Diamond formation in the system MgO–SiO2–H2O–C at 7.5 GPa and 1,600°C

Diamond crystallization has been studied in the SiO₂–H₂O–С, Mg₂SiO₄–H₂O–С and H₂O–С subsystems at 7.5 GPa and 1,600°C. We found that dissolution of initial graphite is followed by spontaneous nucleation of diamond and growth of diamond on seed crystals. In 15-h runs, the degree of graphite to diamond transformation [α = M_Dm/(M_Dm + M_Gr)100, where M_Dm is mass of obtained diamond and M_Gr mass of residual graphite] reached 100% in H₂O-rich fluids but was only 35–50% in water-saturated silicate melts. In 40-h runs, an abrupt decrease of α has been established at the weight ratio H₂O/(H₂O + SiO₂) ≤ 0.16 or H₂O/(H₂O + Mg₂SiO₄) ≤ 0.15. Our results indicate that α is a function of the concentration of water, which controls both the kinetics of diamond nucleation and the intensity of carbon mass transfer in the systems. The most favorable conditions for diamond crystallization in the mantle silicate environment at reliable PT-parameters occur in the fluid phase with low concentration of silicates solute. In H₂O-poor silicate melts diamond formation is questionable.     

新宁—灰汤断裂带花岗斑岩与煌斑岩脉的分布特征及其找矿意义

新宁一灰汤断裂南东盘,分布有数条北西向的花岗斑岩脉带,每条岩脉带内,产出有相同的矿床组合及类似的分带规律,构成一个北东向的多金属成矿带。成矿带北西部分为锑金成矿区,南东侧是铅锌铜钨成矿区。新宁—灰汤断裂与桃江—城步断裂控制的煌斑岩脉,具有明显区别,在某些地段具有钾镁煌斑岩主要特征,它对我省金刚石原生矿的突破,是一条重要线索与新途征。     

Geodynamic controls on diamond deposits: Implications for Australian occurrences

Conventional diamond exploration guidelines predict that economic diamond occurrences will be restricted to Archaean cratons, where the lithosphere is thick and cool, and diamond is the stable form of carbon in the lower portions of the lithosphere. However, Australia's current economic diamond deposits are not well predicted by these conventional exploration guidelines. Tomographic images show that Australia's economic diamond deposits lie at step changes in lithospheric thickness within dominantly cratonized Proterozoic provinces with thick (≥ 200 km) lithosphere. The thickest portions of the seismic lithosphere in Australia occur not under the major Archaean cratons, rather the central Proterozoic regions of the continent. We use a numerical code to show that such features are stable, and that the longevity of the diamond stability field is dependent on distance to the continent–ocean boundary, local depth of the chemical boundary layer (CBL), and proximity to changes in CBL depth. We also show that abrupt changes in lithospheric thickness focus lithospheric stress gradients, affecting melt migration paths, and that continental melt production is enhanced in regions adjacent to major cratons. Diamond pipes occur where conditions conducive to diamond stability and deep-seated alkaline volcanism (kimberlite or lamproite) occur simultaneously, and the common confluence of these factors at abrupt changes in lithospheric thickness marks them as potential exploration targets.     

Mantle processes in an Archean orogen: Evidence from 2.67 Ga diamond-bearing lamprophyres and xenoliths

The world's oldest diamond deposits occur in 2.67 Ga dikes and heterolithic breccias emplaced into greenstone belts of the Wawa and Abitibi Subprovinces, southern Superior Province, Canada. Thousands of white to yellow microdiamonds and macrodiamonds to 5 mm in width have been recovered by non-contaminating fusion techniques. The host rocks exhibit petrographic and compositional features that are characteristic of post-Archean minettes and spessartites of the calc-alkaline or shoshonitic lamprophyre clan. Based on chemical trends and petrographic evidence, host rocks that contain more than 16 wt.% MgO represent lamprophyre magmas that entrained cumulate olivine, probably at the base of the crust. Breccia bodies that are tens of metres wide at the two localities are somewhat atypical of late Archean lamprophyre occurrences in the Superior Province and owe their size to optimum conditions for magma ascent that were required to preserve the diamonds. Abundant altered ultramafic xenoliths occur in the host rocks. The majority of xenoliths studied (10 of 14) display uniform major element compositions similar to websterite cumulate suites derived from crystal fractionation processes at the base of post-Archean volcanic arcs. The xenoliths display highly variable trace element abundances that are characteristic of cryptic metasomatism associated with the flux of an oxidised fluid above a subduction zone.

The tectonic setting of the deposits and the nature of the host rocks indicate that the diamonds may be derived from the asthenospheric wedge and subducted slab at shallow depths (100 to 160 km) rather than the deep keels of Archean cratons associated with traditional diamond deposit types. Models of low-temperature Phanerozoic diamond formation in active subduction zones, or rapid uplift and emplacement of peridotite massif occurrences, can be adapted to the Archean deposits. The stability field of diamonds in most Phanerozoic subduction scenarios, however, may be too deep to be accessed by the lamprophyric magmas. In contrast, shallow subduction, as invoked for the distinctive occurrence of adakitic (slab-melt) type rocks in the southern Superior Province, could generate two different diamond stability windows at sufficiently shallow depths to account for their presence in lamprophyric magmas.

The multiple requirements imposed on Archean tectonic models by occurrences of diamonds in hydrous shoshonitic rock types (spessartite and minette lamprophyres), along with distinctively metasomatised xenoliths, strongly favour plate tectonic subduction models of orogeny. Evidence of slightly earlier mantle plumes, such as 2.7 Ga komatiites, only strengthens the need for a subduction-driven low-temperature thermal anomaly in the Archean mantle prior to lamprophyric magmatism.     

