The Siberian craton in Russia hosts many of the country’s famous diamond mines. The Lomonosov min... more The Siberian craton in Russia hosts many of the country’s famous diamond mines. The Lomonosov mine, however, occurs within the boundaries of a different craton—the Baltic shield, most of which lies in Europe. Unlike many diamond mines in South Africa, Canada, and Siberia, the Lomonosov deposit is not in a stable Archean geologic setting. Similar to the Argyle diamond mine in Australia, Lomonosov is in a younger Proterozoic orogenic (or mountain-building) region. Fancy pink diamonds at both these lo- calities likely relate to these Proterozoic tectonic processes. Along with other diamond mines in Proterozoic geologic regions, the Lomonosov deposit (and its fancy-color diamond inventory) demonstrates that the diamond potential of these regions should not be overlooked.
Ten sulphide inclusions in three diamonds from the Zimmi (West Africa) alluvial diamond locality... more Ten sulphide inclusions in three diamonds from the Zimmi (West Africa) alluvial diamond locality were analysed for their bulk Fe-Cu-Ni-Co contents and Re-Os isotopic compositions. The host diamonds are exceptionally rare, Ib types that still preserve isolated nitrogen (C centres), rather than more common nitrogen pairs (A centres) and nitrogen aggregates (B centres). C centres in Zimmi diamonds require that they did not experience temperatures above 850 C for any extended period. Such diamonds make up less than 0.1 % of natural gem diamonds and have never before been dated. The sulphides are pyrrhotite-rich, have low Ni and Os contents, and radiogenic 187Os/188Os, all features characteristic of eclogitic sulphides. Each diamond has 3 to 4 individual inclusions. 187Re/188Os and 187Os/188Os data fall along three individual ~650 Ma age arrays that represent essentially 3-point or 4-point mineral isochrons for each diamond - unambiguously dating the time of diamond formation. The ~650 Ma age correlates with the timing of Neoproterozoic assembly of Gondwana, recorded in the Rokelide orogen along the SW margin of the West African craton. The initial 187Os/188Os of the three age arrays fall between 1.6 and 2.2 and are highly radiogenic compared to chondritic mantle at 650 Ma. Along with low Re/Os ratios, this data suggests that sulphides were not derived from Neoproterozoic subducting slabs, but rather from older eclogitic material already present in the West African lithospheric mantle. The age of the diamonds and their nitrogen substitutional characteristics, along with their residence in a lithospheric mantle with a normal cratonic geotherm (determined here from Koidu clinopyroxene xenocrysts) suggests that after diamond formation they were rapidly exhumed to shallower depths in the lithosphere. This likely occurred through tectonic uplift following Neoproterozoic continental collision.
Mixed-habit (octahdedral + cuboid) diamonds from the Marange alluvial deposits in the eastern Zim... more Mixed-habit (octahdedral + cuboid) diamonds from the Marange alluvial deposits in the eastern Zimbabwe craton have high nitrogen and hydrogen content that provides an opportunity to evalu- ate diamond growth mechanisms and C-N-H-O bearing fluids in the lithospheric keel. Light grey cuboid sectors with hydrogen-containing defects, trap abundant dispersed CH4 inclusions (Ra- man peaks at 2917 cm−1) associated with graphite (Raman peaks at 1580 cm−1). Clear octahedral sectors are richer in nitrogen and free of any such inclusions. Core to rim co-variations of δ13C- δ15N and N content can be explained by a mixing trend between earlier fluids that are CH4-rich and later fluids that are more CO3- or CO2-rich. Marange diamonds have limited overall δ13C vari- ation, but do show fractionation during growth towards higher δ13C values. This trend can be explained by diamond precipitation from mixed CH4 and CO2 fluids, where isotopic fractionation occurs as the amount of fluid wanes. Calculated δ15N values for diamond source fluids evolving in this manner are between +2.3 and +6.4 . These N isotopic compositions require CH4-rich and CO3-/CO2-rich ’end-member’ fluids to have a recycled metasedimentary component perhaps introduced with subduction of eclogite.
