Peridotitic Inclusions Research Articles

Osmotic Pressure Gradient Effects on Water Diffusion in Porous Rock: Can They Pervert Permeability Tests?

Abstract Generation of a large network of hydraulic cracks is of key importance not only for the success of fracking of shale but also for the recent scheme of sequestration of CO2 in deep formations of basalt and peridotite, which are mafic and ultramafic rocks that combine chemically with CO2. In numerical simulation of the creation of a fracture network in porous rock, an important goal is to enhance the rock permeability. The objective of this article is to calculate the effect of osmotic pressure gradients caused by gradients of concentration of the ions of Ca, Mg, Na, etc. on the effective permeability of the rock. The basic differential equations are formulated, and their explicit solutions for appropriate initial and boundary conditions are obtained under certain plausible simplifications. The main result is explicit approximate formulas for the critical time before which no water permeation through a test specimen can be observed. Depending on various parameters, this time can be unacceptably long, which is manifested as a zero water outflow. The solution may also explain the unreasonably small permeability values reported for some shales.

Serpentinite Diapirs and the Evolution of Oceanic Core Complexes

Serpentinites and peridotites are the predominant lithological components of Oceanic Core Complexes (OCCs), located commonly at triple junctions of slow-spreading oceanic accreting ridges, fracture zones and transform faults. These lithologies differ from the surrounding oceanic brittle lithosphere, built of basalt and gabbro, and the structural evolution of these OCCs is enigmatic. The present investigation suggests that the tectonics of OCCs is derived from the ascent of serpentinic diapirs generated by the unique proximity of shallow asthenosphere, faulted lithosphere and the juxtaposition of oceanic crusts of contrasting densities. Such setting initiated two structural stages in the evolution of the OCC, the first is spontaneous subduction of old and dense oceanic crust under the fresh and lighter basalt at the edge of the MOR across its intersection with transform fault - fracture zone. The sub ducted slab would be affected by the combined effect of the steep thermal gradient and the availability of volatiles there to enhance the alteration of pyroxene into serpentine. Analog and numeric experiments show that spontaneous subduction can initiate spontaneously if the density contrast between the juxtaposed slabs is significant, as is the case between fresh basalt (ρ~2.7 g/cm3 ) and older basalt (ρ~2.9 g/cm3 ). Since the average thermal gradient under the MOR is ca. 130o /km, then at depth of 4-5 km the pyroxene and plagioclase would start their alteration, mostly to serpentine. The abundant faults of MOR normal rifting and strike-slip transform faulting could enable the light and malleable serpentinite to form diapirs, which would ascent to the seafloor at the rift-fracture zone intersections. The friction at the top of the diapirs during their ascent would probably generate breccia at their tops. Highlights a) OCCs are built mostly of serpentinite diapirs that carry peridotite inclusions. b) Serpentinization occurs under low pressure-high temperature conditions. c) OCC source rock is slab of cold oceanic crust sub ducted under fresh crust. d) Such subduction occurs mostly where large density contrast between slabs exists. e) The link between OCCs and detachment normal faults requires support.

Formation of mixed paragenesis diamonds during multistage growth – Constraints from in situ δ13C–δ15N–[N] analyses of Koidu diamonds

Inclusion-bearing diamonds from the Koidu kimberlite complex, Sierra Leone (West African Craton) were analyzed in situ for carbon and nitrogen isotope compositions, nitrogen concentrations and nitrogen aggregation states. In a suite of 105 diamonds, 78% contain eclogitic mineral inclusions, 17% contain peridotitic mineral inclusions, and 5% − an unusually high proportion − contain co-occurring eclogitic and peridotitic mineral inclusions indicating a mixed paragenesis. Major and trace element compositions of mineral inclusions from two mixed paragenesis diamonds (one with omphacite + Mg-chromite, the other with eclogitic garnet + forsteritic olivine) were determined. The presence of positive Eu anomalies in centrally located omphacite and eclogitic garnet inclusions indicates derivation from subducted protoliths, formed as igneous cumulates in lower oceanic crust. Mg-chromite (Cr# 85.5; Mg# 65.2) and olivine (Mg# 94.5) inclusions, located in outer portions of the mixed paragenesis diamonds, have compositions indicative of derivation from strongly depleted cratonic peridotites. Given that the olivine Mg# of 94.5 is the highest reported to date for the West African Craton, the eclogitic and peridotitic inclusions in these mixed paragenesis diamonds cannot have precipitated during infiltration of peridotitic substrates by eclogite-derived fluids, as the consequent fluid-rock interaction should lead to Mg# lower than that for the original peridotitic diamond substrate. The different origins of eclogitic and peridotitic inclusions could be explained by physical transport of their host diamonds from eclogitic into peridotitic substrates, possibly along high-strain shear zones, before renewed diamond growth.Based on the δ13C–δ15N systematics of the entire inclusion-bearing diamond suite from Koidu, three major compositional clusters are identified. Cluster 1 (eclogitic diamond cores; δ13C = –33.2 to −14.4‰ and δ15N = −5.3 to +10.1‰) bears the isotopic signature of recycled crustal material (± a mantle component). Cluster 2 (peridotitic diamonds and including the core of a diamond containing omphacite + Mg-chromite; δ13C = −6.0 to −1.1‰ and δ15N = −4.2 to +9.7‰) likely involves mixing of carbon and nitrogen from subducted and mantle sources. Cluster 3 (rims of eclogitic diamonds and including the eclogitic garnet + olivine included diamond and the rim of the omphacite + Mg-chromite included diamond; δ13C = −7.8 to −3.6‰ and δ15N = −7.9 to −2.1‰) matches convecting mantle-derived fluids/melts. The distinct isotopic signatures of the three diamond clusters, together with differences in nitrogen aggregation and cathodoluminescence response between diamond cores and rims, suggest episodic diamond growth during multiple fluid/melt pulses.

