Experimental studies At 5–12 GPa of the Ondermatjie hypabyssal kimberlite

Liquidus and sub-liquidus phase relationships are reported for melts formed from an aphanitic kimberlite composition crystallized at 5–12 GPa and 900–1400 °C. The liquidus phase over the pressure range investigated is forsteritic olivine. This is followed with decreasing temperature by olivine plus garnet as the initial sub-liquidus solid phase assemblage. Supra-solidus assemblages consist of olivine+garnet+clinopyroxene+Mg-ilmenite+liquid at 5–7 GPa or olivine+garnet+clinopyroxene+hematite–ilmenite solid solutions (+/−perovskite)+liquid at 8–12 GPa. Phlogopite forms as a near-solidus phase only at 900 °C and 6 GPa. Orthopyroxene does not form at any temperature and pressure. All garnets formed at 6–7 GPa are Ti-rich almandine–grossular–pyrope solid solutions and not Cr-pyrope, whereas garnets formed above 8 GPa are Ti- and Fe³⁺-rich and have no natural counterparts. Quenched liquids are represented by magnesite at 10–12 GPa and Mg–Ca-carbonates at lower pressures. In addition to forming discrete crystals, Mg-ilmenite and hematite–ilmenite solid solutions occur as lamellar intergrowths that are identical in texture to naturally occurring intergrowths. Mg-ilmenite compositions at 6–7 GPa are similar to those of the natural occurrences, whereas clinopyroxenes are richer in Ca. The effects of graphite versus platinum capsules on the oxygen fugacity of the experimental charges and the composition of the olivine, clinopyroxene, Fe–Ti-oxides and garnets formed are described. These experimental data are interpreted to indicate that kimberlite magmas are unlikely to be formed by very small degrees of partial melting of a simple homogeneous carbonated garnet lherzolite mantle. It is proposed that kimberlite magmas form by extensive partial melting of metasomatized mantle, i.e. mineralogically complex carbonate-bearing veins in a lherzolitic/harzburgitic substrate, and that lamellar ilmenite–clinopyroxene intergrowths represent the products of non-equilibrium growth in kimberlite magma.     

Gloria survey of the east pacific rise near 3.5°S: Tectonic and volcanic characteristics of a fast spreading mid-ocean rise

A long-range, sidescan sonar (GLORIA) survey of an approximately 100-km square area of the East Pacific Rise crest between 3°S and 4°S extends results obtained in the same area by Lonsdale (1977) using Deep-tow. The axial, linear volcano was found to be continuous over a distance of 75 km. The presence of major inward and outward facing fault scarps was confirmed, but the GLORIA data show several distinct differences between the two fault sets. The inward dipping faults are more numerous, more closely spaced, and longer than the outward dipping ones, and their dip-slopes backscatter sound more extensively than those of the outward dipping faults; moreover most of them appear to be formed within 2 km of the axis, whereas the majority of the outward dipping faults begin to develop between 5 and 8 km from the axis. These differences suggest that the two sets of faults have different origins. The horizontal pattern of inward dipping faults is similar to those observed on other mid-ocean rises at all spreading rates, though the lengths and separations of the scarps are slightly, and their throws considerably, less than on slower spreading rises. This common horizontal pattern suggests that inward dipping faults on all rises have a common mode of formation regardless of spreading rate. Horizontal tension is probably the dominant factor, but an additional mechanism is needed to explain the polarization of fault dips that occurs in the region 2–8 km from the axis. The similarity of major fault spacings on the East Pacific Rise to those on slower spreading rises suggests that faulting is invariant in space, rather than time, and that the lithosphere where these faults are formed (about 2 km from the spreading axis) has a similar, small thickness for all spreading rates. This is attributed to the regulating effect of hydrothermal circulation and plate cooling.     

