Signature Of Melting Research Articles

Consequences of local inter-strand dehybridization for large-amplitude bending fluctuations of double-stranded DNA

The wormlike chain model of DNA bending accurately reproduces single-molecule force-extension profiles of long (kilobase) chains. These bending statistics over large scales do not, however, establish a unique microscopic model for elasticity at the 1-10 basepair (bp) scale, which holds particular interest in biological contexts. Here, we examine a class of microscopic models which allow for disruption of base pairing (i.e., a "melt" or "kink", generically an "excitation") and consequently enhanced local flexibility. We first analyze the effect on the excitation free energy of integrating out the spatial degrees of freedom in a wormlike chain. Based on this analysis, we present a formulation of these models that ensures consistency with the well-established thermodynamics of melting in long chains. Using a new method to calculate cyclization statistics of short chains from enhanced-sampling Monte Carlo simulations, we compute J-factors of a meltable wormlike chain over a broad range of chain lengths, including very short molecules (30 bp) that have not yet been explored experimentally. For chains longer than about 120 bp, including most molecules studied to date in the laboratory, we find that melting excitations have little impact on cyclization kinetics. Strong signatures of melting, which might be resolved within typical experimental scatter, emerge only for shorter chains.

Cambrian bimodal volcanism in the Lhasa Terrane, southern Tibet: Record of an early Paleozoic Andean-type magmatic arc in the Australian proto-Tethyan margin

This paper reports new zircon LA-ICP-MS U–Pb age and Hf-isotope, whole-rock major and trace element, and Sr–Nd isotope data from Cambrian metavolcanic rocks in the central Lhasa subterrane of southern Tibet. These rocks form a bimodal volcanic suite consisting mainly of silicic metavolcanic rocks with subordinate metabasalts. Five silicic metavolcanic samples dated at ca. 492Ma and one metabasalt sample yielding a near-concordant 206Pb/238U age of 492±4Ma indicate that mafic and silicic eruptions were contemporaneous. The metabasalts are mostly high-K calc-alkaline, enriched in Th, U, and light rare earth elements (LREEs), and depleted in Nb, Ta, Ti, Zr, and Hf, geochemically resembling the Andean arc basalts. The silicic metavolcanic rocks are high-K calc-alkaline and low in Nb and Zr. The metabasaltic rocks have negative whole-rock εNd(t) values (−4.7 to −3.5) and varying zircon εHf(t) values (−0.7 to +7.5), differing significantly from those of the silicic metavolcanic rocks, which yield negative whole-rock εNd(t) values of −8.4 to −7.2 and varying zircon εHf(t) values (−13.9 to −4.6). The metabasaltic rocks are interpreted as resulting from partial melting of an enriched lithospheric mantle source that was metasomatized by subduction-related components, whereas the silicic metavolcanic rocks were derived from basaltic melt-induced anatexis of the ancient Lhasa basement with inherited mantle melt signatures. The Western Qiangtang–Amdo–Tethyan Himalaya situated in the Indian proto-Tethyan margin and the Lhasa and other possible microcontinents or terranes (e.g., Gongshan, Baoshan, Tengchong, Burma, and Sibumasu) paleographically located in the Australian proto-Tethyan margin represent an early Paleozoic Andean-type magmatic arc facing the proto-Tethyan Ocean. The emplacement of the bimodal volcanic rocks and the development of the Cambro–Ordovician angular unconformity in the central Lhasa subterrane can be attributed to slab break-off of the proto-Tethyan Ocean lithosphere following the collisional accretion of microcontinents or terranes located outboard of the magmatic arc (possibly Eastern Qiangtang and South China).

