Cr-rich megacrysts of clinopyroxene and garnet from Lac de Gras kimberlites,Slave Craton,Canada – implications for the origin of clinopyroxene and garnet in cratonic lherzolites     

Bussweiler  Yannick  Pearson  D Graham  Stachel  Thomas  Kjarsgaard  Bruce A. 《Mineralogy and Petrology》2018,112(2):583-596

Kimberlites from the Diavik and Ekati diamond mines in the Lac de Gras kimberlite field contain abundant large (>1 cm) clinopyroxene (Cr-diopside) and garnet (Cr-pyrope) crystals. We present the first extensive mineral chemical dataset for these megacrysts from Diavik and Ekati and compare their compositions to cratonic peridotites and megacrysts from the Slave and other cratons. The Diavik and Ekati Cr-diopside and Cr-pyrope megacrysts are interpreted to belong to the Cr-rich megacryst suite. Evidence for textural, compositional, and isotopic disequilibrium suggests that they constitute xenocrysts in their host kimberlites. Nevertheless, their formation may be linked to extensive kimberlite magmatism and accompanying mantle metasomatism preceding the eruption of their host kimberlites. It is proposed that the formation of megacrysts may be linked to failed kimberlites. In this scheme, the Cr-rich megacrysts are formed by progressive interaction of percolating melts with the surrounding depleted mantle (originally harzburgite). As these melts percolate outwards, they may contribute to the introduction of clinopyroxene and garnet into the depleted mantle, thereby forming lherzolite. This model hinges on the observation that lherzolitic clinopyroxenes and garnets at Lac de Gras have compositions that are strikingly similar to those of the Cr-rich megacrysts, in terms of major and trace elements, as well as Sr isotopes. As such, the Cr-rich megacrysts may have implications for the origin of clinopyroxene and garnet in cratonic lherzolites worldwide.

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Evidence for a 200 km thick diamond-bearing root beneath the Central Mackenzie Valley,Northwest Territories,Canada? Diamond indicator mineral geochemistry from the Horn Plateau and Trout Lake regions     

Poitras  St&#;phane P.  Pearson  D. Graham  Hardman  Matthew F.  Stachel  Thomas  Nowell  Geoff M.  Cairns  Scott 《Mineralogy and Petrology》2018,112(2):719-736

The Central Mackenzie Valley (CMV) area of Northwest Territories is underlain by Precambrian basement belonging to the North American Craton. The potential of this area to host kimberlitic diamond deposits is relatively high judging from the seismologically-defined lithospheric thickness, age of basement rocks (2.2–1.7 Ga) and presence of kimberlite indicator minerals (KIMs) in Quaternary sediments. This study presents data for a large collection of KIMs recovered from stream sediments and till samples from two study areas in the CMV, the Horn Plateau and Trout Lake. In the processed samples, peridotitic garnets dominate the KIM grain count for both regions (> 25% each) while eclogitic garnet is almost absent in both regions (< 1% each). KIM chemistry for the Horn Plateau indicates significant diamond potential, with a strong similarity to KIM systematics from the Central and Western Slave Craton. The most significant issue to resolve in assessing the local diamond potential is the degree to which KIM chemistry reflects local and/or distal kimberlite bodies. Radiogenic isotope analysis of detrital kimberlite-related CMV ilmenite and rutile grains requires at least two broad age groups for eroded source kimberlites. Statistical analysis of the data suggests that it is probable that some of these KIMs were derived from primary and/or secondary sources within the CMV area, while others may have been transported to the area from the east-northeast by Pleistocene glacial and/or glaciofluvial systems. At this stage, KIM chemistry does not allow the exact location of the kimberlitic source(s) to be constrained.

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Episodic refertilization and metasomatism of Archean mantle: evidence from an orogenic peridotite in North Qaidam (NE Tibet,China)     

Qing Xiong  William L. Griffin  Jian-Ping Zheng  Suzanne Y. O’Reilly  Norman J. Pearson 《Contributions to Mineralogy and Petrology》2015,169(3):1-24

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Linking continental deep subduction with destruction of a cratonic margin: strongly reworked North China SCLM intruded in the Triassic Sulu UHP belt   总被引：1，自引：0，他引：1  

J. P. Zheng  H. Y. Tang  Q. Xiong  W. L. Griffin  S. Y. O’Reilly  N. Pearson  J. H. Zhao  Y. B. Wu  J. F. Zhang  Y. S. Liu 《Contributions to Mineralogy and Petrology》2014,168(1):1-21

