Major Melting Events Research Articles

Drivers of anomalous surface melting over Ingrid Christensen Coast, East Antarctica

Antarctica contains 90% of the Earth's ice; if it melts, it can significantly contribute to the rise in global sea levels. Over Antarctica, short-term atmospheric warming events have led to significant surface melt in summer. Understanding the conditions of such warming events and subsequent surface melt is highly prioritized in Polar Research. The austral summer of 2016-17 witnessed the largest melt duration of the 21st century over Ingrid Christensen Coast (ICC), East Antarctica. Being situated on the grounded ice near four research stations, understanding the melt over the region has both scientific and operational importance. Here, we investigate the drivers of four major melt events identified over ICC for the austral summer of 2016-17 using the reanalysis dataset, ERA5. The first melt event, coinciding with the season's highest air temperature, was triggered by high turbulent heat flux from strong katabatic winds, while the rest of the events were triggered by low-level, liquid cloud-induced longwave radiation. During the melt events, anomalous high pressure ridges were present over the continent causing low pressure systems to remain stationary for an extended period and direct warm, moist air towards the ICC, facilitating melting. The present study observed melting occurring above the grounding zone, and if such melting extends to a larger scale beyond ice shelves, it could raise significant concerns regarding the hydrodynamics and stability of ice sheets in the future.

Thermochemical lithosphere differentiation and the origin of cratonic mantle.

Cratons record the early history of continental lithosphere formation, yet how they became the most enduring part of the lithosphere on Earth remains unknown1. Here we propose a mechanism for the formation of large volumes of melt-depleted cratonic lithospheric mantle (CLM) and its evolution to stable cratons. Numerical models show large decompression melting of a hot, early Earth mantle beneath a stretching lithosphere, where melt extraction leaves large volumes of depleted mantle at depth. The dehydrated, stiffer mantle resists further deformation, forcing strain migration and cooling, thereby assimilating depleted mantle into the lithosphere. The negative feedback between strain localization and stiffening sustains long-term diffused extension and emplacement of large amounts of depleted CLM. The formation of CLM at low pressure and its deeper re-equilibration reproduces the evolution of Archaean lithosphere constrained by depth-temperature conditions1,2, whereas large degrees of depletion3,4 and melt volumes5 in Archaean cratons are best matched by models with lower lithospheric strength. Under these conditions, which are otherwise viable for plate tectonics6,7, thermochemical differentiation effectively prevents yielding and formation of margins: rifting and lithosphere subduction are short lived and embedded in the cooling CLM as relict structures, reproducing the recycling and reworking environments that are found in Archaean cratons8,9. Although they undergo major melting and extensive recycling during an early stage lasting approximately 500million years, the modelled lithospheres progressively differentiate and stabilize, and then recycling and reworking become episodic. Early major melting and recycling events explain the production and loss of primordial Hadean lithosphere and crust10, whereas later stabilization and episodic reworking provides a context for the creation of continental cratons in the Archaean era4,8.

West Antarctic surface melt triggered by atmospheric rivers

Recent major melting events in West Antarctica have raised concerns about a potential hydrofracturing and ice shelf instability. These events often share common forcings of surface melt-like anomalous radiative fluxes, turbulent heat fluxes and fohn winds. Using an atmospheric river detection algorithm developed for Antarctica together with surface melt datasets, we produced a climatology of atmospheric river-related surface melting around Antarctica and show that atmospheric rivers are associated with a large percentage of these surface melt events. Despite their rarity (around 12 events per year in West Antarctica), atmospheric rivers are associated with around 40% of the total summer meltwater generated across the Ross Ice Shelf to nearly 100% in the higher elevation Marie Byrd Land and 40–80% of the total winter meltwater generated on the Wilkins, Bach, George IV and Larsen B and C ice shelves. These events were all related to high-pressure blocking ridges that directed anomalous poleward moisture transport towards the continent. Major melt events in the West Antarctic Ice Sheet only occur about a couple times per decade, but a 1–2 °C warming and continued increase in atmospheric river activity could increase the melt frequency with consequences for ice shelf stability. Atmospheric rivers associated with blocking events are related to a large fraction of the surface ice melt events in West Antarctica, suggest observation-based analyses of atmospheric dynamics and West Antarctic surface melt.

