Measured and calculated seismic velocities and densities for granulites from xenolith occurrences and adjacent exposed lower crustal sections: A comparative study from the North China craton

Granulites from the Neogene xenolith-bearing Hannuoba alkaline basalt and from the Manjinggou-Wayaokou exposed lower crustal section in the Archean Huai'an terrain, which occurs within and surrounds the Hannuoba basalt, provide a unique opportunity for a comparative study on petrophysical properties and composition of the lower crust represented by these two types of samples. P and S wave velocities and densities of 12 Hannuoba lower crustal xenoliths and one associated spinel lherzolite xenolith as well as nine granulites and granulite-facies metasedimentary rocks from the Archean Huai'an terrain were measured in laboratory at pressures up to 600 MPa and temperatures up to 600°C. Calculations of P and S wave velocities were also made for the same suite of samples based on modal mineralogy and single-crystal velocities whose variations with composition are considered by using microprobe analyses and velocities of end members. The measured and calculated Vp at room temperature and 600 MPa, where the microcrack effect is considered to be almost eliminated, agree within 4% for rocks from the Manjinggou-Wayaokou section and the adjacent Wutai-Jining upper crustal to upper lower crustal section. In contrast, the xenoliths show systematically lower measured Vp by up to 15% relative to calculated velocities, even if decompression-induced products of kelyphite and glass are taken into account. The lower measured velocities for xenoliths are attributed to grain boundary alteration and residual porosity. This implies that although granulite xenoliths provide direct information about lower crustal constitution and chemical composition, they are not faithful samples for studying in situ seismic properties of the lower crust in terms of measured velocities due to alterations during their entrainment to the surface, which changes their physical properties significantly. In this respect, granulites from high-grade terrains are better samples because they are not subjected to significant changes during their slow transport to the surface and because physical properties depend primarily on mineralogy in addition to pressure and temperature. On the other hand, calculated velocities for granulite xenoliths are consistent with velocities for granulites from terrains, suggesting that they can be also used to infer lower crust composition by correlating with results from seismic refraction studies. Copyright 2000 by the American Geophysical Union.published_or_final_versio

First crystal-structure determination of chromites from an acapulcoite and ordinary chondrites

We report the first crystal structure determinations of chromites from an acapulcoite and from ordinary chondrites. Cell edges range from 8.3212 (3) to 8.3501 (1) \uc5, while the oxygen positional parameters are in the range 0.2624 (3) to 0.26298 (9). Their compositions show they are very close to the chromite end-member FeCr2O4 with limited Al and Mg content. Titanium oxide content exceeds 1%, whereas the amount of Fe3+ is negligible. Extraterrestrial chromite is readily distinguished from terrestrial analogues on the basis of cell edge and oxygen positional parameter. These distinctions will facilitate ongoing attempts to reconstruct the paleoflux of meteorites to Earth from resistant extraterrestrial spinel grains recovered from ancient sediments

