A petrogenetic model for the comagmatic origin of chassignites and nakhlites: Inferences from chlorine‐rich minerals,petrology, and geochemistry     

Francis M. McCubbin  Stephen M. Elardo  Charles K. Shearer Jr.  Alexander Smirnov  Erik H. Hauri  David S. Draper 《Meteoritics & planetary science》2013,48(5):819-853

Twelve samples belonging to the chassignite and nakhlite subgroups of Martian meteorites were investigated using a variety of micro‐beam analytical techniques to gain insight into the petrogenesis of these two meteorite classes. There are a striking number of geochemical similarities between the chassignites and nakhlites, including mineralogy and petrology, crystallization age, cosmic‐ray exposure age, and radiogenic isotopic compositions. However, there are also geochemical differences, namely in trace element systematics of pyroxenes, that have led some authors to conclude that the nakhlites are comagmatic with each other, but not comagmatic with the chassignites. On the basis of data presented here, we propose a model in which these differences can be reconciled by the addition of an exogenous Cl‐rich fluid to the chassignite‐nakhlite magma body shortly after the formation of the cumulate horizon that was sampled by the Chassigny meteorite. This model is supported by the textural and chemical associations of the volatile‐bearing minerals apatite, amphibole, and biotite, which record a history starting with the addition of a Cl‐ and LREE‐enriched fluid to the magma body. As the magma continued to crystallize, it eventually reached chloride saturation and degassed a Cl‐rich fluid phase. Depending on the provenance of the Cl‐rich fluid, this model could explain how the chassignites and nakhlites originated from an LREE‐depleted source, yet all exhibit LREE‐enriched bulk‐rock patterns. Additionally, the model explains the range in oxygen fugacity that is recorded by the chassignites and nakhlites because eventual exsolution and loss of Cl‐rich fluid phases near the end of crystallization of the nakhlite sequence leads to auto‐oxidation of the magma body due to the preferential partitioning of Fe²⁺ into the fluid phase.  相似文献   

Shear and compressional velocity models of the mantle from cluster analysis of long-period waveforms     

C. Houser †  G. Masters  P. Shearer  G. Laske 《Geophysical Journal International》2008,174(1):195-212

We present a new technique for the efficient measurement of the traveltimes of long period body wave phases. The technique is based on the fact that all arrivals of a particular seismic phase are remarkably similar in shape for a single event. This allows the application of cross-correlation techniques that are usually used in a regional context to measure precise global differential times. The analysis is enhanced by the inclusion of a clustering algorithm that automatically clusters waveforms by their degree of similarity. This allows the algorithm to discriminate against unusual or distorted waveforms and makes for an extremely efficient measurement technique.
This technique can be applied to any seismic phase that is observed over a reasonably large distance range. Here, we present the results of applying the algorithm to the long-period channels of all data archived at the IRIS DMC from 1976 to 2005 for the seismic phases S and P (from 23° to 100°) and SS and PP (from 50° to 170°). The resulting large data sets are inverted along with existing surface wave and updated differential traveltime measurements for new mantle models of S and P velocity. The resolution of the new model is enhanced, particularly, in the mid-mantle where SS and PP turn. We find that slow anomalies in the central Pacific and Africa extend from the core–mantle boundary to the upper mantle, but their direct connection to surface hotspots is beyond our resolution. Furthermore, we find that fast anomalies that are likely associated with subducting slabs disappear between 1700 and 2500 km, and thus are not continuous features from the upper to lower mantle despite our extensive coverage and high resolution of the mid-mantle.  相似文献   

Habitat suitability curves for benthic macroinvertebrates from a small New Zealand river     