Mineral and fluid inclusions in diamonds from the Urals placers, Russia: Evidence for solid molecular N2 and hydrocarbons in fluid inclusions

The compositions of mineral inclusions from a representative collection (more than 140 samples) of diamonds from the placer deposits in the Ural Mountains were studied to examine their compositional diversity. The overwhelming majority of rounded octahedral and dodecahedral stones typical of placers contain eclogitic (E-type) mineral inclusions (up to 80%) represented by garnets with Mg# 40–75 and Ca# 10–56, including the unique high calcic “grospydite” composition, omphacitic pyroxenes containing up to 65% of jadeite, as well as kyanite, coesite, sulfides, and rutile. Peridotitic (P-type) inclusions are represented by olivine, subcalcic Cr-pyrope, chrome diopside, enstatite and magnesiochromite that are typical for diamonds worldwide. Comparing the chemical composition of olivine, pyrope and magnesiochromite in diamonds of the Urals, north-east of the Siberian platform placers and Arkhangelsk province kimberlites show striking similarity. There are significant differences only in the variations of carbon isotopic composition of the diamonds from the placers of the Urals and north-east of the Siberian platform. One typical rounded dodecahedral diamond was found to contain abundant primary oriented submicrometer-sized (<3.0 µm) octahedral fluid inclusions identified by transmission electron microscopy, which caused the milky color of the entire diamond crystal. The electron energy-loss spectrum of a singular inclusion has a peak at ∼405 eV, indicating that nitrogen is present. The Raman spectra with peaks at 2346–2350 cm−1 confirmed that nitrogen exists in the solid state at room temperature. This means that fossilized pressure inside fluid inclusions may be over 6.0 GPa at room temperature, so the diamond may be considered sublithospheric in origin. However, identification of unique fluid inclusions in one typical placer diamond allows one to expand the pressure limit to at least more than 8.0 GPa. The volatile components of four diamonds from the Urals placers were analyzed by gas chromatography–mass spectrometry (GC–MS). They are represented (rel. %) by hydrocarbons and their derivatives (14.8–78.4), nitrogen and nitrogenated compounds (6.2–81.7), water (2.5–5.5), carbon dioxide (2.8–12.1), and sulfonated compounds (0.01–0.96). It is shown that high-molecular-weight hydrocarbons and their derivatives, including chlorinated, nitrogenated and sulfonated compounds, appear to be stable under upper mantle P-T conditions. A conclusion is drawn that Urals placer diamonds are of kimberlitic origin and are comparable in their high E-type/P-type inclusion ratios to those from the northeastern Siberian platform and in part to diamonds of the Arkhangelsk kimberlite province.

Diamond ages from Victor (Superior Craton): Intra-mantle cycling of volatiles (C, N, S) during supercontinent reorganisation

