Here we study silicate inclusion-bearing diamonds from the Voorspoed Group II kimberlite (or carbonate-rich olivine lamproite—CROL) in the central Kaapvaal Craton. Based on major and trace elements, along with Sm–Nd isotope characteristics, the peridotitic garnet and clinopyroxene inclusions define three compositional groups that are contemporaneous with lithospheric refertilisation and re-healing related to 2.79–2.68 Ga Ventersdorp-related plume magmatism across the central Kaapvaal craton. Three harzburgitic (G10) inclusions define an isochron of 2877 ± 249 Ma, with a low initial εNd = − 15 ± 10 that overlaps the oldest Kaapvaal enriched components as represented by crust in the 3.66–3.22 Ga Ancient Gneiss Complex. This finding suggests that harzburgitic diamond formation occurred in the oldest > 3.2 Ga Kaapvaal lithospheric mantle. Five lherzolitic (G9) garnet and 11 Cr-diopside inclusions with LaN > 1 (where N refers to chondrite normalisation) and low 147Sm/144Nd (0.11–0.41), define a 2606 ± 95 Ma Sm–Nd isochron with an initial εNd = − 6.9 ± 5.4 that is consistent with diamond formation in enriched > 3.0 Ga lithospheric mantle. Four lherzolitic (G9) garnet and five Cr-diopside inclusions with LaN < 1 and higher 147Sm/144Nd (0.45–1.51), define a 2560 ± 51 Ma Sm–Nd isochron with an initial εNd = 9.8 ± 7.8. This initial ratio overlaps with depleted mantle indicating rapid re-healing of the lithospheric mantle after 2.79–2.68 Ga Ventersdorp-related plume magmatism. Sixteen eclogitic inclusions (10 garnets and six omphacites) define an isochron of 2196 ± 61 Ma, with initial εNd = − 9.3 ± 3.9. Growth of Voorspoed eclogitic diamonds is interpreted to be related to magmatism and failed rifting at the time of the 2.25–2.23 Ga Hekpoort LIP and associated magmatism across the Transvaal Basin. Low-pressure protoliths for the eclogite host rocks are supported by the presence of omphacite that is not stable as a high-pressure liquidus phase, along with Eu anomalies in the majority of eclogitic inclusions. The likely host rocks for eclogitic diamond formation are subduction-related eclogites in the lithospheric mantle that may be similar to those sampled nearby at Lace. The Voorspoed peridotitic and eclogitic diamonds document multiple episodes of melt infiltration and modification of older lithospheric mantle, and have allowed us to place diamond formation and preservation in the context of the long history of plume impingement on the Kaapvaal lithospheric mantle.

Re-Os isotope systematics are reported from a suite of eclogitic and websteritic sulphide inclusions extracted from well-characterised diamond growth zones from the Orapa and Jwaneng kimberlite clusters. Re-Os ages (786 +/- 250 Ma) are within uncertainty of previously determined Sm-Nd ages (853 +/- 55 Ma), demonstrating iso-topic equilibrium, at varying levels of completeness, across multiple isotopic systems in different minerals at the time of diamond formation and inclusion encapsulation. These data confirm the concept that inclusion isochron ages, when used with detailed textural/ growth zone control, reflect the timing of diamond crystallisation. Our data substantiate previous Re-Os and Sm-Nd inclusion ages of diamonds from Orapa and Jwaneng, indicating that major tectono-magmatic events formed discrete diamond populations of Paleo- (-2.0 to 1.7 Ga), Meso- (-1.2 to 1.1 Ga) and Neoproterozoic (-0.9 to 0.75 Ga) age. Some of these processes occurred simultaneously across the Kalahari Craton and can be traced over 100's of km illustrating the significance of diamond inclusions for monitoring continental tectonics. Inclusion ages indicating diamond formation that are younger than 300 Ma appear to be more common than pre-viously recognised, consistent with evidence of relatively abundant, young, fluid-rich "fibrous" and polycrystal-line diamonds at Jwaneng and Orapa. The increasingly widespread evidence for Mesozoic diamond-forming events in southern Africa and elsewhere appears closely linked with the kimberlite-related magmatism that af-fected these regions and subsequently transported diamonds to the surface. The inclusion isochron ages empha-sise that diamond formation isa multi-stage and episodic process that can occur contemporaneously in disparate substrates and produce multiple diamond populations in the sub-continental lithospheric mantle. (c) 2021 The Authors. Published by Elsevier B.V. This is an open access article under the CC BY license (http:// creativecommons.org/licenses/by/4.0/).

