Many diagrams conventionally used to classify igneous rocks utilize mobile elements, which commonly renders them unreliable for classifying rocks from the geological record. The K2O–SiO2 diagram, ...used to subdivide volcanic arc rocks into rock type (basalts, basaltic andesites, andesites, dacites and rhyolites) and volcanic series (tholeiitic, calc-alkaline, high-K calc-alkaline and shoshonitic), is particularly susceptible to the effects of alteration. However, by using Th as a proxy for K2O and Co as a proxy for SiO2 it is possible to construct a topologically similar diagram that performs the same task but is more robust for weathered and metamorphosed rocks. This study uses >1000 carefully filtered Tertiary–Recent island arc samples to construct a Th–Co classification diagram. A ‘testing set’ comprising data not used in constructing the diagram indicates a classification success rate of c. 80%. When applied to some hydrothermally altered, then tropically weathered Cretaceous volcanic arc lavas from Jamaica, the diagram demonstrates the presence of a tholeiitic volcanic arc series dominated by intermediate–acid lavas overlain by a calc-alkaline series dominated by basic lavas.
A new ICP‐MS database for glasses from the Mariana Trough, together with published and new ICP‐MS data from the Mariana arc, provides the basis for geochemical mapping of the Mariana arc‐basin ...system. The geochemical maps presented here are based on the graphic representation of spatial variations in geochemical proxies for the principal mantle and subduction components. The focus is on three elements with high and similar partition coefficients but different behavior in subduction systems, namely, Ba, Th, and Nb. Two elements with different partition coefficients, Ta and Yb, are used as normalizing factors. Ratio maps (Ta/Yb, Nb/Ta, Th/Ta, Ba/Ta, Ba/Th) provide the simplest petrogenetic insights, subduction zone addition maps based on deviations from a MORB array provide more quantitative insights, and component maps represent an attempt to isolate the different subduction components. The maps shown here indicate the presence of a variably depleted asthenosphere and three added components: a Nb‐Th‐Ba component, a Th‐Ba deep‐subduction component, and a Ba‐only shallow‐subduction component. The asthenosphere entering the system is enriched relative to N‐MORB and appears to be focused at three sites within the Mariana Trough. The Nb‐Th‐Ba component is present mainly in the north of the arc (the Northern Seamount province and northern Central Island Province), the northern edge of the Mariana Trough, and two locations within the Southern Seamount Province. It has a distinctively high Nb/Ta ratio and a moderate enrichment in Th and Ba relative to Nb. Its composition and distribution indicate that it may not be part of the present subduction system but instead originates in mantle lithosphere previously enriched above the subduction zone by addition of small‐degree, subduction‐modified mantle melts. The Th‐Ba component is present throughout the arc and, in minor amounts, in parts of the back‐arc basin. The Ba‐only component is mainly present in the central part of the arc and at the edges of the back‐arc basin. Overall, the geochemical maps provide a new perspective on the geochemical processes that accompany the evolution of an arc basin system from prerifting lithospheric enrichment, through arc‐rifting to arc volcanism and back‐arc spreading.
Pb isotope systematics have already been used successfully to demonstrate that the lavas of the arc-basin terrains of the SW Pacific are derived from two mantle domains, one of Pacific-like character ...and the other of Indian-like character. However, the mobility of Pb during subduction and alteration has mainly restricted the fingerprinting of domains to fresh lavas of MORB composition. We demonstrate that the less alteration-sensitive Hf–Nd isotope projection also discriminates successfully between ‘Pacific’ and ‘Indian’ domains, and thus enables us to extend mantle domain fingerprinting to the back-arc basin basalts and boninites of the Lau and North Fiji Basins and the volcanic arc lavas of the Kermadec, Tonga and Vanuatu arcs. Fingerprinting is facilitated by the observation that the Hf isotope ratio is independent of subduction-input parameters, indicating that Hf has been essentially conservative during the subduction process. Subducted Nd has been added to the mantle source, but subtracting this numerically using the magnitude of negative Hf anomalies filters out the subduction effect. The data show that the ‘Indian’ domain provides the source for magmas erupted at ridges, and arcs near these ridges, that have propagated southwards following the 12 Ma collision of the Ontong-Java Plateau with the Vitiaz Trench. This indicates that the ‘Indian’ domain is actually derived from SOPITA mantle (South Pacific Isotopic and Thermal Anomaly) — mantle modified by the Samoa and other plumes outboard of the trench which only entered the SW Pacific arc-basin system after the Ontong-Java Plateau collision removed the slab barrier at <
12 Ma. In the west, mantle flows beneath the network of south-propagating ridges in the North Fiji and NW Lau Basins, undergoing progressive depletion until the final loss of plume components produces an N-MORB mantle (Indian MORB Mantle) composition in the south North Fiji Basin and Central Lau Spreading Centre. In the east, newly-depleted Samoan plume mantle provides the source for the boninites and depleted arc tholeiites of the northern Tonga arc.
