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Olivine is a major rock-forming mineral in various magmatic and metamorphic rocks and the upper mantle. In this paper, we present the first high-precision analyses of olivine from 15 samples of magnesian skarns and silicate marbles (MSSM) from the collection of the Fersman Mineralogical Museum (Moscow, Russia). Mg# [Mg/(Mg+Fe2+)·100, mol%] of olivine from the samples studied varies from 86 to nearly 100. The main distinctive features of the olivine are anomalously low contents of Co (<51 μg/g), Cr (<5 μg/g), and Ni (<44 μg/g) and high content of B (23–856 μg/g), which correlate with host-rock compositions. Phosphorus (5–377 μg/g) incorporation in olivine is charge balanced by the incorporation of Li (0.15–61 μg/g) and Na (<14.3 μg/g). Y and REE contents exhibit positive correlations with Na, which suggests that REE incorporation into MSSM olivine could occur via charge-balanced coupled substitution with Na at low temperature and low aSiO2conditions during MSSM formation. The documented compositional features of olivine from magnesian skarns and silicate marbles can help reconstruct the genesis of the host-rocks and identify xenocrysts of MSSM olivine in magmatic rocks.

Water strongly influences the physical properties of the mantle and enhances its ability to melt or convect. Its presence can also be used to trace recycling of surface reservoirs down to the deep mantle1, which makes knowledge of the water content in the Earth's interior and its evolution crucial for understanding global geodynamics. Komatiites (MgO-rich ultramafic magmas) result from a high degree of mantle melting at high pressures2and thus are excellent probes of the chemical composition and water contents of the deep mantle. An excess of water over elements that show similar geochemical behaviour during mantle melting (for example, cerium) was recently found in melt inclusions in the most magnesium-rich olivine in 2.7-billion-year-old komatiites from Canada3and Zimbabwe4. These data were taken as evidence for a deep hydrated mantle reservoir, probably the transition zone, in the Neoarchaean era (2.8 to 2.5 billion years ago). Here we confirm the mantle source of this water by measuring deuterium-to-hydrogen ratios in these melt inclusions and present similar data for 3.3-billion-year-old komatiites from the Barberton greenstone belt. From the hydrogen isotope ratios, we show that the mantle sources of these melts contained excess water, which implies that a deep hydrous mantle reservoir has been present in the Earth's interior since at least the Palaeoarchaean era (3.6 to 3.2 billion years ago). The reconstructed initial hydrogen isotope composition of komatiites is more depleted in deuterium than surface reservoirs or typical mantle but resembles that of oceanic crust that was initially altered by seawater and then dehydrated during subduction. Together with an excess of chlorine and depletion of lead in the mantle sources of komatiites, these results indicate that seawater-altered lithosphere recycling into the deep mantle, arguably by subduction, started before 3.3 billion years ago. Hydrogen isotopes and compositions of melt inclusions in olivine in komatiites indicate a hydrous source produced by recycling of seawater-altered crust into the deep mantle over 3.3 billion years ago.

The strong dependence of vanadium partitioning between olivine and silicate melt (DVO1-M$\begin{array}{} D_{\text V}^{\text{O1-M}} \end{array} $) on redox conditions (fO2) can be used as sensitive oxybarometer in magmatic systems. Here we extend the experimental database on DVO1-M$\begin{array}{} D_{\text V}^{\text{O1-M}} \end{array} $, obtained so far at high temperatures (mainly above 1250 °C), to lower temperatures that are typical for island-arc basalts. Crystallization experiments were performed using a composition from Mutnovsky volcano (Kamchatka), and the investigated temperature, pressure, and oxygen fugacity ranges were 1025–1150 °C, 0.1 and 0.3 GPa, and ΔQFM of –0.5 to +3.2, respectively. The water content in melts ranged from 0.6 to ~6.5 wt% H2O. The data demonstrate a strong negative correlation between DVO1-M$\begin{array}{} D_{\text V}^{\text{O1-M}} \end{array} $and oxygen fugacity, similar to the behavior observed previously at higher temperatures and in MgO-rich compositions. The correlation between DVO1-M$\begin{array}{} D_{\text V}^{\text{O1-M}} \end{array} $and ΔQFM in the range from –0.5 to +3.2 is described for melts with MgO < 12 wt% and Na2O < 4 wt% at temperatures ≤1250 °C by the empirical equation: ΔQFM = −3.07−0.29+0.26log⁡DVO1-M−3.34−0.49+0.40$\begin{array}{} -3.07_{-0.29}^{+0.26}\,\log D_{\text V}^{\text{O1-M}}-3.34_{-0.49}^{+0.40} \end{array} $with the standard error (SE) as a function of log DVO1-M$\begin{array}{} D_{\text V}^{\text{O1-M}} \end{array} $: 2SE(ΔQFM) = –0.275log DVO1-M$\begin{array}{} D_{\text V}^{\text{O1-M}} \end{array} $+ 0.4.