The Victor Mine, the first diamond mine in the Archaean Superior craton in Ontario, provides the ... more The Victor Mine, the first diamond mine in the Archaean Superior craton in Ontario, provides the unique opportunity to study the association of a diamond deposit with a post-Archaean rift system (the 1.1 Ga Keweenawan Midcontinent Rift). Victor forms part of the Attawapiskat kimberlite cluster, which was emplaced at ~170–180 Ma (Kong et al., 1999; Heaman and Kjarsgaard, 2000), subsequent to the Midcontinent Rift. Relatively little is known about the details of the history and composition of the lithospheric mantle below the Superior craton, due to the scarcity of large suites of mantle xenoliths. In particular, the impact of the 1.1 Ga Keweenawan Midcontinent Rift on diamond-bearing lithospheric mantle beneath the Superior Craton is poorly constrained. Preliminary results from garnet xenocrysts from the Kyle Lake (~1.1 Ga) and Victor kimberlites (Armstrong et al., 2004; Scully et al., 2004) indicate that the local lithospheric mantle was modified to lherzolitic compositions after em...
TheJurassic Attawapiskat kimberlites allow the study of the associ- ation of a world-class primar... more TheJurassic Attawapiskat kimberlites allow the study of the associ- ation of a world-class primary diamond deposit (Victor Mine) with a post-Archaean rift system, the Midcontinent Rift, which affected the southern Superior Craton at ~1·1 Ga. Peridotite xenoliths and xenocrysts from the Attawapiskat kimberlites have been analysed to understand the processes of craton formation and modification in the Superior lithospheric mantle. Chondrite-normalized platinum group element (PGEN) signatures in olivine are complex and highly variable, but correlate with Os isotopic compositions.The existence of a depleted mantle reservoir beneath the Attawapiskat area since the Palaeoarchaean is indicated by ~3·6 Ga TRD ages preserved in peridotitic olivine. An Mg# up to 93·6 in these olivines requires that protolith formation involved high degrees of partial melting, leading to harzburgitic and dunitic residues. Cr-rich garnets with positive slopes in depleted chondrite-normalized heavy rare earth elements (HREEN) are consistent with fractional polybaric melt extraction continuing from the garnet into the spinel stability field. TRD ages of ~2·7 Ga in olivines with residual PGEN patterns probably reflect residual PGE alloy or refractory PGE sulphide inclusions and indicate that additional melting occurred in the mantle at the time of subduction-accretion, with hydrous melts infiltrating the overlying mantle wedge leading to iridium-group PGE (I-PGE) alloy formation. Metasomatic melts related to the Midcontinent Rift (1·1 Ga) interacted with variably depleted peridotite, leading to platinum-group PGE (P-PGE) enrichment and Mesoproterozoic TRD ages. Older depleted domains are, however, preserved (e.g. sinusoidal REEN patterns in lherzolitic garnet). After the thermal impact of the rift subsided, diamond-stable conditions were extended to shallower depths in the lithosphere via cooling, and diamonds sampled by post-rift kimberlites, such as Victor (~180 Ma), must have formed after the Midcontinent Rift. These diamonds are likely to be of mixed parageneses: high-pressure compositions indicative of diamond stability are observed in both lherzolitic and high-Mg eclogitic to pyroxenitic garnets at Victor.
Here, we compare nitrogen aggregation characteristics and carbon isotopic compositions in diamond... more Here, we compare nitrogen aggregation characteristics and carbon isotopic compositions in diamonds from Mesoproterozoic (T1) and Jurassic (U2) kimberlites in the Attawapiskat area—the first diamond-producing area on the Superior craton. The T1 kimberlite sampled dia- monds from the lithospheric mantle at 1.1 Ga, at the same time as the major Midcontinent Rift event. These diamonds have a narrow range in d13C (mode of -3.4 %), with compositions that overlap other diamond localities on the Superior craton. Some diamond destruction must have occurred during the Mesoproterozoic in response to the thermal impact of the Midcontinent Rift—the associated elevated geotherm caused a narrow diamond window (< 30 km) close to the base of the lithosphere, compared to a wide diamond window of ~ 85 km following thermal relaxation (sampled by Jurassic kimberlites, such as U2). T1 diamonds have highly aggregated nitrogen, possibly due to the thermal effect of the rift. Diamond-favourable conditions were re-established in the lithospheric mantle after the thermal impact of the Midcontinent Rift dissi- pated. The poorly aggregated nature of nitrogen in U2 diamonds—compared to highly aggregated nitrogen in diamonds from T1—indicates that renewed diamond for- mation must have occurred only after the thermal impact of the Midcontinent Rift at 1.1 Ga had subsided and that these newly formed diamonds were subsequently sampled by Jurassic kimberlites. The overall d13C distribution for U2 diamonds is distinct to T1 and other Superior diamonds, further suggesting that U2 diamonds are not related to the older pre-rift diamonds.
