Variable features on Mars. VI. An unusual crater streak in Mesogaea     

J. Veverka  Carl Sagan  Ronald Greeley 《Icarus》1976,27(2):241-253

An unusual, prominent dark streak located in Mesogaea (near 8°N, 191°W) is described. Its appearance is unlike that of most dark streaks on Mars, many of which have ragged outlines, are variable on short time-scales, and are presumed to be erosional. The Mesogaea streak has a tapered, smooth outline, and no changes within it were observed. We suggest that this streak is depositional and that the low-albedo material originated within the associated crater itself. The source area is identified with a compact, low-albedo region on the crater floor. Two possible origins for the dark material are suggested: (1) deflation from a recently exposed, relatively unconsolidated subsurface deposit, and (2) production of ash by a volcanic vent.  相似文献   

Phase equilibria and melt productivity in the pelitic system: implications for the origin of peraluminous granitoids and aluminous granulites   总被引：26，自引：1，他引：25  

Alberto E. Patiño Douce  A. Dana Johnston 《Contributions to Mineralogy and Petrology》1991,107(2):202-218

Peraluminous granitoid magmas are a characteristic product of ultrametamorphism leading to anatexis of aluminous metasedimentary rocks in the continental crust. The mechanisms and characteristic length-scales over which these magmas can be mobilized depend strongly on their melt fraction, because of their high viscosities. Thus, it is of fundamental importance to understand the controls exerted by pressure, temperature and bulk composition of the source material on melt productivity. We have studied experimentally the vapour-absent melting behaviour of a natural metapelitic rock and our results differ greatly from those of previous experimental and theoretical investigations of melt productivity from metamorphic rocks. Under H₂O-undersaturated conditions, bulk composition of the source material is the overriding factor controlling melt fraction at temperatures on the order of 850–900° C. Granitoid melts formed in this temperature interval by the peritectic dehydration-melting reaction: $$\begin{gathered} Biotite + plagioclase + aluminosilicate + quartz \hfill \\ = melt + garnet \hfill \\ \end{gathered} $$ have a restricted compositional range. As a consequence, melt fractions will be maximized from protoliths whose modes coincide with the stoichiometry of the melting reaction. This “optimum mode” (approximately 38% biotite, 32% quartz, 22% plagioclase and 8% aluminosilicate) reflects the fact that generation of low-temperature granitoid liquids requires both fusible quartzo-feldspathic components and H₂O (from hydrous minerals). Metapelitic rocks rich in mica and aluminosilicate and poor in plagioclase contain an excess of refractory material (Al₂O₃, FeO, MgO) with low solubility in low-temperature silicic melts, and will therefore be poor magma sources. Melt fraction varies inversely with pressure in the range 7–13 kbar, but the effect is not strong: the decrease (at constant temperature) over this pressure range is of at most 15 vol% (absolute). The liquids produced in our experiments are silicarich (68–73 wt% SiO₂), strongly peraluminous (2–5 wt% normative corundum) and very felsic (MgO+FeO^* +TiO₂ less than 3 wt%, even at temperatures above 1000° C). The last observation suggests that peraluminous granitoids with more than 10% mafic minerals (biotite, cordierite, garnet) contain some entrained restite. Furthermore, because liquids are also remarkably constant in composition, we believe that restite separation is more important than fractional crystallization in controlling the variability within and among peraluminous granitoids. We present liquidus phase diagrams that allow us to follow the phase relationships of melting of silica-and alumina-saturated rocks at pressures corresponding to the mid- to deep-continental crust. Garnet, aluminosilicate, quartz and ilmenite are the predominant restitic phases at temperatures of about 900° C, but Ti-rich biotite or calcic plagioclase can also be present, depending on the bulk composition of the protolith. At temperatures above 950–1050° C (depending on the pressure) the restitic assemblage is: hercynitic spinel+ilmenite+quartz±aluminosilicate. Our results therefore support the concept that aluminous granulites (garnet-spinel-plagioclase-aluminosilicate-quartz) can be the refractory residuum of anatectic events.  相似文献   