A survey of the atmospheric physical processes key to the onset of Arctic sea ice melt in spring

September sea ice concentration (SIC) is found to be most sensitive to the early melt onset over the East Siberian Sea and Laptev Sea (73°–84°N, 90°–155°) in the Arctic, a region defined here as the area of focus (AOF). The areal initial melt date for a given year is marked when sea ice melting extends beyond 10% of the AOF size. With this definition, four early melting years (1990, 2012, 2003, 1991) and four late melting years (1996, 1984, 1983, 1982) were selected. The impacts and feedbacks of atmospheric physical and dynamical variables on the Arctic SIC variations were investigated for the selected early and late melting years based on the NASA MERRA-2 reanalysis. The sea ice melting tends to happen in a shorter period of time with larger magnitude in late melting years, while the melting lasts longer and tends to be more temporally smooth in early melting years. The first major melting event in each year has been further investigated and compared. In the early melting years, the positive Arctic Oscillation (AO) phase is dominant during springtime, which is accompanied by intensified atmospheric transient eddy activities in the Arctic and enhanced moisture flux convergence in the AOF and consequently enhanced northward transport of moist and warm air. As a result, positive anomalies of air temperature, precipitable water vapor (PWV) and/or cloud fraction and cloud water path were found over the AOF, increasing downward longwave radiative flux at the surface. The associated warming effect further contributes to the initial melt of sea ice. In contrast, the late melt onset is usually linked to the negative AO phase in spring accompanied with negative anomalies of PWV and downward longwave flux at the surface. The increased downward shortwave radiation during middle to late June plays a more important role in triggering the melting, aided further by the stronger than normal cloud warming effects.

Seasonal monitoring of melt and accumulation within the deep percolation zone of the Greenland Ice Sheet and comparison with simulations of regional climate modeling

Abstract. Increasing melt over the Greenland Ice Sheet (GrIS) recorded over the past several years has resulted in significant changes of the percolation regime of the ice sheet. It remains unclear whether Greenland's percolation zone will act as a meltwater buffer in the near future through gradually filling all pore space or if near-surface refreezing causes the formation of impermeable layers, which provoke lateral runoff. Homogeneous ice layers within perennial firn, as well as near-surface ice layers of several meter thickness have been observed in firn cores. Because firn coring is a destructive method, deriving stratigraphic changes in firn and allocation of summer melt events is challenging. To overcome this deficit and provide continuous data for model evaluations on snow and firn density, temporal changes in liquid water content and depths of water infiltration, we installed an upward-looking radar system (upGPR) 3.4 m below the snow surface in May 2016 close to Camp Raven (66.4779∘ N, 46.2856∘ W) at 2120 m a.s.l. The radar is capable of quasi-continuously monitoring changes in snow and firn stratigraphy, which occur above the antennas. For summer 2016, we observed four major melt events, which routed liquid water into various depths beneath the surface. The last event in mid-August resulted in the deepest percolation down to about 2.3 m beneath the surface. Comparisons with simulations from the regional climate model MAR are in very good agreement in terms of seasonal changes in accumulation and timing of onset of melt. However, neither bulk density of near-surface layers nor the amounts of liquid water and percolation depths predicted by MAR correspond with upGPR data. Radar data and records of a nearby thermistor string, in contrast, matched very well for both timing and depth of temperature changes and observed water percolations. All four melt events transferred a cumulative mass of 56 kg m−2 into firn beneath the summer surface of 2015. We find that continuous observations of liquid water content, percolation depths and rates for the seasonal mass fluxes are sufficiently accurate to provide valuable information for validation of model approaches and help to develop a better understanding of liquid water retention and percolation in perennial firn.

January 2016 extensive summer melt in West Antarctica favoured by strong El Ni\xf1o

Over the past two decades the primary driver of mass loss from the West Antarctic Ice Sheet (WAIS) has been warm ocean water underneath coastal ice shelves, not a warmer atmosphere. Yet, surface melt occurs sporadically over low-lying areas of the WAIS and is not fully understood. Here we report on an episode of extensive and prolonged surface melting observed in the Ross Sea sector of the WAIS in January 2016. A comprehensive cloud and radiation experiment at the WAIS ice divide, downwind of the melt region, provided detailed insight into the physical processes at play during the event. The unusual extent and duration of the melting are linked to strong and sustained advection of warm marine air toward the area, likely favoured by the concurrent strong El Niño event. The increase in the number of extreme El Niño events projected for the twenty-first century could expose the WAIS to more frequent major melt events.