The Micron Scale Pb Isotopic Systematics of the Lower Continental Crust

The thermochronology, genetic relationships, and U-Th-Pb history of the deep crust in the North China Craton, the Tanzanian Craton, and the Mozambique Belt have been interrogated using the Pb isotopic system in feldspars from lower crustal xenoliths measured via a novel LA-MC-ICP-MS method developed at the University of Maryland, College Park. Individual feldspars analyzed from the lower crust beneath the Tanzanian Craton show a large spread in Pb isotopic compositions. The spread in isotopic data reflects radiogenic Pb that accumulated in a closed feldspar system that was below 600°C since 2.4 Ga. The entire suite of feldspar Pb isotopic compositions overlaps the common Pb measurements from the present day surface of the Tanzanian Craton. The combined surface and lower crust Pb isotopic compositions indicate a mantle extraction age of 2.7 Ga. The feldspar thermochronometry of the lower crust, along with other geochemical data, indicate that the crust of the Tanzanian Craton was extracted from the mantle ca. 2.7 Ga, in an arc setting, and cooled to a temperature below 600°C by 2.4 Ga, and has remained undisturbed since that time. Common Pb isotopic compositions in feldspars analyzed from the surface and deep crust of the Mozambique belt, which sits adjacent to the Tanzanian Craton show that common Pb isotopic compositions can be used to determine provenance of granulite belts at depth. Common Pb modeling of the lower, middle, and upper crust of the Western Granulites indicate a 2.5x enrichment of U in the upper crust occurred around the time of mantle extraction ca. 2.7 Ga. Additionally, the 238U/204Pb in the cratonic lower crust underlying the Mozambique Belt increases with distance away from the surface craton margin. The enrichment of U in the basement away from the surface margin of the craton, could be the reason that the Mozambique belt has been continually re-worked since the Archean, while the craton has been stable. The Pb isotopic compositions of feldspars analyzed in granulite xenoliths from two different volcanic provinces in the North China Craton (Hannuoba and Nushan) reveal that there was a widespread crustal generation event ca. 2.5 Ga, followed by metamorphism at 1.8 Ga, as well as basaltic underplating ca. 0.15 Ga. The basaltic underplates show a composition that is a mixture between new basalt and ancient material. Felsic granulites, were extracted from the mantle ca. 2.9 Ga, and were emplaced in the crust ca. 1.8 Ga, and re-equilibrated ca. 0.15 Ga. Intermediate granulites, which were the product of the large 2.5 Ga crustal generation event, have internal feldspar isochrons of 1.7 Ga, which indicates while the lower crust was largely overprinted by the Mesozoic underplating, the lower crust was not elevated above 600°

Syn-kinematic hydration reactions, grain size reduction, and dissolution-precipitation creep in experimentally deformed plagioclase-pyroxene mixtures

Source at https://doi.org/10.5194/se-9-985-2018 .It is widely observed that mafic rocks are able to accommodate high strains by viscous flow. Yet, a number of questions concerning the exact nature of the involved deformation mechanisms continue to be debated. In this contribution, rock deformation experiments on four different water-added plagioclase–pyroxene mixtures are presented:(i) plagioclase(An60–70)–clinopyroxene–orthopyroxene,(ii) plagioclase(An60)–diopside,(iii) plagioclase(An60)–enstatite,and iv) plagioclase(An01)–enstatite. Samples were deformed in general shear at strain rates of 3×10−5 to 3×10−6 s−1, 800°C, and confining pressure of 1.0 or 1.5GPa. Results indicate that dissolution–precipitation creep (DPC) and grain boundary sliding (GBS) are the dominant deformation mechanisms and operate simultaneously. Coinciding with sample deformation, syn-kinematic mineral reactions yield abundant nucleation of new grains; the resulting intense gray size reduction is considered crucial for the activity of DPC and GBS. In high strain zones dominated by plagioclase, a weak, nonrandom, and geometrically consistent crystallographic preferred orientation (CPO) is observed. Usually, a CPO is considered a consequence of dislocation creep, but the experiments presented here demonstrate that a CPO can develop during DPC and GBS. This study provides new evidence for the importance of DPC and GBS in mid-crustal shear zones within mafic rocks, which has important implications for understanding and modeling mid-crustal rheology and flow

Avon Alkaline Igneous Province, Missouri: Characterization of subcontinental mantle source and evolution via chemical analysis of olivine

This article presents the crystallization age of, and composition of olivine phenocrysts within an alnoite of, the Avon Alkaline Igneous Province (AAIP) of Ste. Genevieve County, Missouri. The AAIP is an ultramafic igneous province consisting of approx. 80 known intrusives of diverse lithology and texture. 40Ar/39 Ar geochronology indicates an emplacement age of 386 +/- 1 Ma, which establishes the AAIP as the only known Devonian-age ultramafic igneous body in the Midwestern U.S. Study of the AAIP provides a unique opportunity to characterize the Devonian-age subcontinental mantle and the processes that generated the suite of ultramafic rocks present in the province. The compositions of 52 olivine crystals are characterized using electron microprobe analysis. Olivine major element compositions are homogeneous, Mg-rich (Fo86.9-Fo89.9), and exhibit variation in trace element (e.g., Ni, Cr, Co, Ti, P) abundances consistent with fractional crystallization. These results indicate that AAIP olivines are phenocrysts rather than mantle xenocrysts. Olivine geothermometry indicates derivation at temperatures of approx. 1500-1750⁰C at pressures of 1.6 to 5.4 GPa. Olivine trace element discrimination diagrams indicate AAIP magmas were derived from mantle sources with a compositional alkalic affinity, similar to other continental alkaline rocks and kimberlite. A mantle origin via partial melting of peridotite mantle is suggested due to the high Mg content, results of geothermometric modeling, and high Ca and Ti abundance within olivine phenocrysts. Disequilibrium textures observed in alnöite olivine are consistent with resorption of magmatic olivine as a result of decompression and fractional crystallization --Abstract, page iv

