Subduction related to the ancient supercontinent cycle is poorly constrained by mantle samples. Sublithospheric diamond crystallization records the release of melts from subducting oceanic lithosphere at 300–700 km depths 1 , 2 and is especially suited to tracking the timing and effects of deep mantle processes on supercontinents. Here we show that four isotope systems (Rb–Sr, Sm–Nd, U–Pb and Re–Os) applied to Fe-sulfide and CaSiO 3 inclusions within 13 sublithospheric diamonds from Juína (Brazil) and Kankan (Guinea) give broadly overlapping crystallization ages from around 450 to 650 million years ago. The intracratonic location of the diamond deposits on Gondwana and the ages, initial isotopic ratios, and trace element content of the inclusions indicate formation from a peri-Gondwanan subduction system. Preservation of these Neoproterozoic–Palaeozoic sublithospheric diamonds beneath Gondwana until its Cretaceous breakup, coupled with majorite geobarometry 3 , 4 , suggests that they accreted to and were retained in the lithospheric keel for more than 300 Myr during supercontinent migration. We propose that this process of lithosphere growth—with diamonds attached to the supercontinent keel by the diapiric uprise of depleted buoyant material and pieces of slab crust—could have enhanced supercontinent stability.

Diamonds and their inclusions are some of the most scientifically valuable samples of the Earth (Haggerty 1999; Shirey et al. 2019). Among the analytical techniques used to study diamonds, Raman spectroscopy offers several advantages that make it an appealing tool for characterizing inclusions. It is a relatively low-cost, rapid, and non-destructive option, requiring minimal sample preparation, if any. Inclusions can often be characterized in-situ, while still fully enclosed in their diamond host, which ensures that no material is inadvertently lost (e.g., fluid) and the remnant pressure of the inclusion–host system is preserved. The pressure within inclusions can be on the order of several gigapascals (e.g., Nasdala et al. 2003) and is especially important for stabilizing the crystal structure of certain highpressure minerals, such as ringwoodite, in sublithospheric diamonds (Pearson et al. 2014). Ideally, Raman spectroscopy can be complemented by other in-situ methods, such as infrared spectroscopy (FTIR), micro-beam X-ray diffraction (XRD), X-ray computed tomography (CT), and synchrotron X-ray fluorescence (XRF). Raman spectroscopy can serve as a first step to help characterize inclusions before employing more time-consuming or destructive analytical techniques, but it can also serve as a powerful tool in its own right for diamond research (e.g., Liu et al. 1990; Gillet et al. 2002; Nasdala et al. 2003, 2005; Brenker et al. 2005; Walter et al. 2011; Howell et al. 2012; Pearson et al. 2014; Nimis et al. 2016; Smit et al. 2016; Smith et al. 2016b, 2018; Anzolini et al. 2018; Kemppinen et al. 2018). This chapter is the first Raman spectroscopy review specifically applied to inclusions in diamond.

The geographic origin of gem corundum has emerged as one of its major value factors. Combined with gemological observations, trace element analysis is a powerful tool for the determination of corundum provenance. However, owing to similar properties and features of gem corundum from different localities, but similar geological settings, and very low levels of many trace elements in gem corundum, the determination of geographic origin remains challenging. In this study, we present trace elements compositions determined by laser ablation inductively coupled plasma mass spectrometry (LA-ICP-MS) for rubies and blue sapphires from several different localities of geologically similar deposits: high-Fe amphibolite-type rubies, low-Fe marble-type rubies, and metamorphic blue sapphires. In addition, we determined Sr and Pb isotopic ratios by offline laser ablation sampling followed by thermal ionization mass spectroscopy (TIMS). By applying new and existing elemental discrimination schemes and the multivariate statistical method linear discriminant analysis (LDA), we show that, in addition to the commonly used discriminators Mg, Fe, V, Ti, and Ga, the elements Ni, Zr, Cr, and Zn show potential for geographic origin determination. Amphibolite-type rubies from different localities can be discriminated using Sr and Pb isotope ratios, whereas the discrimination of marble-type ruby and metamorphic blue sapphires is limited. Our results re-emphasize the challenge of geographic origin determination and the need for a more powerful discriminatory tool.

