The use of electron backscatter diffraction and orientation contrast imaging as tools for sulphide textural studies: example from the Greens Creek deposit (Alaska)

The Greens Creek polymetallic massive sulphide deposit is hosted in a typical polyphase deformed lower greenschist facies orogenic setting. The structure of the host rocks is well constrained, exhibiting a series of three superimposed ductile deformations followed by two brittle episodes. The ore is found both in fold hinges where early-formed depositional features are preserved and in fold limbs where primary features are typically strongly modified or obliterated. Samples from both settings have been investigated using electron backscatter diffraction (EBSD) coupled with forescatter orientation contrast (OC) imaging in order to observe the effects of deformation and lower greenschist facies metamorphism on pyrite. Results suggest that colloform pyrite may preserve information relevant to palaeoenvironment, that apparently simple textures are generally more complex, and that pyrite can deform plastically by dislocation glide and creep processes at lower temperatures and/or strain rates than generally accepted. This analysis indicates that EBSD and OC imaging provide powerful tools for observing textural relationships in pyrite that are not shown by more traditional methods. They should become routine tools for pyrite texture analysis.Editorial handling: D. Lentz     

A strontium isotopic study of formation waters from the Illinois basin, U.S.A.

The isotopic composition of Sr has been measured in 73 formation-water samples from Paleozoic strata in the Illinois basin; ⁸⁷Sr/⁸⁶Sr ratios range from 0.7079 to 0.7108. With the exception of four samples, the waters are more radiogenic than corresponding Paleozoic sea-water values. The relatively narrow range of slightly elevated ⁸⁷Sr/⁸⁶Sr rations is uniformly distributed in waters throughout the stratigraphic column and in Silurian waters across the basin. Isotopic analyses of core samples from reservoir rocks show an absence of water-rock Sr isotopic equilibration. Basin lithology and analyses of detrital rock units indicate that clay minerals in shales and in quartz sandstone matrices represent the only significant source of radiogenic Sr for the waters. Silurian and Devonian water show a two-component mixing relation which suggests that they comprise a single hydrogeological system that evolved when radiogenic water from New Albany shales entered Silurian-Devonian carbonate rocks and mixed with marine interstitial water. Regional migration of the waters and associated petroleum within the Silurian-Devonian strata, proposed in other studies, is consistent with the Sr isotopic data. Under favorable circumstances subsurface waters are capable of retaining a Sr isotopic recor of their evolution.     

The Miocene-Pliocene Macusani Volcanics,SE Peru

The Miocene-Pliocene Macusani volcanics, SE Peru, outcrop in three separate tectonic intermontane basins developed on a Paleozoic-Mesozoic volcano-sedimentary sequence. Several ignimbrite sheets are recognized and K-Ar dates record at least semi-continuous volcanic activity from 10 to 4 Ma in the Macusani field. The volcanics in the Macusani basin comprise crystal-rich (45% crystals) ash-flow tuffs and rare obsidians glasses, both with unusual mineralogy, similar to two-mica peraluminous leucogranites. The mineralogical assemblage (quartz, sanidine Or_69–75, plagioclase, biotite, muscovite and andalusite (both coexisting in the entire volcanic field), sillimanite, schörl-rich tourmaline, cordierite-type phases, hercynitic spinel, fluor-apatite, ilmenite, monazite, zircon, niobian-rutile) is essentially constant throughout the entire Macusani field. Two distinct generations of plagioclase are recognized, viz. group I (An_10–20) and group II (An_30–45). Sillimanite forms abundant inclusions in nearly all phases and is earlier than andalusite which occurs as isolated phenocrysts. Biotite (Al-, Ti-, Fe- and F-rich) shows pronounced deficiencies in octahedral cations. Muscovite is also F-rich and displays limited biotitic and celadonitic substitutions. There is no systematic variation in mineral chemistry with stratigraphic position. The mineralogical data provide a basis for distinction between an early magmatic and a main magmatic stage. The early stage corresponds to the magmatic evolution at or near the source region and includes both restites and early phenocrysts. Some biotites (with textures of disequilibrium melting to Fe — Zn spinel), part of the sillimanite, apatite and monazite, possibly some tourmaline and cordierite-type phases are restites. However, the restite content of the magma was low (5 vol. % maximum). The group II plagioclase are interpreted as early phenocrysts. During this stage, temperatures were as high as 800° C, pressure was no more than 5–7.5 kbar, was intermediate between WM and QFM and was low. The biotite melting textures and the coexistence of restites and early phenocrysts imply fast heating rates in the source region. The transition between the early and the main magmatic stage was abrupt (andalusite crystallization in place of sillimanite, group I vs. group II plagioclases) and suggests rapid ascent of the magma from its source region. During the main crystallization stage, temperature was 650° C or lower at a pressure of 1.5–2 kbar. (calculated from equilibrium between muscovite, quartz, sanidine and andalusite) are around 1, suggesting conditions close to H₂O-saturation. f _HF is around 1 bar but the ratios are significantly different between samples. ranges between 138 and 225 bar. This study shows that felsic, strongly peraluminous, leucogranitic magmas having andalusite and muscovite phenocrysts may be generated under H₂O-undersaturated conditions.CRPG Contribution n 769     