A suite of 30 eclogite and pyroxenite xenoliths recovered from the Jurassic Victor kimberlite in ... more A suite of 30 eclogite and pyroxenite xenoliths recovered from the Jurassic Victor kimberlite in the western Superior Province are investigated to determine their formation and emplacement in the sub-continental lithospheric mantle (SCLM). The samples have a wide compositional range, including low-Mg and high-Mg varieties. The low-Mg eclogites have a shallow origin as plagioclase-bearing protoliths that were subsequently subducted and emplaced into the SCLM. This is supported by their generally flat MREE to HREE compositions, the presence of kyanite and a positive Eu anomaly in the kyanite-bearing sample, as well as d18O in three low-Mg eclogites that are higher than the pristine mantle value. LREE depletion in the low-Mg eclogites, along with unradiogenic 87Sr/86Sr indicate that they were not affected by widespread metasomatism after emplacement in the SCLM. The high-Mg eclogites and pyroxenites have compositional characteristics that require a distinct origin to the low-Mg eclogites. Their bulk compositions, LREE-enriched trace element patterns and in particular, occurrence of unradiogenic 187Os/188Os in pyroxenite, is consistent with formation by reaction of broadly siliceous melts (generated from the melting of low-Mg eclogites) with depleted peridotite. A subduction origin of the eclogites studied here is consistent with seismic and field-based studies that have reported terrane accretion by successive subduction of the west–east orientated terranes in the western Superior Province. Although the timing of eclogite and pyroxenite formation could not be constrained, radiogenic 187Os/188Os require long-term isolation from the convecting mantle and supports a Neorchaean age for their formation.
The Siberian craton in Russia hosts many of the country’s famous diamond mines. The Lomonosov min... more The Siberian craton in Russia hosts many of the country’s famous diamond mines. The Lomonosov mine, however, occurs within the boundaries of a different craton—the Baltic shield, most of which lies in Europe. Unlike many diamond mines in South Africa, Canada, and Siberia, the Lomonosov deposit is not in a stable Archean geologic setting. Similar to the Argyle diamond mine in Australia, Lomonosov is in a younger Proterozoic orogenic (or mountain-building) region. Fancy pink diamonds at both these lo- calities likely relate to these Proterozoic tectonic processes. Along with other diamond mines in Proterozoic geologic regions, the Lomonosov deposit (and its fancy-color diamond inventory) demonstrates that the diamond potential of these regions should not be overlooked.
Ten sulphide inclusions in three diamonds from the Zimmi (West Africa) alluvial diamond locality... more Ten sulphide inclusions in three diamonds from the Zimmi (West Africa) alluvial diamond locality were analysed for their bulk Fe-Cu-Ni-Co contents and Re-Os isotopic compositions. The host diamonds are exceptionally rare, Ib types that still preserve isolated nitrogen (C centres), rather than more common nitrogen pairs (A centres) and nitrogen aggregates (B centres). C centres in Zimmi diamonds require that they did not experience temperatures above 850 C for any extended period. Such diamonds make up less than 0.1 % of natural gem diamonds and have never before been dated. The sulphides are pyrrhotite-rich, have low Ni and Os contents, and radiogenic 187Os/188Os, all features characteristic of eclogitic sulphides. Each diamond has 3 to 4 individual inclusions. 187Re/188Os and 187Os/188Os data fall along three individual ~650 Ma age arrays that represent essentially 3-point or 4-point mineral isochrons for each diamond - unambiguously dating the time of diamond formation. The ~650 Ma age correlates with the timing of Neoproterozoic assembly of Gondwana, recorded in the Rokelide orogen along the SW margin of the West African craton. The initial 187Os/188Os of the three age arrays fall between 1.6 and 2.2 and are highly radiogenic compared to chondritic mantle at 650 Ma. Along with low Re/Os ratios, this data suggests that sulphides were not derived from Neoproterozoic subducting slabs, but rather from older eclogitic material already present in the West African lithospheric mantle. The age of the diamonds and their nitrogen substitutional characteristics, along with their residence in a lithospheric mantle with a normal cratonic geotherm (determined here from Koidu clinopyroxene xenocrysts) suggests that after diamond formation they were rapidly exhumed to shallower depths in the lithosphere. This likely occurred through tectonic uplift following Neoproterozoic continental collision.
