Lithification of gas-rich meteorites

High-voltage electron microscopy has been used to study the fine structure of four gas-rich meteorites, with particular reference to the cementation and compaction processes that have affected the fine-grained matrix. The observed features are compared with similar effects in lunar breccias. Lithification is attributed to the passage of shock waves through porous aggregates, causing deformation whose intensity varied spatially on a small scale, the most intense deformation and heating effects being concentrated at the edges of large grains and in the matrix between them. It is inferred that relatively mild shocks have produced amorphous cement between matrix grains in the achondrite Khor Temiki and the chondrite Weston. Relatively intense shock is inferred for specimens of the chondrites St. Mesmin and Pantar. These have non-porous, completely crystalline matrices, and fine-grained black veins which fill cracks in relatively large deformed grains. Recrystallization of some deformed material is attributed to shock-heating, which was not sustained long enough to erase the irradiation record from all the relatively large grains. Matrix recrystallization without extensive melting constitutes a metamorphic event, and the observations indicate that shock was responsible for the metamorphism experienced by these chondrites at relatively late stages of the evolution of their parent bodies.     

Incipient melting in and shock classification of L-group chondrites

Thirty-three of 52 type L4 to L6 chondrites that we have examined in thin section contain closed bodies of crystal-laden glass or devitrified glass (melt pockets) that testify to in-situ melting. A close correlation between the distribution of melt pockets and shock intensity as inferred from the characteristics of olivine and plagioclase indicates that the pockets reflect shock melting. The appearance of pockets coincides with a sharp decrease of⁴⁰Ar in L-group chondrites, suggesting that shock melting was responsible for loss of argon and raising the possibility that this process redistributed other volatile elements as well. The use of three criteria for shock intensity — olivine and plagioclase characteristics, and the presence or absence of melt pockets — leads to a refined shock classification for equilibrated chondrites that is based entirely on petrographic observations.     

In quest of lunar regolith breccias of exotic provenance: a uniquely anorthositic sample from the Fra Mauro (Apollo 14) highlands

Polymict samples can be used to establish mass-balance constraints regarding the bulk composition of the lunar crust, and to gauge the degree of regional heterogeneity in the composition of the lunar crust. The most ideally polymict type of sample is finely-mixed regolith (lunar soil), or its lithified equivalent, regolith breccia. Fortunately, lunar regolith breccias can occasionally be found at great distances from their points of origin — most of the known lunar meteorites are regolith breccias. We are searching for examples of exotic regolith samples among the Apollo regolith breccia collection. Most of the 21 Apollo regolith breccias analyzed for this study strongly resemble the local soils over which they were collected. Nine regolith breccias from Apollo 16 are surprisingly mature compared to previously-analyzed Apollo 16 regolith breccias, and six of the seven from Apollo 16 Station 5 have lower, more local-soil-like,mg ratios than previously analyzed regolith breccias from this station. Several of the Apollo 14 regolith breccias investigated show significantly highermg, and lower Al, than the local soils.The most interesting sample we have investigated is 14076,1, from a lithology that constitutes roughly half of a 2.0-g pebble. The presence of spherules indicates a regolith derivation for 14076,1, yet its highly aluminous (30 wt.% Al₂O₃) composition is clearly exotic to the 1.6-km traverse surface over which the Apollo 14 samples were collected. This sample resembles soils from the Descartes (Apollo 16) highlands far more than it does any other polymict sample from the Fra Mauro (Apollo 14) region. The I/_sFeO maturity index is extremely low, but this may be a result of thermal annealing. A variety of siderophile elements occur in 14076,1 at typical regolith concentrations. The chemistry of the second most aluminous regolith sample from Apollo 14, 14315, can only be roughly approximated as a mixture of local regolith and 14076,1-like material. However, the low a priori statistical probability for long-distance horizontal transport by impact cratering, along with the relatively high contents of incompatible elements in 14076,1 (despite its high Al content), suggest that this regolith breccia probably originated within a few hundred kilometers of the Appollo 14 site. If so, its compositional resemblance to ferroan anorthosite tends to suggest that the regional crust is, or originally was, far richer in ferroan anorthosite than implied by the meager statistics for pristine rocks from this site. Thus, 14076,1 tends to strengthen the hypothesis that ferroan anorthosite originated as the flotation crust of a global magmasphere.     

