Pegmatite/wallrock interactions,Black Hills,South Dakota: Progressive boron metasomatism adjacent to the Tip Top pegmatite     

C.K. Shearer  J.J. Papike  S.B. Simon  J.C. Laul  R.P. Christian 《Geochimica et cosmochimica acta》1984,48(12):2563-2579

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Oxygen isotopic composition of minerals in the Kenna ureilite     

Robert N. Clayton  Toshiko K. Mayeda  Naoki Onuma  John Shearer   《Geochimica et cosmochimica acta》1976,40(12):1475-1476

The δ¹⁸O and δ¹⁷O values of olivine from Kenna are 7.6 and 3.0%, respectively, relative to SMOW. These values are typical of ureilites which form a unique group on a δ¹⁷O -δ¹⁸O graph. The ureilites are related to, but not directly derived from, the anhydrous phases of C2 and C3 meteorites. The ¹⁸O/¹⁶O fractionation between pyroxene and olivine is 0.60, indicating a temperature of last equilibration of 1000 ± 100°C.  相似文献   

The global short-period wavefield modelled with a Monte Carlo seismic phonon method   总被引：5，自引：0，他引：5  

Peter M. Shearer  Paul S. Earle 《Geophysical Journal International》2004,158(3):1103-1117

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Imaging mantle transition zone thickness with SdS-SS finite-frequency sensitivity kernels     

Jesse F. Lawrence  Peter M. Shearer 《Geophysical Journal International》2008,174(1):143-158

We invert differential SdS-SS traveltime residuals measured from stacked waveforms and finite-frequency sensitivity kernels for topography on the 410- and 660-km discontinuities. This approach yields higher resolution images of transition zone thickness than previous stacking methods, which simply average/smooth over topographic features. Apparent structure measured using simple stacking is highly dependent upon the bin size of each stack. By inverting for discontinuity topography with a variety of bin sizes, we can more accurately calculate the true structure. The inverted transition zone model is similar to simple stack models with an average thickness of 242 km, but the lateral variations in thickness are larger in amplitude and smaller in scale. Fast seismic velocities in 3-D mantle models such as SB4L18 correlate with areas of thicker transition zone. The elongated curvilinear regions of thickened transition zone that occur near subduction zones are narrow and high amplitude, which suggests relatively little lateral spreading and warming of subducted lithosphere within the transition zone. The anomalously thin transition zone regions are laterally narrow, and not broadly continuous. If these variations in transition zone thickness are interpreted as thermal in nature, then this model suggests significant temperature variations on small lateral scales.  相似文献   

Analysis of Foreshock Sequences in California and Implications for Earthquake Triggering   总被引：1，自引：0，他引：1  

Xiaowei Chen  Peter M. Shearer 《Pure and Applied Geophysics》2016,173(1):133-152

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Relationships between sedimentary diatom assemblages and lakewater pH values in the Experimental Lakes Area     

D. L. Findlay  J. A. Shearer 《Journal of Paleolimnology》1992,7(2):145-156

Relationships between sedimentary diatom assemblages and lakewater pH values from 36 lakes and experimentally acidified Lake 223, in the Experimental Lakes Area, were analyzed. The relationships were used to assign diatoms in the 36 lakes into pH preference groupings. Based on their regional distribution Cyclotella stelligera and Tabellaria flocculosa strain IIIp were categorized as pH indifferent, in comparison to other areas were they have been categorized as acidophilic. Two models were then applied to calculate transfer coefficients which were used to calculate diatom-inferred pH values for Lake 223. Index B and a multiple linear regression of the pH groupings yielded similar correlations (r² 0.82 and 0.84 respectively, p=0.0001). The multiple linear regression inferred a pH of 5.36 for acidified Lake 223 compared to a measured pH of 5.46.  相似文献   

Initial shear wave particle motions and stress constraints at the Anza Seismic Network   总被引：1，自引：0，他引：1  

Richard C. Aster  Peter M. Shearer 《Geophysical Journal International》1992,108(3):740-748

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High speed photon-counting imager using a position sensitive diode     

Oliver Ryan  Mike Redfern  Andrew Shearer 《Experimental Astronomy》2006,21(1):13-22