Mixed-habit (octahdedral + cuboid) diamonds from the Marange alluvial deposits in the eastern Zim... more Mixed-habit (octahdedral + cuboid) diamonds from the Marange alluvial deposits in the eastern Zimbabwe craton have high nitrogen and hydrogen content that provides an opportunity to evalu- ate diamond growth mechanisms and C-N-H-O bearing fluids in the lithospheric keel. Light grey cuboid sectors with hydrogen-containing defects, trap abundant dispersed CH4 inclusions (Ra- man peaks at 2917 cm−1) associated with graphite (Raman peaks at 1580 cm−1). Clear octahedral sectors are richer in nitrogen and free of any such inclusions. Core to rim co-variations of δ13C- δ15N and N content can be explained by a mixing trend between earlier fluids that are CH4-rich and later fluids that are more CO3- or CO2-rich. Marange diamonds have limited overall δ13C vari- ation, but do show fractionation during growth towards higher δ13C values. This trend can be explained by diamond precipitation from mixed CH4 and CO2 fluids, where isotopic fractionation occurs as the amount of fluid wanes. Calculated δ15N values for diamond source fluids evolving in this manner are between +2.3 and +6.4 . These N isotopic compositions require CH4-rich and CO3-/CO2-rich ’end-member’ fluids to have a recycled metasedimentary component perhaps introduced with subduction of eclogite.
The Victor Mine, the first diamond mine in the Archaean Superior craton in Ontario, provides the ... more The Victor Mine, the first diamond mine in the Archaean Superior craton in Ontario, provides the unique opportunity to study the association of a diamond deposit with a post-Archaean rift system (the 1.1 Ga Keweenawan Midcontinent Rift). Victor forms part of the Attawapiskat kimberlite cluster, which was emplaced at ~170–180 Ma (Kong et al., 1999; Heaman and Kjarsgaard, 2000), subsequent to the Midcontinent Rift. Relatively little is known about the details of the history and composition of the lithospheric mantle below the Superior craton, due to the scarcity of large suites of mantle xenoliths. In particular, the impact of the 1.1 Ga Keweenawan Midcontinent Rift on diamond-bearing lithospheric mantle beneath the Superior Craton is poorly constrained. Preliminary results from garnet xenocrysts from the Kyle Lake (~1.1 Ga) and Victor kimberlites (Armstrong et al., 2004; Scully et al., 2004) indicate that the local lithospheric mantle was modified to lherzolitic compositions after em...
