The Cayconi district of the Cordillera de Carabaya, SE Peru, exposes a remnant of an upper Oligocene–Lower Miocene (22.2–24.4 Ma) volcanic field, comprising a diverse assemblage of S-type silicic and calc-alkaline basaltic to andesitic flows, members of the Picotani Group of the Central Andean Inner Arc. Basaltic flows containing olivine, plagioclase, clinopyroxene, ilmenite and glass, and glassy rhyolitic agglutinates with phenocrystic quartz, cordierite, plagioclase, sanidine, ilmenite and apatite, respectively exhibit mineralogical and geochemical features characteristic of medium-K mafic and Lachlan S-type silicic lavas. Cordierite-bearing dacitic agglomerates and lavas, however, are characterized by dispersed, melanocratic micro-enclaves and phenocrysts set in a fine-grained quartzo-feldspathic matrix. They contain a bimodal mica population, comprising phlogopite and biotite, as well as complexly zoned, sieve-textured plagioclase grains, sector-zoned cordierite, sanidine, quartz, irregular patches of replaced olivine, clinopyroxene and orthopyroxene and accessory phases including zircon, monazite, ilmenite and chromite. The coexistence of minerals not in mutual equilibrium and the growth/dissolution textures exhibited by plagioclase are features indicative of magmatic commingling and mixing. Trachytic-textured andesite flows interlayered with olivine+plagioclase–glomerophyric, calc-alkaline basalts have a phenocrystic assemblage of resorbed orthopyroxene and plagioclase and exhibit melanocratic groundmass patches of microphenocrystic phlogopite, Ca-rich sanidine, ilmenite and aluminous spinel. The mineralogical and mineral chemical relationships in both the dacites and the trachytic-textured andesites imply subvolcanic mixing between distinct ultrapotassic mafic melts, not represented by exposed rock types, and both the S-type silicic and calc-alkaline mafic magmas. Such mixing relationships are commonly observed in the Oligo-Miocene rocks of the Cordillera de Carabaya, suggesting that the S-type rocks in this area and, by extension, elsewhere derive their unusually high K2O, Ba, Sr, Cr and Ni concentrations from commingling and mixing with diverse, mantle-derived potassic mafic magmas. 相似文献
Previously proposed methods of area selection for diamond-prospective regions have predominantly relied on till geochemistry, airborne geophysics, and/or an appraisal of tectonic setting. Herein we suggest that a novel, deep-probing geophysical technique—electromagnetic studies using the natural-source magnetotelluric (MT) method—can contribute to such an activity. Essentially, diamondiferous regions must have (1) old lithosphere, (2) thick lithosphere, and (3) lithosphere that contains high concentrations of carbon. Deep-probing MT studies are able to address all three of these. The second and the third of these can be accomplished independently using MT, but for the first the geometries produced from modelling the MT observations must be interpreted with appropriate interaction with geologists, geochemists and other geophysicists. Examples are given from the Slave and Superior cratons in North America, with a brief mention of an area of the Rae craton, and general speculations about possible diamondiferous regions. 相似文献
A finite element model (namely TELEMAC) with a range of mesh refinements and assumptions of coastal water depths is used to
determine an optimal mesh for computing the M2 tide in a region of significant geographical extent. The region adopted is the west coast of Britain covering the Irish and
Celtic Seas. The nature of the spatially varying topography and tidal distribution, together with a comprehensive set of measurements
and existing accurate finite difference model makes it ideal for such a study. Calculations show that a water-depth dependent
criterion for determining element size gives an optimal distribution over the majority of the region. However, local refinements
in narrow channels such as the North Channel and Bristol Channel are required. Although the specification of a zero coastal
water depth, leads to a fine near coastal grid, this does not yield the most accurate solution. In addition the computational