KA Shearer  JW Hayes  IG Jowett  DA Olsen 《新西兰海洋与淡水研究杂志》2013,47(2):178-191

We developed habitat suitability curves (HSC) using generalised additive models (GAMs) for nine benthic macroinvertebrate taxa from a small New Zealand river for hydraulic-habitat modelling assessments of instream flow requirements. We included interaction terms between the primary variables (water depth, velocity, substrate) when significant, to address a longstanding criticism of univariate HSC. To date, only large-river univariate HSC have been available and these have been used in hydraulic-habitat applications on small rivers, despite doubt over the transferability of HSC between rivers of different size and type. We tested the outcome on the predicted abundance–flow relationship of applying the small-river habitat suitability GAMs versus large-river GAMs for two taxa on the same small river. We found the effects of flow allocation were overestimated by the large-river GAMs relative to the small-river GAMs. Further research to develop general HSC for categories of river size and type is needed to better inform hydraulic-habitat modelling applications.  相似文献   

Jarosite growth zoning as a recorder of fluid evolution     

Paul V. Burger  James J. Papike  Charles K. Shearer  Jim M. Karner 《Geochimica et cosmochimica acta》2009,73(11):3248-318

Growth zoning in jarosite, an Fe³⁺-bearing sulfate mineral, can generally be characterized by variation of Na⁺ for K⁺ in the crystallographic A site or by Al³⁺ for Fe³⁺ in the octahedral (B) site. Growth zoning in a sample from Post Pit, NV, however, is more complicated than has been observed in other jarosite samples examined in this study and by previous work, and is characterized by varying Ba, Sr, P, and As. In this sample, these elements define a coupled substitution in which Ba²⁺ and Sr²⁺ substitute for K⁺ in the crystallographic A site, and are balanced by the substitution of P⁵⁺ and As⁵⁺ for S⁶⁺ in the tetrahedral (X) site. The Post Pit sample also exhibits a high concentration of V, which does not appear to participate in the aforementioned coupled substitution. Analysis of mineral stoichiometry and charge balance reveal that V is likely tetravalent, and represents a charge excess in the B site. The occurrence of V-bearing goethite in this sample records fluctuating fluid pH, resulting in the alternating stability of jarosite and goethite. Measurement of the REEs in jarosite by SIMS, indicate that Ce is trivalent in the Post Pit sample; reduced relative to Ce⁴⁺ found in the remainder of the sample suite. The presence of V⁴⁺ and Ce³⁺ indicate that the Post Pit sample was deposited in a less oxidizing environment than the remainder of the sample suite. This study illustrates how jarosite crystal chemistry can be used to place relative constraints on fluid conditions (pH, Eh, chemistry, etc.) in the terrestrial environment, and characterizes possible major and trace element coupled substitutions into the jarosite crystal structure. Furthermore, this study provides a framework for future studies to examine mineral-fluid partition coefficients (D values) which may place more absolute constraints on fluid chemistry in the jarosite depositional environment.  相似文献   

Magmatic fractionation of Hf and W: constraints on the timing of core formation and differentiation in the Moon and Mars   总被引：1，自引：0，他引：1  

K. Righter  C.K. Shearer 《Geochimica et cosmochimica acta》2003,67(13):2497-2507

Excesses of ¹⁸²W have previously been measured in samples from the Moon and Mars, and can be derived from high Hf/W regions in their interiors during their early histories. Although planetary mantles will have superchondritic Hf/W after core formation, the extent to which high Hf/W regions could be generated by magmatic fractionation has not been evaluated. In order to address the latter possibility, we have carried out experiments from 100 MPa to 10.0 GPa, 1150 to 1850°C, at oxygen fugacities near the IW (iron-wüstite) buffer, and measured partition coefficients for W and Hf for plagioclase-liquid, olivine-liquid, orthopyroxene-liquid, clinopyroxene-liquid, garnet-liquid, and metal-liquid pairs. Clinopyroxene and garnet are both capable of fractionating Hf from W during magmatic crystallization or mantle melting, and minor variations in the measured D’s can be attributed to crystal chemical effects. Excesses of ¹⁸²W and ¹⁴²Nd in lunar samples can be explained by fractionation of Hf from W, and Sm from Nd (by ilmenite and clinopyroxene) during crystallization of the latest stages of a lunar magma ocean. Correlations of ε_W with ε_Nd in martian samples could be a result of early silicate fractionation in the martian mantle (clinopyroxene and/or garnet).  相似文献   

