Lunar Magma Ocean crystallization revisited: Bulk composition, early cumulate mineralogy, and the source regions of the highlands Mg-suite     

Stephen M. Elardo  David S. Draper  Charles K. Shearer Jr. 《Geochimica et cosmochimica acta》2011,75(11):3024-5093

Crystallization of the Lunar Magma Ocean (LMO) has been numerically modeled and its products inferred from sample observations, but it has never been fully tested experimentally. This study is a reexamination of the LMO hypothesis by means of the first experimental simulation of lunar differentiation. Two end-member bulk Moon compositions are considered: one enriched in refractory lithophile elements relative to Earth and one with no such enrichment. A “two-stage” model of magma ocean crystallization based on geophysical constraints is simulated and features early crystal suspension and equilibrium crystallization followed by fractional crystallization of the residual magma ocean. An initially entirely molten Moon is assumed. Part 1 of this study, presented here, focuses on stage 1 of this model and considers the early cumulates formed by equilibrium crystallization, differences in mantle mineralogy resulting from different bulk Moon compositions, and implications for the source regions of the highlands Mg-suite.Refractory element enriched bulk Moon compositions produce a deep mantle that contains garnet and trace Cr-spinel in addition to low-Ca pyroxene and olivine. In contrast, compositions without refractory element enrichment produce a deep dunitic mantle with low-Ca pyroxene but without an aluminous phase. The differences in bulk composition are magnified in the residual melt; the residual LMO from the refractory element enriched composition will likely produce plagioclase and ilmenite earlier and in greater quantities. Both compositions produce Mg-rich early cumulate piles that extend from the core-mantle boundary to ∼355 km depth, if 50% equilibrium crystallization and whole Moon melting are assumed. These early LMO cumulates provide good fits for the source regions for a component of the high-Mg^∗, Ni- and Co-poor parental magmas of the Mg-suite cumulates, if certain conditions are called upon. The olivine in early LMO cumulates produced by either bulk Moon composition is far too rich in Cr to be reasonable for the source regions of the Mg-suite, meaning either core formation in the presence of S and/or C must be invoked to deplete the LMO and the crystallizing olivine in Cr, or that current estimates of the bulk lunar Cr content are too high. We infer that melts meeting the criteria of the Mg-suite parents could be produced from early LMO cumulates by solid state KREEP and plagioclase hybridization near the base of the crust and subsequent partial melting. Additionally, we propose a revised model for Mg-suite petrogenesis.  相似文献   

U-Pb and Pb-Pb apatite ages for Antarctic achondrite Graves Nunataks 06129     

Qin Zhou  Qing-Zhu Yin  Charles K. Shearer  Xian-Hua Li  Qiu-Li Li  Yu Liu  Guo-Qiang Tang  Chun-Lai Li 《Meteoritics & planetary science》2018,53(3):448-466

The Antarctic achondrite Graves Nunataks 06128 (GRA 06128) and Graves Nunataks 06129 (GRA 06129) represent a unique high-temperature, nonbasaltic magmatism in the early solar system. These objects have been interpreted as products of low-degree partial melting of volatile-rich chondritic material, which may have been the asteroid parent bodies of brachinite. Previous studies have investigated their crystallization and metamorphic history with various isotope systematics. Here, we report the U-Pb intercept age of 4466 ± 29 Ma and the weighted-average ²⁰⁷Pb-²⁰⁶Pb age of 4460 ± 30 Ma for the Cl-apatite grains from GRA 06129. Our apatite ages are obviously younger than that of the ²⁶Al-²⁶Mg model age (4565.9 ± 0.3 Ma; Shearer et al. 2010a ), but are the same as the ⁴⁰Ar-³⁹Ar age obtained via step-heating of the bulk rock (4460 ± 28 Ma; Fernandes and Shearer 2010 ; Shearer et al. 2010a ). Based on petrographic observations, merrillites are usually rimmed by apatite and exist as inclusions in apatite. Therefore, the apatite U-Pb age from GRA 06129 probably records a metamorphic event of replacing merrillite with apatite, caused by Cl-rich melts or fluids on their parent body. A collisional event has provided the impact heating for this metamorphic event. Increasing amounts of geochronologic evidence show that the giant impact of the Moon-forming event has affected the asteroid belt at 4450–4470 Ma (Bogard and Garrison 2009 ; Popova et al. 2013 ; Yin et al. 2014 ; Zhang et al. 2016 ). Considering the contemporary metamorphic events for GRA 06129 (4460 ± 30 Ma), it is likely that the asteroid parent body of GRA 06129 was also affected by the same giant impact as the Moon-forming event.  相似文献   

Martian meteorite Dhofar 019: A new shergottite     

L. A. Taylor  M. A. Nazarov  C. K. Shearer  H. Y. McSween  J. Cahill  C. R. Neal  M. A. Ivanova  L. D. Barsukova  R. C. Lentz  R. N. Clayton  T. K. Mayeda 《Meteoritics & planetary science》2002,37(8):1107-1128