Rheology of fault rocks - an experimental study on the brittle-viscous transition in mafic rocks

In this thesis, the rheology, deformation mechanisms and microstructural evolution of a mafic fault rock are investigated, over a range of conditions where a transition from dominant brittle to dominant viscous deformation is observed. A Griggs-type deformation apparatus was used to perform experiments at elevated confining pressures (Pc) of 0.5 to 1.5 GPa, and over a temperature (T) range from 300 - 900 °C. The sample material is a ’simulated’ fault rock of mafic composition, fabricated from pre-crushed plagioclase-pyroxene mixtures. 0.18 or 0.11 wt.-% H2O is added to the samples to allow for solution-mass transport and mineral reactions during the experiments. Most experiments were performed in a general shear set-up, at constant displacement rates of ∼ 1e-4 down to 1e-6 mm/s. Mechanical data and microstructural observations are used to determine the rheology and identify the deformation mechanisms. Microstructural descriptions mainly base on observations made from scanning electron microscopy (SEM) and transmission electron microscopy (TEM). Different methods of image analysis were used for microstructural quantifications. In the experiments, the brittle-viscous transition (BVT) occurs within the temperature range of 600 °C ≤ T ≤ 800 °C, at confining pressures of 0.5 GPa ≤ Pc ≤ 1.5 GPa. The transition is effected by a switch from brittle fracturing and cataclastic flow, to dissolution-precipitation creep and grain boundary sliding (GBS). Viscous flow is enabled by intense grain size refinement resulting from heterogeneous nucleation during syn-kinematic mineral reactions. The reaction products are fine-grained (< 1 µm) neo-crystallized plagioclase and the new mineral phases amphibole, quartz and zoisite. In the BVT regime, the mechanical response of the sample is a mixed-mode between brittle and viscous rheology and microstructures associated with both brittle and viscous de- formation are observed. Microstructural evolution is seen to be a crucial factor determining the bulk sample rheology, highlighting the importance of considering strain dependent rheological changes for natural fault zones. Especially in the BVT regime, the initially more brittle dominated rheology is suggested to be only transient, evolving with increasing strain to a more viscous rheology. In the viscous dominated 800 °C experiments, the following findings are furthermore made: 1) A method was developed to study amphibole reaction corona thicknesses as a function of direction around their pyroxene host clasts. During the initial ∼ hydrostatic stage of the experiments (during lead run-in) reaction coronas grow isotropically in all directions. Upon applying a differential stress however, corona thicknesses decrease in directions correlating with high stress sites and increase in low stress sites. The results are interpreted to show the ability of amphibole to deform by dissolution-precipitation creep - a mechanism that is frequently described for naturally deformed amphibole but so far has seldom been reproduced in deformation experiments. 2) Electron backscatter diffraction (EBSD) orientation mapping revealed a weak but consistent crystallographic preferred orientation (CPO) of fine-grained albite within shear bands. No indicators for the activity of dislocation glide or creep are observed and the CPO is interpreted to form during deformation by dissolution-precipitation creep and GBS. In the low-T experiments at T of 300 - 600 °C, samples deform in a dominant semi-brittle manner by cataclastic flow. Within these experiments, the syn-kinematic formation of amorphous material in high strain zones was observed. The amorphous material is interpreted to evolve from an ultra-cataclastic pre-stage by mechanical wear, where plagioclase is seen to be particularly susceptible to this form of amorphisation. Flow structures within the amorphous material indicate its potential viscous flow behaviour. The mechanical data does show significantly lower strengths for samples deformed at higher temperatures and based on microstructural observations and Brillouin spectroscopy measurements it is seen as likely that the amorphous material is causing this temperature sensitivity. The occurrence of amorphous material in natural brittle fault zones might thus bear the potential to introduce a viscous contribution to fault rock rheology

