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11.
We report here on a survey of distal fine-grained ejecta deposits on the Moon, Mars, and Venus. On all three planets, fine-grained ejecta form circular haloes that extend beyond the continuous ejecta and other types of distal deposits such as run-out lobes or ramparts. Using Earth-based radar images, we find that lunar fine-grained ejecta haloes represent meters-thick deposits with abrupt margins, and are depleted in rocks ?1 cm in diameter. Martian haloes show low nighttime thermal IR temperatures and thermal inertia, indicating the presence of fine particles estimated to range from ∼10 μm to 10 mm. Using the large sample sizes afforded by global datasets for Venus and Mars, and a complete nearside radar map for the Moon, we establish statistically robust scaling relationships between crater radius R and fine-grained ejecta run-out r* for all three planets. On the Moon, r* ∼ R−0.18 for craters 5-640 km in diameter. For Venus, radar-dark haloes are larger than those on the Moon, but scale as r* ∼ R−0.49, consistent with ejecta entrainment in Venus’ dense atmosphere. On Mars, fine-ejecta haloes are larger than lunar haloes for a given crater size, indicating entrainment of ejecta by the atmosphere or vaporized subsurface volatiles, but scale as R−0.13, similar to the ballistic lunar scaling. Ejecta suspension in vortices generated by passage of the ejecta curtain is predicted to result in ejecta run-out that scales with crater size as R1/2, and the wind speeds so generated may be insufficient to transport particles at the larger end of the calculated range. The observed scaling and morphology of the low-temperature haloes leads us rather to favor winds generated by early-stage vapor plume expansion as the emplacement mechanism for low-temperature halo materials. 相似文献
12.
R. Douglas Cruickshank Edward D. Ghent 《Contributions to Mineralogy and Petrology》1978,65(3):333-339
Aluminous pelitic rocks of the Late Precambrian Horsethief Creek Group of southeastern British Columbia contain the assemblage chloritoidmuscovite-paragonite-quartz-chlorite (biotite zone). Additional members of the assemblage may include graphite, Fe-Mg carbonate, rutile, ilmenite and pyrite. No albite was detected. Lower grade pelitic rocks (chlorite zone) contain muscovite-chlorite and rare paragonite.Chloritoids from carbonate-free assemblages show a narrow range of composition (85±5 mol % Fe-chtd) and most porphyroblasts are zoned with higher Mn in cores and higher Mg in rims. For eight chloritoid-chlorite pairs, K
D = (Mg/Fe chtd/Mg/Fe chl) = 0.188±0.0234.Correlation of these mineral assemblages with experimental and computed phase equilibria and oxygen isotope temperatures suggest a minimum pressure near 4.5 Kbar, a minimum temperature near 335 ° C and an upper limit on temperature near 460 ° C. Variation in X
CO
2 content of fluids attending metamorphism is inferred from the alternate appearance of either Fe-Mg carbonate + rutile or ilmenite-bearing assemblages. The assemblage paragonite-chloritoid-quartz-Fe-Mg carbonate-rutile is inferred to be stable at a T near 360 ° C, an X
CO
2 near 0.9 and P near 5 Kbar. 相似文献
13.
The assemblage, plagioclase-biotite-garnet-muscovite is widespread in garnet to lower sillimanite zone metasedimentary rocks. The equilibria, (1) pyr+gr+mu=3an+phl and (2) alm+gr+mu=3an+ann, involve a change in Al coordination from 6 to 4 and in Mg-Fe coordination from 8 to 6 and should be strongly pressure dependent. Using an ionic solution model we can define solid activity products for (1) and (2). Using a linear least squares analysis of activity products and estimated P-T from natural assemblages we can derive equilibrium constant equations for (1) and (2). Combining either of these equations with that for Mg-Fe exchange for garnet-biotite from Ferry and Spear (1978) allows estimation of P and T of metamorphism. Pressures estimated from these equilibria compare favorably to pressures estimated from garnet-plagioclase-Al2SiO5-quartz equilibria. 相似文献
14.
