Discriminating assimilants and decoupling deep- vs. shallow-level crystal records at Mount Adams using 238U–230Th disequilibria and Os isotopes     

Brian R. Jicha  Clark M. Johnson  Wes Hildreth  Brian L. Beard  Garret L. Hart  Steven B. Shirey  Brad S. Singer 《Earth and Planetary Science Letters》2009,277(1-2):38-49

A suite of 23 basaltic to dacitic lavas erupted over the last 350 kyr from the Mount Adams volcanic field has been analyzed for U–Th isotope compositions to evaluate the roles of mantle versus crustal components during magma genesis. All of the lavas have (²³⁰Th/²³⁸U) > 1 and span a large range in (²³⁰Th/²³²Th) ratios, and most basalts have higher (²³⁰Th/²³²Th) ratios than andesites and dacites. Several of the lavas contain antecrysts (crystals of pre-existing material), yet internal U–Th mineral isochrons from six of seven lavas are indistinguishable from their eruption ages. This indicates a relatively brief period of time between crystal growth and eruption for most of the phenocrysts (olivine, clinopyroxene, plagioclase, magnetite) prior to eruption. One isochron gave a crystallization age that is ~ 20–25 ka older than its corresponding eruptive age, and is interpreted to reflect mixing of older and juvenile crystals or a protracted period of magma storage in the crust. Much of the eruptive volume since 350 ka consists of lavas that have small to moderate ²³⁰Th excesses (2–16%), which are likely inherited from melting of a garnet-bearing intraplate (“OIB-like”) mantle source. Following melt generation and subsequent migration through the upper mantle, most Mt. Adams magmas interacted with young, mafic lower crust, as indicated by ¹⁸⁷Os/¹⁸⁸Os ratios that are substantially more radiogenic than the mantle or those expected via mixing of subducted material and the mantle wedge. Moreover, Os–Th isotope variations suggest that unusually large ²³⁰Th excesses (25–48%) and high ¹⁸⁷Os/¹⁸⁸Os ratios in some peripheral lavas reflect assimilation of small degree partial melts of pre-Quaternary basement that had residual garnet or Al-rich clinopyroxene. Despite the isotopic evidence for lower crustal assimilation, these processes are not generally recorded in the erupted phenocrysts, indicating that the crystal record of the deep-level ‘cryptic’ processes has been decoupled from shallow-level crystallization.  相似文献   

Zircon U–Pb strain chronometry reveals deep impact-triggered flow     

D.E. Moser  W.J. Davis  S.M. Reddy  R.L. Flemming  R.J. Hart 《Earth and Planetary Science Letters》2009,277(1-2):73-79

Large (> 100 km) meteorite impact cratering events play important roles in surface and biosphere evolution, however, their potential for widespread ductile modification of the lithosphere has been difficult to assess, due partly to our inability to isotopically age-correlate deep mineral fabrics with surface records. We have integrated benchmark U–Pb zircon dating methods (ID-TIMS, SHRIMP) with new microstructural techniques (EBSD, µXRD) to demonstrate that crystal–plastic deformation can cause rapid out-diffusion of radiogenic Pb and accompanying trace element alteration in crystalline zircon. We have used this phenomenon to directly date fabric in Archean zircons and xenoliths of the lower crust of South Africa at 2023 ± 15 million years, coeval with the 2020 ± 3 million year old Vredefort cratering event at surface, with extent ≥ 20,000 km². Our findings indicate that regional exogenic fabrics, similar to high-temperature tectonic fabrics, exist in ancient crust. Moreover, our results establish that crystal-plastic deformation in the lithosphere can now be directly dated and linked to planetary evolution by zircon U–Pb strain chronometry.  相似文献   

Book Review: Earth's glacial record. Eds. M. Deynoux,J.M.G. Miler,E.W. Domack,N. Eyles,I.J. Fairchild and G.M. Young. Cambridge University Press,Cambridge 1994. 266pp. £60.00 (hardback) ISBN 0-521-42022-9     

J. K. Hart 《第四纪科学杂志》1996,11(1):93-94

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Morphological and textural diversity of the Steens Basalt lava flows,Southeastern Oregon,USA: implications for emplacement style and nature of eruptive episodes     