Chronology, petrogenesis and heat sources for successive Carboniferous magmatic events in the Southern–Central Variscan Vosges Mts (NE France)

Plutonic bodies of the Central and Southern Vosges Mts can be assigned to two major early Carboniferous magmatic events: a Visean Mg–K event ( c. 345 and 340–336 Ma) and a younger S-type event (329–322 Ma). New petrological, geochemical and Sr–Nd isotopic data highlight the existence of two groups of Mg–K intrusions that might be related to the nature of their primary magma sources; that is, CHUR-like and enriched mantle, which interacted with juvenile and mature crustal material, respectively. The differences between these two groups are explained by a geodynamic scenario involving deep subduction and relamination of the Saxothuringian continental crust under the Moldanubian continent. The relaminated radiogenic Saxothuringian material is thought to have been responsible for dehydration melting of both subducted crust and underlying metasomatized mantle, thereby generating the Mg–K magma subsequently emplaced at middle crustal depth. During their ascent, the mafic magmas interacted with crustally derived felsic melts. Significantly later ( c . 10–15 myr) a widespread mid-crustal anatexis occurred, generating voluminous granite intrusions from mixed crustal sources (paragneisses and/or immature felsic–intermediate metaigneous rocks mixed with Mg–K plutons). The principal heat source for such a major melting event is related to the presence of Mg–K plutons rich in heat-producing elements, which were responsible, after the time lag specified, for a temperature increase at mid-crustal levels by in situ radiogenic heat production. The current study underlines the importance of deep continental crust subduction and relamination for the magmatism and development of collisional orogens. Supplementary material: Analytical methods and data, and supplementary figures, are available at http://www.geolsoc.org.uk/SUP18795 .

Variability in the surface temperature and melt extent of the Greenland ice sheet from MODIS

AbstractSatellite‐derived moderate‐resolution imaging spectroradiometer (MODIS) ice‐surface temperature (IST) of the Greenland ice sheet shows a positive trend and two major melt events from 2000 to present. IST increased by ~0.55 ± 0.44°C/decade, with the greatest increase (~0.95 ± 0.44°C/decade) found in northwestern Greenland where coastal temperatures and mass loss are also increasing and outlet glaciers are accelerating. IST shows the highest rates of increase during summer (~1.35 ± 0.47°C/decade) and winter (~1.30 ± 1.53°C/decade), followed by spring (~0.60 ± 0.98°C/decade). In contrast, a decrease in IST was found in the autumn (~−1.49 ± 1.20°C/decade). The IST trends in this work are not statistically significant with the exception of the trend in northwestern Greenland. Major surface melt (covering 80% or more of the ice sheet) occurred during the 2002 and 2012 melt seasons where clear‐sky measurements show a maximum melt of ~87% and ~95% of the ice sheet surface, respectively. In 2002, most of the extraordinary melt was ephemeral, whereas in 2012 the ice sheet not only experienced more total melt, but melt was more persistent, and the 2012 summer was the warmest in the MODIS record (−6.38 ± 3.98°C). Our data show that major melt events may not be particularly rare during the present period of ice sheet warming.

Successive reef depositional events along the Marquesas foreslopes (French Polynesia) since 26 ka

Bathymetric surveys of submarine platforms and terraces of the tropical Marquesas archipelago (French Polynesia), coupled with sedimentological and paleoecological analyses and radiometric dating of dredged rocks, reveal the occurrence of several distinct intervals of reef deposition during the last deglacial sea-level rise (i.e. the last 26,000 yr or 26 ky). Five facies related to reef, shallow-water and deeper depositional zones have been identified, on the basis of their respective biological assemblages. Facies 1, 2 and 3 consist of in situ scleractinian corals (mainly Porites) associated with thick crusts of coralline algae species, Hydrolithon onkodes, H. reinboldii, Lithophyllum pustulatum group, and widespread vermetids and victoriellinid foraminifers. These assemblages are typical of high-energy, shallow-water reef settings (i.e. upper forereef, reef crest and outer reef flat, and inner reef flat respectively). Facies 4 is characterized by the prevalence of Halimeda plates, mixed with debris of Amphiroa and Amphistegina spp.; these algae generally inhabit the inner parts of the reef flat and backreef zones but also the relatively protected, deeper parts of outer reef slopes. Facies 5 is composed of large broken mollusc shells (oysters), embedded in fine-grained sediments with significant numbers of deep-living coralline algae Mesophyllum sp., and planktonic foraminifers. Locally, mollusc shells are covered with microbialite crusts. The replacement of shallower assemblages by deeper ones is interpreted as an upward-deepening sequence. Identification of morphological and biological features combined with coral dating provide evidence of 4 reef generations (RG) between 26 and 9 ka, now ranging from 138 to 58 m in depth: 1) RG 0 from 26 to 19 ka in the 125–115 m depth interval; 2) RG 1 from 18 to 15 ka between 110 and 95 m; 3) RG 2 from 14 to 11.5 ka between 80 and 68 m; and 4) RG 3 younger than 11.5 ka, at around 55 to 60 m. The first three units exhibit clear zonation, from upper reef slope to reef flat. Their development was probably interrupted by abrupt rises in sea-level and/or drastic climate changes. Such a scenario is similar to observations reported in Barbados and Hawai'i where rapid rises in sea-level, interpreted as reflecting major melting events of continental ice, caused the drowning of reefs. The younger unit refers to coral banks similar to those in modern Marquesas environments. The replacement of typical coral reefs by coral banks may reflect an increase El Niño-Southern Oscillation (ENSO) frequency and intensity inimical to reef development in the last 12 ka.