Quantitative trace element data from high-purity gem diamonds from the Victor Mine, Ontario, Canada as well as near-gem diamonds from peridotite and eclogite xenoliths from the Finsch and Newlands mines, South Africa, acquired using an off-line laser ablation method show that we see the same spectrum of fluids in both high-purity gem and near-gem diamonds that was previously documented in fibrous diamonds. “Planed” and “ribbed” trace element patterns characterize not only the high-density fluid (HDF) inclusions in fibrous diamonds but also in gem diamonds. Two diamonds from two Finsch harzburgite xenoliths show trace element patterns similar to those of saline fluids, documenting the involvement of saline fluids in the precipitation of gem diamonds, further strengthening the link between the parental fluids of both gem and fibrous diamonds. Differences in trace element characteristics are evident between Victor diamonds containing silicate inclusions compared with Victor diamonds containing sulphide inclusions. The sulphide-bearing diamonds show lower levels of inter-element fractionation and more widely varying siderophile element concentrations - indicating that the silicate and sulphide-bearing diamonds likely formed by gradations of the same processes, via melt-rock reaction or from a subtly different fluid source. The shallow negative LREEN-HREEN slopes displayed by the Victor diamonds establish a signature indicative of original derivation of the diamond forming agent during major melting (~10% melt). Consequently, this signature must have been passed on to HDFs separating from such silicate melts.

Trace element characteristics of rubies from the Aappaluttoq deposit, SW Greenland, were measured using laser ablation-inductively coupled plasma-mass spectrometry (LA-ICP-MS), laser ablation - inductively coupled plasma-time of flight-mass spectrometry (LA-ICP-TOF-MS) and offline laser ablation followed by solution ICPMS. LA-ICP-TOF-MS - applied to rubies for the first time - effectively maps trace element spatial variation in these gems. With the exception of a small number of elements that can substitute for Al3+ in the crystal structure (e.g., Ti, Fe, V, Cr, Mg), trace element mapping clearly demonstrates that most elements such as Th, U, Sr and Rb are hosted in mineral and fluid inclusions or are present along fractures. Primitive mantle normalized trace element patterns show characteristics that are broadly correlative to mineral inclusions within the analysed rubies. These minerals include rutile (enrichment of HFSE over LREE, high Ta/Nb and Hf/Zr ratios and low Th/U ratios), phlogopite (enrichment in Rb and Ba and positive Sr anomalies), and zircon (extreme enrichment in Zr-Hf, U and Th, HREE enrichment over LREE and positive Ce anomalies). The sample suite analysed here is derived from a bulk sample of ore composed of three different rock types (sapphirine-gedrite, leucogabbro and phlogopitite). Two different populations of ruby were identified at Aappaluttoq; these can be defined on the basis of their different V content within the corundum lattice. Therefore, V content may be able to geochemically define rubies from different host rocks within the same deposit. Using offline laser ablation followed by thermal ionization mass spectrometry (TIMS) we measured the radiogenic isotope compositions in ruby for the first time. A Pb-Pb isochron age of 2686 + 300/- 74 Ma, was defined for gem formation at Aappaluttoq. We believe that this is the first ever direct age determined on a ruby suite, independent of associated minerals, derived by bulk sampling sub-micron to micron sized inclusions in the corundum lattice. This age likely reflects the re-crystallization and re-setting of the ruby (and its U-Pb system) during the Neoarchean in SW Greenland, due to regional granulite to upper-amphibolite facies metamorphism.

Ten individual gem-quality monocrystalline diamonds of known peridotite/eclogite paragenesis from Southern Africa (Koffiefontein, Letlhakane, Orapa) were studied for trace element concentrations and He and Ar abundances and isotopic compositions. In addition, two samples, consisting of pooled fragments of gem-quality peridotitic diamonds from Finsch and DeBeers Pool respectively, were analysed for noble gases. Previous studies (Richardson et al., 1984; Pearson et al., 1998; Gress et al., 2017; Timmerman et al., 2017) provided age constraints of 0.09, 1.0-1.1, 1.7, 2.3, and 3.2-3.4 Ga on mineral inclusions in the studied diamonds, allowing us to study trace elements and noble gases over 3 Gyr of geological time. Concentrations of trace elements in the diamonds are very low - a few hundred ppt to several tens of ppbs - and are likely dependent on the amount of sub-micron inclusions present. Trace element patterns and trace element/He-3 ratios of the studied monocrystalline diamonds are similar to those in fibrous diamonds, suggesting that trace elements and stable noble gas isotopes reside within the same locations in diamond and track the same processes that are reflected in the trace element patterns. We cannot discern any temporal differences in these geochemical tracers, suggesting that the processes generating them have been occurring over at least the past 2.3 Ga. He-3/(4) He ratios decrease and He-4 and Ar-40* contents increase with increasing age of peridotitic and some eclogitic diamonds, showing the importance of in-situ radiogenic He-4 and Ar-40 ingrowth by the decay of U-Th-Sm and K respectively. For most gem-quality monocrystalline diamonds, uncertainties in the He-3/He-4 evolution of the continental lithospheric mantle combined with large analytical uncertainties and possible spatial variability in U-Th-Sm concentrations limit our ability to provide estimates of diamond formation ages using He-4 ingrowth. However, the limited observed He-4 ingrowth (low U + Th/He-3) together with a R/Ra value of 5.3 for peridotitic diamond K306 is comparable to the present-day sub-continental lithospheric mantle value and supports the young diamond formation age found by Re-Os dating of sulphides in the same diamond by Pearson et al. (1998). After correction for in-situ radiogenic He-4 produced since diamond formation a large variation in He-3/He-4 remains in similar to 1 Ga old eclogitic diamonds that is suggested to result from the variable influence of subducted altered oceanic crust that has low He-3/He-4. Hence, the He-3/He-4 isotope tracer supports an origin of the diamond-forming fluids from recycled oceanic crust for eclogitic diamonds, as indicated by other geochemical proxies. (C) 2019 Elsevier Ltd. All rights reserved.

