sieve-textured, fish-hook to symplectic and amoeboidal to atoll-like chromites, are believed to have been formed due to coupled dissolution-reprecipitation of the earlier cumulus or xenocrystic chromite during interaction with chromite-undersaturated evolved melt. The electron backscattered diffraction data confirm the same single-crystal crystallographic orientation of all domains of the reaction-textured chromites as well as their clustered semi-dissolved relics. Therefore, Bushveld inclusion-rich chromite might have captured different populations of melt inclusions during its discontinuous out-of-equilibrium growth with fast episodic resorption and regeneration. The occurrence of reaction-textured chromites indicates a zone of interaction between dynamic magmatic influxes where chemical equilibrium was not achieved whereas a complete re-equilibration between chromite and the stagnant and sequestered interstitial liquid was attained during the formation of the massive chromitites.