Genotyping from saliva with a one-step microdevice

This paper presents a disposable microfluidic device for on-chip lysing, PCR, and analysis in one continuous-flow process. Male-female sex determination was performed with human saliva in less than 20 min from spit to finish, and requiring only seconds of manual sample handling. This genetic analysis was based on the amplification and detection of the DYZ1 repeat region unique to the Y-chromosome. The flow-through microfluidic chip consisted of a single serpentine channel designed to guide samples through 42 heating and cooling cycles. Cycling was performed by matching the local channel geometry to a steady-state temperature gradient established across the microfluidic chip. 38 channel segments were designed for rapid low volume PCR, and four were optimized for spatial DNA melting analysis. Fluorescence detection was used to monitor the amplification and to capture the melting signature of the amplicon was performed with a basic 8-bit CCD camera. The microfluidic device itself was fabricated from microscope slides and a double-sided tape. The simplicity of the system and its robust performance combine in an elegant solution for lab-on-a-chip genetic analysis.

Signature of ice melt over the Greenland derived from MSMR (OCEANSAT-1) data

In order to monitor the impact of global warming phenomena over the Polar Regions, it is necessary to monitor snow/ice melt on the Greenland and the Antarctic ice sheets. Using MSMR data, it is possible to differentiate sea ice at different concentration levels. On the basis of microwave emissivities of continental ice and sea ice, useful information on the formation and melting of the ice can be derived.
 The paper discusses different strategies to derive a melt signal from the MSMR observations for the continental ice sheets in Greenland. The Polarization Difference (PD) for 21 GHz, available from MSMR data, is studied and an appropriate threshold is selected to detect the presence of melt signal. The results of the present study have bearing on climate changes.

Plume–subduction interaction in southern Central America: Mantle upwelling and slab melting

The volcanic front in southern Central America is well known for its Galapagos OIB-like geochemical signature. A comprehensive set of geochemical, isotopic and geochronological data collected on volumetrically minor alkaline basalts and adakites were used to better constrain the mantle and subduction magma components and to test the different models that explain this OIB signature in an arc setting. We report a migration of back-arc alkaline volcanism towards the northwest, consistent with arc-parallel mantle flow models, and a migration towards the southeast in the adakites possibly tracking the eastward movement of the triple junction where the Panama Fracture Zone intersects the Middle America Trench. The adakites major and trace element compositions are consistent with magmas produced by melting a mantle-wedge source metasomatized by slab derived melts. The alkaline magmas are restricted to areas that have no seismic evidence of a subducting slab. The geochemical signature of the alkaline magmas is mostly controlled by upwelling asthenosphere with minor contributions from subduction components. Mantle potential temperatures calculated from the alkaline basalt primary magmas increased from close to ambient mantle (~1380–1410°C) in the Pliocene to ~1450°C in the younger units. The calculated initial melting pressures for these primary magmas are in the garnet stability field (3.0–2.7GPa). The average final melting pressures range between 2.7 and 2.5GPa, which is interpreted as the lithosphere–asthenosphere boundary at ~85–90km. We provide a geotectonic model that integrates the diverse observations presented here. The slab detached after the collision of the Galapagos tracks with the arc (~10–8Ma). The detachment allowed hotter asthenosphere to flow into the mantle wedge. This influx of hotter asthenosphere explains the increase in mantle potential temperatures, the northwest migration in the back-arc alkaline lavas that tracks the passage of the hotter asthenosphere, and the presence of a slab melting signature in the volcanic front caused by recycling of Galapagos Hotspot tracks.

Heterogeneous mantle sources of potassium-rich magmas in central-southern Italy: Melt inclusion evidence from Roccamonfina and Ernici (Mid Latina Valley)