The central Superior Craton hosts both the diamondiferous 1.1 Ga Kyle Lake and Jurassic Attawapiskat kimberlites. A major thermal event related to the Midcontinent Rift at ca. 1.1 Ga induced an elevated geothermal gradient that largely destroyed an older generation of diamonds, raising the question of when, and how, the diamond inventory beneath Attawapiskat was formed. We determined Re–Os isotope systematics of sulphides included in diamonds from Victor by isotope dilution negative thermal ionisation mass spectrometry in order to obtain insights into the age and nature of the diamond source in the context of regional tectonothermal evolution. Regression of the peridotitic inclusion data (n=14 of 16) yields a 718 ± 49 Ma age, with an initial 187Os/188Os ratio of 0.1177 ± 0.0016, i.e. depleted at the time of formation (γOs −3.7 ± 1.3). Consequently, Re depletion model ages calculated for these samples are systematically overestimated. Given that reported 187Os/188Os in olivine from Attawapiskat xenoliths varies strongly (0.1012–0.1821), the low and nearly identical initial Os of sulphide inclusions combined with their high 187Re/188Os (median 0.34) suggest metasomatic formation from a mixed source. This was likely facilitated by percolation of amounts of melt sufficient to homogenise Os, (re)crystallise sulphide and (co)precipitate diamond; that is, the sulphide inclusions and their diamond host are synchronous if not syngenetic.The ∼720 Ma age corresponds to rifting beyond the northern craton margin during Rodinia break-up. This suggests mobilisation of volatiles (C, N, S) and Os due to attendant mantle stretching and metasomatism by initially oxidising and S-undersaturated melts, which ultimately produced lherzolitic diamonds with high N contents compared to older Kyle Lake diamonds. Thus, some rift-influenced settings are prospective with respect to diamond formation. They are also important sites of hidden, intra-lithospheric volatile redistribution that can be revealed by diamond studies. Later emplacement of the Attawapiskat kimberlites, linking the carbon cycle to the surface, was associated with renewed disturbance during passage of the Great Meteor Hotspot. Lherzolitic diamond formation from oxidising small-volume melts may be the expression of an early and deep stage of the lithospheric conditioning required for the successful eruption of kimberlites, which complements the late and shallow emplacement of volatile-rich metasomes after upward displacement of a redox freezing front.

Mineral inclusions in diamonds from Chapada Diamantina, Bahia, Brazil: a Raman spectroscopic characterization

Abstract The Chapada Diamantina, located in the central region of the State of Bahia, is of important historical significance due to its diamond occurrences. Discovered in the nineteenth century, comprehensive research about the regional diamonds and their origins are still limited, demanding more investigation in the matter. Looking for insights about their genesis, mineral inclusions in 23 alluvial diamonds from 4 garimpos located in the Chapada Diamantina were analyzed through the use of Raman micro spectroscopy. Additionally, the characteristics of nitrogen aggregation of the host diamonds were measured using Fourier-transform infrared spectroscopy (FTIR). The diamonds from Chapada Diamantina consist mainly of well-formed crystals, with dominant dodecahedral habits, characterized by faint to very light yellow body colors, typically with green and brown radiation spots on their surface. The main surface textures observed are related to processes that took place in the late stage resorption and during the residence of the diamonds in placer environments. The diamonds are predominantly type IaAB, with a significant occurrence of poorly aggregated nitrogen (Type IaA diamond). The main mineral assemblages of the studied peridotitic inclusions refer to a harzburgitic paragenesis.

Diamonds and their mineral inclusions from the Renard kimberlites in Quebec

Mineral inclusions in diamonds from the Renard kimberlites, on the eastern Superior Craton, document an almost exclusively peridotitic inclusion suite with an unusually high abundance of SiO2 phases (coesite and presumably retrograde quartz). Whilst coesite is often associated with eclogitic diamond sources, there is an almost complete absence of unequivocally eclogitic mineral inclusions at Renard. Consequently, the presence of abundant SiO2 inclusion phases is likely caused by influx of CO2 within localised regions of the lithospheric mantle with very high fluid/rock ratios. This caused a progression of carbonation reactions within the host peridotite, likely along veins, locally consuming all olivine, orthopyroxene, and clinopyroxene present and producing free SiO2.Nitrogen contents and aggregation states of the diamonds indicate normal mantle residence temperatures, predominantly between 1100 and 1200°C. Co-variations of carbon isotopic composition and nitrogen content suggest formation from both reducing (CH4) and oxidising (CO32− or CO2 bearing) melts/fluids. The observation of single diamonds with non-isothermal co-variations of nitrogen content and aggregation state, or containing disequilibrium inclusion parageneses (e.g., variations in olivine Mg# up to 0.5 within single diamonds), indicates that Renard diamonds grew during a number of precipitation events within a physically and chemically changing environment.

Composition and thermal structure of the lithospheric mantle beneath kimberlite pipes from the Catoca cluster, Angola

Garnet, clinopyroxene and ilmenite xenocrysts from three Angolan kimberlite pipes belonging to the Catoca cluster (Angola Caquele, Camitongo I and II, and Catoca) from the SW part of the Congo–Kasai craton, reveal similar features which suggest a similarity of mantle structure. PT estimates for pyropes, Cr-diopsides and picroilmenites reveal similar geothermal conditions of ~37–40mW/m2. This is slightly higher than the values determined for the Catoca pipe. Higher temperature conditions ~45mW/m2 were determined for low-Cr pyroxenes and omphacites. The similar general mineralogy and suggested mantle lithology, as well as reconstructed layering of the sub-continental lithospheric mantle (SCLM), are similar for Camitongo I–II as well as for Caquele and Catoca pipes. Heating at depths of 7.5–4.5GPa (240–140km) is a general feature of the SCLM beneath the field. The high temperature trend for low-Cr and hybrid pyroxenes from the base of the SCLM up to 30GPa (100km) represents the PT path of the protokimberlite melts. PT conditions for ilmenites mainly correspond to colder conditions of crystallization in wall rocks and the outer parts of magmatic channels.Individual geochemical features of the minerals for each SCLM suggest pervasive metasomatism in lower part of the SCLM. Clinopyroxene trace element patterns from the Caquele pipe reveal a lherzolitic affinity; they are LILE-enriched with Ba peaks due to phlogopite melting, while those from Camitongo I–II show Ta–Nb enrichment and Pb troughs. The ilmenite trends trace the mantle column from deep to shallow mantle, evolving to Fe-ilmenites due to advanced AFC of protokimberlite magma that also produced abundant Fe-rich clinopyroxenes. The rise of calculated fO2 correlates with the position of protokimberlites. Comparison with the thermal gradient derived from peridotitic inclusions from Catoca cluster is lower than for Lesotho possibly related to the thicker lithospheric roots beneath the Congo–Kasai craton.