The Sm–Nd isotope systematics and geochemistry of eclogitic, websteritic and peridotitic garnet and clinopyroxene inclusions together with characteristics of their corresponding diamond hosts are presented for the Letlhakane mine, Botswana. These data are supplemented with new inclusion data from the nearby (20–30 km) Orapa and Damtshaa mines to evaluate the nature and scale of diamond-forming processes beneath the NW part of the Kalahari Craton and to provide insight into the evolution of the deep carbon cycle. The Sm–Nd isotope compositions of the diamond inclusions indicate five well-defined, discrete eclogitic and websteritic diamond-forming events in the Orapa kimberlite cluster at 220 ± 80 Ma, 746 ± 100 Ma, 1110 ± 64 Ma, 1698 ± 280 Ma and 2341 ± 21 Ma. In addition, two poorly constrained events suggest ancient eclogitic (> 2700 Ma) and recent eclogitic and websteritic diamond formation (< 140 Ma). Together with sub-calcic garnets from two harzburgitic diamonds that have Archaean Nd mantle model ages ( T CHUR ) between 2.86 and 3.38 Ga, the diamonds studied here span almost the entire temporal evolution of the SCLM of the Kalahari Craton. The new data demonstrate, for the first time, that diamond formation occurs simultaneously and episodically in different parageneses, reflecting metasomatism of the compositionally heterogeneous SCLM beneath the area (~ 200 km 2 ). Diamond formation can be directly related to major tectono-magmatic events that impacted the Kalahari Craton such as crustal accretion, continental breakup and large igneous provinces. Compositions of dated inclusions, in combination with marked variations in the carbon and nitrogen isotope compositions of the host diamonds, record mixing arrays between a minimum of three components (A: peridotitic mantle; B: eclogites dominated by mafic material; C: eclogites that include recycled sedimentary material). Diamond formation appears dominated by local fluid–rock interactions involving different protoliths in the SCLM. Redistribution of carbon during fluid–rock interactions generally masks any potential temporal changes of the deep carbon cycle.

Constraining the formation age of individual diamonds from incorporated mineral inclusions and assessing the host diamonds' geochemical characteristics allows determination of the complex history of diamond growth in the sub-continental lithospheric mantle (SCLM). It also provides the rare opportunity to study the evolution of the deep cycling of volatiles over time. To achieve these aims, Sm-Nd isotope systematics are presented for 36 eclogitic garnet and clinopyroxene inclusions from 16 diamonds from the Jwaneng mine, Botswana. The inclusions and host diamonds comprise at least two compositional suites that record different 'mechanisms' of diamond formation and define two isochrons, one Paleoproterozoic (1.8 Ga) and one Neoproterozoic (0.85 Ga). There are indications of at least three additional diamond-forming events whose ages currently cannot be well constrained. The Paleoproterozoic diamond suite formed by large-scale (>100's km), volatile-rich metasomatism related to formation and re-working of the Proto-Kalahari Craton. In contrast, the heterogeneous composition of the Neoproterozoic diamond suite indicates diamond formation on a small-scale, through local (<10 km) equilibration of compositionally variable diamond-forming fluids in different eclogitic substrates during the progressive breakup of the Rodinia supercontinent. The results demonstrate that regional events appear to reflect the input of volatiles (i.e., carbon-bearing) derived from the asthenospheric mantle, whereas local diamond-forming events mainly promote the redistribution of volatiles within the SCLM. The occurrence of isotopically light carbon analysed in distinct growth zones from samples of this study (delta C-13 < -21.1 parts per thousand) provides further indication of a recycled origin for surface-derived carbon in some diamonds from Jwaneng. Determining Earth's long-term deep carbon cycle using diamonds, however, requires an understanding of the nature and scale of specific diamond-forming events. (C) 2020 Elsevier Ltd. All rights reserved.