We present a comprehensive set of data on compositions of melt inclusions and earliest crystallized mineral phases from mafic lavas of Roccamonfina and Ernici, situated in a central sector of the string of Pliocene–Quaternary potassic volcanic centres along the Tyrrhenian border of peninsular Italy. Studied samples of mafic lavas (4.4–7 wt.% MgO) cover a wide spectrum of potassium levels, and represent magmas considered to be parental to the ultrapotassic leucite-bearing high-K series (HKS, 4–8 wt.% K 2O) and to shoshonitic (1.5–5%) and subalkaline (< 1.5%) series, here collectively referred to as medium-low potassic series (M-LKS). Highly variable compositions of melt inclusions in olivine hosts (Fo = 89–91.5) from single lava samples indicate that all parental magmas are composed of diverse collections of primary melts, consistent with extraction from heterogeneous vein-type mantle lithologies. Major and trace-element systematics provide evidence that primitive HKS and M-LKS magmas originated from separate domains and not through changes in the proportion of vein and wall-rock peridotite during progressive melting of a common source. We infer that parental HKS magmas are largely derived from the vein portion of a heterogeneous phlogopite (± amphibole ± apatite) wehrlitic mantle. Contrasting major element, volatile and trace-element signatures of melt inclusions from medium-K lavas point to an amphibole-bearing wehrlitic source for shoshonitic magmas, probably with a subordinate role of phlogopite. Finally, a population of silica-undersaturated potassium-poor melt inclusions with extreme CaO/Al 2O 3 ratios (> 1.2) and fluid-depleted signatures suggests that subalkaline magmas originate either from the same source following the exhaustion of amphibole, or from a separate wehrlitic–pyroxenitic (± apatite ± carbonate?) assemblage. Our melt inclusion data are consistent with a mixed metasomatic imprint by siliceous potassium-rich and carbonate-rich (carbonatitic) potassium-poor melts. Siliceous melt components dominated in the HKS sources, but a minor group of potassium-poor melt inclusions in HKS samples, with compositions not represented by erupted products, carries trace-element signatures pointing to metasomatism by carbonatite-like melts. The melt inclusions show a general inverse relationship between fO 2 and potassium enrichment, suggesting that primitive potassium-poor magmas are the most oxidized (NNO + 0.5), whereas primitive HKS magmas have the lowest oxidation state (NNO − 1.6). Mineral chemistry of (near-)liquidus assemblages varies systematically according to the signature of the primary melts from which they crystallized. Calcium contents of Mg-rich olivines and clinopyroxenes are unrelated to the calcium content of the melts but are higher in HKS than in M-LKS samples. Forsterite-rich olivines contain Cr-spinel inclusions with Cr/(Cr + Al) ratios that tend to be higher in M-LKS (0.58–0.80) than in HKS (0.48–0.67) samples. We infer that this parameter, particularly at the more elevated values, is predominantly controlled by the metasomatic veins, and does not necessarily reflect the (‘refractory’) nature of the pre-metasomatic peridotite component. We surmise that primitive HKS and M-LKS melts of Roccamonfina–Ernici are derived by lower degrees of partial melting of vein lithologies with a stronger metasomatic imprint of the carbonatitic component and more subordinate involvement of pristine wall-rock peridotite than their equivalents of the northern Roman Province. The distinctive HKS and M-LKS sources point to a layered vein-permeated mantle column. Melting possibly occurred in response to a heat pulse induced by exposure to hot asthenospheric mantle, facilitated by post-collisional slab tearing and shallow break-off.

Ridge subduction and porphyry copper-gold mineralization: An overview

Many large porphyry Cu-Au deposits are connected to adakitic rocks known to be closely associated with ridge subduction. For example, there are several subducting ridges along the east Pacific margin, e.g., in Chile, Peru, and South America, most of which are associated with large porphyry Cu-Au deposits. In contrast, there are much fewer ridge subductions on the west Pacific margin and porphyry Cu-Au deposits are much less there, both in terms of tonnage and the number of deposits. Given that Cu and Au are moderately incompatible elements, oceanic crust has much higher Cu-Au concentrations than the mantle and the continental crust, and thus slab melts with their diagnostic adakitic chemistry have systematically higher Cu and Au, which is favorable for mineralization. Considering the geotherm of subducting slabs in the Phanerozoic, ridge subduction is the most favorable tectonic setting for this. Therefore, slab melting is the likely link in the spatial association between ridge subduction and Cu-Au deposits. Geochemical signatures of slab melting and hence maybe ridge subduction in less eroded regions in eastern China, the central Asian orogenic belt etc. may indicate important exploration targets for large porphyry Cu-Au deposits.