Ninad R. Bondre  William K. Hart 《Bulletin of Volcanology》2008,70(8):999-1019

This study focuses on Middle Miocene tholeiitic flood basalt lava flows from the Oregon Plateau, northwestern USA (Steens Basalt), and is the first to comprehensively document and evaluate their morphology. Field observations of flows from several sections within and proximal to the main exposures at Steens Mountain have been supplemented with textural and geochemical data, and are used to offer preliminary insights into their emplacement. Compound pahoehoe flows of variable thickness appear to be common throughout the study area, laterally and vertically. These tend to be plagioclase phyric and the morphology and disposition of constituent flow lobes are quite similar to those from other provinces such as Hawaii and the Snake River Plain. Classic a’a flows with brecciated upper and basal crusts are not abundant, but by no means uncommon. Flows with characters different from typical pahoehoe and a’a are also common. Such flows display a range in morphology; flows with preserved upper crusts but brecciated basal crusts, as well as those displaying well-developed flow-top breccias and preserved basal crusts (rubbly pahoehoe) are observed. The Steens Basalt appears to display greater morphological and textural diversity at the outcrop scale than that described for some other flood basalt provinces. The abundant compound pahoehoe flows (often rich in plagioclase phenocrysts) were likely emplaced during slow but sustained eruptive episodes; their constituent lobes show clear evidence for endogenous growth. The relatively aphyric flows with brecciated surfaces (including a’a) hint at higher strain rates and/or higher viscosity, probably caused by higher effusion rates. A couple of sections are characterized by compositionally similar, but morphologically different flows that were possibly part of the same eruption. While differences in pre-eruptive topography could explain this, it is also possible that certain physical parameters changed substantially and abruptly during eruption and that such changes were accompanied by differentiation processes within the plumbing system. It is possible that such observations indicate temporal fluctuations within complex magmatic and eruptive systems, and deserve closer scrutiny.  相似文献   

Determining flow patterns and emplacement dynamics from tsunami deposits with no visible sedimentary structure          下载免费PDF全文

Claire Kain  Patrick Wassmer  James Goff  Catherine Chagué‐Goff  Christopher Gomez  Deidre Hart  Daniela Fierro  Geraldine Jacobsen  Atun Zawadzki 《地球表面变化过程与地形》2017,42(5):763-780

In the absence of eyewitness reports or clear sedimentary structures, it can be difficult to interpret tsunami deposits or reconstruct tsunami inundation patterns. The emplacement dynamics of two historical tsunami deposits were investigated at seven transects in Okains Bay, New Zealand, using a combined geospatial, geomagnetic and sedimentological approach. The tsunami deposits are present as layers of sand and silt intercalated between soils and become finer and thinner with distance inland. The deposits are attributed to the 1960 and possibly the 1868 tsunamis, based on radiometric dating and correlation with historical records. Measurements of Magnetic Fabric (MF: Anisotropy of Magnetic Susceptibility) and particle size were used to reconstruct the evolution of flow dynamics laterally and vertically. A combination of statistical methods, including spatial autocorrelation testing, Spearman's rank order correlation, Principal Component Analysis (PCA) and K‐means cluster analysis, was applied to examine relationships between MF parameters and sediment texture, and infer depositional hydrodynamics. Flow patterns deduced from MF show the estuary channel acted as a conduit for inundation, with flow commonly aligned sub‐perpendicular to the estuary bed. MF and sediment data suggest deposition occurred from settling during laminar flow. Evidence of both uprush and backwash deposition, as well as wave reflection from infrastructure, was found. Statistical analysis of data showed significant relationships between grain size parameters and MF parameters associated with flow speed and magnetic fabric type. PCA and cluster analysis differentiated samples into two primary hydrodynamic groups: (1) samples deposited from laminar flow; and (2) samples deposited close to the limit of inundation, which includes samples deposited further inland, those affected by flow convergence, and those in the upper part of tsunami deposits. This approach has potential as a tool for reconstructing hydrodynamic conditions for palaeotsunamis and by combining spatial and statistical analyses, large‐scale investigations can be more easily performed. Copyright © 2016 John Wiley & Sons, Ltd.  相似文献   

Geochemistry of Suspended Particulate Matter (SPM) in the Murray-Darling River System: A Conceptual Isotopic/Geochemical Model for the Fractionation of Major, Trace and Rare Earth Elements   总被引：1，自引：0，他引：1  

G.B. Douglas  B.T. Hart  R. Beckett  C.M. Gray  R. L. Oliver 《Aquatic Geochemistry》1999,5(2):167-194