We present new geochemical and isotopic data for rock samples from two island arc volcanoes, Erromango and Vulcan Seamount, and from a 500 m thick stratigraphic profile of lava flows exposed on the SW flank of Vate Trough back‐arc rift of the New Hebrides Island Arc (NHIA). The basalts from the SW rift flank of Vate Trough have ages of ~0.5 Ma but are geochemically similar to those erupting along the active back‐arc rift. The weak subduction component in the back‐arc basalts implies formation by decompression melting during early rifting and rifting initiation by tectonic processes rather than by lithosphere weakening by arc magma. Melting beneath Vate Trough is probably caused by chemically heterogeneous and hot mantle that flows in from the North Fiji Basin in the east. The melting zone beneath Vate Trough back‐arc is separate from that of the arc front, but a weak slab component suggests fluid transport from the slab. Immobile incompatible element ratios in South NHIA lavas overlap with those of the Vate Trough depleted back‐arc basalts, suggesting that enriched mantle components are depleted by back‐arc melting during mantle flow. The slab component varies from hydrous melts of subducted sediments in the Central NHIA to fluids from altered basalts in the South NHIA. The volcanism of Erromango shows constant compositions for 5 million years, that is, there is no sign for variable depletion of the mantle or for a change of slab components due to collision of the D'Entrecasteaux Ridge as in lava successions further north.

To fully understand the implications of the compositional information recorded by inclusions in diamond it is vital to know if their growth was syn- or protogenetic and the extent to which they have equilibrated with diamond forming agents. The current paradigm is that the majority of inclusions in diamond are syngenetic but recently this assumption has been questioned. This study presents an integrated cathodoluminescence (CL) and electron backscatter diffraction (EBSD) study of 8 diamonds containing eclogitic inclusions: 19 pyrope-almandine garnets, 12 omphacitic clinopyroxenes, 4 sulphides, 1 coesite and 1 rutile from the Jwaneng diamond mine, Botswana. Diamond plates were sequentially polished to expose inclusions at different levels and CL imaging and EBSD were performed to constrain the relationship between diamond and inclusion growth. Despite complex growth and resorption, individual diamonds are single crystals with a homogeneous crystallographic orientation. All individual inclusions have homogeneous crystallographic orientation and no resolvable compositional zonation. The combined CL and EBSD data suggest that epitaxial inclusion-diamond growth is rare (none of 24 inclusions) and that the imposition of cubo-octahedral faces on inclusions does not necessarily result in epitaxy. Individual diamonds contain inclusions that record evidence of both syngentic and protogenetic relationships with the host diamond and in one case an inclusion appears syngenetic to the diamond core but protogenetic to the growth zone that surrounds 70% of the inclusion. These findings emphasise that inclusions in diamonds have multiple modes of origin and that in order to validate the significance of geochronological studies, further work is needed to establish that there is rapid chemical equilibration of protogenetic inclusions with diamond forming agents at mantle temperatures.

Episodic eclogitic diamond genesis at Jwaneng diamond mine, Botswana M.U. GRESS D.G. PEARSON I.L. CHINN J.M. KOORNNEEF A.S.M. PALS E.A.S. VAN DER VALK G.R. DAVIES 1 Vrije Universiteit, De Boelelaan 1085, 1081 HV Amsteram, The Netherlands m.u.gress@vu.nl 2 University of Alberta, Edmonton T6G 23, Canada gdpearso@ualberta.ca 3 De Beers Exploration, Private Bag X01, Southdale 2135, South Africa ingrid.chinn@debeersgroup.com

Changing recycling budgets of surface materials and volatiles by subduction of tectonic plates influence the compositions of Earth’s major reservoirs and affect climate throughout geological time. Fluids play a key role in processes governing subduction recycling, but quantifying the exact fate of volatiles introduced into the mantle at ancient and recent destructive plate boundaries remains difficult.