Resolution Control for Balancing Overview + Detail in Spatial, Multivariate Analysis

Parallel coordinates, re-orderable matrices, and dendrograms are widely used for visual exploration of multivariate data. This research proposes an approach to systematically integrate the methods in a complementary manner for supporting multi-resolution visual data analysis with an enhanced overview+detail exploratory strategy. The paper focuses on three topics: (1) dynamic control across resolutions at which data are explored; (2) coordination and color mapping among the views; and (3) enhanced features of each view designed for the overview+detail exploratory tasks. We contend that systematically coordinating the views through user-controlled resolutions within a highly interactive analysis environment will boost productivity for exploration tasks. We offer a case study analysis to demonstrate this potential. The case study is focused on a complex, geographically referenced dataset including public health, demographic and environmental components.     

Strontium isotope and alkali element abundances in ultramafic rocks

The concentrations of the trace elements Na, K, Rb and Sr and the isotopic composition of Sr have been measured in a suite of ultramafic rocks, including alpine-type intrusions, inclusions in basalts and kimberlite pipes, zones from stratiform sheets, and a mica peridotite. From these data and those available in the literature the following conclusions can be drawn. Alpine-type ultramafic material appears to be residual in nature and can be neither the source material for the derivation of basalts nor the refractory residue of modern basalts. Alpine-type ultramafic intrusions appear to have no relationship with ultramafic zones in stratiform sheets and were probably derived from the upper mantle. A genetic relationship exists between basalts and their ultramafic inclusions, but it is extremely doubtful that this inclusion material could give rise to basalts by partial fusion. There is a possible genetic relationship between basalts and ultramafic inclusions in kimberlite pipes, and this ultramafic material is a potential source for the derivation of basalts. Ultramafic inclusions in basalts are probably not fragments of an alpine-type ultramafic zone in the mantle. An attempt has been made to synthesize the data and interpretations of this study by way of speculations on the role of ultramafic rocks in the differentiation history of the earth.     

The influence of groundwater abstraction on interpreting climate controls and extreme recharge events from well hydrographs in semi-arid South Africa

There is a scarcity of long-term groundwater hydrographs from sub-Saharan Africa to investigate groundwater sustainability, processes and controls. This paper presents an analysis of 21 hydrographs from semi-arid South Africa. Hydrographs from 1980 to 2000 were converted to standardised groundwater level indices and rationalised into four types (C1–C4) using hierarchical cluster analysis. Mean hydrographs for each type were cross-correlated with standardised precipitation and streamflow indices. Relationships with the El Niño–Southern Oscillation (ENSO) were also investigated. The four hydrograph types show a transition of autocorrelation over increasing timescales and increasingly subdued responses to rainfall. Type C1 strongly relates to rainfall, responding in most years, whereas C4 notably responds to only a single extreme event in 2000 and has limited relationship with rainfall. Types C2, C3 and C4 have stronger statistical relationships with standardised streamflow than standardised rainfall. C3 and C4 changes are significantly (p <?0.05) correlated to the mean wet season ENSO anomaly, indicating a tendency for substantial or minimal recharge to occur during extreme negative and positive ENSO years, respectively. The range of different hydrograph types, sometimes within only a few kilometres of each other, appears to be a result of abstraction interference and cannot be confidently attributed to variations in climate or hydrogeological setting. It is possible that high groundwater abstraction near C3/C4 sites masks frequent small-scale recharge events observed at C1/C2 sites, resulting in extreme events associated with negative ENSO years being more visible in the time series.

