Trace element abundances in andesites

Data for 8 major and 45 trace elements in 6 central North Island andesites and 8 Japanese andesites ofKuno’s hypersthenic series are presented. The andesite compositions are compared with that of average continental crustal material, and differences and similarities are noted. The rare earth abundance patterns are similar to those of well mixed crustal material. Present data for the absolute abundances of Rb, K, Ba, U and Th in particular appear to be too low for derivation of andesitic magma by mixing of average basaltic and granitic material. Implications of the data for the origin of andesite magma and for continental growth are discussed.     

Streamflow routing in the indian arid zone

A lumped model for streamflow routing in arid ephemeral channels has been developed. The governing equations for movement of flood waves subjected to transmission losses are simplified through a time averaging process to develop an ordinary differential equation describing transmission losses as a function of distance, inflow, channel width, time parameters of flow and effective hydraulic conductivity. The resulting equation has an analytical solution and simulates runoff volume and peak discharge rates for individual storm events. The outflow hydrograph is fairly well approximated with a triangular approximation. The model is simplified and constructed to require a minimum of observed data for calibration. It can also be used for ungauged basins in arid regions through parameterization.     

A new potassium-iron-nickel sulphide from a nodule in kimberlite

A new K-rich sulphide phase is described from a clinopyroxene-ilmenite intergrowth from the Frank Smith kimberlite diatremes. The mineral occurs as ovoid blebs, which also contain pyrrhotite and pentlandite, in close spatial association with the ilmenite lamellae, and as individual grains in cracks in the clinopyroxene host. Chemical analyses from twelve grains show the following ranges of composition (wt.%): K, 8.12–13.01; Cu, 1.24–2.99; Fe, 38.47–41.76; Ni, 11.65–15.04; S, 32.09–34.34. Electron diffraction data indicate a primitive cubic unit cell,a = 10.29 ± 0.03Å, with no systematically absent reflections. Three models for the origin of this sulphide, liquid immiscibility, multiple exsolution, and metasomatism, are considered. The discovery of potassic sulphide in a kimberlite nodule has important implications, not only as a potential source of K in the upper mantle, but also for those models which propose removal of K and S from the lower mantle into the core.     

Faeroe-Iceland plume: Rare-earth evidence

Rare-earth abundances through the 3000-m thick Faeroes Plateau basalt monitor the Faeroe-Iceland plume activity with time. An abrupt change from light Re-enriched to depleted patterns is observed naer the boundary of the middle and upper series of the Faeroes Plateau basalt. The discontinuity seems to reflect a change of volcanic regime from plume-derived to more akin to oceanic type. The change also coincides with field evidence for beginning of subsidence of the plateau. Using existing spreading rate history for the North Atlantic during the 50–60-m.y. B.P. period, which is one of deceleration, the change of volcanic regime suggests a decline of intensity of the Faeroe-Iceland plume during the late part of the period. Rising blobs, rather than a continuous plume, appears preferable for modeling the temporal plume activity.     

On the geodynamic setting of kimberlite genesis

The emplacement of kimberlites in the North American and African continents since the early Palaeozoic appears to have occurred during periods of relatively slow motion of these continents. The distribution of kimberlites in time may reflect the global pattern of convection, which forces individual plates to move faster or slower at different times. Two-dimensional numerical experiments on a convecting layer with a moving upper boundary show two different regimes: in the first, when the upper boundary velocity is high, heat is transferred by the large-scale circulation and in the second, when the upper boundary velocity is lower, heat is predominantly transferred by thermal plumes rising from the lower boundary layer. For a reasonable mantle solidus, this second regime can give rise to partial melting beneath the moving plate, far from the plate boundaries. The transition between these modes takes place over a small range of plate velocities; for a Rayleigh number of 10⁶ it occurs around 20 mm yr^?1. We suggest that the generation of kimberlite magmas may result from thermal plumes incident on the base of a slowly moving plate.     