Abstract— Dhofar 019 is a new martian meteorite found in the desert of Oman. In texture, mineralogy, and major and trace element chemistry, this meteorite is classified as a basaltic shergottite. Olivine megacrysts are set within a groundmass composed of finer grained olivine, pyroxene (pigeonite and augite), and maskelynite. Minor phases are chromite‐ulvöspinel, ilmenite, silica, K‐rich feldspar, merrillite, chlorapatite, and pyrrhotite. Secondary phases of terrestrial origin include calcite, gypsum, celestite, Fe hydroxides, and smectite. Dhofar 019 is most similar to the Elephant Moraine (EETA) 79001 lithology A and Dar al Gani (DaG) 476/489 shergottites. The main features that distinguish Dhofar 019 from other shergottites are lack of orthopyroxene; lower Ni contents of olivine; the heaviest oxygen‐isotopic bulk composition; and larger compositional ranges for olivine, maskelynite, and spinel, as well as a wide range for pyroxenes. The large compositional ranges of the minerals are indicative of relatively rapid crystallization. Modeling of olivine chemical zonations yield minimum cooling rates of 0.5‐0.8 °C/h. Spinel chemistry suggests that crystallization took place under one of the most reduced conditions for martian meteorites, at an fO₂ 3 log units below the quartz‐fayalite‐magnetite (QFM) buffer. The olivine megacrysts are heterogeneously distributed in the rock. Crystal size distribution analysis suggests that they constitute a population formed under steady‐state conditions of nucleation and growth, although a few grains may be cumulates. The parent melt is thought to have been derived from partial melting of a light rare earth element‐ and platinum group element‐depleted mantle source. Shergottites, EETA79001 lithology A, DaG 476/489, and Dhofar 019, although of different ages, comprise a particular type of martian rocks. Such rocks could have formed from chemically similar source(s) and parent melt(s), with their bulk compositions affected by olivine accumulation.  相似文献   

2D modeling of overpressure in a salt withdrawal basin,Gulf of Mexico,USA     

Jeffrey R. Allwardt  G. Eric MichaelChuck R. Shearer  Philip D. HeppardHongxing Ge 《Marine and Petroleum Geology》2009

This study demonstrates the utilization of 2D basin models to address overpressure development due to compaction disequilibrium in supra-allochthonous salt mini-basins with very high sedimentation rates in the Gulf of Mexico. By properly selecting 2D line sections with moderate stratigraphic resolution, it is possible to predict timing of overpressure development and approximate present-day overpressure distributions in the mini-basin. This study shows that even low resolution models with approximate information on the net-to-gross (sand:shale ratio) can average ±0.4 ppg with a maximum error of 1.0 ppg relative to pressure measurements in sandstones. The models based on age, depth, approximate lithology and an interpretation of complicated salt movement are adequate to evaluate pressure to address issues around trap containment and may be used for preliminary well planning. This study tested the results of overpressure prediction utilizing different stratigraphic resolutions and shows the sensitivity of overpressure modeling to 2D line selection. Also, three models were built to investigate how the permeability of salt welds affects overpressure development in an adjacent salt mini-basin. These results indicate that even a salt weld permeability reduction of 1.5 log mD results in a pressure difference between neighboring mini-basins. Additionally, these models qualitatively reproduced the seismic velocity volume which is supporting evidence that the salt welds in this mini-basin are at least partially sealing.  相似文献   

Mobile element analysis by secondary ion mass spectrometry (SIMS) of impactite matrix samples from the Yaxcopoil‐1 drill core in the Chicxulub impact structure     

Horton E. Newsom  Melissa J. Nelson  Charles K. Shearer  Burkhard O. Dressler 《Meteoritics & planetary science》2006,41(12):1929-1945

Abstract— The concentrations of the fluid mobile trace elements lithium, beryllium, boron, and barium were measured in samples of the altered matrix of several impactite breccias of the Yaxcopoil‐1 drill core using secondary ion mass spectrometry (SIMS) to determine the extent of transport due to aqueous or hydrothermal processes. Three of the elements, Li, Be, and B, have higher concentrations in the upper suevite impact breccias than in the lower impact melt deposits by factors of 3.5, 2.2, and 1.5, respectively. Lithium and B are the most enriched elements up section, and appear to have had the greatest mobility. The similar fractionation of Li and B is consistent with fluid transport and alteration under low‐temperature conditions of less than 150 °C based on published experimental studies. In contrast to Li, Be, and B, the concentration of Ba in the altered matrix materials decreases upward in the section, and the concentration of Ba in the matrix is an order of magnitude less than the bulk concentrations, likely due to the presence of barite. The origin of the elemental variations with depth may be related to different protolith compositions in the upper versus the lower impactite units. A different protolith in the altered matrix is suggested by the Mg‐rich composition of the lower units versus the Al‐rich composition of the upper units, which largely correlates with the mobile element variations. The possibility that vertical transport of mobile elements is due to a postimpact hydrothermal system is supported by published data showing that the sediments immediately overlying the impactites are enriched in mobile elements derived from a hydrothermal system. However, the mobile elements in the sediments do not have to originate from the underlying impactites. In conclusion, our data suggests that the impactites at this location did not experience extensive high‐temperature hydrothermal processing, and that only limited transport of some elements, including Li, Be, and B, occurred.  相似文献   