cost is high. In practice in a large area model the use of a non-zero coastal water depth yields optimum accuracy at minimal
computational cost. However, calculations show that accuracy is critically dependent upon nearshore water depths. Comparison
with the finite difference model shows that the bias in elevation amplitude in the finite difference solution is removed in
the finite element calculation. 相似文献
A three-dimensional baroclinic finite element model with a coarse and fine (i.e. local refinement along the shelf edge) grid
is used to examine the influence of shelf edge grid refinement upon the internal tide generation and propagation off the west
coast of Scotland. Comparisons are made with observations in the region and with a published solution using a finite difference
model. The calculations show that provided that the finite element grid is refined in the internal tide generation area and
the adjacent region through which the internal tide propagates, then a numerically accurate solution is obtained. In the regions
of strong internal tide generation with a local grid refinement, internal wave energy can accumulate at small scales and must
be removed by a scale-selective filter. 相似文献
The Fe isotope composition of samples from the Moon, Mars (SNC meteorites), HED parent body (eucrites), pallasites (metal and silicate) and the Earth's mantle were measured using high mass resolution MC-ICP-MS. These high precision measurements (δ56Fe ≈ ± 0.04‰, 2 S.D.) place tight constraints on Fe isotope fractionation during planetary differentiation.Fractionation during planetary core formation is confined to < 0.1‰ for δ56Fe by the indistinguishable Fe isotope composition of pallasite bulk metal (including sulfides and phosphides) and olivine separates. However, large isotopic variations (≈ 0.5‰) were observed among pallasite metal separates, varying systematically with the amounts of troilite, schreibersite, kamacite and taenite. Troilite generally has the lightest (δ56Fe ≈ − 0.25‰) and schreibersite the heaviest (δ56Fe ≈ + 0.2‰) Fe isotope composition. Taenite is heavier then kamacite. Therefore, these variations probably reflect Fe isotope fractionation during the late stage evolution and differentiation of the S- and P-rich metal melts, and during low-temperature kamacite exsolution, rather than fractionation during silicate-metal separation.Differentiation of the silicate portion of planets also seems to fractionate Fe isotopes. Notably, magmatic rocks (partial melts) are systematically isotopically heavier than their mantle protoliths. This is indicated by the mean of 11 terrestrial peridotite samples from different tectonic settings (δ56Fe = + 0.015 ± 0.018‰), which is significantly lighter than the mean of terrestrial basalts (δ56Fe = + 0.076 ± 0.029‰). We consider the peridotite mean to be the best estimate for the Fe isotope composition of the bulk silicate Earth, and probably also of bulk Earth. The terrestrial basaltic mean is in good agreement with the mean of the lunar samples (δ56Fe = + 0.073 ± 0.019‰), excluding the high-Ti basalts. The high-Ti basalts display the heaviest Fe isotope composition of all rocks measured here (δ56Fe ≈ + 0.2‰). This is interpreted as a fingerprint of the lunar magma ocean, which produced a very heterogeneous mantle, including the ilmenite-rich source regions of these basalts.Within uncertainties, samples from Mars (SNC meteorites), HED (eucrites) and the pallasites (average olivine + metal) have the same Fe isotope compositions as the Earth's mantle. This indicates that the solar system is very homogeneous in Fe isotopes. Its average δ56Fe is very close to that of the IRMM-014 standard. 相似文献
Mineralogically zoned and unzoned discordant bodies composed predominately of plagioclase with up to 35% olivine, occur at
three different levels in Olivine-Bearing zones III and IV of the Middle Banded series of the Stillwater complex. The discordant
bodies are elongate perpendicular to the layering of the host cumulates with slender concordant apophyses. Although the host
olivine-gabbros are foliated with tabular plagioclase, the discordant bodies lack a discernible fabric and have blocky plagioclase.