TheJurassic Attawapiskat kimberlites allow the study of the associ- ation of a world-class primar... more TheJurassic Attawapiskat kimberlites allow the study of the associ- ation of a world-class primary diamond deposit (Victor Mine) with a post-Archaean rift system, the Midcontinent Rift, which affected the southern Superior Craton at ~1·1 Ga. Peridotite xenoliths and xenocrysts from the Attawapiskat kimberlites have been analysed to understand the processes of craton formation and modification in the Superior lithospheric mantle. Chondrite-normalized platinum group element (PGEN) signatures in olivine are complex and highly variable, but correlate with Os isotopic compositions.The existence of a depleted mantle reservoir beneath the Attawapiskat area since the Palaeoarchaean is indicated by ~3·6 Ga TRD ages preserved in peridotitic olivine. An Mg# up to 93·6 in these olivines requires that protolith formation involved high degrees of partial melting, leading to harzburgitic and dunitic residues. Cr-rich garnets with positive slopes in depleted chondrite-normalized heavy rare earth elements (HREEN) are consistent with fractional polybaric melt extraction continuing from the garnet into the spinel stability field. TRD ages of ~2·7 Ga in olivines with residual PGEN patterns probably reflect residual PGE alloy or refractory PGE sulphide inclusions and indicate that additional melting occurred in the mantle at the time of subduction-accretion, with hydrous melts infiltrating the overlying mantle wedge leading to iridium-group PGE (I-PGE) alloy formation. Metasomatic melts related to the Midcontinent Rift (1·1 Ga) interacted with variably depleted peridotite, leading to platinum-group PGE (P-PGE) enrichment and Mesoproterozoic TRD ages. Older depleted domains are, however, preserved (e.g. sinusoidal REEN patterns in lherzolitic garnet). After the thermal impact of the rift subsided, diamond-stable conditions were extended to shallower depths in the lithosphere via cooling, and diamonds sampled by post-rift kimberlites, such as Victor (~180 Ma), must have formed after the Midcontinent Rift. These diamonds are likely to be of mixed parageneses: high-pressure compositions indicative of diamond stability are observed in both lherzolitic and high-Mg eclogitic to pyroxenitic garnets at Victor.
Here, we compare nitrogen aggregation characteristics and carbon isotopic compositions in diamond... more Here, we compare nitrogen aggregation characteristics and carbon isotopic compositions in diamonds from Mesoproterozoic (T1) and Jurassic (U2) kimberlites in the Attawapiskat area—the first diamond-producing area on the Superior craton. The T1 kimberlite sampled dia- monds from the lithospheric mantle at 1.1 Ga, at the same time as the major Midcontinent Rift event. These diamonds have a narrow range in d13C (mode of -3.4 %), with compositions that overlap other diamond localities on the Superior craton. Some diamond destruction must have occurred during the Mesoproterozoic in response to the thermal impact of the Midcontinent Rift—the associated elevated geotherm caused a narrow diamond window (< 30 km) close to the base of the lithosphere, compared to a wide diamond window of ~ 85 km following thermal relaxation (sampled by Jurassic kimberlites, such as U2). T1 diamonds have highly aggregated nitrogen, possibly due to the thermal effect of the rift. Diamond-favourable conditions were re-established in the lithospheric mantle after the thermal impact of the Midcontinent Rift dissi- pated. The poorly aggregated nature of nitrogen in U2 diamonds—compared to highly aggregated nitrogen in diamonds from T1—indicates that renewed diamond for- mation must have occurred only after the thermal impact of the Midcontinent Rift at 1.1 Ga had subsided and that these newly formed diamonds were subsequently sampled by Jurassic kimberlites. The overall d13C distribution for U2 diamonds is distinct to T1 and other Superior diamonds, further suggesting that U2 diamonds are not related to the older pre-rift diamonds.
A suite of 30 eclogite and pyroxenite xenoliths recovered from the Jurassic Victor kimberlite in ... more A suite of 30 eclogite and pyroxenite xenoliths recovered from the Jurassic Victor kimberlite in the western Superior Province are investigated to determine their formation and emplacement in the sub-continental lithospheric mantle (SCLM). The samples have a wide compositional range, including low-Mg and high-Mg varieties. The low-Mg eclogites have a shallow origin as plagioclase-bearing protoliths that were subsequently subducted and emplaced into the SCLM. This is supported by their generally flat MREE to HREE compositions, the presence of kyanite and a positive Eu anomaly in the kyanite-bearing sample, as well as d18O in three low-Mg eclogites that are higher than the pristine mantle value. LREE depletion in the low-Mg eclogites, along with unradiogenic 87Sr/86Sr indicate that they were not affected by widespread metasomatism after emplacement in the SCLM. The high-Mg eclogites and pyroxenites have compositional characteristics that require a distinct origin to the low-Mg eclogites. Their bulk compositions, LREE-enriched trace element patterns and in particular, occurrence of unradiogenic 187Os/188Os in pyroxenite, is consistent with formation by reaction of broadly siliceous melts (generated from the melting of low-Mg eclogites) with depleted peridotite. A subduction origin of the eclogites studied here is consistent with seismic and field-based studies that have reported terrane accretion by successive subduction of the west–east orientated terranes in the western Superior Province. Although the timing of eclogite and pyroxenite formation could not be constrained, radiogenic 187Os/188Os require long-term isolation from the convecting mantle and supports a Neorchaean age for their formation.