The Geochemistry of the Arabian Lithospheric Mantle--a Source for Intraplate Volcanism?

We present trace element and Sr–Nd–Hf–Pb isotope compositions for clinopyroxenes from anhydrous spinel peridotite and garnet ± spinel pyroxenite xenoliths of Pan-African lithospheric mantle from Jordan, including the first high-precision double-spike Pb isotope measurements of mantle clinopyroxene. Clinopyroxenes from the peridotites are variably Th–U–LILE–LREE enriched and display prominent negative Nb, Zr and Ti anomalies. MREE–HREE abundances can generally be modelled as partial melting residues of spinel lherzolite with primitive-mantle-like composition after extraction of 5–10% melt, whereas the enrichments in Th–U–LILE–LREE require a Pan-African or later metasomatic event. The large range of Nd, Sr, Pb and Hf isotope ratios in both peridotites and pyroxenites (e.g. εNd ∼ 1·4–17·5; 206Pb/204Pb ∼ 17·2–20·4; εHf ∼ 0·6–164·6) encompasses compositions more radiogenic than mid-ocean ridge basalt (MORB), and Pb isotopes cover almost the entire range of oceanic basalt values. εHf values are some of the highest ever recorded in mantle samples and are decoupled from εNd in the same samples. Marked correlations between Sr–Nd–Pb isotopes, LILE–LREE enrichments and HFSE depletion suggest that the metasomatizing agent was a carbonatitic-rich melt and isotopic data suggest that metasomatism may have been related to Pan-African subduction. The metasomatic melt permeated depleted upper mantle (<16 kbar) during Pan-African subduction at ∼ 600–900 Ma, and the variably metasomatized material was then incorporated into the Arabian lithospheric mantle. There is no evidence for recent metasomatism (<30 Ma) related to the Afar plume like that postulated to have affected southern Arabian lithospheric mantle. Hf isotopes in the mantle clinopyroxenes are unaffected by metasomatism, and even some strongly overprinted lithologies record ancient (>1·2 Ga) pre-metasomatic Lu–Hf signatures of the depleted upper mantle that was the protolith of the Arabian lithospheric mantle. The ‘resistance’ of the Lu–Hf isotopic system to later metasomatic events resulted in the development of extremely heterogeneous Hf isotopic signatures over time that are decoupled from other isotopic systems. No mantle sample in this study exactly matches the chemical and isotopic signature of the source of Jordanian intraplate basalts. However, the xenolith compositions are broadly similar to those of the source of Arabian intraplate basalts, suggesting that the numerous Cenozoic intraplate volcanic fields throughout Arabia may be the product of melting upper mantle wedge material fertilized during Pan-African subduction and incorporated into the Arabian lithospheric mantle. We propose a model whereby the proto-Arabian lithospheric mantle underwent a major melting event in early Proterozoic–late Archean times (at the earliest at ∼1·2 Ga). Island-arc volcanism and major crust formation occurred during the Pan-African orogeny, which liberated fluids and possibly small-degree melts that migrated through the mantle creating zones of enrichment for certain elements depending upon their compatibility. Immobile elements, such as Nb, were concentrated near the base of the mantle wedge providing the source of the Nb-rich Jordanian volcanic rocks. More mobile elements, such as LILE and LREE, were transported up through the mantle and fertilized the shallow mantle source of the Jordanian xenoliths. Following subduction, the mantle wedge became fossilized and preserved distinct enriched and depleted zones. Lithospheric rifting in the Miocene triggered partial melting of spinel-facies mantle in the lower lithosphere, which mixed with deeper asthenospheric garnet-facies melts as rifting evolved. These melts entrained segments of variably carbonatite-metasomatized shallow lithospheric mantle en route to the surface.