The origins of high-grade hydrothermal ore deposits are debated, but active geothermal systems provide important clues to their formation. The highest concentrations of gold are found in geothermal systems with direct links to island arc magmatism. Yet, similar concentrations have also been found in the absence of any input from arc magmas, for example, in the Reykjanes geothermal field, Iceland. Here we analyse brine samples taken from deep wells at Reykjanes and find that gold concentrations in the reservoir zone have increased over the past seven years from an average of 3 ppb to 14 ppb. The metal concentrations greatly exceed the maximum solubility of gold in the reservoir under saturated conditions and are now nearly two orders of magnitude higher than in mid-ocean ridge black smoker fluids—the direct analogues of Reykjanes deep liquids. We suggest that ongoing extraction of brine, the resulting pressure drop, and increased boiling have caused gold to drop out of solution and become trapped in the reservoir as a colloidal suspension. This process may explain how the stock of metal in the reservoirs of fossil geothermal systems could have increased over time and thus become available for the formation of gold-rich ore deposits.

Archaean komatiites are shown to originate in hot mantle plumes that entrained hydrous material from deep in the mantle.

Here we present the first record of Sr/Ca variability in a massive Porites luteacoral from the Lakshadweep Archipelago, Arabian Sea. The annual mean sea surface temperature (SST) in this region and the surrounding areas has increased steadily in the recent past. During some major El Niño events, SSTs are even higher, imposing additional thermal‐stress on corals, episodically leading to coral bleaching. We infer from the coral‐Sr/Ca record (1981–2008) that during some of these events high and persistent SSTs lead to a dampening of the temperature signal in coral‐Sr/Ca, impairing the coral's ability to record full scale warming. Thus, coral‐Sr/Ca may provide a history of past El Niño Southern‐Oscillation (ENSO) induced thermal‐stress episodes, which are a recurrent feature also seen in cross‐spectral analysis between coral‐Sr/Ca and the Nino3.4 index. Despite the impact of episodical thermal‐stress during major El Niño events, our coral proxy faithfully records the seasonal monsoon‐induced summer cooling on the order of ∼2.3°C. Calibration of coral‐Sr/Ca with instrumental grid‐SST data shows significant correlation to regional SST and monsoon variability. Hence, massive Poritescorals of this region are highly valuable archives for reconstructing long‐term changes in SST, strongly influenced by monsoon variability on seasonal scales. More importantly, our data show that this site with increasing SST is an ideal location for testing the future effects of the projected anthropogenic SST increase on coral reefs that are already under thermal‐stress worldwide. First coral‐Sr/Ca record from this region, which has strong monsoonal influenceCoral‐Sr/Ca records monsoon‐induced summer cooling of surface water temperaturesThermal‐stress during some major El Niño years suppresses the summer Sr/Ca peaks