Strength effects in diamond under shock compression from 0.1 to 1 TPa

A two-wave shock structure---elastic precursor followed by an inelastic compression wave---is observed in single crystal and polycrystalline diamond laser shock compressed to peak stresses as high as 800 GPa. The Hugoniot elastic limits are measured to be 80 $(\ifmmode\pm\else\textpm\fi{}12)$, 81 $(\ifmmode\pm\else\textpm\fi{}6)$, and 60 $(\ifmmode\pm\else\textpm\fi{}3)\text{ }\text{GPa}$ for the $⟨100⟩$, $⟨110⟩$, and $⟨111⟩$ orientations of single crystals with the directional dependence attributable to the relative increase in strength under confining stress. These values imply a single crystal yield strength approximately 1/3 of theoretical predictions. The measurements reveal clear deviations from an elastic-plastic response upon dynamic yielding with significant relaxation toward an isotropic stress state for shock stresses of at least 160 GPa. Previously reported signatures of melting at 700--800 GPa along the diamond Hugoniot may be related to the transition from a two-wave to a single-wave structure, supporting the interpretation that melting begins at lower stresses $(\ensuremath{\sim}600\text{ }\text{GPa})$ with the appearance of an optically reflecting phase of carbon.

Longer growing seasons lead to less carbon sequestration by a subalpine forest

AbstractAs global temperatures increase, the potential for longer growing seasons to enhance the terrestrial carbon sink has been proposed as a mechanism to reduce the rate of further warming. At the Niwot Ridge AmeriFlux site, a subalpine forest in the Colorado Rocky Mountains, we used a 9‐year record (1999–2007) of continuous eddy flux observations to show that longer growing season length (GSL) actually resulted in less annual CO2 uptake. Years with a longer GSL were correlated with a shallower snow pack, as measured using snow water equivalent (SWE). Furthermore, years with a lower SWE correlated with an earlier start of spring. For three years, 2005, 2006, and 2007, we used observations of stable hydrogen isotopes (δD) of snow vs. rain, and extracted xylem water from the three dominant tree species, lodgepole pine, Engelmann spruce, and subalpine fir, to show that the trees relied heavily on snow melt water even late into the growing season. By mid‐August, 57% to 68% of xylem water reflected the isotopic signature of snow melt. By coupling the isotopic water measurements with an ecosystem model, SIPNET, we found that annual forest carbon uptake was highly dependent on snow water, which decreases in abundance during years with longer growing seasons. Once again, for the 3 years 2005, 2006, and 2007, annual gross primary productivity, which was derived as an optimized parameter from the SIPNET model was estimated to be 67% 77%, and 71% dependent on snow melt water, respectively. Past studies have shown that the mean winter snow pack in mountain ecosystems of the Western US has been declining for decades and is correlated with positive winter temperature anomalies. Since climate change models predict continuation of winter warming and reduced snow in mountains of the Western US, the strength of the forest carbon sink is likely to decline further.

Cryptic metasomatism in clino- and orthopyroxene in the upper mantle beneath the Pannonian region

Abstract Clino- and orthopyroxenes in anhydrous spinel peridotite xenoliths from Pliocene alkali basalts of the western Pannonian Basin have been analysed for trace elements by laser ablation inductively coupled plasma mass spectrometry (LA-ICP-MS). Clinopyroxenes show highly variable mantle normalized REE (rare earth elements) patterns but basically can be classified into three major groups: LREE-depleted, LREE-enriched and U-shaped patterns. As the REE patterns of clinopyroxenes usually reflect the REE patterns of the host peridotite, the three major REE patterns define three geochemically different groups of xenoliths. LREE-depleted xenoliths generally have undeformed protogranular textures, while the more deformed xenoliths with porphyroclastic and equigranular textures have LREE-enriched trace element patterns. The U-shaped pattern is very distinctive and is generally associated with poikilitic textures. The HREE content of the clinopyroxenes suggest that most of the xenoliths experienced less than15% partial melting, with the lowest degree occurring in the LREE-depleted xenoliths, and the highest degree in LREE-enriched xenoliths. Cryptic metasomatism frequently accompanies deformation. Metasomatic enrichment of incompatible trace elements can be observed not only in clinopyroxenes but also in coexisting orthopyroxenes. The metasomatic agents were probably alkaline mafic melts of asthenospheric origin and some may relate to upper Cretaceous alkali lamprophyre magmatism. Geochemical signatures of subduction-related melts or fluids have not been found in the anhydrous LREE-enriched xenoliths, although poikilitic xenoliths with U-shaped normalized REE patterns may indicate the influence of subduction-related melts.

