Alan Greig

Variations in the geochemistry of magmatism on the East Pacific Rise at 10?30'N since 800 ka

Samples of volcanic rock, collected from the flanks of the East Pacific Rise at 10o30 0 N, were used to investigate changes in the geochemistry of magmatism at the ridge axis, over the past 800 ka at this location. We show that there have been large variations in the major element chemistry of the lavas erupted at the spreading axis on this ridge segment over this period. For example, the average MgO content of lavas erupted at the ridge axis increased from about 3.0% at 600 ka, to about 7.0% at 300 ka. Since 300 ka the average MgO content has systematically decreased, and the average MgO content of lavas collected from within the neovolcanic zone at 10o30 0 N is 6.0%. These temporal changes in major element chemistry are not accompanied by systematic changes in isotope composition or incompatible trace element ratios, and are interpreted to reflect changes in the average rate of supply of melt to the ridge axis during this period. The data support previous arguments that changes in melt supply rate over periods of 100‐1000 ka have an important influence on the major element chemistry of the lavas erupted at fast spreading ridges. At 10o30 0 N, the melt supply rate appears to have been relatively low for much of the past 800 ka. Samples younger than 50 ka, collected from within 3 km of the ridge axis at 10o30 0 N (inside the neovolcanic zone), have a smaller range in major element chemistry compared to the samples dredged from the ridge flanks. Variations in the chemistry of lavas erupted over periods of less than about 100 ka may be controlled by the geometry of the magma plumbing system beneath the ridge axis.

A refined solution to Earth’s hidden niobium: implications for evolution of continental crust and mode of core formation

New high-precision niobium (Nb) and tantalum (Ta) concentration data are presented for early Archaean metabasalts, metabasaltic komatiites and their erosion products (mafic metapelites) from SW Greenland and the Acasta gneiss complex, Canada. Individual datasets consistently show sub-chondritic Nb/Ta ratios averaging 15.1+/-11.6. This finding is discussed with regard to two competing models for the solution of the Nb-deficit that characterises the accessible Earth. Firstly, we test whether Nb could have sequestered into the core due to its slightly siderophile (or chalcophile) character under very reducing conditions, as recently proposed from experimental evidence. We demonstrate that troilite inclusions of the Canyon Diablo iron meteorite have Nb and V concentrations in excess of typical chondrites but that the metal phase of the Grant, Toluca and Canyon Diablo iron meteorites do not have significant concentrations of these lithophile elements. We find that if the entire accessible Earth Nb-deficit were explained by Nb in the core, only ca. 17% of the mantle could be depleted and that by 3.7 Ga, continental crust would have already achieved ca. 50% of its present mass. Nb/Ta systematics of late Archaean metabasalts compiled from the literature would further require that by 2.5 Ga, 90% of the present mass of continental crust was already in existence. As an alternative to this explanation, we propose that the average Nb/Ta ratio (15.1+/-11.6) of Earths oldest mafic rocks is a valid approximation for bulk silicate Earth. This would require that ca. 13% of the terrestrial Nb resided in the Ta-free core. Since the partitioning of Nb between silicate and metal melts depends largely on oxygen fugacity and pressure, this finding could mean that metal/silicate segregation did not occur at the base of a deep magma ocean or that the early mantle was slightly less reducing than generally assumed. A bulk silicate Earth Nb/Ta ratio of 15.1 allows for depletion of up to 40% of the total mantle. This could indicate that in addition to the upper mantle, a portion of the lower mantle is depleted also, or if only the upper mantle were depleted, an additional hidden high Nb/Ta reservoir must exist. Comparison of Nb/Ta systematics between early and late Archaean metabasalts supports the latter idea and indicates deeply subducted high Nb/Ta eclogite slabs could reside in the mantle transition zone or the lower mantle. Accumulation of such slabs appears to have commenced between 2.5 and 2.0 Ga. Regardless of these complexities of terrestrial Nb/Ta systematics, it is shown that the depleted mantle Nb/Th ratio is a very robust proxy for the amount of extracted continental crust, because the temporal evolution of this ratio is dominated by Th-loss to the continents and not Nb-retention in the mantle. We present a new parameterisation of the continental crust volume versus age curve that specifically explores the possibility of lithophile element loss to the core and storage of eclogite slabs in the transition zone

Multistage evolution of Australian subcontinental mantle: Re-Os isotopic constraints from Victorian mantle xenoliths