Although wetlands were remarkable habitats with their fauna and flora diversity, few studies have been devoted to the study of amphipod distribution in this type of environment. To study both qualitatively and quantitatively amphipod community, surveys were conducted during the spring season in ten coastal lagoons ranging from subhumid to arid bioclimatic stage. At each station, eight quadrats of 50 × 50 cm were randomly placed. Amphipods were preserved in alcohol 70°C. In the laboratory, the specimens collected were identified and counted. Meanwhile, analyses of organic matter, particle size, and heavy metals from the soil taken from each station were made. A total of 1,340 specimens of amphipods were collected, and eight species belonging to Talitridae family were identified. Species richness ranges from one species collected in the supralittoral zone of El Bcherliya (Ghar El Melh lagoon) and eight species in the supralittoral zone of Bizerte lagoon. In this last station, the relative abundance of amphipods was significantly higher (36.04%, N= 483). In addition, the diversity indices of Simpson, Shannon-Weaver, and equitability shows that the highest species diversity characterizes this same station while the community was more balanced in opposite El Boughaz (Ghar El Melh lagoon) (J″ = 0.996). The spatial distribution of different amphipod species depends on edaphic (heavy metals, granulometry, organic matter) and climatic (temperature, humidity) factors.

Due to its inaccessibility, no complete and coherent data set exists for the composition of modern fast‐spreading oceanic crust. We sampled outcrops through 6,500 m of fossil oceanic crust in the Oman Ophiolite (Wadi Gideah Transect) that is regarded as best analogue of fast‐spreading crust on land. Here we report a complete set of whole‐rock major and trace element data displaying systematic and contrasting compositional trends in lower and upper gabbros being correlated with stratigraphic depth. A significant discontinuity in crystallization regime is observed at ∼3,525 m above the mantle‐crust boundary: gabbros below ∼3,525 m have in general very low incompatible element mass fractions which develop upwards in a barely noticeable way to more differentiated compositions while Mg# decreases. More pronounced trends indicating progressive fractionation of ascending melts can be observed for incompatible elements and their element ratios as a consequence of in situ crystallization. Locally, more variable compositions within narrow depth intervals testify for advanced differentiation in situ within individual sills. Gabbros above ∼3,525 m become significantly more evolved and show considerable variations in composition. Fractional crystallization and mixing processes in a transient axial melt lens control the composition of isotropic “varitextured” gabbros and sheeted dike basalts where fractionation of high field strength elemental ratios is minor. New average compositions of fast‐spread (paleo) oceanic crust are reported for major and 38 trace elements. Comparison with new data from Wadi Khafifah close to Wadi Gideah suggests robustness of crustal accretion processes in both space and time. Oceanic crust covers more than two‐thirds of Earth's surface but, due to limited accessibility, no single coherent compositional profile that would allow for understanding the processes how oceanic crust is formed has been obtained up to now. We sampled a transect through 6.5 km of fossil lower oceanic crust of the Samail ophiolite (Sultanate of Oman, Wadi Gideah) that is regarded as best analogue of fast‐spreading oceanic crust on land, and analyzed all samples with a wide selection of techniques. Here, we present whole rock data and see systematic compositional trends correlating with stratigraphic depth. Distribution trends of incompatible trace elements suggest that lower, layered gabbros are made of cumulus crystals that crystallize in situ, separating from the ascending parental magma. We also find gabbros with variable compositions at intermediate depth intervals that could originate from melts crystallizing and differentiating in sills. In contrast, compositions of upper non‐layered gabbros above ∼3,525 m above the mantle‐crust boundary are controlled by fractional crystallization and mixing processes in transient melt lenses. New average compositions of (paleo) oceanic crust are reported for major and 38 trace elements. Comparison with new data from Wadi Khafifah close to Wadi Gideah suggests robustness of accretion processes in both space and time. A coherent compositional transect through fossil oceanic crust for major and 38 trace elements, compositions for Samail oceanic crustSystematic but contrasting compositional trends correlate with stratigraphic depth over 6,500 m of plutonic crustA discontinuity at ～3,525 m above Moho separates lower cumulate gabbros crystallized in situ from upper gabbros fractionated in melt lens A coherent compositional transect through fossil oceanic crust for major and 38 trace elements, compositions for Samail oceanic crust Systematic but contrasting compositional trends correlate with stratigraphic depth over 6,500 m of plutonic crust A discontinuity at ～3,525 m above Moho separates lower cumulate gabbros crystallized in situ from upper gabbros fractionated in melt lens