Disease of forest tree seedlings (Article Review)

Nursery are one of the most important reasons for the success and progress of the agricultural renaissance, as it depends on the application of various advanced scientific methods and the use of Nursery of various types in the propagation and production of forest seedlings and others. A greenhouse is a place where small plants are produced to plant them in gardens, fields, and trees. The plants produced by the greenhouse vary between ornamental plants and flowers, fruit plants, vegetables, and forest trees, and Nursery are often held for commercial purposes.The incidence of Nursery diseases increases from what happens in the forests through the increase in service operations and attention to irrigation and fertilization and not sterilizing the soil and seeds, as this appears on the resulting seedlings, and the seedlings resulting from Nursery can be a source of transmission of infection when artificial afforestation is performed. Therefore, we must monitor and ensure the safety of the productive seedlings. The disease of Root rot and seedling disease Damping-off are one of the most critical diseases found in forest Nursery and the most widespread throughout the hot and cold world. As these diseases affect many forest plants, it is preferable to study them to avoid infecting forest species through the correct management of these Nursery by sterilizing the soil and seeds, as well as regular irrigation and correct fertilization

The geochemistry of Cenozoic basalts and peridotite xenoliths from Hannuoba region, eastern China : implication for their petrogenesis and subcontinental mantle heterogeneity

Thesis (Ph. D.)--Massachusetts Institute of Technology, Dept. of Earth, Atmospheric, and Planetary Sciences, 1989.Includes bibliographical references (v. 2, leaves 273-291).by Yan Song.Ph.D

Sublithosphere mantle crystallization and immiscible sulphide melt segregation in continental basal magmatism: evidence from clinopyroxene megacrysts in the Cenozoic basalts of eastern China

This study explores the effects of high-pressure crystallization and immiscible sulphide melt segregation under mantle conditions on the compositional variation of basaltic magmas, using clinopyroxene megacrysts in the Cenozoic basalts of eastern China. These clinopyroxene megacrysts are large (up to > 10 cm in size) and homogeneous at the grain scale. They were crystallized from variably evolved parental magmas and then captured by their host basalts. The large and systematic variations of [Sm/Yb]N, Lu/Hf, Fe/Mn, Sc/La, Ni and Cu with Mg# in the clinopyroxene megacrysts suggest their co-precipitation with garnet and with immiscibility between sulphide and silicate melts. This is consistent with the appearance of garnet megacrysts in the host basalts and abundant sulphide globules in the clinopyroxene megacrysts. The covariation between Ni contents of sulphide globules and Mg# of the clinopyroxene megacrysts suggests a genetic relationship between sulphide globules and clinopyroxene megacrysts. High-pressure crystallization of clinopyroxene and garnet results in decrease of Mg# and concentrations of CaO, MnO and heavy rare earth elements (e.g., Yb) and increase of Fe/Mn and [Sm/Yb]N in the residual melts. Therefore, geochemical characteristics of low Mg#, low CaO and MnO contents and high Fe/Mn and [Sm/Yb]N in basalts do not necessarily indicate a pyroxenite mantle source. In addition, caution is needed when applying the olivine addition method to infer the primary compositions of alkali basalts without considering the effects of highpressure crystallization of clinopyroxene and garnet. The calculated P-T conditions of the clinopyroxene megacrysts are close to those of the lithosphere-asthenosphere boundary (LAB) beneath eastern China, and the low primitive [Sm/Yb]N (~ 4.0) of melts parental to the clinopyroxene megacrysts suggests final equilibration at relatively low pressures most likely beneath the LAB. Hence, a melt-rich layer is expected close beneath the LAB. Melt pools in this melt-rich layer provide a stable and closed environment for the growth of compositionally homogeneous clinopyroxene megacrysts. As a result, melts in these melt pools are compositionally evolved with low and variable Mg#. Subsequent pulses of melt aggregation/supply from depths with primitive compositions and high Mg# will disturb these melt pools, cause magma mixing and trigger the eruption of magmas carrying clinopyroxene and garnet megacrysts