High-pressure differential thermal analysis measurements of the melting curve of lithium

The pressure effect on the melting behavior of lithium has been measured by observing the latent heat signatures of melting and freezing using differential thermal analysis (DTA). Samples were repeatedly melted and recrystallized at selected pressures up to 15 GPa in a multianvil press. Despite the weak DTA signals due to small sample sizes at high pressure, the melting and freezing temperatures were clearly determined from the derivatives of the DTA traces. We measured a drop in the melting temperature between 9 and 12 GPa, yielding a maximum at 10(2) GPa and 245(2) ∘C. This work highlights both the successes and failures of recent theoretical models for the melting behavior of this and related systems.

Effect of external drive on strongly coupled Yukawa systems: A nonequilibrium molecular dynamics study

Using nonequilibrium molecular dynamics (MD) simulations behavior of three-dimensional (3D) Yukawa system has been studied in the presence of a small amplitude drive along one direction (say z[over ] ). This drive has the general form V=V_{0} cos(k_{L}z)Theta(t-t_{0}) , where Theta(t-t_{0}) is a Heaviside step function in time at t=t_{0} and k_{L}=2pi/L , L being the size of the system; V0 is considered small compared to average interparticle potential energy. In particular, a 3D equilibrated Yukawa crystal (bcc) near solid-liquid transition is subjected to an external drive at times t> or =t_{0} at the largest possible scale. For a given k_{L} it is observed that there exists a critical amplitude (V_{0};{c}) of the external drive below which the crystalline order is preserved and above which (V_{0}> or =V_{0};{c}) the transition from bcc to strongly coupled Yukawa liquid is observed. This critical amplitude (V_{0};{c}) is sensitive to the location of the Yukawa solid in the (kappa,Gamma) phase space. Various signatures of melting, transients, and steady state in the presence of this drive are elucidated using extensive MD diagnostics such as loss of long-range crystalline order, change in diffusion from subnormal to normal, and the fall of transversal shear peak in the Fourier transform of the velocity autocorrelation function. The mechanism of heating in the transient state is attributed to the local heating of the system where the forces are maximum. It is shown that these local hot regions dissipate heat into surrounding regions ultimately leading to a uniform temperature throughout the system. Ion streaming due to external field has been neglected.

Estimation of Molten Content of the Spray Stream from Analysis of Experimental Particle Diagnostics

Particle melting is one of the key issues in air plasma spray processing of high temperature ceramics such as Yttria Stabilized Zirconia (YSZ). The significance of assessing, monitoring, and controlling the molten content in spray stream on achieving an efficient process and reproducible coating characteristics and properties is known. This study aims to estimate the molten content of the spray stream (as an ensemble) from experimental measurement of in-flight (individual) particle characteristics. In a previous study by Streibl et al. the presence of melting signature in the particle temperature distribution was observed, which has been confirmed by simulation and through independent experimental observation by Mauer et al. Based on this observation, the particle temperature distribution could be delineated into the different achievable particle states in-flight (unmolten, partially molten, and completely molten) to a first approximation. This in-turn would enable estimation of the molten content in the spray stream. Thus obtained percentage molten content (referred in this study as Spray Stream Melting Index—SSMI) has been observed to correlate well with the experimentally measured deposition efficiency for a wide range of process conditions and feedstock characteristics. The implications of estimating SSMI for other materials and processes are also discussed.