The lithification and metamorphism of lunar breccias

Experimental data have been used to establish the temperature coordinates for Warner's [1] scale of lunar metamorphic grades. A model of an ejecta blanket with a hot base layer overlain by a cold top layer can produce the observed features of the Apollo 14 breccias. The breccia formation process probably encompassed temperatures between 700°C and at least 1100°C and was a reasonably rapid process (hours to days).     

Strong slope-aspect control of regolith thickness by bedrock foliation

The porous near-surface layer of the Earth's crust – the critical zone – constitutes a vital reservoir of water for ecosystems, provides baseflow to streams, guides recharge to deep aquifers, filters contaminants from groundwater, and regulates the long-term evolution of landscapes. Recent work suggests that the controls on regolith thickness include climate, tectonics, lithology, and vegetation. However, the relative paucity of observations of regolith structure and properties at landscape scales means that theoretical models of critical zone structure are incompletely tested. Here we present seismic refraction and electrical resistivity surveys that thoroughly characterize subsurface structure in a small catchment in the Santa Catalina Mountains, Arizona, USA, where slope-aspect effects on regolith structure are expected based on differences in vegetation. Our results show a stark contrast in physical properties and inferred regolith thickness on opposing slopes, but in the opposite sense of that expected from environmental models and observed vegetation patterns. Although vegetation (as expressed by normalized difference vegetation index [NDVI]) is denser on the north-facing slope, regolith on the south-facing slope is four times thicker (as indicated by lower seismic velocities and resistivities). This contrast cannot be explained by variations in topographic stress or conventional hillslope morphology models. Instead, regolith thickness appears to be controlled by metamorphic foliation: regolith is thicker where foliation dips into the topography, and thinner where foliation is nearly parallel to the surface. We hypothesize that, in this catchment, hydraulic conductivity and infiltration capacity control weathering: infiltration is hindered and regolith is thin where foliation is parallel to the surface topography, whereas water infiltrates deeper and regolith is thicker where foliation intersects topography at a substantial angle. These results suggest that bedrock foliation, and perhaps by extension sedimentary layering, can control regolith thickness and must be accounted for in models of critical zone development. © 2020 John Wiley & Sons, Ltd.     

Uranium-lead age of the Bruderheim L6 chondrite and the 500-Ma shock event in the L-group parent body

Measured Pb-Pb whole meteorite data for the Bruderheim L6 chondrite scatter slightly about a line passing above Can?on Diablo lead and yielding an age of 4.482 + 0.017 Ga, using the terrestrial²³⁸U/²³⁵U ratio of 137.88. The measured U isotopic composition for Bruderheim, using the dissolution procedures employed for these U-Pb studies, is near the terrestrial composition. In the concordia diagram the U-Pb data chiefly plot above the concordia curve and define a line which intersects the concordia curve at 4.536 + 0.006 Ga and 0.495 Ga, but the data for Bruderheim cannot be understood at all in terms of the more usual two- or three-stage episodic U-Pb models involving a fixed μ-value in the first stage. Most samples show an apparent excess of radiogenic lead for single-stage (closed system) evolution when Can?on Diablo troilite is used for the initial lead composition. Evidence is presented to show that the apparent excess radiogenic lead cannot be explained by terrestrial contamination alone.A different U-Pb model is presented which describes qualitatively and quantitatively most features of the U-Pb data for Bruderheim. If this model correctly describes the U-Pb evolution of Bruderheim then the “formation” age is given as 4.536 Ga by both U-Pb and Pb-Pb data, the meteorite U-Pb system was disturbed by a later (shock?) event at about 500 Ma ago, and the data are consistent with (though do not require) a Can?on Diablo initial lead composition. This interpretation suggests that the classical phenomenon of apparent excess radiogenic lead reflects the application of a single-stage model to a meteorite that has evidently experienced at least a two-stage history. The explanation of the observation that in the concordia diagram most meteorite samples (corrected for Can?on Diablo lead) plot in the lead excess region remains obscure, though this may be due to the wrong choice of initial lead for Bruderheim.     