Light lithophile elements in martian basalts: Evaluating the evidence for magmatic water degassing     

Christopher D.K. Herd  Allan H. Treiman  Charles K. Shearer 《Geochimica et cosmochimica acta》2005,69(9):2431-2440

Whether water has played a role in the petrogenesis of martian basalts remains a subject of significant debate. Estimates of pre-eruptive water concentrations in martian magmas are impeded by the effects of degassing and shock. However, zoning trends of light lithophile elements (LLE) in pyroxene have been interpreted as evidence for the degassing of magmatic water, on the basis of the soluble behavior of these elements in hydrothermal fluids. We provide ion microprobe analyses of LLE in pyroxene in the Zagami and Shergotty martian basalts, complemented by detailed electron microprobe analyses and major-element X-ray maps, to independently verify the zoning of LLE and its relationship to texture and major-element variations. Our results corroborate previous results; specifically, that Li concentrations are lower in rims than cores of Shergotty and Zagami pyroxene. In contrast, we see no evidence for a core-to-rim decrease in B. In the absence of further data, we interpret the decrease in Li as reflecting either loss after crystallization of pyroxene cores, consistent with magmatic degassing, or the diffusive preferential loss of Li from pyroxene rims, possibly as a result of shock. Because the partitioning behavior of Li between hydrous fluid, minerals, and melt under relevant conditions of pressure, temperature, and melt composition is unknown, the viability of the water degassing hypothesis depends on experiments establishing the compatibility of Li in hydrous fluid associated with martian basaltic melt and the incompatibility of Li in pyroxene at elevated pressures.  相似文献   

Delineation of the distinctive nature of tertiary coal beds     

Jane C. Shearer  Timothy A. Moore  Thomas D. Demchuk 《International Journal of Coal Geology》1995,28(2-4)

Floral character in mires has changed progressively through time. In the Carboniferous, pteridophytes, sphenophytes and lycophytes were dominant but by the Permian gymnosperms were an important component of mire flora. During the early Mesozoic gymnosperms remained the characteristic mire vegetation, together with pteridophytes, and conifers became dominant during the Jurassic. Cretaceous and Paleocene vegetation are similar, with taxodiaceous flora being important in mire vegetation. From the Eocene onwards, however, angiosperms were increasingly dominant in mire communities and in the Miocene herbaceous vegetation began to play a significant role. Together with these changes in floral character at least three aspects of coal character also appear to vary sequentially with time and are distinctive in the Tertiary: (1) proportions and thickness of vitrain banding, (2) coal bed thickness and (3) proportions of carbonised material. A compilation has been made of data from the coal literature comparing older coals with those of the Tertiary, in order to give a perspective in which to examine Tertiary coals. It was found that only Tertiary coals contain significant proportions of coal devoid of vitrain bands. In addition, Tertiary coals are the thickest recorded coal beds and generally contain low percentages of carbonised material (many less than 5%) as compared to older coals. It is interesting to note that Paleocene coal beds are similar to Cretaceous coals in that they tend to be thinner and contain higher proportions of carbonised material than do younger Tertiary coals.The absence of vitrain bands in some Tertiary coal beds is thought to result from the floras dominated by angiosperms, which are relatively easily degraded as compared to gymnosperms. The thickness of Tertiary coals may be related to an increase in biomass production from the Carboniferous through to the Tertiary, as plants made less investment in producing lignin, an energy-intensive process. In addition, with less lignin in plants, easier degradation of biomass may have facilitated nutrient recycling which, in turn, led to greater biomass production. Increased biomass production may have also ‘diluted’ the carbonised material present in some Tertiary peats, leading to lower proportions in the coal. Another possible cause of decreased carbonised components in Tertiary coal is that decreasing lignin content resulted in decreased charring during fires, as lignin is particularly prone to charring. A third possibility is that the carbonised component of peat may be concentrated during coalification so that Tertiary coals, generally of lower rank than Mesozoic or Paleozoic coals, contain a smaller fraction of carbonised plant material. It is not at present clear which of these mechanisms may have affected carbonised material in peat and coal but it is clear that lignin type and content has had an important role in determining peat and coal character since the Paleozoic.  相似文献   