Geochemical and Re-Os isotopic data for refractory spinel peridotite xenoliths from the Newer Volcanics Province of western Victoria demonstrate that parts of the subcontinental lithospheric mantle (SCLM) beneath southeastern Australia underwent melt depletion during the Late Proterozoic as well as the early Paleozoic. Late Proterozoic Re-Os model ages (as old as 900 Ma) for refractory harzburgites are significantly older than the exposed Cambrian crust. In addition, our Re-Os melt-depletion mantle model ages ( T MA ) are consistent with Sm-Nd depleted-mantle model ages ( T DM ) and older zircon U-Pb ages for I-type granites from southeastern Australia, suggesting that this region is underlain by Proterozoic continental lithosphere. In contrast, a fertile spinel lherzolite xenolith has a radiogenic Os isotopic composition (γ Os > +8) consistent with derivation from an “enriched-mantle” volume within the SCLM. Ancient subduction of Re-rich oceanic crust or melt metasomatism of the lithospheric mantle may have generated enriched-mantle volumes similar to those sampled by some ocean-island basalts (OIB) and komatiites.

CellSpace: A module for creating spatially registered laser ablation images within the Iolite freeware environment

We present a novel approach to creating compositional images using a module created for use with the freely distributed software package Iolite. The module creates images by synchronising the state of the laser (e.g., whether the laser is firing or not) and the position on the sample, which are recorded in laser log files, with concurrently collected mass spectrometer data. When these two data sources are synchronised, mass spectrometer data which are recorded temporally can then be displayed versus ablation position (i.e., spatially). Each mass spectrometer reading is then plotted as a circular spot representing the size of the area ablated. This approach has many advantages. CellSpace takes advantage of Iolites ability to manipulate data from various mass spectrometers and to reduce data of different types. Laser ablation data can be plotted over other images, such as those produced by scanning electron microscopes, where the image has been transformed into cell coordinates using third party software. This allows the analyst to visualise laser ablation data in context and to correlate sample data from multiple sources and/or techniques. The code also has the advantage of averaging data spatially, rather than just temporally, and faithfully presents the data as a corresponding laser spot, rather than a simple rectangular pixel. Here we provide an example of a fish otolith, where trace element concentrations and Sr-isotopic compositions are overlain on microscope images, providing information on migration patterns that are applicable to population studies and fisheries conservation.

The zircon ‘matrix effect’: evidence for an ablation rate control on the accuracy of U–Pb age determinations by LA-ICP-MS

Many studies now acknowledge the occurrence of systematic discrepancies between U–Pb ages determined in zircons in situ by LA-ICP-MS and the benchmark analytical method ID-TIMS. In this study, we present detailed investigations into the ablation characteristics of zircons that suggest an underlying mechanism responsible for these age biases relative to ID-TIMS. Confocal laser scanning microscopy of laser ablation pits reveals that there are small but significant differences in the amount of material removed by the laser between different zircons. Based on numerous pit depth and LA-ICP-MS 206Pb/238U ratio measurements of a suite of natural zircon reference materials and samples, we demonstrate that a systematic age bias is strongly correlated with the offset in ablation rates between the primary reference material and sample zircons. We offer further insights concerning the effects of thermal annealing on the ablation behaviour of zircons and demonstrate that, although there is a change in laser ablation rates for annealed zircons, the variations between different zircons are not eliminated. Finally, we show that slight variations in laser focus also influence the ablation behaviour of zircons and may further degrade the accuracy of U–Pb age determinations.

Application of ICP-MS trace element analysis in study of ancient Chinese ceramics

Thirty-nine trace elements of the Song-Yuan period (960–1368 AD) porcelain bodies from Cizhou, Jizhou and Longquanwu kilns were analyzed with ICP-MS, a technique rarely used in Chinese archaeometry, to investigate its potential application in such studies. Trace element compositions clearly reflect the distinctive raw materials and their mineralogy at the three kilns and allow their products to be distinguished. Significant chemical variations are also observed between Yuan and Song-Jing dynasties samples from Cizhou as well as fine and coarse porcelain bodies from Longquanwu. In Cizhou, porcelains of better quality which imitate the famous Ding kiln have trace element features distinctive from ordinary Cizhou products, that indicates geochemically distinctive raw materials were used and which possibly also underwent extra refining prior to use. The distinct trace element features of different kilns and the various types of porcelains from an individual kiln can be interpreted from a geochemical perspective. ICP-MS can provide a large amount of valuable information about ancient Chinese ceramics as it is capable of analyzing >40 elements with a typical of precision < 2%.