A conceptual isotopic/geochemical model is presented to explain the variation of major, trace and rare earth element (REE) geochemistry and Sr isotope systematics in suspended particulate matter (SPM) as a function of particle/colloid size. This conceptual model is an extension of a previous investigation of the origin of SPM in the Murray-Darling River system (MDRS) that utilised Sr isotope systematics to examine aspects of SPM (particle/colloid) origin, structure and mineralogy. The geochemical processes that give rise to the often coherent trends in major, trace and REE geochemistry and Sr isotopic signature as a function of particulate (<1 μm) and colloidal (>1 μm) size in the MDRS have been identified using an enhanced SPM size fractionation technique as a basis to not only obtain a broad range of particle/colloid size ranges, but also to provide sufficient material for subsequent geochemical and isotopic analysis. The conceptual isotopic/geochemical model proposed here contains three major components: (i) the differential weathering of micas and alkali (K-) feldspars to form the majority of the particulate (<1 μm) fractions (high 87Sr/86Sr ratio), which have a geochemical and Sr isotopic signature that closely resembles precursor mineralogies, (ii) the differential weathering of Na, Ca-feldspars (plagioclase) which decompose to form clay minerals in the colloidal (>1 μm) fractions (low 87Sr/86Sr ratio), with a range of geochemical signatures related to the relative proportions of inorganic and organic constituents, and (iii) the presence of natural organic matter as coatings on the particulate (<1 μm) and colloidal (>1 μm) matter and possibly as organo-colloids which exert an increasing influence in particular on bulk colloid geochemistry with decreasing colloid size. This conceptual isotopic/geochemical model also accounts for the distinct variation in major, trace and REE geochemistry and Sr isotopic systematics between the particulate (<1 μm) and colloidal (>1 μm) fractions, the variation being primarily a function of the distinctly different precursor mineralogies of the SPM fractions and geochemical fractionation during the weathering and transport. Additionally, this model explains a systematic fractionation of REE apparent within colloidal (>1 μm) fractions. Statisitcal (hierachical cluster) analysis of two particulate and three colloidal fractions from 23 samples from the MDRS is used as a basis to investigate geochemical and mineralogical associations within the particulate and colloidal size fractions and to provide additional supporting evidence for the conceptual isotopic/geochemical model. This revised version was published online in July 2006 with corrections to the Cover Date.  相似文献   

Does one size fit all? An evaluation of length–weight relationships for New Zealand's freshwater fish species     

PG Jellyman  DJ Booker  SK Crow  ML Bonnett  DJ Jellyman 《新西兰海洋与淡水研究杂志》2013,47(4):450-468

Length–weight relationships are a fundamental tool for assessing populations and communities in fisheries science. Many researchers have collected length–weight data throughout New Zealand, yet parameters describing these relationships remain unpublished for many species of freshwater fish. We compiled 285,124 fish records from researchers and institutions across New Zealand to parameterise length–weight equations, using both power and quadratic models, for 53 freshwater species belonging to 13 families. The influence of location and sex on length–weight relationships was also assessed. Location, in particular, generated different length–weight relationships for 65% of the species examined. Length–weight equations were validated by comparing predicted weights against independently measured weights from 25 electrofished sites across New Zealand and the equations were highly accurate (R²>0.99). Recommendations are made about how to robustly apply this new resource which should assist freshwater fisheries researchers throughout New Zealand.  相似文献   

Estimating periphyton standing crop in streams: a comparison of chlorophyll a sampling and visual assessments     

C Kilroy  DJ Booker  L Drummond  JA Wech  TH Snelder 《新西兰海洋与淡水研究杂志》2013,47(2):208-224

Periphyton standing crop is often measured as chlorophyll a but there is increasing interest in using visual assessments of periphyton cover. Visual methods are rapid and require no sample analysis, but can be regarded as subjective with high inter-operator variability. We investigated variability in periphyton standing crop across operators, rivers and time, as determined visually and from chlorophyll a measurements. We found that visual assessments (comprising percentage cover estimates of up to eight periphyton categories) distinguished sites and occasions as effectively as chlorophyll a. Furthermore, an estimate of chlorophyll a could be derived from the visual assessments. Because our surveys were conducted in only three rivers, general applicability of the derivation of estimated chlorophyll a from the visual assessments warrants further investigation. The current recommendation of 20 views was sufficient for realistic visual assessments of average cover. Overall, our results indicated that inter-operator variability in visual assessments need not be a major concern, given adequate training.  相似文献   

Anatomy of the stratigraphical boundary between the Arnager Greensand and Arnager Limestone (Upper Cretaceous) on Bornholm,Denmark     

Malcolm B. Hart  Richard G. Bromley  Stephen R. Packer 《Proceedings of the Geologists' Association. Geologists' Association》2012,123(3):471-478

The complex boundary between the Arnager Greensand Formation and the Arnager Limestone Formation on the island of Bornholm (Denmark) has been studied for almost a century. Despite this effort, the hiatus represented by the boundary remains poorly constrained. Using a considerable number of processed samples and thin sections the uppermost Arnager Greensand Formation is confirmed as Thalmanninella reicheli Zone age (early Middle Cenomanian) and the overlying Arnager Limestone Formation is probably early Coniacian in age. No evidence of macrofossil and microfossil assemblages indicative of the late Cenomanian or the Turonian have been recorded and there is no palaeontological or sedimentological evidence of the global late Cenomanian (Bonarelli or OAE 2) anoxic event. The significant mid-Cenomanian to early Coniacian hiatus reflects a period of sediment starvation along the line of the Sorgenfrei-Tornquist Zone in the vicinity of Bornholm.  相似文献   

A Review of “Fundamentals of Tree-Ring Research”     

Sarah J. Hart 《The Professional geographer》2013,65(1):150-151

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