Gamma-ray bursts and other sources of giant lightning discharges in protoplanetary systems

Lightning in the solar nebula is considered to be one of the probable sources for producing the chondrules that are found in meteorites. Gamma-ray bursts (GRBs) provide a large flux of γ-rays that Compton scatter and create a charge separation in the gas because the electrons are displaced from the positive ions. The electric field easily exceeds the breakdown value of ≈1 V m-1 over distances of order 0.1 AU. The energy in a giant lightning discharge exceeds a terrestrial lightning flash by a factor of ~1012. The predicted post-burst emission of γ-rays from accretion into the newly formed black hole or spin-down of the magnetar is sufficiently intense to cause a lightning storm in the nebula that lasts for days and is more probable than the GRB because the radiation is beamed into a larger solid angle. The giant outbursts from nearby soft gamma-ray repeater sources (SGRs) are also capable of causing giant lightning discharges. The total amount of chondrules produced is in reasonable agreement with the observations of meteorites. Furthermore in the case of GRBs most chondrules were produced in a few major melting events by nearby GRBs and lightning occurred at effectively the same time over the whole nebula, and provide accurate time markers to the formation of chondrules and evolution of the solar nebula. This model provides a reasonable explanation for the delay between the formation of calcium aluminium inclusions (CAIs) and chondrules.

Geochemistry of near-EPR seamounts: importance of source vs. process and the origin of enriched mantle component

Niu and Batiza [Earth Planet. Sci. Lett. 148 (1997) 471–483] show that lavas from the seamounts on the flanks of the East Pacific Rise (EPR) between 5° and 15°N vary from extremely depleted tholeiites to highly enriched alkali basalts. The extent of depletion and enrichment exceeds the known range of seafloor lavas in terms of the abundances and ratios of incompatible elements. New Sr–Nd–Pb isotope data for these lavas show variations (87Sr/86Sr=0.702362–0.702951; 206Pb/204Pb=18.080–19.325 and 143Nd/144Nd 0.512956–0.513183) larger than observed in lavas erupted on the nearby EPR axis. These isotopic ratios correlate with each other, with the abundances and ratios of incompatible elements, with the abundances of measured major elements such as MgO, CaO, Na2O and TiO2 contents, and with the abundances and ratios of major elements corrected for crystal fractionation to Mg#=0.72 (Ti72, Al72, Fe72, Ca72, Na72, and Ca72/Al72). These coupled correlations and the spatial distribution of seamounts require an EPR mantle source that has long-term (>1 Ga) lithological heterogeneities on very small scales [Niu and Batiza, Earth Planet. Sci. Lett. 148 (1997) 471–483]. Mid-ocean ridge basalt (MORB) major element systematics are, to a great extent, inherited from their fertile sources, which requires caution when using major element data to infer melting conditions. The significant correlations in elemental and isotopic variability (defined as RSD%=1σ/mean×100) between seamount and axial lavas suggest that both seamount and axial volcanisms share a common heterogeneous mantle source. We confirm previous interpretations [Niu and Batiza, Earth Planet. Sci. Lett. 148 (1997) 471–483; Niu et al., J. Geophys. Res. 104 (1999) 7067–7087] that the geochemical variability of lavas from the broad northern EPR region results from melting-induced mixing of a two-component mantle with the enriched (easily melted) component dispersed as physically distinct domains in a more depleted (refractory) matrix prior to the major melting events. The data also allow the conclusion that recycled oceanic crust cannot explain elevated abundances of elements such as Ba, Rb, Cs, Th, U, K, Pb, Sr etc. in enriched MORB and many ocean island basalts. These elements will be depleted in recycled oceanic crust that has passed through subduction zone dehydration reactions. We illustrate that deep portions of recycled oceanic lithosphere are important geochemical reservoirs hosting these and other incompatible elements as a result of metasomatism taking place at the interface between the low velocity zone and the cooling and thickening oceanic lithosphere.

