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101.
R. Arvidson G. Crozaz R. J. Drozd C. M. Hohenberg C. J. Morgan 《Earth, Moon, and Planets》1975,13(1-3):259-276
Cosmic ray exposure ages of lunar samples have been used to date surface features related to impact cratering and downslope movement of material. Only when multiple samples related to a feature have the same rare gas exposure age, or when a single sample has the same81Kr-Kr and track exposure age can a feature be considered reliably dated. Because any single lunar sample is likely to have had a complex exposure history, assignment of ages to features based upon only one determination by any method should be avoided. Based on the above criteria, there are only five well-dated lunar features: Cone Crater (Apollo 14) 26 m.y., North Ray Crater (Apollo 16) 50 m.y., South Ray Crater (Apollo 16) 2 m.y., the emplacement of the Station 6 boulders (Apollo 17) 22 m.y., and the emplacement of the Station 7 boulder (Apollo 17) 28 m.y. Other features are tentatively dated or have limits set on their ages: Bench Crater (Apollo 12) ?99 m.y., Baby Ray Crater (Apollo 16) ?2 m.y., Shorty Crater (Apollo 17) ≈ 30 m.y., Camelot Crater (Apollo 17) ?140 m.y., the emplacement of the Station 2 boulder 1 (Apollo 17) 45–55 m.y., and the slide which generated the light mantle (Apollo 17) ?50 m.y. 相似文献
102.
Pseudunciola obliquua is the most abundant subtidal amphipod of the near-shore macrobenthos in the New York Bight south of Fire Island. It has an annual life-cycle. Breeding occurs in March and April, and a single brood of between 8 and 20 eggs per female is produced. Fecundity is linearly proportional to maternal length. The eggs take about two months to develop; the juveniles emerge in June and July. Initial recruitment in the study area was estimated to be 2980 and 5850 per m2 for the 1979–1980 and 1980–1981 year classes, respectively. Of the initial recruits only about 10% survive to form the reproductive stock of the following spring.Females and males grow at similar rates and are equally abundant as juveniles and initially as adults, until they reach sexual maturity. After breeding, the abundance of males decreases rapidly due to post-reproductive death. Females continue to live, carrying the developing eggs in their brood pouches. Mean female growth increases throughout the brooding period until the young are released, shortly after which adult females also die. The entire generation of reproductive adults has died by September. 相似文献
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Properties of fluids attending variable recrystallization of quartzite during contact metamorphism in the White‐Inyo Range,California 下载免费PDF全文
P. I. Nabelek S. K. Stephenson S. S. Morgan J. J. Student 《Journal of Metamorphic Geology》2017,35(3):357-369
Fluid inclusions in the metamorphic aureole of the Eureka Valley‐Joshua Flat‐Beer Creek (EJB) pluton in the White‐Inyo Range, California, reveal the compositions and origin of fluids that were present during variable recrystallization of quartzite with sedimentary grain shapes to metaquartzite with granoblastic texture. Metamorphosed sedimentary formations, including quartzites, marbles, calcsilicates and schists, became ductile and strongly attenuated in the aureole during growth of the magma chamber. The microstructures of quartzites have an unusual distribution in that within ~250 m from the pluton, where temperatures exceeded 650 °C, they exhibit relict sedimentary grain shapes, only small amount of grain boundary migration (GBM), and crystallographic preferred orientations (CPOs) dominated by <a> slip. At distances >250 m, quartzites were completely recrystallized by GBM and CPOs are indicative of prism [c] slip, characteristics that are typically associated with H2O‐assisted, high‐T recrystallization. The lack of extensive GBM in the inner aureole can be attributed to rapid replacement of H2O by CO2 produced by reaction of quartz grains with calcite cement that also produced interstitial wollastonite. Fluid inclusions in the inner aureole generally occur in margins of quartz grains and are either wholly aqueous (Type 1) or also contain H2S, CO2 and CH4 (Type 2). Type 2 inclusions occur only in some stratigraphic layers. In both inclusion types, NaCl and CaCl2, in variable proportions, dominate the solutes in the aqueous phase, whereas FeCl2 and KCl are less abundant solutes. The solutes indicate attainment of a degree of equilibrium with carbonates and schists that are interbedded with the quartzites. Some Types 1 and 2 inclusions in the inner aureole show evidence of decrepitation due to high amounts of strain and/or heating suffered by the host rocks, which suggests that they represent pore fluids that existed in the rocks prior to contact metamorphism. In addition to Type 1 inclusions, outer aureole quartzites also contain inclusions that contain CO2 vapour bubbles in addition to aqueous phase (Type 3). These inclusions only occur in interiors of granoblastic quartz that was produced by large amounts of GBM. The aqueous phase has identical ranges of first melting and final ice melting temperatures as Type 1 inclusions, suggesting that they have the same solute compositions. These inclusions are thought to represent the interstitial pore H2O that promoted recrystallization of quartz and reacted with graphite to produce CO2. Absence of significant amounts of CH4 in Type 3 inclusions is attributed to elevated fO2 that was buffered by mineral assemblages in interbedded schists. As opposed to the large amount of CO2 that was produced by the wollastonite‐forming reaction in the inner aureole to inhibit GBM, the amount of CO2 produced in the outer aureole by reaction between H2O and graphite was apparently insufficient to inhibit recrystallization of quartz. 相似文献
106.