First crystal-structure determination of olivine in diamond: Composition and implications for provenance in the Earth's mantle

We report for the first time a complete X-ray diffraction in-situ crystal-structure refinement of a single crystal of olivine still trapped in a diamond (Udachnaya kimberlite, Siberia). A two-step experimental procedure, consisting of accurate crystal centering using a four-circle diffractometer equipped with a point detector and subsequent collection of complete intensity data using a second diffractometer equipped with a CCD detector, allowed us to overcome previously reported experimental problems and to refine the crystal structure without extracting the inclusion from the diamond host. The data allowed us to obtain the cation distribution over the two crystallographic M2 and M1 sites, which provided composition of olivine inclusion as Fo 92.7(4). A novel experimental calibration of the pressure–volume equation of state for such composition was obtained using new in situ high-pressure X-ray data on a Fo 92 olivine single-crystal and new established compositional effects on olivine unit-cell volume. Such equation of state allowed us to determine the internal pressure at the olivine inclusion, P i = 0.40(1) GPa. The value for the internal pressure compares well with but has lower uncertainty than estimates obtained using micro-Raman spectrometry for similar olivine inclusions in diamonds from the same kimberlite. Taking into account elastic relaxation of the diamond–olivine pair to ambient P– T, we determined formation pressures of 3.5 GPa to 4.9 GPa, depending on the assumed temperature (800 °C to 1300 °C). These values suggest formation near the graphite–diamond boundary and are comparable to estimates from conventional and Raman thermobarometry for other peridotitic inclusions in Siberian diamonds.

Nature and genesis of Kalimantan diamonds

The origin of alluvial diamonds from the four main diamond mining districts in Kalimantan was studied through characterisation of their properties, and determination of PT and age of formation of representative collections of diamonds from four localities of the island. The diamonds are mostly colourless, yellow or pale brown, shiny surfaced, dodecahedroids, octahedron/dodecahedroids, and more rarely cube combination forms. They are intensively resorbed. They have surface radiation damage and show abrasion features indicative of fluvial transportation and crustal recycling. The diamonds were polished down to expose internal structures and mineral inclusions. The majority of the diamonds are internally homogeneous or have simple octahedral zonation and show plastic deformation. Analysis by Fourier transform infra red spectroscopy of their N content and aggregation characteristics shows that many diamonds are well-aggregated type IaB implying a long-term, mantle residence time and/or high temperatures of formation. Identified inclusion parageneses are 68% peridotitic and 32% eclogitic. The peridotitic inclusions are represented by olivine, chromite, garnet, orthopyroxene and pentlandite. Olivines (Fo 92–93) belong to the dunite–harzburgite paragenesis, with one at Fo 90 identified as lherzolitic. Chromite inclusions with 65–66 wt.% Cr 2O 3 and < 1 wt.% TiO 2 are typical of chromite diamond inclusions world-wide. Two garnet inclusions identified are a subcalcic high chrome harzburgite “G10” and a mildly subcalcic type transitional between “G9” and “G10”. The eclogitic inclusions are represented by omphacite, rutile, kyanite and coesite. Re/Os dating of a high Ni sulphide inclusion from one peridotitic diamond gave an Archean model age of 3.1 Ga ± 0.2 (2 sigma). In terms of their external and internal morphology, N aggregation characteristics and paragenesis the Kalimantan diamonds resemble those transported to surface by kimberlite or lamproite from sources in the subcontinental lithospheric mantle. Five distinctive genetic groups of diamond have been recognised but occur mixed in each of the four diamond mining districts due to a presumed long history of sedimentary recycling and alluvial transportation. Thermobarometry calculations from diamond inclusion chemistry (930 °C to 1250 °C; > 4.2 GPa) are consistent with a paleo-heat flow of 38 to 40 mW/m 2 and derivation from 120 to 160 km depth, i.e. subcontinental mantle lithosphere conditions similar to diamonds from African and Yakutian cratonic situations.