The partitioning of Na between melilite and liquid: part II. applications to Type B inclusions from carbonaceous chondrites

The zoning of Na in the melilite of Type B1 Ca-, Al-rich inclusions from carbonaceous chondrites reflects a combination of factors including the maximum temperature, T max , for melting, bulk composition, crystal dimensions and the degree of diffusive relaxation. We used experimentally determined partition coefficients for Na between melilite and liquid together with modes and compositions of phases in meteoritic inclusions to calculate Na zoning produced in melilite by crystallization from Na-bearing liquids. We then used a simple model for the diffusive relaxation of an originally igneous zoning profile to constrain the thermal history. For Allende Type B1 inclusion TS-34, the melilite compositions are consistent with an initial melting event (T max ∼ 1450°C) during which essentially Na-free melilite crystallized, followed by the introduction of Na during a hiatus, a second melting event with T max ∼ 1290°C in which Na-bearing melilite with X Ak Mel ≥ 0.47 grew on relict cores of melilite from the first melting event, and partial diffusive relaxation. For the Type B1 inclusion Vigarano 1623-8, previously published bulk and melilite compositions lead to the conclusion that T max did not exceed ∼1280°C during the last major melting event. A survey of literature data for other inclusions suggests that Na zoning in melilite may be a generally useful indicator of thermal history and that Type B1 inclusions were commonly subjected to multiple melting events with intervening periods of alteration. At any given value of X Ak Mel in TS-34, calculated melilite/liquid partition coefficients for Na are lower than values determined experimentally for melilite of similar composition because of liquid composition effects. The difference is more than a factor of two for X Ak Mel > 0.5, and it is likely, based on the activities of major oxides in the melt, that liquid composition effects are also important for other elements. On the other hand, modeling of diffusive relaxation for Na in melilite suggests that the diffusion coefficients are not strongly dependent on either X Ak Mel or Na concentration.

Crystal chemical control of clinopyroxene-melt partitioning in the Di-Ab-An system: implications for elemental fractionations in the depleted mantle

The partitioning of fifteen trace elements (Rb, Sr, Zr, Nb, Ba, La, Ce, Nd, Sm, Gd, Yb, Hf, Ta, Pb, and Th) between clinopyroxene and synthetic melt has been studied in two compositions along an isotherm in the diopside-albite-anorthite ternary at 1 bar pressure. The two compositions correspond to ∼Di 65An 35 and ∼Di 55Ab 45 and produce clinopyroxenes distinct in chemistry while melt compositions range from 49 wt% SiO 2 to 61 wt% SiO 2. The partition coefficients of high field strength elements (HFSE) increase by factors of 2–8 in Di-An experiments relative to Di-Ab experiments while other elements show very little change (±20%) between compositions. The change in HFSE partitioning correlates with increases in tetrahedral Al 20 3 ( IVAl) content of clinopyroxenes in the anorthite-bearing experiments. Changes in D Ta/D Nb also correlate with IVAl based on a survey of previously published determinations. Tests of models of trace element substitution energetics produce values for Young’s modulus (E) and optimum D (D o) consistent with previous results for clinopyroxene for mono-, di-, and trivalent cations. The wide variations in partitioning behavior for tetra- and pentavalent cations are also consistent with these models because the high values for E make partition coefficients and relative HFSE partitioning sensitive to small changes in composition. The overall increase in HFSE partitioning and D o for the DiAn composition is consistent with D o increasing as a function of IVAl, consistent with the role of IVAl in charge balancing HFSE. However, IVAl must also cause lattice changes that affect the ability of the clinopyroxene to discriminate between Nb and Ta. There are two important implications to the observed dependence of HFSE partition coefficients on clinopyroxene aluminum content. First, HFSE will be fractionated from their adjacent REE within ultramafic samples during melting of spinel lherzolite as clinopyroxene Al 2O 3 content decreases. Second, fractionations between Nb and Ta and Zr and Hf observed in mantle-derived magmas are consistent with extraction of melt in equilibrium with spinel lherzolite having clinopyroxenes with ∼5 wt% Al 2O 3. The fractionations among HFSE (Nb/Ta, Hf/Zr) and between HFSE and REE observed in both arc magmas and upper mantle peridotites may simply reflect prior depletion by major melting events (F > 10%) which left clinopyroxene as a residual phase. We speculate that the peridotitic sources for MORB and arc lavas are similar in composition with both having significant HFSE anomalies. However, MORB do not typically record HFSE anomalies because of the complementary contribution of HFSE from enriched mafic veins interspersed within the peridotite.