Accessory phase control on the trace element signature of sediment melts in subduction zones

Phase and melting relations in deeply subducted sediments, the main reservoir of large ion lithophile elements (LILE) in subduction zones, are of first order importance to understand the general trace element signature of arc lavas. Here we present results from an experimental study on trace element liberation from metapelites in the range of 2.5–4.5 GPa and 750–1050 °C, i.e., conditions relevant for the slab at sub-arc depth. Apart from the common phases garnet, clinopyroxene, quartz/coesite and phengite, accessory phases such as rutile, apatite, allanite, monazite and zircon occur in the run products. The hydrous melts produced during wet melting of sediments have been analysed with laser ablation ICP-MS. The minor phase phengite is the main host of LILE in the residue and exerts a strong control on Rb and K in the coexisting melt. The presence of accessory phases profoundly influences the trace element characteristics of the hydrous melts because these phases control several trace elements (Ti by rutile, P by apatite, Zr by zircon, Th and light rare earth elements (Th, LREE) by monazite/allanite). The concentration of these elements in the hydrous melts is primarily a function of temperature and is independent of the amounts of the mineral and melt present. The saturation levels for these elements are used to predict that, in a common subducted sediment, accessory phases are likely to persist up to 850–900 °C at up to 50% melting. Trace element concentrations in hydrous melts are at least one order of magnitude higher than in aqueous fluids. Therefore, sediment melts provide an efficient way of extracting incompatible elements from the subducted slab. Accessory phases also influence the fractionation of elements with similar geochemical behaviour such as Th/La, Th/U, Ce/Pb (allanite/monazite), Nb/Ta (rutile) and Zr/Hf (zircon). Moreover, phengite is able to retain Rb while Cs is incompatible. These accessory phases therefore control characteristic element fractionation in the subduction component of arc magmas.

Influence of crustal cumulates on 210Pb disequilibria in basalts

In historical basalts from a wide range of tectonic settings, 210Pb is often found to have an activity deficit relative to its predecessor 226Ra. Several processes have been hypothesized as causes of 210Pb deficits in basalts. In subduction zone and ocean island environments, 210Pb deficits have often been attributed to shallow magmatic degassing. At mid-ocean ridges, 210Pb deficits have been inferred to result from mantle melting, limiting the time between melt production and eruption to 100 years or less. Here we present an alternative mechanism for producing 210Pb deficits in magmas, by diffusive exchange between a melt and cumulate minerals (plagioclase and/or clinopyroxene) in the crust. The deficit in 210Pb develops in response to its decay toward secular equilibrium with 226Ra within the mineral grains; decay provides an internal sink for 210Pb that drives continuous diffusive exchange with the melt. Deficits in 210Pb develop under a broad range of conditions, in enriched and depleted melts and during interaction with young or old cumulate minerals. The magnitude of the deficit depends mainly on the equilibrium mineral/melt partition coefficients for Pb and Ra and on the melt/rock ratio during diffusive interaction, and is only weakly dependent on the relative diffusivities of Ra and Pb in the minerals or the trace element disequilibrium between the melt and cumulate minerals. Plagioclase in the crust has greater leverage on the 210Pb– 226Ra system than any silicate mineral present during mantle melting, and is capable of inducing significant 210Pb deficits in the melt even at melt fractions above 50%. Its influence on the melt is also rapid, with a substantial 210Pb deficit developing in less than a year and approaching a steady state value after several decades or less. The strong control crustal cumulates are capable of exerting on 210Pb– 226Ra fractionation in melts indicates that they may have a significant role in a wide range of tectonic environments, and suggests caution in interpreting 210Pb deficits as a signature of mantle melting, or as a product of 222Rn degassing.

The two-component model for the genesis of granitic rocks in southeastern Australia — Nature of the metasedimentary-derived and basaltic end members

Granitic magmas in three areas of southeastern Australia are the product of melting of quartzofeldspathic metasedimentary rock (high-silica end member) combined with varying degrees of incorporation of mafic (dominantly basaltic) magma (low-silica end member). The link between granitic magma and local metasedimentary rock is established by identifying shared geochemical signatures in Sr, Na 2O and K 2O contents. High-silica end members have compositions formed in a genetic spectrum from total source mobilisation to partial melt segregation from metasedimentary rock. The position of a granitic end member in this spectrum is monitored by titanium contents varying from normal metasedimentary to the strongly depleted induced by partial melting. The titanium signature of partial melting is established in a type area with unequivocal links between migmatite mesosomes and leucosomes and the high-silica end members of granitic variation systems that extend to lower silica. The low-silica end members approximate high-alumina basalts on the basis of observed gabbros and projection of granitic variation lines to equivalent gabbroic compositions. High-alumina basalt is postulated to be a fundamental ingredient in the petrogenesis of orogenic granitic rocks.