Diamonds from the V. Grib pipe, Arkhangelsk kimberlite province, Russia

A large collection (717 samples) of diamonds from the V. Grib deposit, discovered in 1996 in the Verhotinskoe field of the Arkhangelsk kimberlite province, was studied. The diamond crystals are characterized by high transparency and preservation. The collection consists of complete crystals (71%), chipped and damaged crystals (preservation > 50%; 14%), and fragments (preservation < 50%; 15%). Resorption is generally moderate resulting in a dominance of octahedral and mixed octahedral–dodecahedral shapes. Moderate resorption points to rapid ascent of the transporting kimberlite magma. A characteristic feature of V. Grib is a large number of green diamonds. This may relate to the close proximity of radioactive deposits. Microscopic green surface spots have no cathodoluminescence. The internal diamond morphology was studied by UV- and cathodoluminescence. The main typomorphic feature of diamond crystals from the V. Grib pipe is a high percentage of crystals without UV-luminescence. Presence of sectorial growth was also identified by luminescence. Analysis of mineral inclusions, carbon isotopic composition, nitrogen content and nitrogen aggregation state provided important genetic information. Peridotitic inclusions (olivine, chromian spinel and pyrope) predominate, sulfides are almost completely absent. The carbon isotopic composition of the host diamonds is typical for peridotitic diamonds worldwide. IR-spectroscopy suggests the presence of two diamond populations with low and high nitrogen concentrations. Three sub-populations may be identified based on a combination of morphology, nitrogen and hydrogen defects. Residence temperatures ( T Nitrogen), based on a mantle residence time of 3 Ga, fall between 1050 and 1170 °C. Diamond crystallization in V. Grib occurred in multiple stages. This is documented through luminescence patterns, data on nitrogen concentration and aggregation state, and the presence of “diamond-in-diamond” inclusions.

Petrogenesis and Geological Implications of the Tianheyong Cenozoic Basalts, Inner Mongolia China

The Tianheyong basalts are basanites, which at least can be subdivided into three types of mineral assemblages. In general, the Tianheyong basalts are characteristics of low silica ( w(SiO 2)=43.97%–45.68%), enriched alkali ( w(K 2O+Na 2O) =5.91%–7.65%), enriched potassium ( w(K 2O)=2.04%–2.89%), high contents of titanium ( w(TiO 2)=2.18%–2.37%), high magnesium value (Mg #=68–76), and high contents of REE (ΣREE=(246.62–329.82)×10 −6). The chondrite normalized patterns of REE show right inclined smooth lines, extremely enriched in LREE, significantly fractionated from LREE and HREE, without obvious anomaly of Eu (δEu=0.90–1.02) and Ce (δCe=0.96–1.00), and the high enrichment of incompatible elements, especially, the peak value in high field strength elements (HFSE) of Nb, Ta, and Th indicate the OIB-like distribution patterns of multi-elements. The Tianheyong basalts are also high in compatible elements: Co ((39.1–48.9)×10 −6), Ni ((130–257)×10 −6), and Cr ((138–320)×10 −6). All of the above characteristics as well as the low degree of crystallization, the abundance in peridotite inclusions, and minor amount of xenocrysts, and the variation trend of elements indicate that the basalts were solidified from primitive basaltic magma. Trace element ratio of Ba/Rb (12–35) and variation of alkali metals might indicate the metasomatism of fluids in a source region of enriched lherzolites. Petrogenesis simulation indicates that the Tianheyong primitive basaltic magma might be the product of polybaric melting column with non-modal partial melting processes of enriched lherzolite, which straddle the boundary between garnet- and spinel-facies lherzolites. Magma might originate from low partial melting of source rocks(∼<5%), which have∼1% partial melting of garnet lherzolite and 2%–5% partial melting of spinel lherzolite. The integrative analysis indicates that the triggering mechanism of source rock partial melting might be the edge-driven convection of mantle, which results in that the Tianheyong basalts have the deepest origin depth as compared with those of eastern Jining and Hannuoba basalts.

Garnet–clinopyroxene and clinopyroxene geothermobarometry of deep mantle and crust eclogites and peridotites

Eclogite is one of the major types of xenoliths in kimberlite. An investigation of such xenoliths provides important constraints on the distribution of P and T within the upper mantle and its chemical composition. Garnet–clinopyroxene and clinopyroxene barometers are proposed for mantle eclogites and peridotites on the basis of CaTs solubility in clinopyroxene coexisting with garnet. Such barometers permit estimation of P– T conditions for eclogite and peridotite garnet–clinopyroxene assemblages, including those involving omphacite and Cr-diopside pyroxenes for the ranges 700 ≤ T ≤ 2100 °C and 1.5 ≤ P ≤ 22 GPa. The barometers were used to determine formation conditions of eclogitic and peridotitic inclusions in diamonds, diamond-bearing, diamond–graphite-bearing and diamond-free eclogites and peridotites from kimberlite pipes and metamorphic complexes.

