Showing papers on "Incompatible element published in 1998"

Peridotites from the Izu-Bonin-Mariana Forearc (ODP Leg 125): Evidence for Mantle Melting and Melt-Mantle Interaction in a Supra-Subduction Zone Setting

Abstract: Ocean Drilling Program Leg 125 recovered serpentinized harzburgites and dunites from a total of five sites on the crests and flanks of two serpentinite seamounts, Conical Seamount in the Mariana forearc and Torishima Forearc Seamount in the Izu–Bonin forearc. These are some of the first extant forearc peridotites reported in the literature and they provide a window into oceanic, supra-subduction zone (SSZ) mantle processes. Harzburgites from both seamounts are very refractory with low modal clinopyroxene (<4%), chrome-rich spinels (cr-number = 0.40–0.80), very low incompatible element contents, and (with the exception of amphibole-bearing samples) U-shaped rare earth element (REE) profiles with positive Eu anomalies. Both sets of peridotites have olivine–spinel equilibration temperatures that are low compared with abyssal peridotites, possibly because of water-assisted diffusional equilibration in the SSZ environment. However, other features indicate that the harzburgites from the two seamounts have very different origins. Harzburgites from Conical Seamount are characterized by calculated oxygen fugacities between FMQ (fayalite–magnetite–quartz) – 1.1 (log units) and FMQ + 0.4 which overlap those of mid-ocean ridge basalt (MORB) peridotites. Dunites from Conical Seamount contain small amounts of clinopyroxene, orthopyroxene and amphibole and are light REE (LREE) enriched. Moreover, they are considerably more oxidized than the harzburgites to which they are spatially related, with calculated oxygen fugacities of FMQ – 0.2 to FMQ + 1.2. Using textural and geochemical evidence, we interpret these harzburgites as residual MORB mantle (from 15 to 20% fractional melting) which has subsequently been modified by interaction with boninitic melt within the mantle wedge, and these dunites as zones of focusing of this melt in which pyroxene has preferentially been dissolved from the harzburgite protolith. In contrast, harzburgites from Torishima Forearc Seamount give calculated oxygen fugacities between FMQ + 0.8 and FMQ + 1.6, similar to those calculated for other subduction-zone related peridotites and similar to those calculated for the dunites (FMQ + 1.2 to FMQ + 1.8) from the same seamount. In this case, we interpret both the harzburgites and dunites as linked to mantle melting (20–25% fractional melting) in a supra-subduction zone environment. The results thus indicate that the forearc is underlain by at least two types of mantle lithosphere, one being trapped or accreted oceanic lithosphere, the other being lithosphere formed by subduction-related melting. They also demonstrate that both types of mantle lithosphere may have undergone extensive interaction with subduction-derived magmas.

Trace element modeling of aqueous fluid - peridotite interaction in the mantle wedge of subduction zones