The impact of climate change on tribal communities in the US: displacement, relocation, and human rights   总被引：1，自引：1，他引：0  

Julie Koppel Maldonado  Christine Shearer  Robin Bronen  Kristina Peterson  Heather Lazrus 《Climatic change》2013,120(3):601-614

Tribal communities in the United States, particularly in coastal areas, are being forced to relocate due to accelerated rates of sea level rise, land erosion, and/or permafrost thaw brought on by climate change. Forced relocation and inadequate governance mechanisms and budgets to address climate change and support adaptation strategies may cause loss of community and culture, health impacts, and economic decline, further exacerbating tribal impoverishment and injustice. Sovereign tribal communities around the US, however, are using creative strategies to counter these losses. Taking a human rights approach, this article looks at communities’ advocacy efforts and strategies in dealing with climate change, displacement, and relocation. Case studies of Coastal Alaska and Louisiana are included to consider how communities are shaping their own relocation efforts in line with their cultural practices and values. The article concludes with recommendations on steps for moving forward toward community-led and government-supported resettlement programs.  相似文献   

High-speed, multicolour optical photometry of the anomalous X-ray pulsar 4U 0142+61 with ULTRACAM   总被引：1，自引：0，他引：1  

V. S. Dhillon  T. R. Marsh  F. Hulleman  M. H. van Kerkwijk  A. Shearer  S. P. Littlefair  F. P. Gavriil  V. M. Kaspi 《Monthly notices of the Royal Astronomical Society》2005,363(2):609-614

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Short time scale pulse stability of the Crab pulsar in the optical band     

S. Karpov  G. Beskin  A. Biryukov  V. Debur  V. Plokhotnichenko  M. Redfern  A. Shearer 《Astrophysics and Space Science》2007,308(1-4):595-599

The fine structure and the variations of the optical pulse shape and phase of the Crab pulsar are studied on various time scales. The observations have been carried out on 4-m William Hershel and 6-m BTA telescopes with APD photon counter, photomultiplier based 4-channel photometer and PSD based panoramic spectrophotopolarimeter with 1 μs time resolution in 1994, 1999, 2003 and 2005–2006 years. The upper limit on the pulsar precession on Dec 2, 1999 is placed in the 10 s–2 hr time range. The evidence of a varying from set to set fine structure of the main pulse is found in the 1999 and 2003 years data. No such fine structure is detected in the integral pulse shape of 1994, 1999 and 2003 years. The drastic change of the pulse shape in the 2005–2006 years set is detected along with the pulse shape variability and quasi-periodic phase shifts. This work has been supported by the Russian Foundation for Basic Research (grant No 04-02-17555), Russian Academy of Sciences (program “Evolution of Stars and Galaxies”), by the Russian Science Support Foundation, and by INTAS (grant No 04-78-7366).  相似文献   

Ray tracing in anisotropic media with a linear gradient     

P. M. Shearer  C. H. Chapman 《Geophysical Journal International》1988,94(3):575-580

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The Galway astronomical Stokes polarimeter: an all-Stokes optical polarimeter with ultra-high time resolution     

Patrick Collins  Gillian Kyne  David Lara  Michael Redfern  Andy Shearer  Brendan Sheehan 《Experimental Astronomy》2013,36(3):479-503

Many astronomical objects emit polarised light, which can give information both about their source mechanisms, and about (scattering) geometry in their source regions. To date (mostly) only the linearly polarised components of the emission have been observed in stellar sources. Observations have been constrained because of instrumental considerations to periods of excellent observing conditions, and to steady, slowly or periodically-varying sources. This leaves a whole range of interesting objects beyond the range of observation at present. The Galway Astronomical Stokes Polarimeter (GASP) has been developed to enable us to make observations on these very sources. GASP measures the four components of the Stokes Vector simultaneously over a broad wavelength range 400–800 nm., with a time resolution of order microseconds given suitable detectors and a bright source - this is possible because the optical design contains no moving or modulating components. The initial design of GASP is presented and we include some preliminary observational results demonstrating that components of the Stokes vector can be measured to $<1$ % in conditions of poor atmospheric stability. Issues of efficiency and stability are addressed. An analysis of suitable astronomical targets, demanding the unique properties of GASP, is also presented.  相似文献