The origin of cratonic diamonds — Constraints from mineral inclusions

The origin of cratonic diamonds is reviewed on the basis of nearly 5000 analyses of silicate, oxide and sulphide inclusions in diamonds. Compositional fields are defined for common minerals of the peridotitic, eclogitic and websteritic inclusion suites and used to establish the characteristics of diamond source rocks in the subcratonic lithospheric mantle. Peridotitic inclusion compositions overlap with the record established from cratonic garnet peridotite xenoliths and xenocrysts but reflect overall higher levels of depletion in basaltic components. The interior of the Kaapvaal Block (Kalahari Craton) is by far the best studied diamond source region in the world but appears to be unique rather than representative because of extreme levels of chemical depletion preserved in the peridotitic inclusion suite. Major and trace element characteristics of peridotitic diamond sources indicate polybaric melt extraction proceeding from the garnet into the spinel stability fields, most readily explained by protolith depletion in Archean mid-oceanic ridge environments. Eclogitic mineral inclusions broadly reflect basaltic source compositions and show chemical trends that are indicative of igneous fractionation and cumulate enrichment in magmatic precursors. In agreement with mounting evidence from xenolith studies, eclogitic diamond sources are linked to subducted oceanic protoliths. A more mafic character relative to present-day MORB may relate to (i) higher degrees of partial melting in Archean or early Proterozoic spreading centres and (ii) secondary melt depletion during subduction, or after emplacement in the subcratonic lithosphere. In line with a subduction origin of eclogitic diamond source rocks, mines with predominantly eclogitic diamond populations are generally (but not invariably) associated with craton margin settings or lithosphere with a post-Archean tectonothermal history. The websteritic suite is poorly defined and reflects a range of broadly pyroxenitic source rocks intermediate between peridotite and eclogite. Geothermometry, based on inclusions and nitrogen aggregation in diamonds, indicates that crystallization and mantle storage of peridotitic, eclogitic and websteritic diamonds occurred under the same thermal conditions. Geobarometry for peridotitic inclusions shows that the majority of diamonds formed at depths of less than 200 km along model geotherms corresponding to 38 to 42 mW/m 2 surface heat flow. Lower geothermal gradients observed for diamonds from the Kalahari and Slave cratons likely represent re-equilibration of touching inclusion pairs to cooling ambient conditions, suggesting that diamond formation was accompanied by transient heating events. Diamond precipitation is interpreted to have occurred during metasomatic events under super-solidus (melt dominated) and sub-solidus (CHO-fluid dominated) conditions. Increasing evidence for a reduced character of the subcratonic lithospheric mantle implies that diamond precipitation through redox reactions requires upward migration of carbonate-bearing melts/fluids. In such a redox scenario high solubility of sulphate relative to sulphide in melts/fluids may provide an explanation for a high abundance of sulphide inclusions as a consequence of co-precipitation with diamond in response to decreasing oxygen fugacity. Such comparatively oxidized metasomatic agents cannot derive from the reduced deep upper mantle and, therefore, likely relate to recycling of oceanic lithosphere.

Diamonds from Jagersfontein (South Africa): messengers from the sublithospheric mantle

The diamond population from the Jagersfontein kimberlite is characterized by a high abundance of eclogitic, besides peridotitic and a small group of websteritic diamonds. The majority of inclusions indicate that the diamonds are formed in the subcratonic lithospheric mantle. Inclusions of the eclogitic paragenesis, which generally have a wide compositional range, include two groups of eclogitic garnets (high and low Ca) which are also distinct in their rare earth element composition. Within the eclogitic and websteritic suite, diamonds with inclusions of majoritic garnets were found, which provide evidence for their formation within the asthenosphere and transition zone. Unlike the lithospheric garnets all majoritic garnet inclusions show negative Eu-anomalies. A narrow range of isotopically light carbon compositions (δ13C −17 to −24 ‰) of the host diamonds suggests that diamond formation in the sublithospheric mantle is principally different to that in the lithosphere. Direct conversion from graphite in a subducting slab appears to be the main mechanism responsible for diamond formation in this part of the Earth’s mantle beneath the Kaapvaal Craton. The peridotitic inclusion suite at Jagersfontein is similar to other diamond deposits on the Kaapvaal Craton and characterized by harzburgitic to low-Ca harzburgitic compositions.

Inclusions in diamonds from the K14 and K10 kimberlites, Buffalo Hills, Alberta, Canada: diamond growth in a plume?