The collision between the North and South China cratons in Middle Triassic time (240–225 Ma) created the world’s largest belt of ultrahigh-pressure (UHP) metamorphism. U–Pb ages, Hf isotope systematics and trace element compositions of zircons from the Xugou, Yangkou and Hujialing peridotites in the Sulu UHP terrane mainly record a ~470 Ma tectonothermal event, coeval with the Early Paleozoic kimberlite eruptions within the North China craton. This event is interpreted as the result of metasomatism by fluids/melts derived from multiple sources including a subducting continental slab. The peridotites also contain zircons with ages of ~3.1 Ga, and Hf isotope data imply a component ≥3.2 Ga old. Most zircon Hf depleted mantle model ages are ~1.3 Ga, suggesting that the deep subcontinental lithospheric mantle beneath the southeastern margin of the North China craton experienced a intense mid-Mesoproterozoic metasomatism by asthenospheric components, similar to the case for the eastern part of this craton. Integrating data from peridotites along the southern margin of the craton, we argue that the deep lithosphere of the cratonic margin (≥3.2 Ga old), from which the Xugou, Yangkou and Hujialing peridotites were derived, experienced Proterozoic metasomatic modification, followed by a strong Early Paleozoic (~470 Ma) tectonothermal event and the Early Mesozoic (~230 Ma) collision and northward subduction of the Yangtze craton. The Phanerozoic decratonization of the eastern North China craton, especially along its southern margin, was not earlier than the Triassic continental collision. This work also demonstrates that although zircons are rare in peridotitic rocks, they can be used to unravel the history of specific lithospheric domains and thus contribute to our understanding of the evolution of continental cratons and their margins.  相似文献   

Suspended-Sediment Flux and Retention in a Backwater Tidal Slough Complex near the Landward Boundary of an Estuary     

Tara?L.?Morgan-KingEmail author  David?H.?Schoellhamer 《Estuaries and Coasts》2013,36(2):300-318

Backwater tidal sloughs are commonly found at the landward boundary of estuaries. The Cache Slough complex is a backwater tidal region within the Upper Sacramento–San Joaquin Delta that includes two features that are relevant for resource managers: (1) relatively high abundance of the endangered fish, delta smelt (Hypomesus transpacificus), which prefers turbid water and (2) a recently flooded shallow island, Liberty Island, that is a prototype for habitat restoration. We characterized the turbidity around Liberty Island by measuring suspended-sediment flux at four locations from July 2008 through December 2010. An estuarine turbidity maximum in the backwater Cache Slough complex is created by tidal asymmetry, a limited tidal excursion, and wind-wave resuspension. During the study, there was a net export of sediment, though sediment accumulates within the region from landward tidal transport during the dry season. Sediment is continually resuspended by both wind waves and flood tide currents. The suspended-sediment mass oscillates within the region until winter freshwater flow pulses flush it seaward. The hydrodynamic characteristics within the backwater region such as low freshwater flow during the dry season, flood tide dominance, and a limited tidal excursion favor sediment retention.  相似文献   

Note on kelvin wave reflection in a channel with an arbitrary end wall     

Carl E. Pearson 《地球物理与天体物理流体动力学》2013,107(1):303-309

Abstract

Taylor's analysis of the reflection of a Kelvin wave by the perpendicular end wall of a rotating channel is extended to the case of an arbitrary end wall. A mirror imaging theorem applicable to such situations is derived.  相似文献   

Ida Susser: AIDS, sex, and culture: global politics and survival in Southern Africa     

Amber L. Pearson 《GeoJournal》2012,77(4):583-584

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Spatial variability of initial 230Th/232Th in modern Porites from the inshore region of the Great Barrier Reef     

Tara R. Clark  Jian-xin Zhao  Yue-xing Feng  Terry J. Done  Stacy Jupiter  Janice Lough  John M. Pandolfi 《Geochimica et cosmochimica acta》2012

The main limiting factor in obtaining precise and accurate uranium-series (U-series) ages of corals that lived during the last few hundred years is the ability to constrain and correct for initial thorium-230 (²³⁰Th₀), which is proportionally much higher in younger samples. This is becoming particularly important in palaeoecological research where accurate chronologies, based on the ²³⁰Th chronometer, are required to pinpoint changes in coral community structure and the timing of mortality events in recent time (e.g. since European settlement of northern Australia in the 1850s). In this study, thermal ionisation mass spectrometry (TIMS) U-series dating of 43 samples of known ages collected from living Porites spp. from the far northern, central and southern inshore regions of the Great Barrier Reef (GBR) was performed to spatially constrain initial ²³⁰Th/²³²Th (²³⁰Th/²³²Th₀) variability. In these living Porites corals, the majority of ²³⁰Th/²³²Th₀ values fell within error of the conservative bulk Earth ²³⁰Th/²³²Th atomic value of 4.3 ± 4.3 × 10^?6 (2σ) generally assumed for ²³⁰Th₀ corrections where the primary source is terrestrially derived. However, the results of this study demonstrate that the accuracy of ²³⁰Th ages can be further improved by using locally determined ²³⁰Th/²³²Th₀ values for correction, supporting the conclusion made by Shen et al. (2008) for the Western Pacific. Despite samples being taken from regions adjacent to contrasting levels of land modification, no significant differences were found in ²³⁰Th/²³²Th₀ between regions exposed to varying levels of sediment during river runoff events. Overall, 39 of the total 43 ²³⁰Th/²³²Th₀ atomic values measured in samples from inshore reefs across the entire region show a normal distribution ranging from 3.5 ± 1.1 to 8.1 ± 1.1 × 10^?6, with a weighted mean of 5.76 ± 0.34 × 10^?6 (2σ, MSWD = 8.1). Considering the scatter of the data, the weighted mean value with a more conservative assigned error of 25% (i.e. 5.8 ± 1.4 × 10^?6) that encompasses the full variation of the 39 ²³⁰Th/²³²Th₀ measurements is recommended as a more appropriate value for initial ²³⁰Th corrections for U-series dating of most Porites samples from inshore regions of the GBR. This will result in significant improvement in both the precision and accuracy of the corrected ²³⁰Th ages related to those based on the assumed bulk Earth ²³⁰Th/²³²Th₀ value of 4.3 ± 4.3 × 10^?6. However, several anomalously high ²³⁰Th/²³²Th₀ values reaching up to 28.0 ± 1.6 × 10^?6 occasionally found in some coral annual bands coinciding with El Niño years imply high ²³⁰Th/²³²Th₀ sources and highlight the complexities of understanding ²³⁰Th/²³²Th₀ variability. For U-series dating of young coral samples from such sites where anomalous ²³⁰Th/²³²Th₀ values occur, we suggest replicate dating of multiple growth bands with known age difference to verify age accuracy.  相似文献   