A low-cost radiometric temperature sensor system is described, with emphasis on the use of a 12.5 GHz near-field focused microstrip array to provide a low-profile compact product suitable for ...commercial and industrial applications. An overview of the temperature sensor system is given, followed by design details of the focused microstrip array, including near-field patterns and antenna efficiency measurements. Temperature measurements made with the radiometric system in a food-processing plant field test show that the antenna can resolve individual hamburger patties at a practical measurement distance
Neogene potassic lavas in northern and southern Tibet have different isotopic (εNd(i) north, −5·5 to −10·3; south −8·8 to −18·1) and major element signatures suggesting derivation from separate ...sub-continental lithospheric mantle (SCLM) sources. Inverse trace-element modelling shows that the southern Tibet magmas were derived by 1–2% partial melting of a phlogopite and amphibole peridotite, and that the northern samples were derived by 3–4% partial melting of a phlogopite peridotite. In both cases, melting is inferred to take place in the spinel stability field. Both sources show large ion lithophile element (LILE) enrichment relative to the high field strength elements (HFSE), and heavy rare earth element (HREE) depletion relative to primitive mantle. LILE/HFSE enrichment suggests subduction-related metasomatism; HREE depletion is indicative of prior melt extraction. Extension postdates the earliest magmatism in southern and north–central Tibet by 7 Myr and 5 Myr, respectively, which, in combination with the shallow depths of melting inferred for the Tibetan samples, supports geodynamic models invoking thinning of the SCLM. The northern Tibetan magmatism and extension can be explained by convective removal of the lower SCLM; the older ages and arcuate distribution of the southern magmas are most consistent with the SCLM erosion following slab break-off.
Ocean Drilling Program Leg 125 recovered serpentinized harzburgites and dunites from a total of five sites on the crests and flanks of two serpentinite seamounts, Conical Seamount in the Mariana ...forearc and Torishima Forearc Seamount in the Izu-Bonin forearc. These are some of the first extant forearc peridotites reported in the literature and they provide a window into oceanic, suprasubduction zone (SSZ) mantle processes. Harzburgites from both seamounts are very refractory with low modal clinopyroxene (<4%), chrome-rich spinels (cr-number = 0 times 40-0 times 80), very low incompatible element contents, and (with the exception of amphibolebearing samples) U-shaped rare earth element (REE) profiles with positive Eu anomalies. Both sets of peridotites have olivine-spinel equilibration temperatures that are low compared with abyssal peridotites, possibly because of water-assisted diffusional equilibration in the SSZ environment. However, other features indicate that the harzburgites from the two seamounts have very different origins. Harzburgites from Conical Seamount are characterized by calculated oxygen fugacities between FMQ (fayalite-magnetite-quartz) - 1 times 1 (log units) and FMQ + 0 times 4 which overlap those of mid-ocean ridge basalt (MORB) peridotites. Dunites from Conical Seamount contain small amounts of clinopyroxene, orthopyroxene and amphibole and are light REE (LREE) enriched. Moreover, they are considerably more oxidized than the harzburgites to which they are spatially related, with calculated oxygen fugacities of FMQ - 0 times 2 to FMQ + 1 times 2. Using textural and geochemical evidence, we interpret these harzburgites as residual MORB mantle (from 15 to 20% fractional melting) which has subsequently been modified by interaction with boninitic melt-within the mantle wedge, and these dunites as zones of focusing of this melt in which pyroxene has preferentially been dissolved from the harzburgite protolith. In contrast, harzburgites from Torishima Forearc Seamount give calculated oxygen fugacities between FMQ + 0 times 8 and FMQ + 1 times 6, similar to those calculated for other subduction-zone related peridotites and similar to those calculated for the dunites (FMQ + 1 times 2 to FMQ + 1 times 8) from the same seamount. In this case, we interpret both the harzburgites and dunites as linked to mantle melting (20-25% fractional melting) in a supra-subduction zone environment. The results thus indicate that the forearc is underlain by at least two types of mantle lithosphere, one being trapped or accreted oceanic lithosphere, the other being lithosphere formed by subduction-related melting. They also demonstrate that both types of mantle lithosphere may have undergone extensive interaction with subduction-derived magmas.
Mantle preconditioning may be defined as the extraction of small melt fractions from mantle asthenosphere during its flow to the site of magma generation. Equations may be written for mantle ...preconditioning, assuming that the mantle comprises enriched ‘plums’ in a depleted matrix. The equations take into account variations in mass fraction of plums, the relative rate of melting of plums and matrix, the temperature and pressure of melt extraction, the mass fraction of melt extracted, the extent of chemical exchange between plums and matrix, and the efficiency of melt extraction. Monitoring mineralogical changes and variations in partition coefficients along the inferred P–T–t path of the mantle asthenosphere allows the equations to be correctly applied to the conditions under which melt extraction takes place. Numerical experiments demonstrate the influence of petrogenetic variables on the shape of melt extraction trajectories and provide new criteria for distinguishing between melt extraction and mixing as the cause of regional geochemical gradients. Representative examples of arc–back-arc systems (Scotia), continental break-up (Afar) and plume–ridge interaction (Azores) indicate that the compositions of the mantle sources of mid-ocean ridge basalts and island arc basalts may be determined, at least in part, by the melt extraction histories of their asthenospheric sources.
Petrographic and geochemical studies of peridotites from the South Sandwich forearc region provide new evidence for the evolution of the South Sandwich arc-basin system and for the nature of ...interactions between arc magma and oceanic lithosphere. Peridotites from the inner trench wall in the north-east corner of the forearc vary from clinopyroxene-bearing harzburgites, through samples transitional between harzburgites and dunites or wehrlites, to dunites.
Corporate social responsibility has become a vital part of the business conversation. The issue for most companies is no longer whether to engage in socially responsible activities but how to achieve ...the maximum benefit from the resources available for social projects while still increasing shareholder value. In this article, the authors draw on years of quantitative and case-based studies of major corporations to conclude that CSR activities work best for society and the corporate participants when they are managed strategically and in collaboration with an array of commercial and noncommercial partners. The authors cite exemplars such as Avon Products, whose name is synonymous with responses to womens healthcare issues, and The Home Depot, whose foundation involves suppliers and government agencies in large-scale efforts to combat housing problems in the United States. The authors point to five core principles behind effective CSR strategies, from the need to contribute what we do to the importance of accommodating governments regulatory and taxation influences. PUBLICATION ABSTRACT
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