Oceanic gabbros are the most abundant rocks close to Earth's surface. Here we present new data from a consistent profile through the paleocrust of the Samail Ophiolite (Oman), which is thought to provide the best analog for modern fast‐spreading oceanic crust. Incompatible trace elements of co‐existing plagioclase and clinopyroxene fractionate from the mineral core to rim and up section from layered to foliated to varitextured gabbros. Layered gabbro parental melts correspond to mid‐ocean‐ridge basalts (MORB), and plagioclase Ca# shows a pronounced inverse zonation. Likely, they crystallized in situ from hydrous melts, compositionally buffered by replenishment at equilibrium to MORB and near steady‐state boundary conditions. Further upsection, the compositional variability increases. Foliated gabbro rim and core compositions indicate increased fractionation and disequilibrium to MORB, triggered by open‐system fractional crystallization within a heterogeneous magma plumbing structure, characterized by magma mixing, varying ambient water activities, and boundary conditions. Varitextured gabbros are chemically diverse with parental melts partially more primitive than MORB, suggesting that primitive melts directly reach the axial melt lens (AML). REE‐in‐plagioclase‐clinopyroxene thermometry compared to and supported by anorthite‐in‐plagioclase thermometry reveals a relationship of TREEAn${T}_{\text{REE}}^{\text{An}}$[°C] = 6.1 ± 0.2 × An + 706 ± 19. Crystallization temperatures of the layered gabbros cover a narrow range of 1216 ± 14°C. Considerable temperature variability of 1077–1231°C is observed further upsection, featuring a thermal minimum within the foliated gabbros. This minimum is assumed to represent a zone where the fractionated descending crystal mushes originating from the AML meet with evolved liquids expelled from deeper crustal levels. Our findings suggest hybrid accretion of fast‐spread crust. Oceanic crust, the crystalline basement beneath the ocean‐floor sediments is difficult to be sampled by shipboard drilling operations, but can be obducted onto continents by tectonic movement of lithospheric crustal fragments to form ophiolites. These rock formations serve as an analog to in‐situ crust. They can be samples along a profile from extrusive basaltic rocks to gabbros to the upper mantle giving access to the earth's interior. The geochemical and thermal evolution along a profile of paleocrust is crucial for gaining knowledge of the crust's accretional dynamics and formation processes. Our data suggest that the lower part the layered gabbros, are formed by a fundamentally different process than the upper foliated, and varitextured gabbros. While geochemical signatures systematically evolve upsection and the temperature structure is homogeneous for the lower part, the upper part is characterized by much higher dynamics and lower crystallization temperatures. Therefore, the crust forms in a hybrid process, where the lower part is dominated by near steady‐state in situ crystallization in equilibrium with the parental melt. The upper part is formed in a more heterogeneous environment, dominated by subsiding crystal mushes, which mix with more primitive melts under disequilibrium conditions, yielding enhanced chemical variability. Geochemical profiles reveal continuous evolution of incompatible elements for the layered, and diverse fractionation for the upper gabbrosREE and anorthite thermometry shows a homogenous temperature structure for the lower, and a variable temperature record for the upper crustBoth suggests a hybrid model, with lower crust replenished in situ accretion and upper crust melt mixing and variable equilibrium dynamics Geochemical profiles reveal continuous evolution of incompatible elements for the layered, and diverse fractionation for the upper gabbros REE and anorthite thermometry shows a homogenous temperature structure for the lower, and a variable temperature record for the upper crust Both suggests a hybrid model, with lower crust replenished in situ accretion and upper crust melt mixing and variable equilibrium dynamics

We have investigated experimentally the partitioning of Au between solid and liquid sulfide phases and basaltic melts at 200 MPa, at redox conditions close to the sulfide-sulfate transition, over temperatures between 1050 and 1200 °C, which span the monosulfide solid solution (MSS) - sulfide liquid (SuL) solidus. The measured MSS/basalt partition coefficient of Au (DMSS-silAu) is about 100-200, whereas the partition coefficient of sulfide liquid/basalt (DMSS-silAu) is approximately 10 times larger at 2200. Although we find that temperature, pressure, and oxygen fugacity (ƒO2) exert relatively weak controls on Au partitioning, they exert major indirect influences on Au behavior by controlling the identity of the condensed sulfide phase and by affecting S solubility. These observations have important implications for the behavior of Au in the processes of partial melting in the mantle and magma crystallization in the crust. The occurrence of natural magmas with elevated concentrations of Au and presumably other highly siderophile and chalcophile elements requires predominance of MSS over SuL in the source or/and oxidizing conditions close to or above the sulfide-sulfate transition in the magma.

