摘要:

The electrical conductivity of (Mg0.9Fe0.1)SiO3perovskite has been measured at pressures of 0–8 GPa and ambient temperature using a DIA‐6 cubic‐anvil apparatus. At ambient conditions, the electrical conductivity of the (Mg0.9Fe0.1)SiO3perovskite is found to be 7.6 (±2.0) ×10−9S/m. The conductivity of the perovskite increases with increasing pressure, and the activation volume for electrical conduction is determined to be −0.12 (±0.04) cm3/mol. As a cross‐check with previous electrical conductivity measurements, an assemblage of the perovskite and magnesiowüstite synthesized from the (Mg0.89Fe0.11)2SiO4olivine has been examined in the present study using a laser‐heated diamond anvil cell at ∼ 48 GPa and in the temperature range of 295–3500 K. The results for this assemblage are consistent with those for the pure (Mg0.9Fe0.1)SiO3perovskite. The present experimental data show that the electrical conductivity of the crystalline (Mg0.9Fe0.1)SiO3perovskite remains below 10−5S/m at 300 K and

ISSN:0148-0227    

DOI:10.1029/92JB01816

年代:1993

数据来源: WILEY

摘要:

The spatial and temporal variation of electrical conductivity in saturated sands during steam injection has been measured and modelled. Experiments consisted of introducing steam into one end of a tube filled with sand saturated with a slightly saline solution. A steam condensation front formed, separating the mixed‐phase steam zone from the liquid zone. Measurements of electrical conductivity were made at 10 locations along the tube using a four‐electrode technique. Results show that conductivity starts at a constant value, decreases before the steam front arrives and then, immediately prior to the steam front arrival, goes through a maximum before dropping by a factor of about 25. These variations can be explained by first, a dilution of the interstitial solution causing the initial drop in conductivity; second, an increase in temperature of the solution immediately prior to the arrival of the steam front causing the conductivity maximum; and finally, the large drop in conductivity due to the combined effects of a decrease in saturation and dilution of the residual liquid in the two‐phase zone. Mathematical solutions of a set of differential equations that take into consideration all of these effects are presented. These solutions reproduce the significant features of the conductivity data. This study suggests that the measurement of changes in the subsurface conductivity field during steam injection operations may indicate the location of ionic concentration, temperature, and steam saturation f

ISSN:0148-0227    

DOI:10.1029/92JB01694

年代:1993

数据来源: WILEY

摘要:

Precise pressure‐volume‐temperature (PVT) data for water exist to pressures of only 8.9 kbar and temperatures of 900°C. Since aqueous fluids are important in many processes at much higher pressures and temperatures, there is a need to develop reliable extrapolations of the low pressures experimental data. In this paper we present the results of molecular dynamics simulations of water properties over a wide P‐T range. It has been shown that at a density of 1.0 g/cm3, the TIP4P intermolecular potential (Jorgensen et al., 1983) very accurately reproduces the available thermodynamic and structural properties of water at pressures of 1 bar to 10 kbar (Brodholt and Wood, 1990). This is extended here by testing TIP4P at temperatures to 2500 K and 350 kbar. The PVT predictions are compared to the experimental data of Bumham et al. (1969) at low pressures, and with the data of Kormer (1968) and Bridgman (1942) at higher pressures. We conclude that there is good reason to be confident of the predictive powers of the TIP4P intermolecular potential in the high‐pressure and high‐temperature regime. Heat capacities and structural properties have also been calculated and compared to available data. High‐pressure simulations have also been made with two other intermolecular potentials which accurately represent water properties at low pressures, SPC/E (Berendsen et al., 1987) and WK (Watanabe and Klein, 1989). SPC/E predicts PVT properties that are similar to those of TIP4P, while WK is accurate only at densities of about 1.0 g/cm3. At high pressures the spherical potential of Belonoshko and Saxena (1991) predicts volumes that are within 0.5 m of those predicted by TIP4P. A modified Redlich Kwong (MRK) type equation of state based on the molecular dynamics simulations is presented which can be used at pressures in excess of 10 kbar. This is used to predict the location of the brucite dehydration reaction which has recently been determined to 120 kbar (Johnson e

ISSN:0148-0227    

DOI:10.1029/92JB01407

年代:1993

数据来源: WILEY

摘要:

Stratigraphically controlled sequences of in situ lavas were collected from Loihi Seamount using theAlvinsubmersible to evaluate the volcano's temporal geochemical evolution. Three sections with up to 370 m of relief were sampled from the two pit craters at the summit of Loihi. All of the analyses were done on glass separates. Our results indicate that tholeiitic and alkalic volcanism at the summit of Loihi has been coeval. The tholeiitic and alkalic lavas have similar incompatible element patterns and O, Pb, Sr, and Nd isotope ratios but are distinct in some incompatible element ratios. These results are consistent with the different Loihi rock types being derived by variable degrees of melting from a common source. The crossing and light‐rare‐earth‐enriched rare earth element patterns and variable Sc/Yb ratios of the tholeiites indicate that their source was a garnet lherzolite. The relatively low δ18O values (∼4.9 ‰) for Loihi lavas are interpreted to be characteristic of the Hawa

ISSN:0148-0227    

DOI:10.1029/92JB01707

年代:1993

数据来源: WILEY

摘要:

Volcanic gases contain ultrafine aerosol particles in the nanometer size range; typical concentrations are 105to 106cm−3. Photoelectric charging of particles (PCP) is an in situ method for the material specific detection of very small particles in a gas. Field studies at degassing lava flows of Mount Etna, Sicily, and Kilauea, Hawaii, show that the chemistry of the ultrafine aerosols depends strongly on the degassing state of the lava. Heating of a relatively undegassed lava sample in the laboratory reveals the chemical nature of the particles that form by nucleation and condensation in the cooling gas. In the initial stages of degassing, the particles are mainly NaCl and KCl nanocrystals that contain iron oxide and copper chloride. Cu is in the monovalent state, which is stable even in an oxidizing environment due to a redox mechanism with the Fe ions. The fraction of Cu(I) in the aerosols is determined by PCP. The evolution of the Cu(I) fraction is considered an effective tracer of chlorides in the magmatic gases and thereby of magma degassing. The PCP technique allows a fingerprint to be obtained of the magmatic gas by an aerosol measurement in the diluted plum

ISSN:0148-0227    

DOI:10.1029/92JB01870

年代:1993

数据来源: WILEY

摘要:

Granulometric analysis of ejecta from Misers Gold high explosive cratering experiment demonstrates that atmospheric dust dispersal can be evaluated by particle‐size distribution data. From size analyses of the Misers Gold preshot test bed alluvium, ejecta, and sweep‐up materials collected out to 35 crater radii (1227 m), we find that approximately 5.9 × 106kg (∼11% of the total crater ejecta mass) was depleted from the crater ejecta deposits and likely represents the portion of the cratered mass initially lofted into atmospheric suspension. The dominant size range of this lofted dust was 88 to 2000 μm with a mean diameter by mass of 384 μm,. In addition to the dust lofted in the explosion column, dust in the size range of 100–800 μm was swept up from the ground by the explosive air blast and base surge dominantly between ranges of 10 and 18 crater radii (360 and 650 m from ground zero). This sweep‐up dust was convectively drawn into the column and contributed up to 2% of the total mass of lofted. Based on the measured abundance of coarse (>250 μm diameter) dust particles in the estimated lofted dust, it is likely that about 70% of this lofted mass fell out within about 1 hour, such that the remaining dust cloud mass was ∼1.8 × 106kg, which is equivalent to a dust lofted per unit blast yield of 0.5 kT/kT or about 4% of the crater ejecta mass. This study supports the hypothesis that if initial distributions can be constrained, the volume of dust lofted into atmospheric suspension from large surface explosions can be estimated from analysis of particle‐size distributions of ejecta deposited near the explosion. This result may have particular applications to study of the atmospheric effects of historic and prehistoric

ISSN:0148-0227    

DOI:10.1029/92JB01888

年代:1993

数据来源: WILEY

摘要:

A two‐dimensional finite element code was used to investigate the effect of changes in surface air pressure on radon flux from soil with parallel, partially penetrating cracks. A sensitivity analysis investigates the effects of various crack dimensions, soil characteristics, and surface air pressure on radon flux from the soil surface to the atmosphere. Simulation results indicate that radon flux is most sensitive to soil properties; the diffusion coefficient is most important, followed by permeability and porosity. Radon flux is also sensitive to changes in barometric pressure, which cause variations in radon flux above and below the average diffusive flux. Sinusoidal variations in barometric pressure cause a net increase in the average radon flux from the soil, because increases in flux during periods of decreasing pressure are greater than the decreases in flux during periods of decreasing pressure of equal magnitude. Cracks were found to significantly increase radon flux from soils of low permeabilit

ISSN:0148-0227    

DOI:10.1029/92JB01845

年代:1993

数据来源: WILEY

摘要:

Crustal xenoliths occur in a variety of Carboniferous and Tertiary alkali basaltic rocks across Scotland and provide indications of the nature of the deep crust beneath the major lithotectonic zones from the Archean Lewisian of the far NW to the Paleozoic terranes in the SE. Sr, Nd, and Pb isotopic and representative chemical compositions are presented for 35 mafic xenoliths from the different age and tectonic zones of Scotland and for 21 felsic, gneissose xenoliths, from the Midland Valley and Southern Uplands in the south. The coexistence of mafic granulite and spinel lherzolite xenoliths at several of these localities, together with seismic velocity data, provide evidence that the mafic xenoliths, and possibly some of the felsic xenoliths, may be derived from 20–30 km depth. The absence of garnet from the mafic assemblages suggests that in most areas the crust was probably not much more than 30 km thick in the Permo‐Carboniferous. The extremely variable Al2O3(up to 27 wt %) and MgO contents at any one location, together with positive Eu and Sr anomalies in chondrite‐ and mantle‐normalized diagrams, provide evidence that many of the mafic xenoliths represent cumulates from basaltic magmas. The Loch Roag mafic xenoliths, erupted through the depleted high‐grade metamorphic Lewisian Complex, are especially rich in K (average 1.3 % K2O) and Rb (average 54 ppm) with K/Rb extending as low as 100. The K/U ratios for the mafic xenoliths are high, ranging to over 100,000 at Loch Roag and in the SW Midland Valley. The relative concentrations of U, K, Rb, Pb, Nd, Sm, Eu and Sr in most of the mafic Xenoliths, while displaying regional characteristics, display intra‐site variations that are consistent with crystal‐liquid fractionation. There is little evidence of U or Rb depletion via fluid or melt extraction during metamorphism. The concentrations of U in the mafic xenoliths display as much intrasite uniformity as the rare earth elements and correlate with Zr concentrations. U/Zr ratios are similar to those found in ocean island basalts. The mafic xenoliths display a marked decrease in ϵNdand206Pb/204Pb with increasing age of the upper crust from SE to NW, as also observed in the ∼400 Ma Caledonian granites. Sm‐Nd crustal residence ages for the Loch Roag suite are particularly variable ranging from 1.6 to 3.6 Ga. However, these isotopic compositions and model ages appear to be, in part, the product of crustal assimilation by the igneous precursors to the mafic xenoliths. Model ages, mineral ages and scattered whole rock isochrons indicate that many of the precursors to these Xenoliths were in fact formed and metamorphosed during the late Proterozoic and Paleozoic and provide evidence of widespread deep crustal magma reservoirs, underplating, crustal assimilation and arc accretion at these times. Nd and Sr isotopic data for coexisting pyroxenite xenoliths at Loch Roag indicate that they may represent contaminated ultramafic cumulates that were cogenetic with the precursors to the mafic xenoliths. The Pb isotopic compositions of the Midland Valley mafic and felsic xenoliths are similar and relatively uniform despite large variations in Nd and Sr isotopic compositions. This is explicable if the crust in this region was relatively uniform and similar in composition to the mantle and/or if crustal Pb dominated in arc environments. The average major element composition of the Scottish mafic xenolith suite is similar to that reported from other regions such as the Sierra Nevada. This is taken as evidence that the processes of crystal fractionation and formation of mafic and ultramafic cumulates during underplating are relatively consistent between different regions. Average U/Pb ratios of mafic and felsic xenoliths are effectively identical (0.068 and 0.064, respectively). The average Sm/Nd ratio (0.20) for the entire suite is lower and average K/U ratio (27,000) higher than estimates of average lower crust based on Queensland mafic xenoliths. These compositions as well as those of basement rocks, sedimentary supracrustal rocks and granites provide evidence of systematic chemical differences between the Scottish and Australian lithosphere. The compositional features of the Paleozoic Scottish crust such as low Sm/Nd and high K/U may have been sustained in part by recycling of Archean and early Proterozoic lithosphere through sedimentary processes, subduction and arc volcanism. The average Sm‐Nd crustal residence age and Pb isotopic composition of the mafic xenoliths at Loch Roag is very similar to that of the host Lewisian. Conservative mass balance models imply an average ratio of Lewisian‐derived Nd to parent magma Nd in the xenoliths of about 2. Such large degrees of contamination indicate that although underplating may be significant process in reconstituting Archean lithosphere, it does not necessarily result in substantial changes to the av

ISSN:0148-0227    

DOI:10.1029/92JB02276

年代:1993

数据来源: WILEY

摘要:

We present thermodynamic estimates of pressures, temperatures, and volatile activities in variably deformed, gabbroic to granitic, Cretaceous (115–100 Ma) batholithic and framework rocks of the Tehachapi Mountains, southernmost Sierra Nevada, California. Al contents of hornblende in granitoids imply igneous emplacement at ∼8 kbar in the southernmost Tehachapi Mountains, with lower pressures (3–7 kbar) to the north. Metamorphic pressures and temperatures for garnet‐bearing paragneisses and metaigneous rocks were estimated on the basis of garnet‐hornblende‐plagioclase‐quartz and garnet‐biotite‐plagioclase‐quartz thermobarometers. Disparate results for the metaigneous rocks from the latter system point to the difficulty of applying pelite‐based thermobarometers to rocks of contrasting composition and mineralogy. Preferred pressures cluster at 7.1–9.4 and 3.6–4.3 kbar. Incomplete knowledge of reaction histories, however, limits our interpretation of the lower pressures because they are minimum estimates. The ∼4‐kbar samples are all from a small area and, if our interpretation is correct, they imply a local, more shallow event superimposed on crust once residing at deeper structural levels. Garnet‐hornblende and garnet‐biotite temperatures are less coherent, likely owing to retrograde Fe‐Mg exchange, and range from 570° to 790°C, The majority of the rocks are igneous and affected by recrystallization and metamorphism during subsolidus cooling; they are not granulites. Country rock paragneisses are typically migmatized at “peak” metamorphic conditions near that of the wet granite solidus (>690°C). Veinlike paragenesis of garnet in the metaigneous rocks suggests formation related to the presence of a fluid phase. Thermodynamic estimates of volatile activities in these garnet‐bearing assemblages suggest variable, mostly CO2‐rich fluid compositions, in the absence of any pervasive fluid flux. The igneous rocks of the Tehachapi Mountains were thus intruded at depths of ∼30 km, making them the deepest known exposed components of the Cretaceous Sierra Nevada batholith. Metamorphism occurred at these great depths and, perhaps, locally after ∼15 km of uplift before ∼87 Ma, implying an uplift rate of 1.2 mm/yr. (A minimum uplift rate is 0.6 mm/yr.) This original uplift and possible subsequent uplift events may have been related to underthrusting of a block of Rand Schist from what is now the southeast, with concomitant widespread ductile deformation. The deduced pressure‐temperature and uplift history is similar to those of high‐pressure/high‐temperature Cretaceous batholithic rocks in Salinia and the San Gabriel Mountains, but direct correlation is not wan‐anted. When compared with higher‐level intrusive rocks from analogous portions of the Sierra Nevada batholith to the north, the Tehachapi rocks reveal a deep batholith that is more heterogeneous and somewhat more mafic on average, but displaying a similar level of isotopic hybridization involving mantle and crustal sources. The batholith is quartz‐rich at these levels, suggestive of a weak, ductile middle crus

ISSN:0148-0227    

DOI:10.1029/92JB01889

年代:1993

数据来源: WILEY

摘要:

In this paper we investigate the value ofbin the Gutenberg‐Richter relation for four teleseismic catalogs of earthquakes: Abe's historical catalog, the Harvard Centroid Moment Tensor (CMT) catalog, the catalog of the International Seismological Centre (ISC), and the Blacknest catalog. An unfortunate result is thatbdiffers by 30% or more when determined in different magnitude ranges, in different catalogs, or using different methods. For global catalogs separated into shallow, intermediate, and deep earthquake groups, all values determined for b lie between 0.72 and 1.34. We can identify no systematic global variation ofbwith depth. For teleseismic catalogs it is difficult to believe measured geographic variations inbbecause systematic errors cause problems of earthquake detection, earthquake location, aftershock identification, and magnitude determination. However, some variations inbare so persistent and large that they must be real. For deep earthquakes in Tonga‐Fiji, for example, various measurements of b He between 1.06 and 1.57, comparable tobfor shallow earthquakes, whereas measurements ofbfor deep earthquakes in the rest of the world are much lower, between 0.53 and 0.96. For shallow earthquakes in the Harvard CMT catalog, earthquakes with thrust and strike slip focal mechanisms have significantly lowerbvalues (0.86 and 0.77) than earthquakes with normal faulting mechanisms (1.06). When we separate the ISC catalog into primary events (mainshocks and earthquakes with no aftershocks or foreshocks) and secondary events (aftershocks and foreshocks), we observe that b for secondary events is nearly always significantly higher thanbfor mainshocks. However, we show that the difference has no physical significance, as it arises simply from the act of choosing mainshocks as the largest earthquake in a foreshock‐mainshock‐aftershock sequence. When we correct for this systematic effect by comparing the real catalogs to identical catalogs with randomly reassigned magnitudes, we find thatbfor secondary events in the real catalog is actually lower than expected. Thus among aftershocks large earthquakes are relatively more common than expected, perhaps because the mainshock rupture loads asperities in adjacent

ISSN:0148-0227    

DOI:10.1029/92JB01891

年代:1993

数据来源: WILEY