Trace element zoning in clinopyroxenes from spinel peridotite xenoliths

Abstract The excellent spatial resolution of the proton microprobe allows trace element zoning profiles to be determined within single grains of mantle minerals. Analyses of clinopyroxenes from two spinel peridotite xenoliths from the young basaltic volcanic field of the Western Districts of Victoria show they have pronounced zoning in Ti, Zr and Sr. These zoning profiles demonstrate that regions of the lithospheric mantle have undergone recent metasomatism by infiltrating basaltic melts resulting in enrichment in Ti, Zr and Sr.

Petrographical and geochemical changes in Bosnian stalagmites and their palaeo-environmental significance

Detailed petrographic observations have been coupled with trace element and δ13C - δ18O analyses in order to investigate their dynamics in two Holocene Bosnian speleothems. The potential of this multiproxy approach in providing a means to extract palaeo-environmental information from stalagmites whose stable isotope signals are noisy and without obvious trends has been tested. The studied stalagmites are mostly characterized by columnar microcrystalline fabric. At the sub-millimetre scale of lamination, different microcrystalline columnar sub-types (open and closed) have been detected and classified on the basis of the observed porosity and the crystallite size. The presence of variations in crystallite arrangement at the lamina scale suggests the occurrence of small-scale environmental changes recorded in the studied samples. A positive correlation was found found between Mg concentration, δ13C and fabric variations, while a negative correlation relates those parameters with Sr concentration. Both δ13C and fabric changes appear to be directly related to changes in hydrology. The detailed observation of calcite fabrics combined with stable isotope and trace element profiles allowed for the interpretation of the conditions under which the speleothems were deposited.

Strong population structure deduced from genetics, otolith chemistry and parasite abundances explains vulnerability to localized fishery collapse in a large Sciaenid fish, Protonibea diacanthus

As pressure on coastal marine resources is increasing globally, the need to quantitatively assess vulnerable fish stocks is crucial in order to avoid the ecological consequences of stock depletions. Species of Sciaenidae (croakers, drums) are important components of tropical and temperate fisheries and are especially vulnerable to exploitation. The black‐spotted croaker, Protonibea diacanthus, is the only large sciaenid in coastal waters of northern Australia where it is targeted by commercial, recreational and indigenous fishers due to its food value and predictable aggregating behaviour. Localized declines in the abundance of this species have been observed, highlighting the urgent requirement by managers for information on fine‐ and broad‐scale population connectivity. This study examined the population structure of P. diacanthus across north‐western Australia using three complementary methods: genetic variation in microsatellite markers, otolith elemental composition and parasite assemblage composition. The genetic analyses demonstrated that there were at least five genetically distinct populations across the study region, with gene flow most likely restricted by inshore biogeographic barriers such as the Dampier Peninsula. The otolith chemistry and parasite analyses also revealed strong spatial variation among locations within broad‐scale regions, suggesting fine‐scale location fidelity within the lifetimes of individual fish. The complementarity of the three techniques elucidated patterns of connectivity over a range of spatial and temporal scales. We conclude that fisheries stock assessments and management are required at fine scales (100 s of km) to account for the restricted exchange among populations (stocks) and to prevent localized extirpations of this species. Realistic management arrangements may involve the successive closure and opening of fishing areas to reduce fishing pressure.

Arsenic in a Speleothem from Central China: Stadial-Interstadial Variations and Implications

In a pilot study, arsenic in a stalagmite (SJ3) collected from Central China was measured, and its association with past climate and environment was explored. Most of the SJ3 arsenic concentrations ranged from 120 to 320 ppb with the highest concentrations associated with relatively warm and humid climatic phases and lowest concentrations with cold and dry phases. The SJ3 arsenic record was very similar to the manganese record of SJ3. Variations of arsenic in SJ3 might be controlled by metal oxides of iron, manganese, and aluminum in karst groundwater at the study site, which in turn were closely related with changes in past climate and environment. A considerable proportion of arsenic was in excess over manganese in SJ3, which might be related with incorporation of arsenic into the calcite lattice during the formation of SJ3. It was speculated that more arsenic was released due to stronger weathering of the surface soils and sequestrated by metal oxides in karst groundwater under warm-humid climatic phases than under cold-dry phases. This suggested that climate shift might alter arsenic balance in sedimentary areas and aquifer systems and potentially exert significant influence on global arsenic contamination.