Mixed-habit diamonds are rare samples that contain two different, simultaneously-grown growth sec... more Mixed-habit diamonds are rare samples that contain two different, simultaneously-grown growth sectors, octahedral and cuboid. The faster-grown cuboid sectors often trap abundant fluid inclusions, in contrast to gem-quality octahedral sectors which, like gem-quality diamonds, rarely retain any direct samples of their source fluids. Mixed-habit diamonds from the Marange locality in the eastern Zimbabwe craton, that contain these co-crystallising inclusion-rich and gem-quality sectors in the same diamond, can potentially reveal source fluids for even gem-quality diamonds. Marange diamonds contain abundant micro-inclusions of CH 4 within their cuboid sectors, the first direct occurrence of reduced CH 4-rich fluids that are thought to percolate through the lithospheric mantle. Detailed source composition modelling based on δ 13 C-δ 15 N-N content of the CH 4-bearing Marange diamonds, show that they did not precipitate through the traditional redox exchange concept of CH 4 oxidation [1]. Rather, we propose that they grew during non-redox growth from cooling CH 4-CO 2 hydrous fluids [2]. If so, then this growth mechanism must apply to both fluid-rich cuboid and gem-like octahedral sectors. Hence a non-redox model for diamond formation from mixed CH 4-CO 2 fluids may be applicable to a wider range of gem-quality lithospheric diamonds. The positive δ 15 N values suggest that these CH 4-bearing diamond source fluids were emplaced into the Zimbabwe cratonic lithosphere from subducted oceanic crust [1]. While there are no age constraints for Marange diamonds, slabs that were fluid sources could have been subducted during Archaean craton amalgamation [3] or during 2.1-1.8 Ga subduction along the western margin of the combined Kaapvaal and Zimbabwe cratons, recorded in the Magondi Belt [4,5]. Trace element analyses of the inclusion-rich cuboid sectors are ongoing to evaluate the composition of these diamond source fluids. Previous studies of oxidised fluid inclusions common in fibrous diamonds reveal that the whole spectrum of observed fluids are derived from seawater-altered subducting slabs that variably interacted with either eclogite or peridotite to produce carbonatitic and silicic fluid compositions [6]. However, whether the fluids responsible for fibrous diamond growth can be applied to gem-quality diamond growth has never been firmly established. Trace element compositions of the CH 4-bearing subduction-derived fluids responsible for Marange mixed-habit growth may provide the link.
