Mineralogy and Petrology最新文献

MARID (mica-amphibole-rutile-ilmenite-diopside) rocks are a suite of mantle xenoliths that provide insight into metasomatic processes in the Earth’s lithospheric mantle. However, many aspects of MARID formation, including the identity of their parental melts, remain uncertain. We present the first set of whole rock Re-Os isotope data from four MARID xenoliths derived from the Kimberley kimberlite cluster, South Africa and combine these results with new in situ zircon U–Pb, Lu–Hf isotope results to provide insight on MARID petrogenesis. MARIDs have highly variable Os isotope signatures (γOs_i = -12 to 116) that can be explained by mixing between a “pure” MARID composition and lithospheric peridotite, clearly indicating MARID formation by melt-rock interaction and “hybridisation”. The highest γOs_i values indicate that their parental magmas are derived from a subduction-related source, stored in cratonic mantle lithosphere. Zircon U–Pb data from MARID sample AJE-2422 yields ²⁰⁶Pb/²³⁸U ages ranging from 86 to 126 Ma (n = 22) with modes at 90.6 Ma, 94.6 Ma, 110.1 Ma, and 124.2 Ma. Zircon Lu–Hf data shows a uniform, non-radiogenic Hf isotope signature (average εHf_i = -17.1 ± 0.9; n = 22) across the span recorded in U–Pb ages. This Hf isotope signature is distinct from typical kimberlites but is similar to South African lamproites and their magmatic source regions. We propose a model where zircons in MARID sample AJE-2422 formed via the passage of a lamproite-like melt coinciding with our oldest observed ages (~ 125 Ma).

We present the first study of 73 micro- and macro-diamonds and their inclusions from the Sequoia kimberlites, Central Lac de Gras kimberlite field, NWT. Nitrogen aggregation systematics show the presence of a diamond population that resided in the mantle at high temperatures (> 1250 ºC; 17% of all Type I diamonds), plus a high proportion of Type II diamonds (26%). Raman analysis of mineral inclusions in 24 diamonds reveals that the Sequoia kimberlites sampled both lithospheric (19 diamonds) and sublithospheric (5 diamonds) mantle sources. Olivine crystals coexisting with minerals characteristic of retrogressed sublithospheric assemblages display an extreme shift in Raman peak positions, indicating high pressure of entrapment. In one diamond, we observed the typical X-ray diffraction reflections of preserved ringwoodite, indicating the coexisting olivine identified by Raman is retrogressed. The preserved ringwoodite was stable in the transition zone. Lithospheric diamonds are of peridotitic affinity and the presence of clinopyroxene inclusions indicates sampling of possibly younger, less depleted portions of the mantle beneath Lac de Gras. Eight percent of the studied diamonds are fibrous and trap mineral micro-inclusions derived from peridotitic substrates. The diamond C and N isotope compositions are generally within typical mantle values, but the δ¹³C variability significantly exceeds the mantle range to both lower and higher values. Diamond internal variations in δ¹³C-δ¹⁵N exceeding the mantle range document the influence of subducted fluids in diamond formation. This first look at Sequoia diamonds underlines the need for exploration approaches that assess the presence of lherzolitic and sublithospheric-derived diamonds during indicator mineral assessment.

The term ‘kimberlite’ describes rocks that span a large mineralogical variety including enrichments in mica, carbonates, perovskite, spinel and/or ilmenite. The origin of these compositional variations is addressed here by comparing the petrography, mineral chemistry and bulk-rock as well as groundmass geochemistry of seven representative kimberlite samples (from Wesselton in South Africa; Karowe in Botswana; Diavik and Gahcho Kué in Canada; Majuagaa in Greenland, and Letšeng in Lesotho). These samples exhibit a broad range of mineral and bulk geochemistry covering the whole kimberlite spectrum. Bulk-groundmass compositions are variously enriched in Si, K, Ti, CO₂ and H₂O depending on the dominant groundmass mineralogy – e.g., high K in mica-rich samples. Interaction with mica and ilmenite-bearing lithospheric mantle appears to be the driving factor of K (± Al) and Ti enrichment, respectively. Degassing controls CO₂, and higher SiO₂ in the melt derived from assimilation of lithospheric pyroxenes leads to a decrease in CO₂ solubility. Serpentinization by deuteric and/or crustal fluids governs H₂O concentrations, generally exceeding the H₂O solubility in kimberlitic melts at upper crustal conditions. Even where the groundmass composition closely approximates predicted kimberlitic melts such as at Majuagaa, the low contents of Na require substantial loss of alkalis via fluids during ascent and emplacement. Thus, compositional variations in erupted kimberlites reflect the combination of asthenospheric source variability, lithospheric assimilation, crystallization, degassing and interaction with deuteric and crustal fluids.