Fragmentation in kimberlite: products and intensity of explosive eruption

The explosive eruption of kimberlite magma is capable of producing a variety of pyroclast sizes, shapes, and textures. However, all pyroclastic deposits of kimberlite comprise two main types of pyroclasts: (1) pyroclasts of kimberlite with or without enclosed olivine crystals and (2) olivine crystals which lack coatings of kimberlite. Here, we propose two hypotheses for how kimberlite magmas are modified due to explosive eruption: (1) olivine crystals break during kimberlite eruption, and (2) kimberlite melt can be efficiently separated from crystals during eruption. These ideas are tested against data collected from field study and image analysis of coherent kimberlite and fragmental kimberlite from kimberlite pipes at Diavik, NT. Olivines are expected to break because of rapid pressure changes during the explosive eruption. Disruption of kimberlite magma, and pyroclast production, is driven by ductile deformation processes, rather than by brittle fragmentation. The extent to which melt separates from olivine crystals to produce kimberlite-free crystals is a direct consequence of the relative proportions of gas, melt and crystals. Lastly, the properties of juvenile pyroclasts in deposits of pyroclastic kimberlite are used to index the relative intensity of kimberlite eruptions. A fragmentation index is proposed for kimberlite eruption based on: (a) crystal size distributions of olivine and on (b) ratios of selvage-free olivine pyroclasts to pyroclasts of kimberlite with or without olivine crystals.     

Evaluation of burst liability in kimberlite using support vector machine

Due to the complex mechanisms of rockburst, there is no current effective method to reliably predict these events. A statistical learning method, support vector machine (SVM), is employed in this paper for kimberlite burst prediction. Four indicators \(\sigma_{\theta } ,\sigma_{c} ,\sigma_{t} ,W_{\text{ET}}\) are chosen as input indices for the SVM, which is trained using 108 groups of rockburst cases from around the world. Data uniformization is used to avoid negative impact of differing dimensions across the original data. Parameter optimization is embedded in the training process of the SVM to achieve optimized predictive ability. After training and optimization, the SVM reaches an accuracy of 95% in rock burst prediction for validation samples. The constructed SVM is then employed in kimberlite burst liability evaluation. The model indicated a moderate burst risk, which matches observed instances of rockburst at a diamond mine in north Canada. The SVM method ignores the focus on rockburst mechanisms, instead relying on representative indicators to develop a predictive model through self-learning. The prediction results show an excellent accuracy, which means this method has a potential application in rockburst prediction.     

Nitrogen abundances and isotopic compositions in stony meteorites

Nitrogen contents range from a few parts per million in ordinary chondrites and achondrites to several hundred parts per million in enstatite chondrites and carbonaceous chondrites. Four major isotopic groups are recognized: (1) C1 and C2 carbonaceous chondrites have δ¹⁵N of+30to+50%.; (2) enstatite chondrites have δ¹⁵N of?30to?40‰; (3) C3 chondrites have low δ¹⁵N with large internal variations; (4) ordinary chondrites have δ¹⁵N of?10to+20‰. The major variations are primary, representing isotopic abundances established at the time of condensation and accretion. Secondary processes, such as spallation reactions, solar wind implantation and metamorphic loss may cause small but observable isotopic variations in particular cases. The large isotopic difference between enstatite chondrites and carbonaceous chondrites cannot be accounted for by equilibrium condensation from a homogeneous nebular gas, and requires either unusually large kinetic effects, or a temporal or spatial variation of isotopic composition of the nebula. Nitrogen isotopic heterogeneity in the nebula due to nuclear processes has not been firmly established, but may be required to account for the large variations found within the Allende and Leoville meteorites. The unique carbonaceous chondrite, Renazzo, has δ¹⁵N of+170%., which is well beyond the range of all other data, and also requires a special source. It is not yet possible, from the meteoritic data, to establish the mode of accretion of nitrogen onto the primitive Earth.     