Donna L. Whitney Helen M. Lang Edward D. Ghent 《Contributions to Mineralogy and Petrology》1995,120(3-4):404-411
Qualitative and quantitative information about metamorphic reaction history and PT paths may be obtained from mineral inclusions in garnet by comparing the mineralogy, distribution, and compositions of paragenetically-related inclusions with minerals in the groundmass assemblage. Using the algebraic technique of singular value decomposition (SVD), we document mass balance relations between inclusion and groundmass assemblages in metapelitic rocks from two metamorphic terranes that experienced different peak metamorphic conditions, and whose transition from inclusion to groundmass assemblage records different PT path segments relative to peak conditions. We calculate mass balances relating an inclusion assemblage consisting in part of armored relics of chloritoid to groundmass mineral assemblages in a kyanite-staurolite mica schist from the Solitude Range, British Columbia, and an inclusion assemblage of kyanite, staurolite, and rutile to groundmass minerals in a sillimanite-cordierite gneiss from the Skagit Gneiss, North Cascade Range, Washington. Mass balances for each rock are consistent with reaction histories inferred from petrographic observations. In the Solitude Range schist, the results of mass balance calculations are consistent with the growth of staurolite and garnet at the expense of chloritoid during prograde metamorphism and suggest that chlorite, although not preserved as an inclusion, was involved in initial staurolite growth. In the Skagit sillimanite gneiss, mass balance relations exist between the inclusion suite, which formed during high pressure metamorphism, and the associated groundmass assemblage, which equilibrated at high temperature but much lower pressure. Mass balance does not exist between the groundmass of the Skagit sillimanite gneiss and the groundmass of a nearby kyanite-staurolite schist that has been proposed as a possible lower-grade equivalent of the sillimanite-bearing rocks. These results indicate that, although compositional modification and selective preservation of minerals must be taken into account, mineral inclusion suites may nevertheless preserve enough compositional information to allow reconstruction of complete or nearly complete pre-existing assemblages. This information may not be retrievable from any other source if no lower-grade equivalents of the rocks of interest are exposed. 相似文献
15.
The activity of TiO2 can be precisely defined as a function of pressure, temperature and activities of other components for common mineral assemblages
in metapelites (ilmenite-quartz-garnet-plagioclase-Al2SiO5) and in metabasites (plagioclase-sphene-ilmenite-quartzgarnet). These mineral assemblages can be modelled by the equilibria:
1) 3ilmenite+Al2SiO5+2quartz=almandine+3TiO2 2) anorthite + 2sphene = grossular + 2TiO2 + quartz 3) 3anorthite+3quartz+6ilmenite = grossular+ 6TiO2+2almandine. These mineral assemblages can be used at
(rutile saturation) and a given T to get maximum pressure limits of some metapelites and metabasites. When electron microprobe
analyses of mineral grains adjacent to Ti-bearing phases are made, these data give maximum pressure estimates in reasonable
agreement with other geobarometers. The activity of TiO2 in many metapelites is very near rutile saturation, but for metabasites the activity of TiO2 in some sillimanite zone rocks is as low as 0.6. The solubility of TiO2 in biotite, hornblende and garnet is a complex function of T, P, the activities of components in coexisting minerals and
crystal chemical constraints in these minerals. At a given P and T the solubility of TiO2 in biotite and hornblende does not appear to be strongly dependent upon
for sphene and ilmenite versus rutile-bearing assemblages. 相似文献
16.
Zeolites, calcite, quartz, kaolinite, chlorite and other authigenic minerals occur in the non-marine sandstones of the Lower Cretaceous Blairmore Group of the southern Alberta Foothills, Canada. Zeolites are restricted to plagioclase-rich sandstones and do not occur with kaolinite. Laumontite and barian-strontian heulandite (containing up to 6.9 weight percent BaO and 4.1 weight percent SrO) generally occur as pore fillings, but laumontite also occurs within albitized plagioclase. About 80 percent of plagioclase grains examined by electron microprobe are partly to completely albitized. A structural-stratigraphic reconstruction of the areas sampled indicate maximum burial depths, including tectonic thickening, in the range 4.7 to 7.8 Km, with P load ～1 to ～2 Kb, respectively. A comparison of mineral assemblages in Blairmore rocks with published experimental data suggest T did not exceed ～250°–280°C, depending on the load pressure. If P H2O∶P load, the presence of laumontite and albite suggest minimum T on the order of 150°–180°C. The lack of lawsonite suggests P-load <3Kb, which is compatible with structural-stratigraphic data. The occurrence of the alternative assemblages calcite-kaolinite-quartz and laumontite implies gradients in f CO2/f HO2. Assemblages containing laumontite presumably equilibrated with fluids having X CO2a Mg++/a (H+)2. X-ray diffraction studies on disordered graphitic material indicate a comparable degree of graphitization between Blairmore graphitic material and that found in zeolite facies assemblages elsewhere. If log f O2 were at least as low as that defined by quartz-magnetite-fayalite buffer, disordered graphite could not have equilibrated with fluids of the same composition as those equilibrated with laumontite or kaolinite. 相似文献
17.
Edward D. Ghent 《Tectonophysics》1980,62(3-4)
18.