A new use for a 2-dimensional position sensitive diode (PSD) is described. A duolateral PSD was used with a microchannel plate image intensifier as a proof-of-concept photon counting (event driven) imager for astronomical imaging and photometry. This produced an imager capable of counting 25–30 kcps over the astronomical bands B, V & R, with an overall efficiency of ∼19%.  相似文献   

The chlorine isotope composition of Martian meteorites 2. Implications for the early solar system and the formation of Mars          下载免费PDF全文

Zachary Sharp  Jeffrey Williams  Charles Shearer  Carl Agee  Kevin McKeegan 《Meteoritics & planetary science》2016,51(11):2111-2126

We determined the chlorine isotope composition of 16 Martian meteorites using gas source mass spectrometry on bulk samples and in situ secondary ion microprobe analysis on apatite grains. Measured δ³⁷Cl values range from ?3.8 to +8.6‰. The olivine‐phyric shergottites are the isotopically lightest samples, with δ³⁷Cl mostly ranging from ?4 to ?2‰. Samples with evidence for a crustal component have positive δ³⁷Cl values, with an extreme value of 8.6‰. Most of the basaltic shergottites have intermediate δ³⁷Cl values of ?1 to 0‰, except for Shergotty, which is similar to the olivine‐phyric shergottites. We interpret these data as due to mixing of a two‐component system. The first component is the mantle value of ?4 to ?3‰. This most likely represents the original bulk Martian Cl isotope value. The other endmember is a ³⁷Cl‐enriched crustal component. We speculate that preferential loss of ³⁵Cl to space has resulted in a high δ³⁷Cl value for the Martian surface, similar to what is seen in other volatile systems. The basaltic shergottites are a mixture of the other two endmembers. The low δ³⁷Cl value of primitive Mars is different from Earth and most chondrites, both of which are close to 0‰. We are not aware of any parent‐body process that could lower the δ³⁷Cl value of the Martian mantle to ?4 to ?3‰. Instead, we propose that this low δ³⁷Cl value represents the primordial bulk composition of Mars inherited during accretion. The higher δ³⁷Cl values seen in many chondrites are explained by later incorporation of ³⁷Cl‐enriched HCl‐hydrate.  相似文献   

Heterogeneous distribution of H2O in the Martian interior: Implications for the abundance of H2O in depleted and enriched mantle sources          下载免费PDF全文

Francis M. McCubbin  Jeremy W. Boyce  Poorna Srinivasan  Alison R. Santos  Stephen M. Elardo  Justin Filiberto  Andrew Steele  Charles K. Shearer 《Meteoritics & planetary science》2016,51(11):2036-2060

We conducted a petrologic study of apatite within 12 Martian meteorites, including 11 shergottites and one basaltic regolith breccia. These data were combined with previously published data to gain a better understanding of the abundance and distribution of volatiles in the Martian interior. Apatites in individual Martian meteorites span a wide range of compositions, indicating they did not form by equilibrium crystallization. In fact, the intrasample variation in apatite is best described by either fractional crystallization or crustal contamination with a Cl‐rich crustal component. We determined that most Martian meteorites investigated here have been affected by crustal contamination and hence cannot be used to estimate volatile abundances of the Martian mantle. Using the subset of samples that did not exhibit crustal contamination, we determined that the enriched shergottite source has 36–73 ppm H₂O and the depleted source has 14–23 ppm H₂O. This result is consistent with other observed geochemical differences between enriched and depleted shergottites and supports the idea that there are at least two geochemically distinct reservoirs in the Martian mantle. We also estimated the H₂O, Cl, and F content of the Martian crust using known crust‐mantle distributions for incompatible lithophile elements. We determined that the bulk Martian crust has ~1410 ppm H₂O, 450 ppm Cl, and 106 ppm F, and Cl and H₂O are preferentially distributed toward the Martian surface. The estimate of crustal H₂O results in a global equivalent surface layer (GEL) of ~229 m, which can account for at least some of the surface features on Mars attributed to flowing water and may be sufficient to support the past presence of a shallow sea on Mars' surface.  相似文献