The physical characteristics and impermeability of diamonds allow them to retain radiogenic He-4 produced in-situ from radioactive decay of U, Th and Sm. This study investigates the U-Th/He systematics of fibrous diamonds and provides a first step in quantification of the uncertainties associated with determining the in-situ produced radiogenic He-4 concentration. Factors determining the total amount of measured helium in a diamond are the initial trapped He-4, the in-situ produced radiogenic He-4, alpha-implantation, alpha-ejection, diffusion, and cosmogenic He-3 production. Alpha implantation is negligible, and diffusion is slow, but the cosmogenic He-3 component can be significant for alluvial diamonds as the recovery depth is unknown. Therefore, samples were grouped based on similar major and trace element compositions to determine possible genetically related samples. A correlation between the He-4 and U-Th concentrations approximates the initial He-4 concentration at the axis-intersect and age as the slope. In this study, the corrections were applied to eight fibrous cubic diamonds from the Democratic Republic of the Congo and two diamonds from the Jwaneng kimberlite in Botswana. A correlation exists between the He-4 and U-Th concentrations of the group ZRC2, 3, and 6, and of the group CNG2, 3, and 4 and both correlations deviate significantly from a 71 Ma kimberlite eruption isochron. The U-Th/He dating method appears a promising new approach to date metasomatic fluid events that result in fibrous diamond formation and this is the first evidence that some fibrous diamonds can be formed 10s to 100s Myr before the kimberlite eruption.

The Murowa and Sese kimberlites erupted through the southern margin of the Zimbabwean craton. These kimberlites provide a unique sample of the continental lithospheric mantle in that area through their entrained mantle xenolith and xenocryst cargo. Mantle xenoliths have only been obtained from the Murowa locality so far and thus they form the focus of this review. Ultradepleted chromite-harzburgite and chromite-dunite rock units dominate the Murowa mantle xenolith inventory. No eclogite xenoliths have been found and eclogitic garnets are extremely scarce in the minerals analyzed from heavy mineral concentrate. The very low bulk rock Al and Ca contents of the Murowa peridotites, along with their extreme Pt and Pd depletions, require extensive melt extraction-to in excess of 40% melting. At the same time, their high bulk rock Cr# (100* Cr/(Cr+Al)) indicates that melting took place at relatively low pressures (<<5 GPa). Such high bulk rock Cr#s (median = 0.60) are considerably higher than those of peridotites from the nearby Venetia mine (median = 0.33) and have only been found elsewhere in cratonic peridotites from the North Atlantic craton (median = 0.89). Similar elevated bulk Cr# values are matched by spinel peridotites derived from Phanerozoic collision zones as ophiolites. This similarity favors a model in which this part of the cratonic lithosphere was formed by the subduction of peridotite that had undergone multistage low-pressure melt extraction, likely in an Archean mantle wedge that underwent flux-melting within a subduction zone prior to lateral compression to form nascent cratonic lithospheric mantle. Preliminary quantitative fitting of mantle geotherms derived from thermobarometry data is hampered by the scarcity of suitable clinopyroxene grains and is highly dependent on assumptions made regarding crustal heat production, especially in the lower crust. Nonetheless, at similar to 540 Ma, the time of kimberlite emplacement, the lithosphere beneath the southern edge of the Zimbabwe craton can be constrained to be approximately 200 km deep, slightly shallower than the 220-km depth estimated for lithosphere beneath the Venetia kimberlite, intruded through the Limpopo Complex. The presence of deep, similar to 200-km-thick lithospheric mantle beneath Murowa and Sese in early Cambrian times agrees with the minimum pressure estimates derived from Cr-Ca relationships in concentrate garnets. This estimate is close to that derived from surface-wave seismic studies and indicates that the thickness of the craton root beneath the southern Zimbabwe craton and the Limpopo Complex has not varied significantly in the last 500 m.y. The ultradepleted nature of the Murowa peridotites, together with the scarcity of eclogite/pyroxenite components, indicates a different petrogenetic history for the lithospheric root beneath the southern Zimbabwe craton compared with the mantle roots beneath the Limpopo Complex (Venetia) and the Kaapvaal craton to the south. The very high fraction (73%; n = 150) of low-Ca, high-Cr harzburgitic "G-10" garnets in the mantle garnet concentrate population at Murowa, along with their Cr-Ca relations, is consistent with the high diamond grade (0.7 ct/metric ton). The likely metasomatic origin for G-10 garnets along with the abundance of ultradepleted chromite-bearing peridotites in the Murowa mantle xenolith suite indicates that this lithology, if present in the lithosphere in the diamond stability field, may be a critical starting component for a variety of diamond-formation events in cratonic lithosphere.