North polar deposits of Mars: Extreme purity of the water ice

The polar layered deposits are the largest reservoir of water on the surface of Mars. The physical properties of the ice and their spatial distribution are largely unknown. 140,000 data points from the sounding radar SHARAD on the Mars Reconnaissance Orbiter were analyzed over the Gemina Lingula region, one‐fourth of the north polar layered deposits area. Maps of the dielectric properties of the bulk ice were drawn up. There is no basal melting signature. A drop of the dielectric constant in north‐west of Gemina Lingula could be explained by an abrupt 250‐meter uplift of the base. The bulk ice of the studied region has an average dielectric constant of 3.10 (σ = 0.12) and a loss tangent &lt;0.0026 (σ = 0.0005). Analytic interpretations shown the volume of ice is pure at ≥95%. The impurities have a radial distribution, with higher concentrations at margins.

Structure and variability of the West Greenland Current in Summer derived from 6 repeat standard sections

Six historical sections across the West Greenland Current are examined. Three sections have been regularly occupied since the late 1950s, while the three southern ones have been taken since 1984. Significant variability is observed for the freshwater core of the coastal current on the shelf, with salinity varying by over 3 units between years. There is also significant variability in the shape and offshore position of the main shelf break front, leading to large variability in Eulerian velocities. Significant presence of Irminger Water is seen during the 1960s and the 2000s, being found right across the sections in recent years. Maximum mean transport relative to the 34.8 isohaline of 5.5 ± 3.9 Sv , relative to 700 db, for 1984–2005, is observed at the Cape Desolation section. Transports decrease to the north, with the majority of the exchange with the interior of the Labrador Sea occurring between Cape Desolation and Fylla Bank. Inter-decadal transport variability is observed at Fylla Bank while a decline in transports since peaks in the early 1990s is seen at Cape Farewell and Cape Desolation. Freshwater transport is largest at Cape Desolation, with a mean Summer transport of 60.2 ± 20.5 mSv . Freshwater transport increases slightly between Cape Farewell and Cape Desolation and we suggest it is related to local discharge by glaciers into Juliannehaab Bight, as well as the melting of sea-ice. We also find that years of high Greenland ice cap melt are consistently associated with years of high freshwater transport at Cape Desolation, suggesting a portion of the freshwater transport of the West Greenland Current may be associated with melt from the Greenland ice sheet. Finally, significantly enhanced freshwater transports (33 mSv at Cape Desolation compared to the long term mean) are seen in 2008, probably a signature of the record Arctic Ocean ice melt and export in 2007.

Gamma-ray constraints on the chemical composition of the martian surface in the Tharsis region: A signature of partial melting of the mantle?

The Gamma-Ray Spectrometer (GRS) on board Mars Odyssey is the first instrument to provide quantitative estimates of the composition of the martian surface at the global scale. At the present time published concentration maps are available across a broad equatorial swath for Ca, Cl, Fe, H, K, Si and Th. However, it remains to be determined to what extent the observed chemical signatures are the result of primary magmatic processes or secondary alteration at the surface. With this question in mind, we have considered in detail the GRS signature on the western part of the Tharsis region including Olympus Mons. This region was chosen as it is clearly of volcanic origin, has had a prolonged magmatic history, and is of spatial extent sufficiently large that potential contributions of surrounding regions to the GRS signature are limited. It also has a characteristic composition, being notably low in silica relative to the average composition of the martian mid-latitudes. The major element chemistry of the surface in this region (SiO 2, FeO, CaO, K 2O) is compared with predictions of thermodynamic calculations of liquids produced by partial melting of the martian mantle composition proposed by Dreibus and Wänke [Dreibus, G., Wänke, H., 1985. Mars, a volatile-rich planet. Meteoritics 20 (No. 2), 267–381] over a range of pressures and temperatures. For all four oxides, the Tharsis compositions are consistent with liquids produced at pressures from 15 to 20 kbar, and degrees of partial melting between 3 and 10%. This pressure of melting implies a magma source depth in the range ∼ 125–170 km, consistent with current estimates of the thickness of the lithosphere in this region. The idea that the surface chemistry in the Tharsis region is dominated by the signature of primary magmatism would therefore appear to be a reasonable working hypothesis, although as more data become available, more stringent and detailed tests of this simple model will be possible.