Nina Morgan 《Geology Today》2016,32(4):154-159
Cemeteries and graveyards not only serve as a peaceful place to commemorate the dead. For geologists—whether amateur, student or professional—almost any urban cemetery provides a valuable opportunity to carry out scientific field work at leisure, right on the doorstep, and at no cost. Cemeteries offer an excellent opportunity to examine and study a wide range of stone types in a small space. They also serve as a useful field area for studying rock weathering. 相似文献
107.
Laboratory and numerical experiments and boundary layer analysis of the entrainment of buoyant asthenosphere by subducting oceanic lithosphere implies that slab entrainment is likely to be relatively inefficient at removing a buoyant and lower viscosity asthenosphere layer. Asthenosphere would instead be mostly removed by accretion into and eventual subduction of the overlying oceanic lithosphere. The lower (hot) side of a subducting slab entrains by the formation of a ∼10–30 km‐thick downdragged layer, whose thickness depends upon the subduction rate and the density contrast and viscosity of the asthenosphere, while the upper (cold) side of the slab may entrain as much by thermal ‘freezing’ onto the slab as by mechanical downdragging. This analysis also implies that proper treatment of slab entrainment in future numerical mantle flow experiments will require the resolution of ∼10–30 km‐thick entrainment boundary layers. 相似文献
108.
The Keisarhjelmen detachment records Silurian–Devonian extensional collapse in Northern Svalbard 下载免费PDF全文
At the junction of the Atlantic and Arctic margins, the crustal‐scale Keisarhjelmen detachment of north‐west Svalbard records previously unrecognised magnitudes of extension. The detachment separates a corrugated metamorphic core complex in the footwall from a mantling Devonian supradetachment basin in the hangingwall. The detachment has a top‐N displacement of more than 50 km, which is aligned with the map‐scale corrugations, and an upwards ductile to brittle transition with shear related footwall retrogression. This configuration has striking similarities to extensional collapse detachments in the paired Scandinavian–Greenland Caledonides, but orientation and position link the detachment with the Ellesmerian orogen. 相似文献
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Improving the way we think about projecting future energy use and emissions of carbon dioxide 总被引:1,自引:1,他引:0
A variety of decision makers need projections of future energy demand, CO2 emissions and similar factors that extend many decades into the future. The past performance of such projections has been
systematically overconfident. Analysts have often used scenarios based on detailed story lines that spell out “plausible alternative
futures” as a central tool for evaluating uncertainty. No probabilities are typically assigned to such scenarios. We argue
that this practice is often ineffective. Rather than expanding people’s judgment about the range of uncertainty about the
future, scenario-based analysis is more likely to lead to systematic overconfidence, to an underestimate of the range of possible
future outcomes. We review relevant findings from the literature on human judgment under uncertainty and discuss their relevance
to the task of making probabilistic projections. The more detail that one adds to the story line of a scenario, the more probable
it will appear to most people, and the greater the difficulty they likely will have in imagining other, equally or more likely,
ways in which the same outcome could be reached. We suggest that scenario based approaches make analysts particularly prone
to such cognitive biases, and then outline a strategy by which improved projections, tailored to the needs of specific decision
makers, might be developed. 相似文献