Major element, trace element and Sr–Nd–Pb isotope data, combined with 40Ar/39Ar age determinations, of volcanic rocks from the Selvagen Islands and neighboring seamounts in the eastern North Atlantic reveal the earlier history of the ≥70Ma old Canary hotspot. A basanitic to phonolitic late shield stage intrusive complex (29Ma) is exposed on Selvagem Pequena. The evolution of Selvagem Grande can be divided into three magmatic phases: a tephritic to phonolitic late shield stage intrusive complex (24–26Ma) and two rejuvenated or post-erosional stages (8–12 and 3.4Ma) consisting of alkali basalt, basanite and rare phonolite. During the early to mid-Miocene volcanic hiatus (12–24Ma), the top of the volcano was beneath sea level as evidenced by marine carbonate sediments (13–24Ma, dated through correlation of 87Sr/86Sr with the seawater Sr isotope curve). The geochemistry of the shield stage lavas indicates that they derive from plume sources, whereas the post-erosional lavas are derived from metasomatized lithospheric sources. Five sampled seamounts to the east and northeast of the islands range in composition from alkali basalt and basanite to phonolite. Samples from Dacia, Conception Bank and Lars were dated at 9, 17 and 68Ma, respectively. Geochemical data suggest that the dredged samples come from the post-erosional stage of volcanism, and therefore, the dates represent minimum ages for the seamount volcanoes. The elevation of erosional platforms formed at wave base decrease from Selvagen Grande (∼100m above sea level) to Lars seamount (∼900m below sea level), suggesting a southwest to northwest age progression and that all of these seamounts are older than the Selvagen Islands. Trace element and Sr–Nd–Pb isotopic composition of the Selvagen Islands and neighboring seamounts are consistent with their origin from the Canary plume. Interaction of the weak Canary mantle plume with a slow moving plate appears to be responsible for generating a 450-km-wide, irregular hotspot track extending 800km from the youngest Canary Island of Hierro in the southwest to Lars seamount in the northeast.

Abstract: This explorative study was performed to assess basic data on the Mo metabolism of premature infants. Premature (n=18, gestational age ≤32 wk, birth weight ≤1500 g) and healthy formula-fed term infants (n=14) were nourished and corrected for gestational age, identically. Plasma was collected at 3, 16, and 52 wk and 72 h balances were performed at 3 wk of age. In the premature infants, these investigations were preceded by two balance studies and an initial plasma collection. Increased Mo intake and low relative urinary excretion resulted in a retention of 4.4 (0.99–7.77) µg Mo/kg initially in premature infants (median, range). Parallel plasma concentrations were 5.5 (2.5–7.3) µg Mo/L, declining to 2.36 (0.73–3.87) µg Mo/L at 4 wk. Term infants rendered 1.49 (0.29–1.7) µg Mo/L (p<0.05), with no significant differences later. It was concluded that the supplementation of formulas for premature infants with Mo should be recinded until there is evidence for its necessity.