Evaporation and recondensation of sodium in Semarkona Type II chondrules

We have investigated the Na distributions in Semarkona Type II chondrules by electron microprobe, analyzing olivine and melt inclusions in it, mesostasis and bulk chondrule, to see whether they indicate interactions with an ambient gas during chondrule formation. Sodium concentrations of bulk chondrule liquids, melt inclusions and mesostases can be explained to a first approximation by fractional crystallization of olivine ± pyroxene. The most primitive olivine cores in each chondrule are mostly between Fa₈ and Fa₁₃, with 0.0022–0.0069 ± 0.0013 wt.% Na₂O. Type IIA chondrule olivines have consistently higher Na contents than olivines in Type IIAB chondrules. We used the dependence of olivine–liquid Na partitioning on FeO in olivine as a measure of equilibration. Extreme olivine rim compositions are ～Fa₃₅ and 0.03 wt.% Na₂O and are close to being in equilibrium with the mesostasis glass. Olivine cores compared with the bulk chondrule compositions, particularly in IIA chondrules, show very high apparent D_Na, indicating disequilibrium and suggesting that chondrule initial melts were more Na-rich than present chondrule bulk compositions. The apparent D_Na values correlate with the Na concentrations of the olivine, but not with concentrations in the bulk melt. We use equilibrium D_Na to find the Na content of the true parent liquid and estimate that Type IIA chondrules lost more than half their Na and recondensation was incomplete, whereas Type IIAB chondrules recovered most of theirs in their mesostases.Glass inclusions in olivine have lower Na than expected from fractionation of bulk composition liquids, and mesostases have higher Na than expected in calculated daughter liquids formed by fractional crystallization alone. These observations also require open system behavior of chondrules, specifically evaporation of Na before formation of melt inclusions followed by recondensation of Na in mesostases. Within this record of evaporation followed by recondensation, there is no indication of a stage with zero Na in the chondrules, which is predicted by models for shock wave cooling at canonical nebular pressures, suggesting high P_T.The high Na concentrations in olivine and mesostases indicate very high P_Na while chondrules were molten. This may be explained by local, very high particle densities where Type II chondrules formed. The high P_T, P_Na and number densities of chondrules implied suggest formation in debris clouds after protoplanetary collisions as an alternative to formation after passage of shock waves through large particle-rich clumps in the disk. Encounters of partially molten chondrules should have been frequent in these dense swarms. However, in many ordinary chondrites like Semarkona, “cluster chondrites”, compound chondrules are not abundant but instead chondrules aggregated into clusters. Chondrule melting, cooling and clustering in dense swarms contributed to rapid accretion, possibly after collision, by fallback on the grandparent body and by reaccretion as a new body downrange.     

中国城市群区域城际轨道交通布局模式

我国城市密集区与城市群的迅速发展,对交通网络化、现代化的需求日益迫切,而城际轨道交通是城市群发展的重要条件和前提。国外发达国家发展的经验表明,城际轨道交通是城市群客流运输最佳的交通方式选择,两者相互影响、互为促进,城市群区域和城际轨道交通之间的空间结构存在着必然的空间映射关系。基于此,本文根据我国城市群发展现状及其区域空间结构发展的5种模式,相应提出放射型、钟摆型、串珠型和网络型等4种城际轨道交通布局形式,并阐述各自特点。在未来一定时期,国内城际轨道交通将蓬勃发展,必须根据城市群区域空间结构发展模式,进行科学规划和布局形式的选择,以有效满足日益庞大的客流需求和推动城市群发展战略的实现。     

On the role of diapirism in the, segregation, ascent and final emplacement of granitoid magmas

Only bodies of magma with a high crystal content and partially molten (crustal) country rocks can ascend as diapirs; once such an envelope is pierced, the diapiric ascent of the pluton is arrested by the high viscosity of a solid aureole. Deformation by shortening of the carapace of these bodies may lead to the expulsion of a magma with a relatively low crystal content, which may then continue ascent via fractures and dykes.