A second generation diamond-in-water laser ablation system combined with mass spectrometry measurements is presented for trace elements and radiogenic isotopic analyses of microinclusion-bearing diamonds. Ablation was conducted using a Nd: YV04 laser (532 nm, 136 µJ/pulse, 25 ns pulse duration, 2000 Hz repetition rate) in a closed ultra-clean glass cuvette filled with milli-Q water (18.2 MΩ cm). Multiple experiments indicate a highly stable and precise ablation process that proceeded at an average rate of 0.75 ± 0.41 mg/h. Triple Quadrupole inductively coupled plasma mass spectrometer (ICP-MS) trace element analyses of the ablated material reveal primitive mantle normalized patterns that are similar to previously analyzed microinclusion-bearing diamonds. Results comparable with previous ablation analyses of individual diamonds, and results of BHVO-2G and BCR-2G standards determined using this new ablation technique confirm its accuracy. Furthermore, the reference material results provide a means to estimate the uncertainty for the elemental concentrations in the diamonds, suggesting that it is generally below 10% for most elements of interest, and likely under 15% for all elements. The new ablation system produces enough material for successful Sr, Nd, and Pb isotope analyses by combined wet-chemistry and thermal ionization mass spectrometer (TIMS) using 10¹¹ or 10¹³ Ω resistors. The Sr–Nd–Pb isotope values of BHVO-2G and BCR-2G ablated, purified using ion chromatography and measured by the same technique validate its accuracy. Low total procedural blank levels (Sr average of 71 ± 21 pg; Nd of 0.42 ± 0.25 pg Nd; Pb of 9.4 ± 3.6 pg) have little impact on the measured isotope values, but a blank correction can be applied if necessary.

This review paper discusses the systematic diamond exploration undertaken over most of the country by De Beers between the mid-1950s and the early-1980s. Seven kimberlite fields and one lamproite field were discovered, the majority of which are diamond-bearing. Most of the fields include both pipes and dykes. The largest lamproite pipe is 45 ha and the largest kimberlite is 23 ha in surface area. The highest concentration of diamonds was found in surface gravels over one of the Kapamba lamproites (8.3 carats from 28 m³). Several of the kimberlite fields were emplaced into Karoo Supergroup sediments and by inference are younger than 250 Ma. The age of lamproite emplacement at Kapamba has been determined at ca.178 Ma. Diamonds have been recorded from many parts of the country that cannot be explained by the known fields. Thus, in northeastern Zambia early work on the Bangweulu Block produced the largest single diamond found in Zambia (1.362 carats) from a stream draining from the Democratic Republic of the Congo near Lake Tanganyika. In Western Zambia many small alluvial diamonds, including some of gem or near gem quality, and some indicator minerals have been recovered away from the known kimberlites. A drilling programme located kimberlitic garnet-bearing Lower Kalahari fluvial beds, confirming a complex detrital history in this part of the country. Since De Beers withdrew from Zambia in the early 1980s a number of other companies have carried out diamond exploration, focussed in the west of the country. To date no economic discoveries have been made.

In the early 1900s diamonds were discovered along the Atlantic coast of southern Africa in Cainozoic alluvial deposits. Most of these were derived from erosion of Mesozoic primary sources on the Kaapvaal Craton. Previous attempts to quantify the amount of diamonds that were eroded from inland lamproites variety Kaapvaal (lamproites vK), and kimberlites varied between 10,000 and 3,000 Mct (million carats). Based on new geomorphological techniques, the post-Gondwana erosion rates and landscape denudation of southern Africa are reviewed. The Early and Middle Cretaceous were peak periods of landscape denudation, which almost coincide with the emplacement of lamproites vK and kimberlites, respectively. This study demonstrates that the eroded lamproites vK and kimberlites, since emplacement, have released 852.7 and 1,336.4 Mct respectively. Assuming that 50% of the carats was lost through breakage during transport and that some 1% was retained in fluvial terraces, it leaves 1,074.6 Mct that were delivered to the West Coast. The palaeo-Orange/Vaal river system has evolved spatially, supplying diamonds via different exit points along the coast. Initially, the transport of diamonds was by the Karoo River delivering some 209.5 Mct eroded from the 120 Ma old lamproites vK to the coast near the present Olifants River. Before the 85 Ma kimberlite event the river outfall to the Atlantic Ocean shifted to the general area of today’s Lower Orange River. This outlet received 865.1 Mct from these Cretaceous lamproites vK and kimberlites, but there are also indications that a small number of diamonds from older primary sources are present.