Platelet development in cuboid diamonds: insights from micro-FTIR mapping   总被引：1，自引：1，他引：0  

D. Howell  C. J. O’Neill  K. J. Grant  W. L. Griffin  S. Y. O’Reilly  N. J. Pearson  R. A. Stern  T. Stachel 《Contributions to Mineralogy and Petrology》2012,164(6):1011-1025

Fourier transform infrared mapping of diamonds can reveal detailed information on impurities, with a spatial context. We apply this technique, combined with in situ isotopic analysis of carbon, to the study of cuboid diamond growth in a sample that exhibits some mixed-habit growth. While there has been some uncertainty in the literature regarding sectoral differences in nitrogen aggregation and subsequent platelet development, the data from this study appear far more conclusive. We show that despite nitrogen being concentrated in octahedral sectors, there is no detectable difference in the concentration-adjusted rate of nitrogen aggregation within octahedral and cuboid sectors. However, the resultant platelet development is significantly reduced in cuboid sectors compared to contemporaneously formed octahedral sectors. This finding has significant implications for the classification of diamonds using the relationship between their platelet intensity and the absorption caused by B centres. It means that cuboid diamonds naturally fall below the linear relationship that has been termed regular, which would lead to them being incorrectly interpreted as having experienced heating or deformation. The data also support earlier suggestions that large hydrogen concentrations in the diamond-forming fluid may be required for cuboid growth. We further suggest that high nitrogen and hydrogen concentrations are required for mixed-habit diamond growth, which might be the product of specific fluid chemistries that occur in reducing mantle environments.  相似文献   

Deserpentinization and high-pressure(eclogite-facies) metamorphic features in the Eoarchean ultramafic body from Isua,Greenland     

Juan Miguel Guotana  Tomoaki Morishita  Ikuya Nishio  Akihiro Tamura  Tomoyuki Mizukami  Kenichiro Tani  Yumiko Harigane  Kristoffer Szilas  D.Graham Pearson 《地学前缘(英文版)》2022,(1):297-309

Discontinuous chains of ultramafic rock bodies form part of the 3800–3700 Ma Isua Supracrustal Belt(ISB),hosted in the Itsaq Gneiss Complex of southwestern Greenland.These bodies are among the world’s oldest outcrops of ultramafic rocks and hence an invaluable geologic record.Ultramafic rocks from Lens B in the northwestern limb of ISB show characteristics of several stages of serpentinization and deserpentinization forming prograde and retrograde mineral assemblages.Ti-rich humite-group minerals such as titanian chondrodite(Ti-Chn)and titanian clinohumite(Ti-Chu)often occur as accessory phases in the metamorphosed ultramafic rocks.The Ti-rich humite minerals are associated with metamorphic olivine.The host olivine is highly forsteritic(Fo96-98)with variable Mn O and Ni O contents.The concentrations of the rare-earth elements(REE)and high-field strength elements(HFSE)of the metamorphic olivine are higher than typical mantle olivine.The textural and chemical characteristics of the olivine indicate metamorphic origin as a result of deserpentinization of a serpentinized ultramafic protolith rather than primary assemblage reflecting mantle residues from high-degrees of partial melting.The close association of olivine,antigorite and intergrown Ti-Chn and Ti-Chu suggests pressure condition between$1.3–2.6 GPa within the antigorite stability field(<700°C).The overall petrological and geochemical features of Lens B ultramafic body within the Eoarchean ISB indicate that these are allochthonous ultramafic rocks that recorded serpentine dehydration at relatively lower temperature and reached eclogite facies condition during their complex metamorphic history similar to exhumed UHP ultramafic rocks in modern subduction zone channels.  相似文献