Analyses of mineral inclusions, carbon isotopes, nitrogen contents and nitrogen aggregation states in 29 diamonds from two Buffalo Hills kimberlites in northern Alberta, Canada were conducted. From 25 inclusion bearing diamonds, the following paragenetic abundances were found: peridotitic (48%), eclogitic (32%), eclogitic/websteritic (8%), websteritic (4%), ultradeep? (4%) and unknown (4%). Diamonds containing mineral inclusions of ferropericlase, and mixed eclogitic-asthenospheric-websteritic and eclogitic-websteritic mineral associations suggests the possibility of diamond growth over a range of depths and in a variety of mantle environments (lithosphere, asthenosphere and possibly lower mantle). Eclogitic diamonds have a broad range of C-isotopic composition ( δ 13C=−21‰ to −5‰). Peridotitic, websteritic and ultradeep diamonds have typical mantle C-isotope values ( δ 13C=−4.9‰ av.), except for two 13C-depleted peridotitic ( δ 13C=−11.8‰, −14.6‰) and one 13C-depleted websteritic diamond ( δ 13C=−11.9‰). Infrared spectra from 29 diamonds identified two diamond groups: 75% are nitrogen-free (Type II) or have fully aggregated nitrogen defects (Type IaB) with platelet degradation and low to moderate nitrogen contents (av. 330 ppm-N); 25% have lower nitrogen aggregation states and higher nitrogen contents (∼30% IaB; <1600 ppm-N). The combined evidence suggests two generations of diamond growth. Type II and Type IaB diamonds with ultradeep, peridotitic, eclogitic and websteritic inclusions crystallised from eclogitic and peridotitic rocks while moving in a dynamic environment from the asthenosphere and possibly the lower mantle to the base of the lithosphere. Mechanisms for diamond movement through the mantle could be by mantle convection, or an ascending plume. The interaction of partial melts with eclogitic and peridotitic lithologies may have produced the intermediate websteritic inclusion compositions, and can explain diamonds of mixed parageneses, and the overlap in C-isotope values between parageneses. Strong deformation and extremely high nitrogen aggregation states in some diamonds may indicate high mantle storage temperatures and strain in the diamond growth environment. A second diamond group, with Type IaA–IaB nitrogen aggregation and peridotitic inclusions, crystallised at the base of the cratonic lithosphere. All diamonds were subsequently sampled by kimberlites and transported to the Earth's surface.

Diamondiferous lithospheric roots along the western margin of the Kalahari Craton?the peridotitic inclusion suite in diamonds from Orapa and Jwaneng

The Orapa and Jwaneng kimberlites are located along the western margin of the Kalahari Craton and the prevalence of eclogitic over peridotitic diamonds in both mines has recently been linked to lower P-wave velocities in the deep mantle lithosphere (relative to the bulk of the craton) to suggest a diamond formation event prompted by mid-Proterozoic growth and modification of preexisting Archean lithosphere (Shirey et al. 2002). Here we study peridotitic diamonds from both mines, with an emphasis on the style of metasomatic source enrichment, to evaluate their relationship with this major eclogitic diamond formation event. In their major element chemistry, the peridotitic inclusions compare well with a world-wide database but reveal differences to diamond sources located in the interior of the Western Terrane of the Kaapvaal block, where the classical mines in the Kimberley region are located. The most striking difference is the relative paucity of low-Ca (<2 wt% CaO in garnet) harzburgites and a low ratio of harzburgitic to lherzolitic garnets (2:1). This suggests that lithospheric mantle accreted to the rim of the Zimbabwe and Kaapvaal blocks was overall chemically less depleted. Alternatively, this more fertile signature may be assigned to stronger metasomatic re-enrichment but the trace element signature of garnet inclusions is not in favor of strong enrichment in major elements. For both mines the majority of lherzolitic and harzburgitic garnet inclusions are characterized by moderately sinusoidal REEN patterns and low Ti, Zr and Y contents, indicative of a metasomatic agent with very high LREE/HREE and low HFSE. This is consistent with metasomatism by a CHO-fluid or, as modeled by Burgess and Harte (2003), a highly fractionated, low-volume silicate melt from the MORB-source. In both cases, changes in the major element chemistry of the affected rocks will be limited. In a few garnets from Orapa preferential MREE enrichment is observed, suggesting that the percolating fluid/melt fractionated a LREE-phyllic phase (such as crichtonite). The overall moderate degree of metasomatism reflected by the inclusion chemistry is in stark contrast to lithospheric sections for Orapa and Jwaneng based on mantle xenocrysts and xenoliths, revealing extensive mantle metasomatism (Griffin et al. 2003). This suggests that the formation of peridotitic diamonds predates the intensive modification of the subcratonic lithosphere during Proterozoic rifting and compression, implying that diamonds may survive major tectonothermal events.