Important implications for the interior workings of the Earth can be drawn by studying diamonds and their inclusions. To better understand the timing and number of diamond forming events beneath the NW margin of the Kaapvaal Craton, a comprehensive reassessment of Jwaneng’s diamond populations has been undertaken. We report new inclusion abundance data from the visual examination of ~130,000 diamonds that validate the predominance of an eclogitic diamond suite (up to 88%) with on average 5% inclusion-bearing diamonds (with inclusions >10 μm in size). From this population, polished plates from 79 diamonds of eclogitic and peridotitic paragenesis have been studied with cathodoluminescence (CL) imaging and infrared spectroscopy (FTIR) traverses. The majority (80%) record major changes in N concentration and aggregation states, as well as sharp boundaries in the CL images of individual plates that are interpreted to demarcate discrete diamond growth events. In addition, bulk FTIR data have been acquired for 373 unpolished diamonds. Silicate inclusions sampled from distinct growth zones define 2 compositional groups of omphacites and pyrope-almandines associated with different N contents in their diamond hosts. These findings reinforce previous observations that at Jwaneng at least seven individual diamond forming events can be identified – 3 peridotitic and 4 eclogitic. The results demonstrate that detailed examination of diamond plates by CL imaging and FTIR traverses is necessary to unveil the complex history recorded in diamonds.

Precise dating of diamond growth is required to understand the interior workings of the early Earth and the deep carbon cycle. Here we report Sm-Nd isotope data from 26 individual garnet inclusions from 26 harzburgitic diamonds from Venetia, South Africa. Garnet inclusions and host diamonds comprise two compositional suites formed under markedly different conditions and define two isochrons, one Archaean (2.95 Ga) and one Proterozoic (1.15 Ga). The Archaean diamond suite formed from relatively cool fluid-dominated metasomatism during rifting of the southern shelf of the Zimbabwe Craton. The 1.8 billion years younger Proterozoic diamond suite formed by melt-dominated metasomatism related to the 1.1 Ga Umkondo Large Igneous Province. The results demonstrate that resolving the time of diamond growth events requires dating of individual inclusions, and that there was a major change in the magmatic processes responsible for harzburgitic diamond formation beneath Venetia from the Archaean to the Proterozoic.

A study of tiny mineral 'inclusions' within diamonds from Botswana has shown that diamond crystals can take billions of years to grow. One diamond was found to contain silicate material that formed 2.3 billion years ago in its interior and a 250 million-year-old garnet crystal towards its outer rim, the largest age range ever detected in a single specimen. Analysis of the inclusions also suggests that the way that carbon is exchanged and deposited between the atmosphere, biosphere, oceans and geosphere may have changed significantly over the past 2.5 billion years.

Evaluation of mineral compositions is a widely used approach in resource exploration strategies where preparation time and cost may prove to be an important factor. In research institutes it is highly beneficial to determine the major element composition of minerals prior to their destructive analysis for trace elements and radiogenic isotopic ratios thus allowing a comprehensive interpretation of mineral petrogenesis. For the analysis of unique and small (submilligram) samples, avoiding sample loss is a key issue in ultimately producing high quality geochemical data. Consequently here we evaluate the precision and accuracy of electron probe microanalysis of unpolished garnet, olivine, orthopyroxene and clinopyroxene grains by comparison of analyses performed on polished thin sections of the same minerals. By utilizing a protocol that focuses on flat mineral surfaces, rejects analyses with low totals (<90%) and major element compositions, magnesium numbers and stoichiometry outside two standard deviation, results had on average a reproducibility of 1.3 times the relative standard deviation of the results of polished thin sections. Major element ratios are indistinguishable from the thin section results. For example, the Mg# for clinopyroxene and olivine is within 0.4% and for garnet within 1–1.5%. Individual analyses of minerals with flat surfaces such as clinopyroxene had a higher rate of success (73%) than minerals with a more variable surface topography such as conchoidally fractured garnet (40%), underlining that a flat topography is the controlling factor in EPMA analyses. These tests establish that accurate and reproducible EPMA analysis can be produced on unpolished minerals that are within error of conventional thin section analyses. The technique is predicted to be of particular use in diamond exploration strategies where knowledge of the geotherm beneath exploration areas is a key parameter. Integrated studies of composition and geochronology of mineral inclusions in diamonds have the potential to significantly improve the understanding of diamond formation processes and the imposed octahedral morphology of the inclusions mean that they have flat crystal faces, ideal for analysis using the proposed methodology.