Type Ib diamonds are rare diamonds (< 0.1% of natural diamonds) that still preserve unaggregated ... more Type Ib diamonds are rare diamonds (< 0.1% of natural diamonds) that still preserve unaggregated nitrogen as single atoms (C centres), rather than the more common nitrogen pairs (A centres) and nitrogen aggregates (B centres). Preservation of unaggregated nitrogen in cratonic Type Ib diamonds requires either extremely short mantle residency times of a few million years and/or storage of diamonds at cooler temperatures than diamonds containing A and B centres (T < 850 °C; [1]). For normal diamond formation scenarios both constraints create geological challenges. The Zimmi alluvial locality on the West African craton is known for its steady supply of Type Ib diamonds [2] and sulphides contained within them provide the first ever opportunity to determine the age of these enigmatic diamonds. Here we present Re-Os age results from Zimmi sulphide inclusions combined with a new West African geotherm obtained using single clinopyroxene xenocrysts from the nearby Koidu kimberlite (146 Ma; [3]). The geotherm provides the minimum temperatures required for diamond stability in the West African lithospheric keel, which combined with the age results allow us to unravel the geodynamic setting for the preservation of unaggregated C centres in natural cratonic diamonds. Ten eclogitic sulphides from 3 Zimmi Ib diamonds, have Re-Os isotopic compositions that fall along Pan-African age arrays (~650 Ma). Timing of Ib diamond formation correlates with the assembly of Gondwana that is recorded in the Rokelide orogen along the SW margin of the Man shield [4]. Initial 187 Os/ 188 Os obtained from age arrays are between 1.5 and 2.2, extremely radiogenic compared to chondritic composition mantle at 650 Ma (187 Os/ 188 Os = 0.12387; [5]). These radiogenic 187 Os/ 188 Os can only evolve in a source with high Re/Os ratios (50 to 100): achieved though long-term isolation from the convecting mantle and typical in MORB [6]. This suggests the sulphides were derived from older eclogitic protoliths that have mafic oceanic crust precursors, possibly Archaean low-Mg eclogites from the nearby Koidu kimberlite [7]. The sulphides were then encapsulated during Pan-African diamond growth from carbon-bearing fluids remobilised during continental collision. The single clinopyroxene geotherm we obtained for the West African craton indicates that diamonds are only stable above 850 ± 100 °C. Conversely, C centres in Zimmi diamonds require that they did not experience temperatures above 700 °C for any extended period. This suggests that after formation, these diamonds were rapidly exhumed to shallower depths in the lithosphere, likely through tectonic uplift following continent collision. Rapid tectonic exhumation is consistent with the presence of abundant deformation lines in Zimmi diamonds, which is also typical of most other natural Ib diamonds [8]. References:
Ophiolites in Tibet and Russia are known to contain abundant microdiamonds [1,2]. Due to their un... more Ophiolites in Tibet and Russia are known to contain abundant microdiamonds [1,2]. Due to their unusual setting in the oceanic lithosphere rather than the cratonic lithosphere, as well as their small size and unaggregated nitrogen (C centres), many researchers have dismissed these microdiamonds as lab-grown HPHT contaminants. We undertook a spectroscopic study of 33 microdiamonds (about 200 µm diameter) from two ophiolitic localities: microdiamonds in peridotite and chromitite from the Luobusa ophiolite (Tibet) and microdiamonds in chromitite from the Ray-Iz ophiolite (Polar Urals, Russia). Our main goal was to distinguish these diamonds from lab-grown HPHT diamonds and confidently establish that they occur naturally within these ophiolites. The diamonds are mostly broken fragments that are translucent and yellow to yellowish-green in color. These ophiolitic diamonds have many characteristics similar to lab-grown HPHT diamonds: 1) well-formed cubo-octahedral faces; 2) majority of nitrogen (>60 %) incorporated as C centres (Type Ib); 3) nickel-related defects, visible as the 882 – 884 nm doublet in their photoluminescence (PL) spectra; and 4) presence of the H2 (NVN-) defect in their PL spectra. In contrast to lab-grown HPHT diamonds, these ophiolitic diamonds contain abundant fluid and mineral micro-inclusions (e.g., water, carbonates, chromite, magnesiochromite, magnetite, hematite, and moissanite). Additionally, step-like resorption features on some diamond faces indicate a natural origin. Our spectroscopic evidence, together with stable isotope and trace element data for microdiamonds from Luobusa [3], indicate that microdiamonds are naturally present within the Luobusa and Ray-Iz ophiolites and should not be considered as contamination. Any geological model for the occurrence of diamonds within these ophiolites should account for their unaggregated nitrogen, the association with high-Cr minerals and crystallisation from water-rich fluids. Temperature constraints for Type Ib diamonds require that they did not experience temperatures over 850 °C for any extended period [4]. C centres in ophiolitic diamonds could be preserved though rapid tectonic exhumation shortly after their formation, as shown for Type Ib diamonds from West Africa [5]. A possible scenario is that these diamonds formed in association with the subduction of oceanic crust to ultra-high pressures that was uplifted during continental collision, similar to Type Ib metamorphic microdiamonds [6]. However, since these ophiolites have no record of subduction-related metamorphism (as seen in the Kockchetav UHP metamorphic rocks [7]), the geodynamic setting for the origin of these ophiolitic Type Ib diamonds needs to be further explored.
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