Azores mantle blob: Rare-earth evidence

Rare earths (RE) in basalts erupted within the rift of the Mid-Atlantic Ridge show a progressive change from light-RE enriched to depleted patterns from the Azores Platform (40°N) down to 33°30′N. South, the pattern remains light-RE depleted as along other “normal ridge” segments. A progressive increase in chemical variability of the basalts towards the Azores is also noted.The latitudinal RE profile and corresponding ΣFeO/ΣFeO + MgO variations, together, indicate that the origin of these basalts cannot be accounted for simply by considering variable extents of partial melting of a single mantle source and subsequent fractional crystallization during the ascent of the magmas. These two processes produce only second-order effects on the RE patterns. The data requires the presence of a distinct, light-RE richer, mantle source beneath the Azores Platform relative to that of south of 33°30′N and an intermediate zone where both mantle types mix. The relative contribution of the Azores mantle source to the mix appears to decrease fairly regularly southward along the ridge and becomes negligible at 33°30′N. Increasing chemical variability of the basalts towards the Azores is probably caused by correspondingly larger extent of fractional crystallization at shallow depth, and/or greater variability in the extent of partial melting, apparently subsequent to, and superimposed on the mixing of the two mantle sources.The combined morphological, geophysical and RE evidence along the profile are consistent with a model suggesting upwelling of a major blob (plume) under the Azores Plateau; and reveal the present extent of the blob's overflow and mixing with the asthenosphere depleted in large ionic lithophile trace elements. The influence of the Azores blob is geochemically detectable up to 1000 km southwestward beneath the ridge axis.     

Geology of the BK9 kimberlite (Damtshaa,Botswana): implications for the formation of dark volcaniclastic kimberlite

The BK9 kimberlite consists of three overlapping pipes. It contains two dark varieties of massive volcaniclastic kimberlite, informally termed dark volcaniclastic kimberlite (DVK). DVK(ns) is present in the north and south pipes and is interbedded with lenses of basalt breccia at the margins of the pipes. DVK(c) is present within the central pipe where it is overlain by a sequence of basalt breccias with interbedded volcanogenic sediments. The features observed within the DVK units of the BK9 kimberlite provide strong evidence for gas fluidisation of the accumulating pyroclastic material. These include the massive interior of the pipes, marginal epiclastic units, well-dispersed country-rock xenoliths and small-scale heterogeneities in lithic clast abundance. The upper portions of the central pipe provide a record of the transition from pyroclastic eruption and infill to passive epiclastic infilling of the crater, after the eruption has ceased. The wall-rock of the BK9 kimberlite dips inwards and is interpreted as post pipe-fill subsidence of the adjacent country rock. The two DVK units contain interstitial, silt-sized pyroclasts. The DVK(ns) has a higher fraction of former melt and displays evidence of incipient welding, as a result of differences in eruption dynamics. These units demonstrate that whilst DVK is comparable in many respects to MVK and forms part of a spectrum of volcaniclastic rocks formed by fluidisation, it differs in frequently containing silt-sized particles and including agglutinated and welded varieties with a high melt fraction. The DVK varieties, studied here, also have a distinctive hydrothermal assemblage, resulting from the abundance of low-silica accidental lithic clasts. Both the hydrothermal alteration and the abundance of silt-sized particles contribute to the DVKs distinctive dark colour.     

The palaeomagnetism of some Upper Cretaceous kimberlite occurrences in South Africa

Palaeomagnetic results are reported from the De Beers, DuToitspan and Wesselton kimberlite pipes (86 ± 3Ma) at Kimberley and from the Finsch and Koffyfontein pipes. The latter are both less than 150 km from Kimberley and although undated are geologically correlated with the Kimberley pipes. Eighty blocks, oriented without recourse to magnetic methods, have been collected from the five pipes. The samples comprise kimberlite, accidental inclusions of widely varying lithologies and wall rock thus enabling inclusion and baked contact tests to be performed. Extensive alternating field and thermal demagnetization experiments show that the magnetization of the kimberlite is primary. Secular variation is not averaged out in any one pipe (with the possible exception of Finsch) and it is suggested the three Kimberley pipes were emplaced simultaneously. The mean pole position of the five pipes (57.2°E, 58.2°S withK = 25.8 andA₉₅ = 15.3°) is believed to be a good estimate of the palaeomagnetic pole86 ± 3Ma ago. It is now possible to state that the African Mesozoic palaeomagnetic pole remained essentially in the same position until at least86 ± 3Ma ago.     