Anhydrous PT phase relations of an Aleutian high-MgO basalt: an investigation of the role of olivine-liquid reaction in the generation of arc high-alumina basalts   总被引：2，自引：2，他引：0  

David S. Draper  A. Dana Johnston 《Contributions to Mineralogy and Petrology》1992,112(4):501-519

We report results of anhydrous 1 atm and piston-cylinder experiments on ID16, an Aleutian high-magnesia basalt (HMB), designed to investigate potential petrogenetic links between arc high-alumina basalts (HABs) and less common HMBs. ID16 is multiply saturated with a plagioclase/spinel iherzolite mineral assemblage (olivine, plagioclase, clinopyroxene, orthopyroxene, spinel) immediately beneath the 12 kbar liquidus. Derivative liquids produced at high temperatures in the 10–20 kbar melting interval of ID16 have compositions resembling those published of many moderate-CaO HABs, although lower-temperature liquids are poorer in CaO and richer in alkalies than are typical HABs. Isomolar pseudoternary projections and numerical mass-balance modeling suggest that derivative melts of ID16 enter into a complex reaction relationship with olivine at 10 kbar and 1,200° C–1,150° C. We sought to test such a mechanism to explain the lack of liquidus olivine in anhydrous experiments on mafic high-alumina basalts such as SSS. 1.4 (Johnston 1986). These derivative liquids, however, do not resemble typical arc high-alumina basalts, suggesting that olivine-liquid reaction does not account for Johnston's (1986) observations. Instead, we suggest that olivine can be brought onto the liquidus of such compositions only through the involvement of H₂O, which will affect the influence of bulk CaO, MgO, and Al₂O₃ contents on the identity of HAB liquidus phases (olivine or plagioclase) at pressures less than 12 kbar.  相似文献   

Experiments In Teaching College Geography: A Report To The Profession     

John M. Ball  Dana G. Kurfman  Leonard M. Lansky  Salvatore J. Natoli 《The Professional geographer》1972,24(4):350-361

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Rising heat wave trends in large US cities     

Dana Habeeb  Jason Vargo  Brian Stone Jr. 《Natural Hazards》2015,76(3):1651-1665

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Circulation on the West Antarctic Peninsula derived from 6 years of shipboard ADCP transects     

Dana K. Savidge  Julie A. Amft 《Deep Sea Research Part I: Oceanographic Research Papers》2009,56(10):1633-1655

Over the past 30 years, shelf circulation on the West Antarctic Peninsula (WAP) has been derived from hydrographic data with a reasonable level of confidence. However, with the exception of a very few drifter tracks and current-meter timeseries from moorings, direct velocity measurements have not previously been available. In this article, shelf and shelf-edge circulation is examined using a new velocity dataset, consisting of several years of acoustic Doppler current profiler transects, routinely collected along the ship tracks of the R/V Gould and the R/V Palmer since the fall of 1997. Initial processing and quality control is performed by Dr. Teresa Chereskin and Dr. Eric Firing, who then place the data in an archive accessible by public website, resulting in the broad availability of the data for a variety of uses. In this study, gridded Eulerian means have been calculated to examine circulation on the shelf and slope off the South Shetland Islands, in Bransfield Strait, and on the shelf and slope south of these regions, including Marguerite Bay and the adjacent shelf and shelf-edge. Shelf-edge flow is northeastward in the study area from the offshore of northern Alexander Island to Smith Island, while a southward flowing shelf-edge feature, probably the shallow component of the polar slope current, appears between Elephant Island and Livingston Island. The shallow polar slope current appears to turn shoreward to pass through Boyd Strait between Smith and Livingston Islands. In Bransfield Strait, there is cyclonic circulation. The previously identified northeastward-flowing South Shetland Island jet is strong and present in all seasons, with a large barotropic component not revealed by the hydrography-based velocities derived in the past. On the shelf seaward of Adelaide, Anvers and Brabant Islands, the strong along-shelf Antarctic Peninsula coastal current flows southwestward, with strongest velocities in winter (June–September) off Anvers and Brabant Islands, but stronger in summer (December–March) off Adelaide Island. Seaward of Marguerite Bay, there is seaward flow in the upper 400 m of the water column over the southwest bank of Marguerite Trough, strongest in summer, and shoreward flow near the northeast bank and adjacent shallower shelf areas.  相似文献   