Global mapping of upper mantle reflectors from long-period SS precursors   总被引：1，自引：0，他引：1  

Peter M. Shearer 《Geophysical Journal International》1993,115(3):878-904

Long-period precursors to SS resulting from underside reflections off upper mantle discontinuities ( SdS where d is the discontinuity depth) can be used to map the global distribution and depth of these reflectors. We analyse 5,884 long-period seismograms from the Global Digital Seismograph Network (1976-1987, shallow sources, transverse component) in order to identify SdS arrivals. Corrections for velocity dispersion, topography and crustal thickness at the SS bounce point, and lateral variation in mantle velocity are critical for obtaining accurate estimates of discontinuity depths. The 410 and 660 km discontinuities are observed at average depths of 413 and 653 km, and exhibit large-scale coherent patterns of topography with depth variations up to 40 km. These patterns are roughly correlated with recent tomographic models, with fast anomalies in the transition zone associated with highs in the 410 km discontinuity and lows in the 660 km discontinuity, a result consistent with laboratory measurements of Clapeyron slopes for the appropriate phase changes. The best resolved feature in these maps is a trough in the 660 km discontinuity in the northwest Pacific, which appears to be associated with the subduction zones in this region. Amplitude variations in SdS arrivals are not correlated with discontinuity depths and probably result from focusing and defocusing effects along the ray paths. The SdS arrivals suggest the presence of regional reflectors in the upper mantle above 400 km. However, only the strongest of these features are above probable noise levels due to sampling inadequacies.  相似文献   

Rb‐Sr and Sm‐Nd isotopic and REE studies of igneous components in the bulk matrix domain of Martian breccia Northwest Africa 7034          下载免费PDF全文

Laurence E. Nyquist  Chi‐Yu Shih  Francis M. McCubbin  Alison R. Santos  Charles K. Shearer  Zhan X. Peng  Paul V. Burger  Carl B. Agee 《Meteoritics & planetary science》2016,51(3):483-498

The bulk matrix domain of the Martian breccia NWA 7034 was examined petrographically and isotopically to better understand the provenance and age of the source material that make up the breccia. Both ¹⁴⁷Sm‐¹⁴³Nd and ¹⁴⁶Sm‐¹⁴²Nd age results for mineral separates from the bulk matrix portion of breccia NWA 7034 suggest that various lithological components in the breccia probably formed contemporaneously ~4.44 Ga ago. This old age is in excellent agreement with the upper intersection ages (4.35–4.45 Ga) for U‐Pb discordia and also concordia defined by zircon and baddeleyite grains in matrix and igneous‐textured clasts. Consequently, we confirm an ancient age for the igneous components that make up the NWA 7034 breccia. Substantial disturbance in the Rb‐Sr system was detected, and no age significance could be gleaned from our Rb‐Sr data. The disturbance to the Rb‐Sr system may be due to a thermal event recorded by bulk‐rock K‐Ar ages of 1.56 Ga and U‐Pb ages of phosphates at about 1.35–1.5 Ga, which suggest partial resetting from an unknown thermal event(s), possibly accompanying breccia formation. The NWA 7034 bulk rock is LREE enriched and similar to KREEP‐rich lunar rocks, which indicates that the earliest Martian crust was geochemically enriched. This enrichment supports the idea that the crust is one of the enriched geochemical reservoirs on Mars that have been detected in studies of other Martian meteorites.  相似文献   

Ray tracing in azimuthally anisotropic media—II. Quasi-shear wave coupling     

C. H. Chapman  P. M. Shearer 《Geophysical Journal International》1989,96(1):65-83

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