Giant gas-rich hydrothermal systems and their role in the generation of vapor-dominated geothermal fields and ore mineralization

This paper characterizes certain unique geological structures on the earth, viz., giant gas-rich hydrothermal systems with major vapor-dominated geothermal fields that are generated beneath them during the present phase of evolution. A review of the relevant literature and materials of our own research are used to show that such systems are formed in zones of deep-seated faults at junctions of oceanic and continental plates, in structures of volcanic island arcs, and in areas of crustal tectono-magmatic activity. The systems extend throughout the crustal thickness and possess enormous geothermal and ore potentials. It was found that in these systems the ascending high-temperature gas-water fluid, as well as all types of mixed waters, and new mineral compounds in the hypergenesis zone of geothermal anomalies, all take part in the transport, accumulation, and rearrangement of complex compounds of many metals (Fe, Al, Ti, Au, Ag, Hg, As, Sb, and others). It was inferred that gas-rich hydrothermal systems and the vapor-dominated geothermal fields that are formed beneath them reflect the conditions for the generation of mesothermal and epithermal gold and complex ores and of Au-Ag-Cu-Mo porphyric deposits.     

The equation-of-states of Jilin ordinary chondrite and Nandan iron meteorite

Shock wave data for Jilin ordinary chondrite and Nandan iron meteorite are measured by electric-pin techniques on the dynamic high-pressure device equipped with a two-stage light gas gun, and then equation-of-states supposedly fit for describing theirP- V-E relations are chosen to evaluate the values of their parameters. Demonstrated from the comparison between P-V curves of equation-of-states and experimental data points, P-V relation of Jilin ordinary chondrite can be described by the universal equation-of-state, of which bulk modulus value of zero-pressure K_os = 48.10 GPa, its pressure derivativeK’ _os = 4.13. That of Nandan iron meteorite can be described by the three-term form of equation-of-state, of which the values of two matter parameters Q = 41.235 31 GPa, q = 12. 27179. The Hugoniot data and equation-of-states for Jilin ordinary chondrite and Nandan iron meteorite are first reported in this paper. The equation-of-state defined by Hugoniot data measurement provides strong empirical support for describing the compression of meteorites to high pressure. Project supported by the National Natural Science Foundation of China.     

Origin of rhyolites by anatectic melting of granitic crustal rocks

Rhyolitic pumice deposits on some East Aegean Islands are the remnants of a tuff sheet which covered formerly more than 2500 km². The rhyolites originated by melting of granitic crustal rocks. The petrological properties leading to this conclusion are as follows:

-mineralogical composition of the pumice being 60 wt % of glass and 40 % of relictic granitic minerals (quartz, plagioclase, potash feldspar, biotite) in «phenocrysts» up to 1 cm Ø.

-phenomena of strong corrosion, resorption and melting of all light colored «phenocrysts» in the rhyolitic pumice.

-mineralogical and chemical identity between pumice and granitic xenolites found in great quantity in the pumice tuff.

-structurally transitional types between pumice and the granites in different state of melting.

-eutectic composition of the pumice glass for a pressure of 2 kb.

     

Rock damage and regolith transport by frost: an example of climate modulation of the geomorphology of the critical zone

In this article we craft process‐specific algorithms that capture climate control of hillslope evolution in order to elucidate the legacy of past climate on present critical zone architecture and topography. Models of hillslope evolution traditionally comprise rock detachment into the mobile layer, mobile regolith transport, and a channel incision or aggradation boundary condition. We extend this system into the deep critical zone by considering a weathering damage zone below the mobile regolith in which rock strength is diminished; the degree of damage conditions the rate of mobile regolith production. We first discuss generic damage profiles in which appropriate length and damage scales govern profile shapes, and examine their dependence upon exhumation rate. We then introduce climate control through the example of rock damage by frost‐generated crack growth. We augment existing frost cracking models by incorporating damage rate limitations for long transport distances for water to the freezing front. Finally we link the frost cracking damage model, a mobile regolith production rule in which rock entrainment is conditioned by the damage state of the rock, and a frost creep transport model, to examine the evolution of an interfluve under oscillating climate. Aspect‐related differences in mean annual surface temperatures result in differences in bedrock damage rate and mobile regolith transport efficiency, which in turn lead to asymmetries in critical zone architecture and hillslope form (divide migration). In a quasi‐steady state hillslope, the lowering rate is uniform, and the damage profile is better developed on north‐facing slopes where the frost damage process is most intense. Because the residence times of mobile regolith and weathered bedrock in such landscapes are on the order of 10 to 100 ka, climate cycles over similar timescales result in modulation of transport and damage efficiencies. These lead to temporal variation in mobile regolith thickness, and to corresponding changes in sediment delivery to bounding streams. Copyright © 2012 John Wiley & Sons, Ltd.     