Peridotitic diamonds from the Slave and the Kaapvaal cratons—similarities and differences based on a preliminary data set

A comparison of the diamond productions from Panda (Ekati Mine) and Snap Lake with those from southern Africa shows significant differences: diamonds from the Slave typically are un-resorbed octahedrals or macles, often with opaque coats, and yellow colours are very rare. Diamonds from the Kaapvaal are dominated by resorbed, dodecahedral shapes, coats are absent and yellow colours are common. The first two features suggest exposure to oxidizing fluids/melts during mantle storage and/or transport to the Earth's surface, for the Kaapvaal diamond population. Comparing peridotitic inclusions in diamonds from the central and southern Slave (Panda, DO27 and Snap Lake kimberlites) and the Kaapvaal indicates that the diamondiferous mantle lithosphere beneath the Slave is chemically less depleted. Most notable are the almost complete absence of garnet inclusions derived from low-Ca harzburgites and a generally lower Mg-number of Slave inclusions. Geothermobarometric calculations suggest that Slave diamonds originally formed at very similar thermal conditions as observed beneath the Kaapvaal (geothermal gradients corresponding to 40–42 mW/m 2 surface heat flow), but the diamond source regions subsequently cooled by about 100–150 °C to fall on a 37–38 mW/m 2 (surface heat flow) conductive geotherm, as is evidenced from touching (re-equilibrated) inclusions in diamonds, and from xenocrysts and xenoliths. In the Kaapvaal, a similar thermal evolution has previously been recognized for diamonds from the De Beers Pool kimberlites. In part very low aggregation levels of nitrogen impurities in Slave diamonds imply that cooling occurred soon after diamond formation. This may relate elevated temperatures during diamond formation to short-lived magmatic perturbations. Generally high Cr-contents of pyrope garnets (inside and outside of diamonds) indicate that the mantle lithosphere beneath the Slave originally formed as a residue of melt extraction at relatively low pressures (within the stability field of spinelperidotites), possibly during the extraction of oceanic crust. After emplacement of this depleted, oceanic mantle lithosphere into the Slave lithosphere during a subduction event, secondary metasomatic enrichment occurred leading to strong re-enrichment of the deeper (>140 km) lithosphere. Because of the extent of this event and the occurrence of lower mantle diamonds, this may be related to an upwelling plume, but it may equally just reflect a long term evolution with lower mantle diamonds being transported upwards in the course of “normal” mantle convection.

Kankan diamonds (Guinea) I: from the lithosphere down to the transition zone

Peridotitic inclusions in alluvial diamonds from the Kankan region of Guinea in West Africa are mainly of lherzolitic paragenesis. Nevertheless, extreme Cr2O3 contents (max. 17 wt%) in some of the exclusively lherzolitic garnets document that the diamond source experienced a previous stage of melt extraction in the spinel stability field. This initial depletion was followed by at least two metasomatic stages: (1) enrichment of LREE and Sr and (2) introduction mainly of MREE–HREE and other HFSE (Ti, Y, Zr, Hf). The Ti- and HFSE-poor character of stage (1) points towards a CHO-rich fluid or carbonatitic melt, the high HFSE in stage (2) favour silicate melts as enriching agent. Eclogitic inclusions are derived from a large depth interval ranging from the lithosphere through the asthenosphere into the transition zone. The occurrence of negative Eu anomalies in garnet and clinopyroxene from both lithosphere and transition zone suggests a possible relationship to subducted oceanic crust. Lithospheric eclogitic inclusions are derived from heterogeneous sources, that may broadly be divided into a low-Ca group with LREE depleted trace element patterns and a high-Ca group representing a source with negative LREE–HREE slope that is moderately enriched in incompatible elements relative to primitive mantle. High-Ca inclusions of majoritic paragenesis are significantly more enriched in incompatible elements, such as in Sr and LREE. Calculated whole rock compositions require metasomatic enrichment even if a derivation from MORB is assumed.

Rare and unusual mineral inclusions in diamonds from Mwadui, Tanzania

Syngenetic diamond inclusions from the Mwadui kimberlite reveal that an unusually fertile section of lithospheric mantle beneath the Central African Craton was sampled. This is shown by a very high ratio of lherzolitic to harzburgitic garnet inclusions (1:2) and low Mg/Fe-ratios in olivine and orthopyroxene. Geothermometry applied to the peridotitic inclusions indicates disequilibrium between non-touching inclusion pairs to be common. Disequilibrium between garnet-olivine and garnet-orthopyroxene pairs suggests successive iron enrichment during diamond formation, e.g. leading to the presence of harzburgitic garnet and lherzolitic olivine in the same diamond. Apart from the dominant peridotitic inclusion suite (88%), rare eclogitic inclusions occur (2%) and a number of uncertain paragenesis. Two diamonds, one with eclogitic garnets with moderate pyroxene solid solution and the other with a single ferro-periclase inclusion, suggest the contribution of a small sub-lithospheric component. The finding of the association Fe-FeO-Fe3O4 in one single diamond indicates diamond formation over a large range of f O2 conditions, possibly along redox fronts. Steep compositional gradients may also be reflected by the joint occurrence of harzburgitic garnet and a SiO2-phase in the same diamond. Alternatively the formation of the SiO2-phase may be due to extreme carbonation of the peridotitic source. Further unusual findings include the exsolution of a silicate phase from magnetite inclusions, (i.e. primary solution of γ-olivine) and an ilmenite inclusion with an eskolaite (Cr2O3) component of 14.5 mol%, the latter together with harzburgitic paragenesis silicate inclusions.