Sulfide inclusions in diamonds from the 90-Ma Jagersfontein kimberlite, intruded into the southern margin of the Kaapvaal
Craton, were analyzed for their Re–Os isotope systematics to constrain the ages and petrogenesis of their host diamonds. The
latter have δ13C ranging between −3.5 and −9.8‰ and nitrogen aggregation states (from pure Type IaA up to 51% total N as B centers) corresponding
to time/temperature history deep within the subcontinental lithospheric mantle. Most sulfides are Ni-poor ([Ni + Co]/Fe = 0.05–0.25
for 15 of 17 inclusions), have elevated Cu/[Fe + Ni + Co] ratios (0.02–0.36) and elemental Re–Os ratios between 0.5 and 46
(12 of 14 inclusions) typical of eclogitic to more pyroxenitic mantle sources. Re–Os isotope systematics indicate two generations
of diamonds: (1) those on a 1.7 Ga age array with initial 187Os/188Os (187Os/188Osi) of 0.46 ± 0.07 and (2) those on a 1.1 Ga array with 187Os/188Osi of 0.30 ± 0.11. The radiogenic initial Os isotopic composition for both generations of diamond suggests that components with
high time-integrated Re–Os are involved, potentially by remobilization of ancient subducted oceanic crust and hybridization
of peridotite. A single sulfide with higher Os and Ni content but significantly lower 187Os/188Os hosted in a diamond with less aggregated N may represent part of a late generation of peridotitic diamonds. The paucity
of peridotitic sulfide inclusions in diamonds from Jagersfontein and other kimberlites from the Kaapvaal craton contrasts
with an overall high relative abundance of diamonds with peridotitic silicate inclusions. This may relate to extreme depletion
and sulfur exhaustion during formation of the Kaapvaal cratonic root, with the consequence that in peridotites, sulfide-included
diamonds could only form during later re-introduction of sulfur. 相似文献
We assess Holocene environmental change at alpine Lake Njulla(68°22N, 18°42E, 999 m a.s.l.) innorthernmost Sweden using sedimentary remains of chironomid head capsules anddiatoms. We apply regional calibration sets to quantitatively reconstruct meanJuly air temperature (using chironomids and diatoms) and lake-water pH(using diatoms). Both chironomids and diatoms infer highest temperatures(1.7–2.3°C above present-day estimates, includinga correction for glacio-isostatic land up-lift by0.6°C) during the early Holocene (c.9,500–8,500 cal. yrs BP). Diatoms suggest a decreasing lake-waterpH trend (c. 0.6 pH units) since the early Holocene. Usingdetrended canonical correspondence analysis (DCCA), we compare the Holocenedevelopment of diatom communities in Lake Njulla with four other nearby lakes(Lake 850, Lake Tibetanus, Vuoskkujávri, Vuolep Njakajaure) locatedalong an altitudinal gradient. All five lakes show similar initial DCCA scoresafter deglaciation, suggesting that similar environmental processes such ashigh erosion rates and low light availability associated with high summertemperature appear to have regulated the diatom community, favouring highabundances of Fragilaria species. Subsequently, the diatomassemblages develop in a directional manner, but timing and scale ofdevelopment differ substantially between lakes. This is attributed primarily todifferences in the local geology, which is controlling the lake-waterpH. Imposed on the basic geological setting, site-specific processessuch as vegetation development, climate, hydrological setting andin-lake processes appear to control lake development in northernSweden. 相似文献
Coordinated geochemical and hydrological calculations guide our understanding of the composition, fluid flow patterns, and thermal structure of near-ridge oceanic crust. The case study presented here illustrates geochemical and thermal changes taking place as oceanic crust ages from 0.2 to 1.0 Myr. Using a finite element code, we model fluid flow and heat transport through the upper few hundred meters of an abyssal hill created at an intermediate spreading rate. We use a reaction path model with a customized database to calculate equilibrium fluid compositions and mineral assemblages of basalt and seawater at 500 bars and temperatures ranging from 150 to 400°C. In one scenario, reaction path calculations suggest that volume increases on the order of 10% may occur within portions of the basaltic basement. If this change in volume occurred, it would be sufficient to fill all primary porosity in some locations, effectively sealing off portions of the oceanic crust. Thermal profiles resulting from fluid flow simulations indicate that volume changes along this possible reaction path occur primarily within the first 0.4 Myr of crustal aging. 相似文献
Wetlands in the Upper South East (USE) of South Australia yield a range of values to their owners and to the wider community. One such value is that generated by duck hunting in the region. Wetland owners receive private benefits from hunting fees, while hunters gain a non-monetary consumers' surplus. These non-monetary benefits received by duck hunters through their enjoyment of hunting in wetlands are the primary focus of this paper. In order to estimate these values, a 'travel cost' survey of duck hunters participating in an organised shoot held by Wetlands and Wildlife in February 2000 was undertaken. The extent of the private recreation benefits so estimated is assessed in terms of their potential to stimulate the provision of wetlands for both private and social benefits. 相似文献
We present a set of 39 new determinations of heat flow and radiogenic heat production for several different geological environments in the State of New Hampshire (U.S.A.). With the extensive data set now available for the state, the linear relation of heat flow and heat production appears as a very useful generalization for the study of the heat flow field of a geological province. Our measurements indicate that the vertical distribution of radiogenic heat production is similar in plutonic and metasedimentary rocks. Our data are compatible with the linear relationship established earlier by F. Birch and his co-workers in 1968. Young granites are markedly enriched in radioactive elements and those which do not outcrop are revealed by anomalies in the general relation of heat flow versus radioactivity.Heat flow is high for plutons by low elsewhere. The mean heat flow through metasedimentary formations is 1.15 μcal/cm2 s (48 mW/m2), a value near the mean heat flow for old cratons. The lowest heat flow measured is 0.76 μcal/cm2 s (32 mW/m2) for a unit poor in radioactivity. The heat flow field grades smoothly into the low heat flow regions of the Canadian Shield.The New Hampshire region is in thermal equilibrium: its heat flow is in secular equilibrium with the heat generated by crustal sources and that supplied from the mantle. In this area, the thermal perturbations due to orogenic events decrease below the detection level in less than 200–275 Ma. The thickness of the layer which is thermally affected during continent-continent collision-type orogenies cannot be greater than about 190 km. 相似文献
A quarterly sampling programme was conducted during 1973–1974 off the coast of Delaware to provide an environmental baseline for two ocean sewage outfalls. Extensive physical measurements and water quality data, together with biological data (fish, invertebrates, bacteriological samples), were collected. Based on this research the design of sewage treatment for one of the outfalls was improved. 相似文献
A rule-based expert system has been built for selection of both retaining wall types and groundwater control methods in deep excavations in Wuhan city. For this expert system, a new type of generation rule is developed in which one condition is able to be defined with a “third state” that not only contributes directly to reaching the conclusion in a rule, but also factors into calculating the reliability of the conclusion. The traditional backward chaining technique has been improved to accommodate the change of a rule type and a fuzzy backward chaining method IRO has been established to increase reasoning flexibility. Using IRO as a fundamental element, it is convenient to form a complicated reasoning network in the inference engine. Finally, two knowledge bases are built from more than 100 case histories and other resources, and the new expert system proves to be effective in case studies. 相似文献
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/m2 surface heat flow), but the diamond source regions subsequently cooled by about 100–150 °C to fall on a 37–38 mW/m2 (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. 相似文献