Nitrogen contents and isotopic ratios of clasts from the enstatite chondrite Abee

Nitrogen contents and isotopic ratios have been determined for three clasts from the enstatite chondrite Abee by stepwise heating. The clasts possess a wide range in nitrogen content, ranging from 254 to 850 ppm, whereas the nitrogen isotopic ratios are nearly identical atδ¹⁵N= ?29.2±0.6‰. A refractory inorganic nitrogen-bearing phase contains about 90% of the nitrogen which is released at temperatures of 1000°C and above. The stepwise heating experiments suggest the possible existence of two other distinct nitrogen components, released at low (770°C) and high (1500°C) temperatures.     

Dynamic response of flexible strip-foundations by boundary and finite elements

A time domain Boundary Element-Finite method is employed to determine the dynamic response of flexible surface two-dimensional foundations under conditions of plane strain placed on an elastic soil medium and subjected either to transient external forces or to obliquely incident seismic waves. The elastic, isotropic, and homogeneous soil medium is treated by the time domain Direct Boundary Element Method, while the flexible foundation is treated by the Finite Element Method. The two methods are appropriately combined through equilibrium and compatibility considerations at the soil-foundation interface. Parametric studies examining the effect of the relative stiffness between the foundation and the soil and the spatial distribution of the dynamic disturbances on the foundation response are presented.     

Composition and origin of clasts and inclusions in the Abee enstatite chondrite breccia

The concentrations of 25 major, minor and trace elements have been determined in four clasts, a metal-rich inclusion and two dark metal-poor inclusions from the Abee enstatite chondrite. The clasts are heterogeneous, displaying 2-fold enrichments or depletions in some elements. The data suggest that there are two generations of metal, one with low, the other with high concentrations of refractory siderophiles. The other elemental patterns can be understood in terms of variations in the abundance of major minerals. We infer that Sc and Mn are located largely in the niningerite ((Fe,Mg)S), V in the troilite (FeS) and rare earth elements in the oldhamite (CaS).Heterogeneities among the clasts are probably primary, resulting from the accretion-agglomeration process, although shock processes in a regolithic setting remain a possibility provided that they were followed by a period of metamorphism sufficient to erase petrologic evidence.In the dark inclusions the concentrations of the rare earths, Eu excepted, are 4 × higher than mean EH levels; this infers enhanced amounts of CaS. The dark inclusions are low in siderophiles, Sc, Mn, K, Na and Al, implying low amounts of metal, niningerite and feldspar. The origin of the dark inclusions is unclear; they do not appear to be the result of a simple, single-stage process.     

Mineralogy and petrology of the Abee enstatite chondrite breccia and its dark inclusions

The Abee E4 enstatite chondrite breccia consists of clasts (many rimmed by metallic Fe, Ni), dark inclusions and matrix. The clasts and matrix were well equilibrated by thermal metamorphism, as evidenced by uniform mineral compositions, recrystallized chondrules, low MnO content of enstatite and high abundance of orthoenstatite. The clasts acquired their metal-rich rims prior to this metamorphic episode. The occurrence in Abee of relatively unmetamorphosed dark inclusions, clasts with nearly random magnetic orientations and a matrix with a uniform magnetic orientation [18,19] indicates that clast and matrix metamorphism occurred prior to the agglomeration of the breccia.The dark inclusions are an unusual kind of enstatite chondritic material, distinguished from the clasts and matrix by their relative enrichments in REE [21–23], low relative abundances of kamacite, total metallic Fe, Ni and silica, lower niningerite/(total sulfide) ratios, high relative abundances of oldhamite and martensite, smaller euhedral enstatite, more heterogeneous enstatite and metallic Fe, Ni, more calcic enstatite and more nickeliferous schreibersite.We propose the following model for the petrogenesis of the Abee breccia: The maximum metamorphic temperature of breccia parent material was?- 840°C (the minimum temperature of formation of Abee niningerite) and perhaps near 950–1000°C (the Fe-Ni-S eutectic temperature). Euhedral enstatite crystals in metallic Fe, Ni- and sulfide-rich areas grew at these metamorphic temperatures into pliable metal and sulfide. Breccia parent material was impact-excavated from depth, admixed with dark inclusions and rapidly cooled (700 to 200°C in about 2 hours) [15]. During this cooling, clast and matrix material acquired thermal remanent magnetization. Random conglomeration of clasts and unconsolidated matrix materials caused the clasts to have random magnetic orientations and the matrix areas to have net magnetic intensities of zero (due to the cancellation of numerous randomly oriented magnetic vectors of equal intensity in the matrix). A subsequent ambient magnetic field imparted a uniform net magnetic orientation to the matrix and caused the magnetic orientations of the clasts to be somewhat less random. The Abee breccia was later consolidated, possibly by shock or by shallow burial and very long-period/low-temperature (< 215°C) metamorphism.     