Emplacement temperatures of pyroclastic and volcaniclastic deposits in kimberlite pipes in southern Africa

Palaeomagnetic techniques for estimating the emplacement temperatures of volcanic deposits have been applied to pyroclastic and volcaniclastic deposits in kimberlite pipes in southern Africa. Lithic clasts were sampled from a variety of lithofacies from three pipes for which the internal geology is well constrained (the Cretaceous A/K1 pipe, Orapa Mine, Botswana, and the Cambrian K1 and K2 pipes, Venetia Mine, South Africa). The sampled deposits included massive and layered vent-filling breccias with varying abundances of lithic inclusions, layered crater-filling pyroclastic deposits, talus breccias and volcaniclastic breccias. Basalt lithic clasts in the layered and massive vent-filling pyroclastic deposits in the A/K1 pipe at Orapa were emplaced at >570°C, in the pyroclastic crater-filling deposits at 200–440°C and in crater-filling talus breccias and volcaniclastic breccias at <180°C. The results from the K1 and K2 pipes at Venetia suggest emplacement temperatures for the vent-filling breccias of 260°C to >560°C, although the interpretation of these results is hampered by the presence of Mesozoic magnetic overprints. These temperatures are comparable to the estimated emplacement temperatures of other kimberlite deposits and fall within the proposed stability field for common interstitial matrix mineral assemblages within vent-filling volcaniclastic kimberlites. The temperatures are also comparable to those obtained for pyroclastic deposits in other, silicic, volcanic systems. Because the lithic content of the studied deposits is 10–30%, the initial bulk temperature of the pyroclastic mixture of cold lithic clasts and juvenile kimberlite magma could have been 300–400°C hotter than the palaeomagnetic estimates. Together with the discovery of welded and agglutinated juvenile pyroclasts in some pyroclastic kimberlites, the palaeomagnetic results indicate that there are examples of kimberlites where phreatomagmatism did not play a major role in the generation of the pyroclastic deposits. This study indicates that palaeomagnetic methods can successfully distinguish differences in the emplacement temperatures of different kimberlite facies.     

Comparison of electric and elastic parameters of indian precambrian rocks

Summary Examination of the relation between electric resistivity, compressional wave velocity and density of Indian Precambrian rocks is presented.

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The abundances of zirconium and hafnium in the solar system

The concentrations of zirconium and hafnium have been determined in Orgueil, Murchison, Allende, Bruderheim and Alais by RNAA. The mean Zr/Hf weight ratio in the first four of these meteorites is 31.3 ± 2.2 indicating no major fractionation of Zr from Hf. Alais contains anomalously high amounts of many refractory lithophile elements, including Zr and Hf. Orgueil contains 3.1 ppm Zr and 0.11 ppm Hf, corresponding to 9.0 and 0.16 atoms, respectively, relative to 10⁶ Si atoms.     

Uranium-lead abundances and isotopic studies in the chondrites Richardton and Farmington

²⁰⁷Pb/²⁰⁴Pb-²⁰⁶Pb/²⁰⁴Pb whole meteorite isochrons for Richardton (H5) and Farmington (L5) are presented and give Pb-Pb ages of 4.545 ± 0.010 and 4.620 ± 0.010 Ga respectively (errors ± 2σ). The Pb-Pb isochron for Farmington passes below the Can?on Diablo troilite composition, which may therefore not be the initial Pb composition for this meteorite.All samples show an apparent excess radiogenic lead for single-stage (closed-system) evolution when Can?on Diablo troilite is used for the initial lead composition. Evidence is presented to show that the apparent excess Pb cannot be explained by terrestrial contamination. There is no unique isotopic composition for initial lead that yields concordant ages at 4.55 Ga for all samples for either meteorite. The data likewise cannot be reconciled to Can?on Diablo initial lead through any of the conventional two- and three-stage evolution models.The apparent excess Pb, with respect to a Can?on Diablo troilite composition and a single-stage closed-system history, and the apparent inhomogeneous initial Pb isotopic compositions, appear to be real. This may be an indication that the U-Pb systems in these meteorites are disturbed, but this disturbance cannot be described consistently by any of the conventional episodic evolution models.     