     

Detrital footprint of the Mozambique ocean: U–Pb SHRIMP and Pb evaporation zircon geochronology of metasedimentary gneisses in eastern Madagascar

The southern East African Orogen is a collisional belt where the identification of major suture zones has proved elusive. In this study, we apply U–Pb isotopic techniques to date detrital zircons from a key part of the East African Orogen, analyse their possible source region and discuss how this information can help in unravelling the orogen.U–Pb sensitive high-mass resolution ion microprobe (SHRIMP) and Pb evaporation analyses of detrital zircons from metasedimentary rocks in eastern Madagascar reveal that: (1) the protoliths of many of these rocks were deposited between 800 and 550 Ma; and (2) these rocks are sourced from regions with rocks that date back to over 3400 Ma, with dominant age populations of 3200–3000, 2650, 2500 and 800–700 Ma.The Dharwar Craton of southern India is a potential source region for these sediments, as here rocks date back to over 3400 Ma and include abundant gneissic rocks with protoliths older than 3000 Ma, sedimentary rocks deposited at 3000–2600 Ma and granitoids that crystallised at 2513–2552 Ma. The 800–700 Ma zircons could potentially be sourced from elsewhere in India or from the Antananarivo Block of central Madagascar in the latter stages of closure of the Mozambique Ocean. The region of East Africa adjacent to Madagascar in Gondwana reconstructions (the Tanzania craton) is rejected as a potential source as there are no known rocks here older than 3000 Ma, and no detrital grains in our samples sourced from Mesoproterozoic and early Neoproterozoic rocks that are common throughout central east Africa. In contrast, coeval sediments 200 km west, in the Itremo sheet of central Madagascar, have detrital zircon age profiles consistent with a central East African source, suggesting that two late Neoproterozoic provenance fronts pass through east Madagascar at approximately the position of the Betsimisaraka suture. These observations support an interpretation that the Betsimisaraka suture separates rocks that were derived from different locations within, or at the margins of, the Mozambique Ocean basin and therefore, that the suture is the site of subduction of a strand of Mozambique Ocean crust.     

Late Pleistocene (MIS 3–4) climate inferred from micromammal communities and δO of rodents from Les Pradelles,France

The middle Paleolithic stratigraphic sequence of Les Pradelles (Charente, France) spans from the end of Marine Isotope Stage (MIS) 4 until the middle of MIS 3. Micromammal remains are present in all the stratigraphic levels, offering a rare opportunity to address the questions of both environmental and climatic fluctuations throughout this period. Climate modes were studied through the taphonomy, biodiversity and oxygen isotope compositions of phosphate (δ¹⁸O_p) from 66 samples of rodent tooth enamel. The δ¹⁸O_p values from the lower sedimentary levels provide summer mean air temperatures of 19 ± 2°C (level 2/1) and of 16 ± 2°C (levels 2A, 2B and 4A). Within the middle of sequence (level 4B), a paleobiodiversity change can be identified with an increase of Dicrostonyx torquatus, which is associated with the largest amplitude in δ¹⁸O_p values and the highest maximal δ¹⁸O_p values. At the top of the sequence (level 5-2), a biodiversity change is observed with the increase of Microtus arvalis, but without any change in δ¹⁸O_p values. The association of cold rodent species with unexpected high and large amplitudes in the δ¹⁸O_p values of their teeth, possibly indicative of aridity, suggests their deposition during a Heinrich event.