Field and paleomagnetic characterization of lithic and scoriaceous breccias at Pleistocene Broken Top volcano, Oregon Cascades

Cirque-wall exposures of cone-forming deposits of Pleistocene Broken Top volcano, Oregon Cascade Range, reveal that the volcano is composed of unconformity-bounded constructional units of coherent lava (lava-flow cores) and breccia. Coarse-grained autoclastic breccias are found above and below lava-flow cores and may extend downslope from coherent lava outcrops where they may or may not be associated with thin lava stringers. Mantle-bedded scoria-fall breccias are recognized by generally good sorting, mantle bedding, and presence of aerodynamically shaped bombs. These breccias vary considerably in thermal oxidation coloration (black, red, orange, purple). Many breccia layers are unsorted mixtures of scoria and lithic (nonvesicular) fragments that grade laterally to unambiguous autoclastic breccia or lava-flow cores. These layers are interpreted as hybrid pyroclastic–autoclastic deposits produced by incorporation of falling or fallen tephra into advancing lava-flow fronts. This latter breccia type is common at Broken Top and offers particular challenges for clast or deposit classification.Progressive thermal demagnetization results for selected examples of different breccia types show that most scoria-fall and autoclastic breccias are emplaced at elevated temperatures (averaging 100–300°C). Clasts within single deposits record different emplacement temperatures ranging, in some cases, from 100 to over 580°C indicating a lack of thermal equilibration within deposits. Magnetization directions for single breccia deposits are more dispersed than data typically reported for lava flows. Settling and rotation of clasts after cooling or incorporation of colder clasts that are not significantly reheated probably accounts for the relatively high dispersion and suggests that paleomagnetic studies demanding low within-site dispersion (e.g., for determining paleomagnetic poles or evaluating tectonic rotation) should avoid volcanic breccias.     

Anhydrous melting and crystallization of peralkaline obsidians

Experimental determinations of the dry liquidus temperatures of two pantellerite, and two pantelleritic trachyte glasses in the pressure range 0–2 kilobars, show minima in the liquidus curves between 0.1 and 0.2 kb. The pantellerite minima are 830°–850° C; the trachyte minima are 920°–940° C. At pressures below the minima a separate vapour phase co-exists with liquid, at higher pressures the intrinsic volatiles are completely soluble in the liquid and the liquidus curves have a positivedT/dP. Similar results have been obtained from a range of other pantelleritic glasses, and together with consistent alkali feldspar compositions (from a wide range of experimental conditions) are indicative of a close approach to equilibrium. The form of the liquidus curves above the minima, if rellecting natural conditions, offers a ready explanation of the near-or super-liquidus aspect of many peralkaline lavas. The temperatures in these anhydrous experiments are 100°–150° C higher than those for similar compositions in the presence of excess water. (Also, in the presence of excess water, the crystallization sequences in the natural glasses are profoundly modified, with pyroxene appearing on the liquidus). At lower pressures, feldspar is the liquidus phase in the dry pantellerites, but is joined by quartz around 1 kb, and superseded by quartz at higher pressures. As pantellerites with quartz phenocrysts are uncommon, low pressure equilibration is perhaps normal in these magmas. Feidspar is the usual liquidus phase in the trachytes, except at very low pressures where it is preceded by iron oxide. Preliminary studies at 5 kb indicate that the pantelleritic and trachytic liquidus curves are converging (in the range 950°–1000° C). Crystallization sequences, and the forms and positions of the solidus curves are therefore of vital importance. These, together with the vapour-present/vapour-absent conditions, are currently under investigation.     