First predictions of the macrodiamond grade of newly discovered kimberlites are commonly obtained using size frequency distributions of microdiamonds. The success of this approach suggests a common origin of microdiamonds and macrodiamonds, an implication not yet conclusively established or disproved. In contrast to previous comparative studies on microdiamonds and macrodiamonds from single deposits, here all diamonds analyzed originate from the same microdiamond samples (558 diamonds, ranging from 0.212 to 3.35 mm). The diamonds were analyzed for their carbon isotope compositions and nitrogen characteristics, and, based on this dataset, statistical comparisons were conducted across the size range to assess cogenesis. As a whole, the Misery diamond suite shows high nitrogen contents (median = 850 at. ppm), a bimodal distribution in time-averaged mantle residence temperatures (two distinct subpopulations in mantle residence temperatures: ≤1,125° and ≥1,175°C), a high degree of platelet degradation, and δ 13 C compositions that are isotopically slightly heavier (median = −4.4‰) than the global median. Statistical comparisons of the various size classes indicate the presence of subtly different subpopulations at Misery; however, the nature and magnitude of these geochemical differences are very small in the context of the global diamond database and are viewed as petrogenetically insignificant. The general geochemical similarity of diamonds from different size fractions at Misery reinforces the use of size-frequency analysis to predict diamond grade in kimberlite diamond deposits.

“Super-deep” diamonds are thought to crystallize between 300 and 800km depth because some of the inclusions trapped within them are considered to be the products of retrograde transformation from lower mantle or transition zone precursors. In particular, single inclusion CaSiO3-walstromite is believed to derive from CaSiO3-perovskite, although its real depth of origin has never been proven. Our aim is therefore to determine for the first time the pressure of formation of the diamond-CaSiO3-walstromite pair by “single-inclusion elastic barometry” and to determine whether CaSiO3-walstromite derives from CaSiO3-perovskite or not.We investigated several single phases and assemblages of Ca-silicate inclusions still trapped in a diamond coming from Juina (Brazil) by in-situ analyses (single-crystal X-ray diffraction and micro-Raman spectroscopy) and we obtained a minimum entrapment pressure of ~5.7GPa (∼180km) at 1500K. However, the observed coexistence of CaSiO3-walstromite, larnite (β-Ca2SiO4) and CaSi2O5-titanite in one multiphase inclusion within the same diamond indicates that the sample investigated is sub-lithospheric with entrapment pressure between ~9.5 and ~11.5GPa at 1500K, based on experimentally-determined phase equilibria. In addition, thermodynamic calculations suggested that, within a diamond, single inclusions of CaSiO3-walstromite cannot derive from CaSiO3-perovskite, unless the diamond around the inclusion expands by ~30% in volume.

Introduction Until recently, only indirect methods based on high-pressure techniques and seismological data were applied to study the composition and conditions of the deep Earth [1,2]. During the last decade, inclusions in ultra-deep natural diamond crystals provided a unique possibility to study the Earth ́s mantle to depths reaching even the lower mantle (> 670km) [3]. During the growth of a natural diamond, high density fluids, high pressure minerals and even small rock fragments (mineral assemblages) can be trapped within. These inclusions are then shielded from the environment during the transport and exhumation of the diamond host towards the Earth ́s surface, preserving their original capture composition and, in some cases, even their high pressure structure [3]. These inclusions provide a uniquely direct way to derive information on the composition and structure of the deep Earth [3]. During an ongoing project at P06 we were able to successfully measure several slices of different cloudy diamonds (Rio Soriso, Machado River, Sao Luiz; Brazil) applying sub-micron and full-field XRF techniques. We were able to demonstrate that even in a single diamond the chemical composition can vary from inclusion to inclusion indicating a complex fluid or melt from which they were separated during the growth of the diamond. Full-field XRF measurements were performed on larger (single) crystals (few 100μm) containing so-called inclusion clouds (large number of small inclusions (≥100nm)).