Petrogenetic implications of uranium abundances in volcanic rocks from southern Peru

The content of uranium of andesitic rocks of southern Peru varies according to the distance from the Peru-Chile trench, as has been observed for other trace elements geochemically associated with K. The highest contents found in these rocks may exceed the estimated U content of the upper crust. This enrichment may be related to a variable degree of partial melting and/or crustal contamination of the same source material.     

Helium isotopic variability within single diamonds from the Orapa kimberlite pipe

The distribution and isotopic composition of helium has been measured in a suite of well-characterized one-carat diamonds from the Orapa kimberlite, Botswana. Crushing of the diamonds in vacuo indicates that most of the helium is contained by the matrix (generally greater than 90%), rather than by the inclusions. Step-heating experiments, performed on inclusion-free fragments remaining after crushing, indicate that the³He/⁴He ratio is variablewithin individual diamonds. The fragments, as small as 10 mg, were heated in two timed steps, both at 2000°C. In every case, lower³He/⁴He ratios are observed in the first graphitization step (0.05–3 × atmospheric), while the last heating step releases helium with systematically higher³He/⁴He ratio (30–80 × atmospheric). We suggest that this internal isotopic variability is the result of stepwise graphitization: the first heating step initiates graphitization, which nucleates around defects, and the second heating step graphitizes the relatively defect-free regions of the diamond. The³He/⁴He ratio measured, using the partial graphitization technique, differs by up to a factor of 100 within a single specimen. The inclusion-free fragments release small quantities of helium below 2000°C, which suggests that helium release is obtained only by graphitization. The³He contents of the monocrystalline diamonds are relatively constant (at 3 × 10⁻¹³ cm³ STP/gram) and indicate that most of the isotopic variability is due to radiogenic⁴He. The variations in⁴He content are either related to zoning of Th and U in the diamonds (i.e., in-situ decay), to zoning of inherited⁴He, or to implantation of α-particles from a Th and U rich environment (i.e., kimberlite). Because the Orapa diamonds were mined from roughly 40 m depth in the kimberlite, spallation reactions from cosmic ray interactions are not a significant source of³He. However, calculations based on the age of the kimberlite (90 m.y.) and reasonable Th and U abundances suggest that most of the³He in the Orapa diamonds could be produced by⁶Li(n, α)T in the diamond. Although this may not be true of all diamonds, nuclear reactions in the crust and mantle (including spallation reactions at the surface) can explain many of the high³He/⁴He ratios previously reported for diamonds.     

Noble gas elemental and isotopic abundances in deep-sea trenches in the western Pacific

Noble gas elemental and isotopic abundances were measured in seven deep-sea water samples from five different sampling sites in the Nankai Trough, the Japan Trench and the Kuril Trench. The samples were obtained by the manned submersible “Nautile”. Most of the sampling sites are associated with clam colonies and/or fluid venting. Excesses both in³He/⁴He ratio and He concentration are observed in a seawater sample collected a few kilometers off the clam colonies which were found at a depth of 3830 m at the mouth of the Tenryu Canyon. Concentrations of noble gases (Ne, Ar, Kr and Xe) in this sample show progressive depletion from Ne to Xe relative to those in 1°C air-saturated seawater, which can be attributed to mixing of hot water ( 15°C) with cold ambient water ( 1°C). Isotopic compositions of Ne, Ar, Kr and Xe in this sample are atmospheric. These observations may reflect venting of hot pore water around the Tenryu Canyon. All the other samples show a significant excess in concentration of all noble gases relative to 1°C air-saturated seawater and the isotopic compositions are atmospheric. This excess of noble gas concentrations may appear to be air contamination in the samples. However, results of hydrocarbon analyses of the Kaiko samples imply that such large amount of air contamination is improbable. Decomposition of gas hydrate in deep-sea sediments is a more likely explanation for the observed excess of noble gas concentration.     

The upper mantle structure of the yakutian kimberlite province,eastern Russia

Deep seismic sounding in the region of the Mirnyi kimberlite field indicates that the boundary velocity of the uppermost mantle is elevated (v _b=8.6–8.8 km/sec) and extremely variable near the Mir kimberlite pipe. These velocity heterogeneities are probably associated with the kimberlite magmatism and may be useful in the identification of other kimberlite fields.