The details of the mechanisms of granitoid magma segregation are still unknown, but it appears that many magmas hegin their ascent through the crust as mushes with at least 50% melt, and that such magmas are rheologically able to ascend through a thickness of crust. This ascent mechanism explains the dearth of structures attributable to the ascent of granitoids, in contrast to the abundance of structures that developed during their final emplacement.

When a magma becomes too crystalline (melt < 25%) to continue its ascent via dykes, it is immobilised. At approximately this stage, a hydrous magma may become saturated with water and release fluids into the aureole, making it particularly susceptible to deformation. Magma that continues to arrive at this level is also immobilised, and the pluton grows as a ballooning diapir. These characteristically deform themselves and their aureoles by bulk shortening.

Magmas that are able to ascend to shallow depths, largely by virtue of lower water contents and higher initial temperatures, tend to become finally accommodated by such brittle processes as stoping and cauldron subsidence. High level intrusions lend to be tabular, are also fed by dykes or conduits, and assemble in tabular batholiths.     

Wave constraints for Titan’s Jingpo Lacus and Kraken Mare from VIMS specular reflection lightcurves

Stephan et al. (Stephan, K. et al. [2010]. Geophys. Res. Lett. 37, 7104-+.) first saw the glint of sunlight specularly reflected off of Titan’s lakes. We develop a quantitative model for analyzing the photometric lightcurve generated during a flyby in which the specularly reflected light flux depends on the fraction of the solar specular footprint that is covered by liquid. We allow for surface waves that spread out the geographic specular intensity distribution. Applying the model to the VIMS T58 observations shows that the waves on Jingpo Lacus must have slopes of no greater than 0.15°, two orders of magnitude flatter than waves on Earth’s oceans. Combining the model with theoretical estimates of the intensity of the specular reflection allows a tighter constraint on the waves: ?0.05°. Residual specular signal while the specular point lies on land implies that either the land is wetted, the wave slope distribution is non-Gaussian, or that 5% of the land off the southwest edge of Jingpo Lacus is covered in puddles. Another specular sequence off of Kraken Mare acquired during Cassini’s T59 flyby shows rapid flux changes that the static model cannot reproduce. Points just 1 min apart vary in flux by more than a factor of two. The present dataset does not uniquely determine the mechanism causing these rapid changes. We suggest that changing wind conditions, kilometer-wavelength waves, or moving clouds could account for the variability. Future specular observations should be designed with a fast cadence, at least 6 points per minute, in order to differentiate between these hypotheses. Such new data will further constrain the nature of Titan’s lakes and their interactions with Titan’s atmosphere.     

Effects of moisture on strata control in coal mines

Moisture has been known to influence strata failures in coal mines for a long time. However, it is only in the last decade that researchers have attempted to quantify this effect. In the laboratory, attempts have been made to study the effects of moisture on the strength of coal-bearing rocks, moisture gain in shales as a function of time and pressures generated due to swelling, slaking and swelling indices and their possible correlation to strata control problems, and effect of humidity variations on anchor creep and bearing plate performance in conjunction with conventional bolts. In the field, researchers have attempted to correlate humidity variations with the incidence and frequency of roof falls and roof convergence, and effects of air tempering on reducing humidity variations in the mine.

A concise review of what is known about effects of moisture on strata control with emphasis on the Illinois basin coal mines is presented in this paper. Some approaches to mitigate the effects of moisture—leaving coal in the roof, guniting and shotcreting, application of sealants—are then briefly discussed.     