Average olivine in the host rocks is slightly more magnesian than that of the discordant bodies (Mg#75.8 ± 0.7 versus Mg#74.6 ± 0.3 respectively) but plagioclase compositions are indistinguishable (An77.6 ± 2.0 versus An76.6 ± 4.3– average host and discordant bodies respectively). Whole-rock major- and trace-element compositions of the discordant bodies
are generally indistinguishable from cumulates with similar modal abundance. However, bulk compositions of anorthositic cores
from the discordant bodies are enriched in K, Na, Ba, Sr and P. We conclude that the discordant bodies formed when cooler
volatile fluids or fluid-rich silicate liquids moved upward and encountered a hotter undersaturated solid-plus-liquid assemblage.
Continued liquid/fluid fluxing increased the permeability along the flow path and focused the flow, allowing the original
bulk compositions to be modified and leaving plagioclase-rich troctolites and anorthosites. The shapes of the discordant bodies
suggest that the cumulus pile had anisotropic permeability during late-stage liquid/fluid flow. Chemical and mineralogical
evidence from other parts of Olivine-Bearing zones III and IV suggests that the processes that formed the discordant bodies
may have influenced other cumulates. In fact, it appears that the same processes that formed the discordant bodies operated
within an anorthositic layer, strongly modifying the chemistry of the rock but leaving no mineralogical or textural evidence.
Received: 10 December 1996 / Accepted: 12 August 1997 相似文献
Hydrous pyrolysis in flexible gold-bag autoclaves was used to study the production of carboxylic acids and light hydrocarbons from two marine type IIb source rocks (New Albany and Phosphoria Shales). Kerogen pyrolysis produced significant amounts of the monocarboxylic acids (acetic > propionic > butyric). The gases were dominated by CO2 and methane, in that order, and progressively smaller amounts of the alkanes (ethane > propane > butane > pentane). Kinetic analyses of production rates for the New Albany Shale suggest mean activation energies (E) of 51-54 kcal/mol for both the light hydrocarbons and acids. Pressure had little effect on measured production rates for either shale over the pressure range investigated. Chemical thermodynamic speciation modeling suggests that in these experiments metal-organic acid anion complexation had little impact on aluminum speciation/solubility, but was important with respect to the alkaline earths. 相似文献
The Upper Permian Castile Formation of the Delaware Basin in northwest Texas and New Mexico consists of up to 600 m of evaporites and is subdivided into units of anhydrite overlain by halite. The Castile Formation has commonly been interpreted as a deep-water, deep-basin deposit in which sediments were laid down in several hundred metres of water or brine. Recent textural observations within anhydrite units, in which the thick-bedded anhydrite horizons have been interpreted as being of shallow-water origin, have challenged this assumption. This geochemical study of the oldest anhydrite unit in the Castile Formation (the Anhydrite 1 Member) attempts to resolve some of the problems regarding brine depth and evolution in the basin. The Anhydrite 1 Member has been subdivided into five major cycles on the basis of the distribution of stratigraphic units of thick-bedded anhydrite.
Stable isotopic analyses of sulphur from anhydrite, and oxygen and carbon from calcite show that the basin waters were chemically homogeneous during precipitation of anhydrite, and do not indicate any significant input of meteoric, continental-derived waters. Throughout the section studied progressive enrichment of 18O upwards within cored intervals indicates continuous evaporation of the water body. Carbon isotopes appear to indicate fluctuations in organic activity within the cycles. Trace elemental analyses of Fe, Mg, Sr, Mn, Al, Ba, Zn, Pb and Cu from the sulphate fraction of the samples show a very high variability. There is a distinct increase in trace elemental abundances at the tops of cycles which may indicate variations in precipitation kinetics. Analyses of texturally defined cycles show that up-core trends for many of the trace elements correlate with changes in δ18O, indicating a progressive increase in the influence of evaporation. In addition, cyclical variations in trace elemental composition indicate changes in basin conditions with around a 350-year cyclicity. These changes are independent of δ18O values. The geochemical data do not provide conclusive proof of water depth during deposition of the Castile Formation. The data are interpreted as reflecting small-scale changes in conditions of deposition, despite the fact that water input remained essentially constant in terms of chemical composition. 相似文献