The study reports textures and compositions of uraninite and brannerite from the Mohuldih uranium deposit of the Singhbhum Shear Zone. Three types of texturally varied uraninite are identified: (i) pre-tectonic (U_G−1), (ii) syn to post-tectonic (U_G−2), and (iii) inclusions (U_G−3). Based on UO₂ and PbO composition, these are further divided into five subgroups (U_G−1a, U_G−2a, U_G−1b, U_G−2b, and U_G−3). The negative correlation between the rare-earth elements including Y (ΣREE₂O₃ + Y₂O₃) and UO₂ of U_G−1a and U_G−2a uraninite indicates substitution of REE and Y for U⁴⁺ in the structure of these grains during primary crystallization, whereas U_G−1b and U_G−2b resulted from REE + Y + U-enrichment during secondary fluid-induced alteration. Compositional trends further reveal post-crystallization incorporation of Na, Si, K, Ca, Ti, and Fe, replacing radiogenic Pb in the structure of uraninite. The U-Th-Pb_Total dating of uraninite yields two clusters: older (~ 1.71−1.64 Ga; preserved by U_G−1a and U_G−2a) and younger (~ 0.93 Ga; U_G−1b and U_G−2b). Additionally, the extrapolated best-fit linear trend for uraninite in U_G−3 population preserves chemical dates ranging from ~ 1.71 to ~ 1.64 Ga. The ~ 1.64 Ga represents a U + Y + HREE + Ca + Fe hydrothermal event, resulting in the precipitation of uraninite, followed by dissolution-reprecipitation of the mineral along the grain boundary and fractures at ~ 0.93 Ga. The oldest mesured date (~ 1.71 Ga) is attributed to partial Pb-loss by older uraninite during the ~ 1.64 Ga hydrothermal event, with no evident geological significance in the area. The dissolution-reprecipitation of the early-formed uraninite also resulted in the formation of brannerite and ilmenite during the ~ 0.93 Ga event. 

The results of a comprehensive study of the Ore Horizon 330 (OH330) sulfide platinum group elements (PGE)-Cu-Ni deposit are presented. OH330 is located within the Sopcha intrusion orthopyroxenites of the layered Monchegorsk pluton (Monchepluton) in the Kola Region (Russia). It is a sill-like body, 3300 m long, 1200 m wide, and 4–6 m thick. The main rock-forming minerals of OH330 were olivine (Fo_87–84) and orthopyroxene (En_84–83). Light rare-earth element (LREE) fractionation is typical for all OH330 rocks, with the (Ce/Sm)_N (where the subscript N indicates normalization to the primitive mantle) values of 1.21 in dunite, 1.33–1.86 in harzburgite, and 1.86–2.15 in orthopyroxenite. The trace elements exhibited positive U and Ta anomalies in dunite and harzburgite and negative Nb and Ta anomalies in orthopyroxenite. The following crystallization parameters of the OH330 rocks were determined: liquidus temperature, 1220 °C; solidus temperature, 1190 °C; and pressure, 2.4 kbar. The SHRIMP (sensitive high-resolution ion microprobe) U–Pb age of magmatic zircon from the OH330 orthopyroxenite is 2492.5 ± 4.1 Ma. This indicates that OH330 was younger than the rocks of the Monchepluton ultramafic subchapter (about 2506 Ma). The calculated parental melt of OH330 has a boninite-like composition with an MgO content of 14 wt% and is contaminated with crustal material. The formation of OH330 occurred as a result of additional magma injection associated with the activation of a mantle diapir. Thus, the OH330 body is probably the ultramafic cumulate part of an intrusion associated with Paleoproterozoic continental rifting.

The presence of a diamond existing freely in an encasing diamond has invoked origins involving the escape of a gas from that diamond, or the dissolution of an impure diamond layer by melts or fluids, access in the second case, being gained by either the dissolution of diamond along dislocations or by breakage. The find of a diamond in which there is an octahedral garnet shell between two stages of diamond growths discerns a more realistic origin for this rare type of diamond. That origin involves the diamond coming to Earth’s surface, when the garnet partially breaks the outside diamond before being dissolved during the acid cleaning processes prior to diamond evaluation and sale. The inner diamond then moves freely in the cavity created.

Despite the economic significance and scientific value of sublithospheric (superdeep) diamonds, useful indicators of their presence in a kimberlite body via traditional indicator mineral approaches remain elusive. The DO-27 kimberlite on the Slave craton hosts abundant (~ 25%) superdeep diamonds, providing an excellent opportunity to examine this issue. Elemental compositions plus thermobarometry of mantle garnets from DO-27 reveal a high fraction (~ 39%) of high-Ti garnets (classes G1 and G11) predominantly derived from the lowermost lithospheric root (~ 170 to 220 km). High-Ti garnets are significantly more abundant at DO-27 than in other kimberlites from the Slave craton where superdeep diamonds are scarce or absent. New density calculations show G11 garnet bearing peridotites are as buoyant as other parts of the lithosphere. Our model invokes the transport of superdeep diamonds to the base of the lithosphere via buoyant, deeply subducted harzburgites rising from transition zone depths. Once accreted to the lithosphere, these protoliths become re-fertilised by Ti–rich asthenosphere-derived melts during periods of melt-rock interaction preceding phases of kimberlite activity. Enhanced sampling of these accreted lithologies in the lowermost lithospheric mantle by kimberlite transports anomalously high proportions of high-Ti garnets along with superdeep diamonds, although there is no direct genetic link between the two. The association between a high proportion of high-Ti garnets and superdeep diamonds at DO-27 could potentially serve as a mineral indicator approach for the presence of superdeep diamonds in other kimberlites.