Mantle source heterogeneity and melting processes beneath seafloor spreading centers: The East Pacific Rise, 18°–19°S

We present new major and trace element and Nd‐Pb‐Sr isotopic data on samples from the East Pacific Rise (EPR) axis and nearby seamounts in the Hump area, 18°–19°S. Most samples studied are normal mid‐ocean ridge basalt (N‐MORB); four samples from the southern seamounts are enriched MORB (E‐MORB). Dredge 52 samples from a southern seamount are depleted in incompatible elements yet possess “enriched” isotopic signatures. Except for the dredge 52 samples, all the samples show significant correlations between isotopic ratios and ratios of incompatible elements; that is, incompatible elements and isotopes are coupled. Sr and Nd isotopic ratios correlate significantly better with ratios of moderately incompatible elements than with ratios involving highly incompatible ones (e.g., Rb, Nb, and K) which appear to be “overenriched”. Both isotopic and incompatible element ratios also correlate with the extent of melting calculated from major elements. We interpret these correlations as mixing trends resulting from melting of a heterogeneous source containing enriched (“plume‐like”) domains of variable sizes. Overenrichment of highly incompatible elements in E‐MORB appears to be recent and is best explained by low‐degree‐melt infiltration in the source region prior to major melting events. The low‐degree melts are primarily derived from isotopically N‐MORB mantle. This low‐degree melt process also explains the incompatible element‐isotope decoupling throughout the EPR between 13° and 23°S. The dredge 52 samples too are consistent with such a process, but their immediate source is a site of low‐degree melt generation.

Chronothermometry of peridotitic and pyroxenitic xenoliths: Implications for the thermal evolution of the Arabian lithosphere

The thermal history of the subcontinental lithosphere beneath Israel has been evaluated by surface heat flow and the chemical and isotopic compositions of mineral phases in xenoliths of spinel lherzolite, garnet pyroxenite and garnet granulite, brought to the surface by Cretaceous and Neogene-Quaternary alkali basalts. The spinel lherzolites show large variations in chemical and isotopic compositions: Al Si = 0.030 to 0.095 , 147Sm 144Nd = 0.11 to 0.24 , 143Nd 144Nd = 0.51289 to 0.51316 and 87Sr 86Sr = 0.70270 to 0.70321 . The isotopically more depleted peridotites lie within the N-MORB field while those with more enriched isotopic compositions are similar to Phanerozoic alkali basalts from the studied area. Xenoliths of garnet pyroxenite from Kerem Maharal volcano (Mt. Carmel) have 143Nd 144Nd of 0.51290 to 0.51292, and 87Sr 86Sr of 0.70296 to 0.70313, within the range of Cretaceous alkali basalts from Israel. These pyroxenites were probably segregated from an alkali basalt magma. The garnet pyroxenite BS-701 from Bat Shlomo volcano (Mt. Carmel) has 143Nd 144Nd of 0.51360, and 87Sr 86Sr of 0.70264, values that are similar to those of some highly depleted peridotites from Arabia. It is postulated that the uppermost mantle beneath the Arabian plate underwent a major melting event during the formation of the Arabian continent in the late Proterozoic. This melting was responsible for the variations in the major and trace element compositions of the peridotites. Pyroxenites, such as BS-701, were probably segregated from magmas extracted from the peridotites. The enrichment of some peridotites in LREE could have occurred during the late Proterozoic magmatic events and/or later during the episodes of Phanerozoic basaltic magmatism. The mineral phases in the xenoliths show homogeneous major element distribution and Neodymium isotopic equilibrium at (or close to) eruption time. Equilibrium temperatures of the peridotites and the pyroxenites range from 900–1100°C, and 990–1190°C, respectively. These temperatures are elevated 300–400°C above the conductive steady state geotherm that characterized the Arabian lithosphere during most of the Phanerozoic. We suggest that the lithosphere beneath Israel was affected by transient heating events which increased the temperatures and reset the chemical and isotopic systems in the xenoliths. The surface low heat flow and the inter-mineral Neodymium isotopic equilibrium in the xenoliths imply that the heating of the lithospheric rocks occurred shortly before (~10–15 Ma) the eruption of the host basalts. Elevated equilibrium temperatures are recorded by other lithospheric xenoliths from the Arabian plate. The thermal perturbations are probably caused by thinning of the Arabian lithosphere in times of extension.