Reconnaissance dating: A new radiocarbon method applied to assessing the temporal distribution of Southern Ocean deep-sea corals     

Andrea Burke  Laura F. Robinson  Ann P. McNichol  William J. Jenkins  Kathryn M. Scanlon  Dana S. Gerlach 《Deep Sea Research Part I: Oceanographic Research Papers》2010,57(11):1510-1520

We have developed a rapid ‘reconnaissance’ method of preparing graphite for ¹⁴C/¹²C analysis. Carbonate (～15 mg) is combusted using an elemental analyzer and the resulting CO₂ is converted to graphite using a sealed tube zinc reduction method. Over 85% (n=45 replicates on twenty-one individual corals) of reconnaissance ages measured on corals ranging in age from 500 to 33,000 radiocarbon years (Ryr) are within two standard deviations of ages generated using standard hydrolysis methods on the same corals, and all reconnaissance ages are within 300 Ryr of the standard hydrolysis ages. Replicate measurements on three individual aragonitic corals yielded ages of 1076±35 Ryr (standard deviation; n=5), 10,739±47 Ryr (n=8), and 40,146±3500 Ryr (n=9). No systematic biases were found using different cleaning methods or variable sample sizes. Analysis of ¹³C/¹²C was made concurrently with the ¹⁴C/¹²C measurement to correct for natural fractionation and for fractionation during sample processing and analysis. This technique provides a new, rapid method for making accurate, percent-level ¹⁴C/¹²C analyses that may be used to establish the rates and chronology of earth system processes where survey-type modes of age estimation are desirable. For example, applications may include creation of sediment core-top maps, preliminary age models for sediment cores, and growth rate studies of marine organisms such as corals or mollusks. We applied the reconnaissance method to more than 100 solitary deep-sea corals collected in the Drake Passage in the Southern Ocean to investigate their temporal and spatial distribution. The corals used in this study are part of a larger sample set, and the subset that was dated was chosen based on species as opposed to preservation state, so as to exclude obvious temporal biases. Similar to studies in other regions, the distribution of deep-sea corals is not constant through time across the Drake Passage. Most of the corals from the Burdwood Bank (continental shelf of Argentina) have ages ranging between 0 and 2500 calendar years, whereas most of the corals from the Sars Seamount in the Drake Passage have ages between 10,000 and 12,500 calendar years. Such differences may be caused in part by sampling biases, but may also be caused by changes in larval transport, nutrient supply, or other environmental pressures.  相似文献   

Eolian Deposition and Soil Fertility in a Prehistoric Agricultural Complex in Central Arizona,USA     

Dana K. Nakase  Anthony S. Hartshorn  Katherine A. Spielmann  Sharon J. Hall 《Geoarchaeology》2014,29(2):79-97

Prehistoric farmers in arid and semiarid ecosystems commonly used rock alignments to concentrate water and sediments on their fields. Previous research has emphasized the importance of runoff from organic matter‐rich uplands as a mechanism for soil nutrient replenishment. However, eolian inputs to these dryland ecosystems might also contribute substantially to mineral‐derived nutrient pools. We explored the relative importance of eolian deposition, prehistoric agriculture, and the presence of rock alignments on soil properties in a semiarid grassland in Arizona. Subsurface soils behind natural rock alignments are finer in texture than soils unbound by rock alignments, while subsurface soils behind agricultural rock alignments coarsen relative to unbound soils. Neither rock alignments nor estimated crop yields had detectable effects on mineral‐derived nutrient pools. In contrast, eolian deposition is an important source of soil mass and nutrients to modern soils. While dust deposition likely reduced soil heterogeneity across this landscape, it could have also contributed to the sustainability of prehistoric agriculture.  相似文献   