The essential trace element molybdenum (Mo) is bound to and required for the function of molybdoenzymes, e.g. sulfite and xanthine oxidase. Dietary recommendations for early infancy are based on limited knowledge about its metabolism. 100Mo was used as an extrinsic tag to study the absorption and kinetics of excretion in infancy. 10 infants with a gestational age of 35 (30–39) weeks, a birth weight of 2.0 (0.9–2.3) kg and a post-natal age of 20 (10–54) days were studied. They received 25 μg 100Mo/kg with a feed of human milk or formula. Fractional urinary and fecal collections were conducted preceding the 100Mo intake and for 48–72 hours afterwards. The materials were analyzed by atomic absorption spectroscopy and inductively coupled plasma mass spectrometry. The median absorption of 100Mo intake was 97.5 (96.3 to 99.1) %. The retention of nutritive Mo intake and 100Mo in the study period was 11.2 (3.8–15.7) μg Mo/kg, equivalent to 35.7 (12.7–55.6) %. The Mo concentration increased to a peak value in urine within 8 (6–13) hours and in feces within 24 (7–48.5) hours. In addition, increases of copper in feces and urine were observed in 8 of 9 infants studied. Mo given orally is well resorbed in premature infants, and predominantly excreted in the urine. Dietary recommendations should prevent excessive intakes in infancy.

An outcrop in the layered gabbro section from the Wadi Wariyah of the Oman ophiolite has been investigated to provide new insights into the hydrothermal activities that cover a large temperature range in the lower crust at fast‐spreading mid‐ocean ridges. Our observations reveal a complex veining system composed of gabbroic dikes, hydrous mineral (amphibole, epidote, and prehnite) veins and associated alteration, which strongly overprinted the layered gabbros shortly after the solidification of the gabbroic cumulates. The general strike of the veins is roughly perpendicular to the magmatic foliation of the layered gabbro. Mineral‐based geothermometry indicates a wide temperature range (ca. 1,000−250°C) of hydrothermal activities, spanning from hydrous suprasolidus stage (VHT) through amphibolite facies (HT) and epidote‐amphibolite facies (MT) down to the prehnite‐pumpellyite facies (LT), which induced significant differences in selective element depletion and enrichment as well as in exchange of Sr and O isotopes. A key observation is the omnipresence of relics from a previous temperature stage in a given hydrothermal paragenesis formed at lower temperatures, implying that the same pathways were used for hydrothermal veining in a broad temperature interval spanning about 750°C. Our observations support a model that focused fluid circulations, in the form of combined microcracking (grain boundary spacing) and macrocracking (veining), developed shortly subsequent to the complete solidification of the layered gabbros, which may have facilitated effective cooling for magma crystallization in the lower crust. The hydrothermal cooling process of the oceanic crust at the spreading center is crucial for understanding its accretion mechanism. In this paper, we present new data, which unambiguously indicate that the same pathways are used for hydrothermal cooling across a broad temperature range. The results and the conclusion of our study has the potential to enhance our understanding on the cooling mechanism occurring at the deep crust significantly and will also aid future works of thermal modeling. hydrothermal alteration beneath mid‐ocean ridges (MOR) starts at very high temperatureThe same pathways were used for hydrothermal veining in a broad temperaturePetrology and Sr‐O isotopes indicate variable fluid/rock ratios hydrothermal alteration beneath mid‐ocean ridges (MOR) starts at very high temperature The same pathways were used for hydrothermal veining in a broad temperature Petrology and Sr‐O isotopes indicate variable fluid/rock ratios

ABSTRACTEpilithic lichen (Xanthoria elegansin Canada, Lecanora muralisin Germany) were gathered from 17 locations in Ontario, Canada (from Lake Ontario to James Bay) and 43 locations in Germany (from the Alps to the North Sea and from the Baltic Sea to the Erzgebirge). Sample aliquots were digested in nitric acid and trace elements were analyzed by inductively coupled plasma-mass spectrom-etry techniques. The organohalogens were determined as absorbable organic halogens (AOXs) by coulometry. Concentration ranges from Germany for trace element samples and AOXs were determined. The lichen showed high spatial resolution in their element enrichment patterns. This allowed for differentiation between natural and anthropogenic dominance in ambient air concentrations. This biomonitoring method has proven to be very sensitive, fast, and reliable. No clear relationship could be found between trace element and AOX concentrations. The AOX values may reflect individual metabolic rates of the fungal partner in lichen symbionts.