First petrographic results on impactites from the Yaxcopoil‐1 borehole,Chicxulub structure,Mexico

Abstract— The ICDP Yaxcopoil‐1 (Yax‐1) borehole located 60 km south‐southwest of the center of the Chicxulub impact structure intercepted an interval of allogenic impactites (depth of 795–895 m). Petrographic analysis of these impactites allows them to be differentiated into five units based on their textural and modal variations. Unit 1 (795–922 m) comprises an apparently reworked, poorly sorted and graded, fine‐grained, clast‐supported, melt fragment‐bearing suevitic breccia. The interstitial material, similar to units 2 and 3, is permeated by numerous carbonate veinlets. Units 2 (823–846 m) and 3 (846–861 m) are groundmass‐supported breccias that comprise green to variegated angular and fluidal melt particles. The groundmass of units 2 and 3 comprises predominantly fine‐grained calcite, altered alkali element‐, Ca‐, and Si‐rich cement, as well as occasional lithic fragments. Unit 4 (861–885 m) represents a massive, variably devitrified, and brecciated impact melt rock. The lowermost unit, unit 5 (885–895 m), comprises highly variable proportions of melt rock particles (MRP) and lithic fragments in a fine‐grained, carbonate‐dominated groundmass. This groundmass could represent either a secondary hydrothermal phase or a carbonate melt phase, or both. Units 1 and 5 contain well‐preserved foraminifera fossils and a significantly higher proportion of carbonate clasts than the other units. All units show diagnostic shock deformation features in quartz and feldspar clasts. Our observations reveal that most felsic and all mafic MRP are altered. They register extensive K‐metasomatism. In terms of emplacement, we suggest that units 1 to 3 represent fallout suevite from a collapsing impact plume, whereby unit 1 was subsequently reworked by resurging water. Unit 4 represents a coherent impact melt body, the formation of which involved a significant proportion of crystalline basement. Unit 5 is believed to represent an initial ejecta/ground‐surge deposit.     

Distribution ofPerkinsus marinus in Gulf Coast oyster populations

Prevalence (percent of oysters infected) ofPerkinsus marinus and infection intensity were measured in oysters from 49 sites in the Gulf of Mexico. Prevalence was less than 50% at only one site. Both prevalence and infection intensity were correlated with condition index, salinity, and a measure of local agricultural activity. The regional distribution ofP. marinus was patchy on spatial scales of 300 km or less and 1,500 km or more. Three regional foci of infection could be distinguished: the north central coast of Texas, central Louisiana west of the Mississippi River, and the southwestern coast of Florida. Lowest infection levels were recorded along the north central and northeastern Gulf, particularly east of the Mississippi River. The spatial distribution of infection varied with the salinity regime; however, other factors also explained part of the regional patterns observed. These included factors associated with man’s activities such as agricultural and industrial activity and the average annual temperature regime.     

Re–Os isotopic constraints on magma mixing in the Peridotite Zone of the Stillwater Complex, Montana, USA

Chromite separates within the Peridotite Zone of the 2,700 Ma-old Stillwater Complex are characterized by low ¹⁸⁷Re/¹⁸⁸Os (0.009 to 1.74) and relatively high Os concentrations (8.74 to 78.2 ppb). Their calculated initial Os isotopic compositions likely reflect the compositions of the magmas from which they crystallized. The chromites show variable initial Os isotopic compositions (%_Os of +2.0 to +16.4) over the vertical extent of the Peridotite Zone, implicating at least two sources of Os. Both the range of %_Os and values of %_Os decrease upsection. These variations in %_Os were caused by mixing of variable proportions of two magmas having different Os isotopic compositions. One of the magmatic components was a more primitive magma with a nearly chondritic Os isotopic composition. The other magma had a radiogenic Os isotopic composition as a result of assimilation of crust, perhaps of sedimentary rocks beneath the Stillwater Complex. The gradual decrease in the initial %_Os values of the chromite layers with increasing stratigraphic height implies a decreasing relative contribution from the contaminated magma throughout the growth of the Peridotite Zone. Small variations in %_Os between different chromite occurrences within the H multicyclic unit reflects the petrologic requirement that chromite layers crystallize from slightly different proportions of the magmas, compared to chromite from olivine- and orthopyroxene-rich layers.