Factors influencing lava-substrate heat transfer and implications for thermomechanical erosion     

Sarah A. Fagents  Ronald Greeley 《Bulletin of Volcanology》2001,62(8):519-532

We develop numerical simulations of basaltic lava flowing laminarly over a basalt substrate in order to examine the details of the lava dynamics and thermal boundary layers and to understand the implications for substrate heating. As the initial stage of a larger study of thermomechanical erosion in different planetary environments, we aim to understand why erosion occurs on Earth, why erosion features are not ubiquitous given the high temperatures involved, and whether it is a plausible mechanism for the formation of planetary channels such as lunar sinuous rilles and venusian canali. Here we confine our attention to terrestrial lavas with well-known properties and eruption parameters. With relatively simple computational fluid dynamic simulations, most closely representing tube-fed hawaiian basalts (for which erosion has been documented), we demonstrate the importance of incorporating several key factors in models of lava flow/substrate heat transfer, which have commonly been neglected in previous treatments. By addressing the interaction of the flow dynamics and heat transfer in the lava, our work suggests that the development of a temperature gradient in the base of the lava, even for undeveloped flow, has a significant influence on substrate temperature. The sensitivity of the lava-substrate interface temperature to the thermophysical properties of the lava and substrate suggests that a delicate balance is required for partial melting to occur. Thus, it might take weeks of continuous flow to initiate partial melting of the substrate at distances of several kilometers from the vent. These durations exceed the periods of stability typical of lava flowing in tubes; pauses, blockages, surges, and breakouts frequently disrupt the flow. However, natural irregularities in the flow dynamics or substrate topography might help to initiate and maintain substrate melting on shorter timescales by disturbing the intimately coupled dynamic and thermal boundary layers. Although a purely thermal mechanism cannot be ruled out, our findings support the premise that mechanical erosion may play a key role in reports of erosion based on field evidence.  相似文献   

Quiescent Reconnection Rate Between Emerging Active Regions and Preexisting Field,with Associated Heating: NOAA AR 11112     

Lucas A. Tarr  Dana W. Longcope  David E. McKenzie  Keiji Yoshimura 《Solar physics》2014,289(9):3331-3349

When magnetic flux emerges from beneath the photosphere, it displaces the preexisting field in the corona, and a current sheet generally forms at the boundary between the old and new magnetic domains. Reconnection in the current sheet relaxes this highly stressed configuration to a lower energy state. This scenario is most familiar and most often studied in flares, where the flux transfer is rapid. We present here a study of steady, quiescent flux transfer occurring at a rate three orders of magnitude lower than that in a large flare. In particular, we quantify the reconnection rate and the related energy release that occurred as the new polarity emerged to form NOAA Active Region 11112 (SOL16 October 2010T00:00:00L205C117) within a region of preexisting flux. A bright, low-lying kernel of coronal loops above the emerging polarity, observed with the Atmospheric Imaging Assembly onboard the Solar Dynamics Observatory and the X-ray Telescope onboard Hinode, originally showed magnetic connectivity only between regions of newly emerged flux when overlaid on magnetograms from the Helioseismic and Magnetic Imager. Over the course of several days, this bright kernel advanced into the preexisting flux. The advancement of an easily visible boundary into the old flux regions allows measuring the rate of reconnection between old and new magnetic domains. We compare the reconnection rate with the inferred heating of the coronal plasma. To our knowledge, this is the first measurement of steady, quiescent heating related to reconnection. We determined that the newly emerged flux reconnects at a fairly steady rate of 0.38×10¹⁶ Mx?s^?1 over two days, while the radiated power varies between (2?–?8)×10²⁵ erg?s^?1 over the same time. We found that as much as 40 % of the total emerged flux at any given time may have reconnected. The total amounts of transferred flux (～?1×10²¹ Mx) and radiated energy (～?7.2×10³⁰ ergs) are comparable to that of a large M- or small X-class flare, but are stretched out over 45 hours.  相似文献