The importance of melt extraction for tracing mantle heterogeneity

Numerous isotope and trace element studies of mantle rocks and oceanic basalts show that the Earth’s mantle is heterogeneous. The isotopic variability in oceanic basalts indicates that most mantle sources consist of complex assemblages of two or more components with isolated long-term chemical evolution, on both global and local scales. The range in isotope and highly incompatible element ratios observed in oceanic basalts is commonly assumed to directly reflect that of their mantle sources. Accordingly, the end-points of isotope arrays are taken to represent the isotopic composition of the different components in the underlying mantle, which is then used to deduce the origin of mantle heterogeneity. Here, a melting model for heterogeneous mantle sources is presented that investigates how and to what extent isotope and trace element signatures are conveyed from source to melt. We model melting of a pyroxenite–bearing peridotite using recent experimental constrains for melting and partitioning of pyroxenite and peridotite. Identification of specific pyroxenite melting signatures allows finger-printing of pyroxenite melts and confirm the importance of lithological heterogeneity in the Earth’s mantle. The model results and the comparison of the calculated and observed trace element–isotope systematics in selected MORB and OIB suites (e.g. from the East Pacific Rise, Iceland, Tristan da Cunha, Gough and St.Helena) further show that factors such as the relative abundance of different source components, their difference in solidus temperature, and especially the extent, style and depth range of melt aggregation fundamentally influence the relationship between key trace element and isotope ratios (e.g. Ba/Th, La/Nb, Sr/Nd, La/Sm, Sm/Yb, 143Nd/ 144Nd). The reason for this is that any heterogeneity present in the mantle is averaged or, depending on the effectiveness of the melt mixing process, even homogenized during melting and melt extraction. Hence to what degree mantle heterogeneity is reflected in the erupted melts is not only a function of source and melting-induced variability. It also depends on the extent of mixing during melting and melt extraction and thus strongly on the relative incompatibility of the elements considered. The observed trace element variation in erupted melts can be greater or smaller than that of their mantle sources, depending on the incompatibility of the elements investigated. The isotopic variability in erupted melts, on the other hand, is generally smaller than that of their mantle source. Melt mixing during melt extraction consequently has an important influence on the relative extent of variation, and hence the degree of correlation between the isotope and trace element ratios. Overall fewer correlations between trace element and isotope ratios are expected whenever melts are extracted from a restricted depth range, as is the case for ocean island basalts, than for cases where melts are extracted over a larger depth interval (mid ocean ridges and especially ridge centered hotspots like Iceland). While the isotopic composition of the most enriched melts may correspond closely to those of the enriched source component, even the most depleted mid ocean ridge basalts are likely to underestimate the isotopic depletion of the depleted mantle component. These observations imply that using the chemical and isotopic range observed in oceanic basalts as directly representative of that in the corresponding mantle source can be misleading, since this assumption is strictly true only for homogeneous mantle sources. In addition to identifying source or partitioning-related differences in melts from different mantle sources, inferring the true composition, origin, and distribution of heterogeneous components in the Earth’s mantle therefore requires detailed knowledge about the mechanisms of melting and melt mixing during the melt extraction process. Only if these processes and their influence on the isotope–trace element relationship are understood, can the composition and origin of the different source components, and thus mantle heterogeneity, be accurately constrained.