Abstract:  Mohns Ridge lavas between 71 and 72°30′N (∼360 km) have heterogeneous compositions varying between alkali basalts and incompatible-element-depleted tholeiites. On a large scale there is a continuity of incompatible element and isotopic compositions between the alkali basalts from the island Jan Mayen and Mohns Ridge tholeiites. The variation in isotopes suggests a heterogeneous mantle which appears to be tapped preferentially by low degree melts (∼5%) close to Jan Mayen but also shows its signature much further north on Mohns Ridge. Three lava types with different incompatible element compositions [e.g. chondrite-normalized (La/Sm)N<1 to >2] occur in the area at 72°N and were generated from this heterogeneous mantle. The relatively depleted tholeiitic melts were mixed with a small degree melt from an enriched source. The elements Ba, Rb and K of the enriched melt were probably buffered in the mantle by residual amphibole or phlogopite. That such a residual phase is stable in this region of oceanic mantle suggests both high water contents and low mantle temperatures, at odds with a hotspot origin for Jan Mayen. Instead we suggest that the melting may be induced by the lowered solidus temperature of a “wet” mantle. Mohns MORB (mid ocean ridge basalt) and Jan Mayen area alkali basalts have high contents of Ba and Rb compared to other incompatible elements (e.g. Ba/La >10). These ratios reflect the signature of the mantle source. Ratios of Ce/Pb and Rb/Cs are normal MORB mantle ratios of 25 and 80, respectively, thus the enrichments of Ba and Rb are not indicative of a sedimentary component added to the mantle source but were probably generated by the influence of a metasomatizing fluid, as supported by the presence of hydrous phases during the petrogenesis of the alkali basalts. Geophysical and petrological models suggest that Jan Mayen is not the product of hotspot activity above a mantle plume, and suggest instead that it owes its existence to the unique juxtaposition of a continental fragment, a fracture zone and a spreading axis in this part of the North Atlantic.

The Easter microplate-Crough Seamount region located between 25° S–116° W and 25° S–122° W consists of a chain of seamounts forming isolated volcanoes and elongated (100–200 km in length) en echelon volcanic ridges oriented obliquely NE (N 065°), to the present day general spreading direction (N 100°) of the Pacific-Nazca plates. The extension of this seamount chain into the southwestern edge of the Easter microplate near 26°30' S–115° W was surveyed and sampled. The southern boundary including the Orongo fracture zone and other shallow ridges (< 2000 m high) bounding the Southwest Rift of the microplate consists of fault scarps where pillow lava, dolerite, and metabasalts are exposed. The degree of rock alternation inferred from palagonitization of glassy margins suggests that the volcanic ridges are as old as the shallow ridges bounding the Southwest Rift of the microplate. The volcanics found on the various structures west of the microplate consist of depleted (K/Ti < 0.1), transitional (K/Ti = 0.11-0.25) and enriched (K/Ti > 0.25) MORBs which are similar in composition to other more recent basalts from the Southwest and East Rifts spreading axes of the Easter microplate. Incompatible element ratios normalized to chondrite values [(Ce/Yb)N = 1-2.5}, {(La/Sm)N = 0.4-1.2} and {(Zr/Y)N = 0.7-2.5} of the basalts are also similar to present day volcanism found in the Easter microplate. The volcanics from the Easter microplate-Crough region are unrelated to other known South Pacific intraplate magmatism (i.e. Society, Pitcairn, and Salas y Gomez Islands). Instead their range in incompatible element ratios is comparable to the submarine basalts from the recently investigated Ahu and Umu volcanic field (Easter hotspot) (Scientific Party SO80, 1993) and centered at about 80 km west of Easter Island. The oblique ridges and their associated seamounts are likely to represent ancient leaky transform faults created during the initial stage of the Easter microplate formation (˜ 5 Ma). It appears that volcanic activity on seamounts overlying the oblique volcanic ridges has continued during their westward drift from the microplate as shown by the presence of relatively fresh lava observed on one of these structures, namely the first Oblique Volcanic Ridge near 25° S–118° W at about 160 km west of the Easter microplate West Rift. Based on a reconstruction of the Easter microplate, it is suggested that the Crough seamount (< 800 m depth) was formed by earlier (7–10 Ma) hotspot magmatic activity which also created Easter Island.