Pressure-induced phase transition in Mg0.8Fe0.2O ferropericlase

Combined X-ray powder diffraction, Mössbauer, and XANES spectroscopy in situ experiments revealed the transformation of cubic (Mg_0.8Fe_0.2)O ferropericlase to a rhombohedrally distorted phase at 35(1) GPa and room temperature. The Mössbauer spectroscopy results show that the rhombohedral distortion does not involve magnetic ordering. Combined with data from the literature, our results imply that the cubic to rhombodedral transition occurs in (Mg,Fe)O under conditions of non-hydrostatic stress over a wide range of composition (0.2≤x _Fe≤1).     

不同结构网囊特性的初步研究

为了测定不同拖网网囊结构对网囊的几何形状、同目张开、网目形状和拖网阻力的影响,设计和制作了4种网囊,一种是传统的菱形同目（DM）网囊,另三种是新型网囊：方形同目（SM）网囊,菱、方形网目混合二片式（DSM2）网囊和菱、方形同目混合四片式（DSM4）网囊。于1996年9月11～12日在中山大学试验水槽进行了同模试验,结果表明：1）各网囊阻力相差不大,且随拖速和渔获的增加而增大。2）新型网囊的几何形状和同目的张开及其形状均优于传统菱形网目网囊。3）空网时,各种网囊均呈圆筒状。当网囊中有了渔获后,传统菱形同目网囊的端部变为球状,除了球状部分的网目有较大张开外,其余同目张开很小,网囊使用直径也小,而新型网囊始终保持筒状,且网目充分张开,网囊使用直径也大。4）新型例翼的几何形状及使用直径和网目的张开及形状几乎不受拖速大小、有无渔获和渔获多少的影响。     

金刚石微粉的用途及其发展趋势

金刚石微粉是一种超硬精细磨料，在科学和工业上的应用越来越广泛。由于高新技术发展的需要，金刚石微粉的生产出现一些值得注意的发展趋势。     

PDC超硬复合材料的研究

在总结前人ＰＤＣ复合材料研究工作基础上，论述了ＰＤＣ材料复合途径及合成过程中几个关键技术问题，阐明了大直径优质ＰＤＣ复合材料合成中存在的问题及其应采取的措施。     

Petrology of the Renard igneous bodies: host rocks for diamond in the northern Otish Mountains region, Quebec

The Renard igneous bodies were discovered in late 2001 as part of a regional diamond exploration program launched by Ashton Mining of Canada and SOQUEM. Nine bodies have been discovered within a 2-km-diameter area, and are comprised of root zone to lower diatreme facies rocks including kimberlitic breccia, olivine macrocrystic hypabyssal material, and brecciated country rock with minor amounts of kimberlitic material. Many mineralogical and petrographic features are common to both kimberlite and melnoite, and strict assignment of the rocks as kimberlite is not possible with these criteria alone. Whole rock trace element compositions suggest a closer affinity to Group I kimberlite, with derivation from a garnet-bearing mantle. Exceptions to conventional classification of the rocks along petrographic or mineralogical lines may be due in part to assimilation of felsic country rock into the Renard magmas at the time of emplacement. The Renard magmas were emplaced into northeastern Laurentia at 630 Ma, when the supercontinent was undergoing a change from convergent margin magmatism to rifting, the latter being associated ultimately with the opening of the Iapetus ocean.     

Syngenetic inclusions of yimengite in diamond from Sese kimberlite (Zimbabwe) — evidence for metasomatic conditions of growth

Syngenetic inclusions of yimengite K (Cr, Ti, Mg, Fe, Al)₁₂O₁₉, a potassium member of the magnetoplumbite mineral group, have been recorded in an octahedral macrodiamond from the Sese kimberlite (50 km south of Masvingo, Zimbabwe). One yimengite inclusion carries lamellae of chromite suggesting peridotitic diamond paragenesis. The diamond and inclusions were studied in situ in a plate polished parallel to (011). Cathodoluminescence (CL) imaging has shown blue colour and octahedral zonation of the diamond, lack of cracks and the location of five yimengites in different growth zones. Nitrogen (N) contents (at. ppm) in the diamond determined by Fourier transform infrared spectroscopy (FTIR) steadily decrease from 576 (core) to 146 (rim). N aggregation (%1aB) is correspondingly 40% in the core and 30% in the rim. Hydrogen (H) content is high in the core, moderate in the intermediate and very high in the rim zones. Four yimengites were dated using the laser ⁴⁰Ar/³⁹Ar method. Three inclusions yielded total gas ages that agree with, or are younger than, or within error of, the Sese kimberlite eruption age (538±11 Ma) but may be compromised by gas loss. One inclusion, with the highest tapped interface gas yield, gave a total gas age of 892±21 Ma that is a likely minimum yimengite age. Time–T °C constraints from N aggregation systematics give a range of possible ages from kimberlite eruption date back to Archean and do not resolve the variable results of the ⁴⁰Ar/³⁹Ar dating. Compared with the published chemistry of yimengite from kimberlites, inclusions from the Sese diamond contain higher Al, Mg, and Sr and have lower concentration of Fe³⁺. The chondrite-normalised REE pattern of the yimengite shows enrichment in LREE and depletion in HREE, but LREE/HREE fractionations are lower than for lindsleyite–mathiasite series mantle titanates and rather similar to the REE concentrations in kimberlite and lamproite rocks. It is suggested that Sese yimengite formed in the lithospheric mantle from metasomatism of chrome spinel by a fluid rich in Ti, K, Ba and LREE.