Edward D. Ghent 《Contributions to Mineralogy and Petrology》1988,98(2):163-168
Estimation of the activity of tremolite component (atr) in calcic amphiboles is an important problem in igneous and metamorphic petrology because equilibria involving tremolite are used in the estimation of the activity of H2O attending crystallization of igneous and metamorphic rocks. Estimated values of atr from hornblende analyses using both ionic and coupled substitution crystalline solution models can be compared to values of atr calculated from vapor-absent mineral equilibria. In addition, these values of atr can be used in calculations of aH2O for rocks for which there is an independent estimate of aH2O. The values of atr calculated from vapor-absent equilibria are generally consistent with those estimated from the different crystalline solution models, but uncertainties in the calculations preclude choosing a preferred solution model. From computed mineral equilibria, it is clear that mineral assemblages with low values of atr can be in equilibrium with high values of aH2O. Consequently, the low values of aH2O estimated from hornblende-bearing high-grade rocks with low values of atr may be real. Rocks from Mica Creek, British Columbia, probably achieved extremely low values of aH2O by vapor-absent metamorphism. 相似文献
19.
Xenoliths collected from Prindle volcano, Alaska (Lat. 63.72°N; Long. 141.82°W) provide a unique opportunity to examine the lower crust of the northern Canadian Cordillera. The cone's pyroclastic deposits contain crustal and mantle-derived xenoliths. The crustal xenoliths include granulite facies metamorphic rocks and charnockites, comprising orthopyroxene (opx)–plagioclase (pl)–quartz (qtz) ± mesoperthite (msp) and clinopyroxene (cpx). Opx–cpx geothermometry yields equilibrium temperatures (T) from 770 to 1015 °C at 10 kbar. Pl–cpx–qtz geobarometry yields pressures (P) of 6.6–8.0 kbar. Integrated mesoperthite compositions suggest minimum temperatures of 1020–1140 °C at 10 kbar using solvus geothermometry. The absence of garnet in these rocks indicates a range of maximum pressure of 5–11.3 kbar, and calculated solidi constrain upper temperature limits. We conclude that the granulite facies assemblages represent relatively dry metamorphism at pressures indicative of crustal thicknesses similar to present day ( 36 km). Zircon separates from a single crustal xenolith yield mainly Early Tertiary (48–63 Ma) U–Pb ages which are considerably younger than the cooling ages of the high-pressure amphibolites exposed at the surface. The distribution of zircon ages is interpreted as indicating zircon growth coincident with at least two different thermal events as expressed at surface: (i) the eruption of the Late Cretaceous Carmacks Group volcanic rocks in western Yukon and adjacent parts of Alaska, and (ii) emplacement of strongly bimodal high level intrusions across much of western Yukon and eastern Alaska possibly in an extensional tectonic regime. The distributions of zircon growth ages and the preservation of higher-than-present-day (> 25 ± 3 °C km− 1) geothermal gradients in the granulite facies rocks demonstrate the use of crustal xenoliths for recovering records of past, lithospheric-scale thermal–tectonic events. 相似文献
20.
Geobarometry of low-temperature eclogites: applications of isothermal pressure-activity calculations 总被引：1，自引：0，他引：1
Estimation of metamorphic pressures in low temperature eclogite (Type C) is difficult because of the high variance mineral assemblages and problems in geothermometry, solution properties of low-temperature omphacite, and the thermodynamic properties of clinozoisite. We have considered equilibria in the CaO–FeO–MgO–TiO2–Al2O3–SiO2–H2O (CFMTASH) system involving the phase components, quartz, rutile, kyanite, ilmenite, almandine, pyrope, grossular, clinozoisite, sphene, diopside, and H2O-fluid There are four linearly independent equilibria involving the phase components in this system. Because kyanite can crystallize as a nearly pure phase, the lack of kyanite in a rock indicates that a
Al2SiO5 is<1.0. If we can estimate temperature independently, we can solve for a
Al2SiO5 and pressure by using two of the equilibria in isothermal pressure-activity diagrams. We have applied this approach to eclogites from New Caledonia and from southwestern Oregon. For the New Caledonia eclogites, calculated pressures range from 11.2 to 13.6 kbar at 500°C, and are consistent with the minimum pressures based upon the presence of jadeitic pyroxene+quartz and the lack of stable albite. Oregon eclogites come from different tectonic blocks and calculated minimum pressures of 11–12 kbar are based upon the presence of jadeitic pyroxene+rutile+garnet and lack of stable albite and ilmenite at reduced values of a
SiO2 (0.7–0.9). 相似文献