Detachment and infiltration variations as consequence of regolith development in a Pyrenean badland system

The Araguás experimental catchment has been monitored to study badland dynamics in the Central Pyrenees. Previous studies of weathering processes within the catchment reported strong regolith dynamics associated with seasonal variations in the temperature and moisture regimes. A preliminary analysis of hydrological response and suspended sediment transport data recorded at a gauging station also demonstrated seasonal trends. The main objective of the present study is to understand the effect of regolith dynamics on sediment detachment and infiltration processes, based on field studies using simulated rainfall. The experiment design was based on seasonal differences in the physical conditions of surface regolith and the general trends of hydro‐sedimentological responses. Rainfall simulations were conducted on small plots using a pressure nozzle. Similar experimental rainfall conditions were set for all plots (rainfall intensity around 45 mm h^–1). The results showed strong variations in the infiltration and detachment responses closely associated with the regolith conditions and crusting development. Infiltration showed seasonal differences in time lag and intensity: average infiltration rates ranged from very low (2·05 mm h^–1) to moderated high values (44·04 mm h^–1) associated to regolith development conditions. Maximum sediment concentration, as an indicator of particles produced by detachment, also ranged from moderate (3 g l^–1) to extreme values (145 g l^–1). Mean and minimum infiltration rates showed negative correlations with initial moisture content. Sediment concentration showed a positive correlation with time lag, ponding, and sealing time, and a negative correlation with initial moisture. In terms of seasonal trends, infiltration and erosion responses were relatively stable during spring and autumn, whereas wide variations were recorded in infiltration rates and sediment detachment during summer and winter. As a general conclusion, the obtained results indicate that seasonal differences in detachment and infiltration depend on the nature of regolith development. Copyright © 2009 John Wiley & Sons, Ltd.     

Modeled steric and mass-driven sea level change caused by Greenland Ice Sheet melting

Meltwater from the Greenland Ice Sheet (GIS) has been a major contributor to sea level change in the recent past. Global and regional sea level variations caused by melting of the GIS are investigated with the finite element sea-ice ocean model (FESOM). We consider changes of local density (steric effects), mass inflow into the ocean, redistribution of mass, and gravitational effects. Five melting scenarios are simulated, where mass losses of 100, 200, 500, and 1000 Gt/yr are converted to a continuous volume flux that is homogeneously distributed along the coast of Greenland south of 75°N. In addition, a scenario of regional melt rates is calculated from daily ice melt characteristics. The global mean sea level modeled with FESOM increases by about 0.3 mm/yr if 100 Gt/yr of ice melts, which includes eustatic and steric sea level change. In the global mean the steric contribution is one order of magnitude smaller than the eustatic contribution. Regionally, especially in the North Atlantic, the steric contribution leads to strong deviations from the global mean sea level change. The modeled pattern mainly reflects the structure of temperature and salinity change in the upper ocean. Additionally, small steric variations occur due to local variability in the heat exchange between the atmosphere and the ocean. The mass loss has also affects on the gravitational attraction by the ice sheet, causing spatially varying sea level change mainly near the GIS, but also at greater distances. This effect is accounted for by using Green's functions.     

耀斑引发的激波初始速度对激波到达时间预测的影响

利用Hakamada-Akasofu-Fry运动学太阳风模型模拟了1981到1985卡林顿周的48个太阳耀斑事件激发的激波到达地球的时间.结果表明,对模式输入参数之一的激波初始速度进行调整,可以使模拟结果和实际观测基本一致.通过对发生在该时期多个事件的统计分析,分别得到日球表面东、西两半球耀斑爆发对应的激波初始速度调整因子和初始速度的统计关系.该关系应用到1986到1990卡林顿周期间发生的耀斑对应的激波到达地球时间的模拟时,对于位于西(东)半球的耀斑,预测行星际激波到达地球所需时间的平均绝对误差从未使用该关系的16(15) h降低到12(11) h.