Abstract: Recently measured partition coefficients for Rb, Th, U, Nb, La (Ce), Pb, Sr, Sm, Zr, and Y between lherzolite assemblage minerals and H2O-rich fluid (Ayers et al 1997; Brenan et al 1995a,b) are used in a two-component local equilibrium model to assess the effects of interaction between slab-derived aqueous fluids and wedge lherzolite on the trace element and isotopic composition of island arc basalts (IAB) The model includes four steps representing chemical processes, with each process represented by one equation with one adjustable parameter, in which aqueous fluid: (1) separates from eclogite in the subducted slab (Rayleigh distillation, mass fraction of fluid released F fluid); (2) ascends through the mantle wedge in isolated packets, exchanging elements and isotopes with depleted lherzolite (zone refining, the rock/fluid mass ratio n); (3) mixes with depleted lherzolite (physical mixing, the mass fraction of fluid in the mixture X fluid); (4) induces melting to form primitive IAB (batch melting, mass fraction of melt F melt) The amount of mantle lherzolite processed by the fluid in step (2) determines its isotopic and trace element signature and the relative contributions of slab and wedge to primitive IAB Assuming an average depleted lherzolite composition and mineralogy (70% olivine, 26% orthopyroxene, 3% clinopyroxene and 1% ilmenite) and using nonlinear regression to adjust parameter values to obtain an optimal fit to the average composition of IAB (McCulloch and Gamble 1991) yields values of F fluid= 020, n= 26, X fluid= 017, and F melt= 015, with r 2= 0995 and the average relative error in trace element concentration = 6% The average composition of IAB can also effectively be modeled with no contribution from the slab other than H2O (ie, skip model step 1): n= 27, X fluid= 021, F melt= 017, with r 2= 0992 By the time the fluid reaches the IAB source, exchange with depleted wedge lherzolite reduces the 87Sr/86Sr ratio isotopic composition to near-mantle values and the slab contribution to <50% for all but the most incompatible elements (eg, Pb) The IAB may retain the slab signature for elements such as B and Be that are highly incompatible and that have very low concentrations in the depleted mantle wedge The relatively high equilibrium D mineral / fluid values measured by Ayers et al (1997), Brenan et al (1995a) and Stalder et al (1998) suggest that large amounts of fluid (>5 wt%) must be added to lherzolite in the IAB source Decreasing X fluid below 005 causes model results to have unacceptably high levels of error and petrologically unreasonable values of F melt That H2O contents of IAB are generally <6 wt% suggests that not all of the H2O that metasomatizes the IAB source remains in the source to dissolve in the subsequently formed melt Modeling of the compositions of specific primitive IAB from oceanic settings with low sediment input and depleted mantle wedges (Tonga, Marianas) shows a generally lower level of fluid-wedge interaction (low n), and therefore a larger slab component in primitive IAB

Melting and metasomatism in the continental lithosphere: laser ablation ICPMS analysis of minerals in spinel lherzolites from eastern Australia

Abstract: Olivine, low-Ca pyroxene, diopside, and spinel from a suite of protogranular lherzolite xenoliths from southeastern Australia have been analysed for their major and trace element compositions using electron microprobe and laser ablation ICPMS. Bulk compositions of the lherzolites range from fertile (12–13% modal diopside) to depleted (2–3% modal diopside), with equilibration temperatures of 850–900 °C indicating entrainment of these lherzolites from relatively shallow depths (probably ≤ 35 km) within the lithosphere. Mineral compositions and abundances indicate a primary control by partial melting, with decreasing abundance of modal diopside accompanied by increasing Mg# of olivine and pyroxene, decreasing Al and Ti contents of diopside, increasing Ni contents of olivine, and increasing Cr/Al of spinel. HREE, Y, and Ga in diopside also follow melting trends, decreasing in concentration with increasing Mg#. In contrast, highly incompatible elements such as LREE, Nb, and Th reveal divergent behaviour that cannot be ascribed entirely to partial melting. Diopsides from the fertile lherzolites have mantle-normalized patterns that are depleted in Th, Nb, and the LREE relative to Y and the HREE, whereas, diopsides from the cpx-poor samples are strongly enriched in Th, Nb and the LREE, and have elevated Sm/Hf and Zr/Hf, and low Ti/Nb. All diopsides have strongly negative Nb anomalies relative to Th and the LREE. Trace element patterns of diopside in the fertile lherzolites can be reproduced by ≤ 5% batch melting of a primitive source. The negative Nb anomalies are a consequence of this melting, and do not require special conditions or tectonic environments. The low concentrations of Y and HREE in diopside from the cpx-poor lherzolites cannot be produced by realistic degrees of batch melting, but can be accomplished by up to ∼20% fractional melting, suggesting multiple episodes of melt depletion. Os isotopic compositions of these lherzolites show that the melt depletion events occurred in the middle and late Proterozoic, demonstrating the long-term stability of lithospheric mantle beneath regions of eastern Australia. The LREE-enriched diopsides are well equilibrated and record metasomatic enrichment events that pre-date the magmatism that entrained these xenoliths. Trace element patterns of these pyroxenes suggest a carbonatitic melt as the metasomatic agent.

Ancient mantle in a modern arc : Osmium isotopes in Izu-Bonin-Mariana forearc peridotites

Abstract: Mantle peridotites drilled from the Izu-Bonin-Mariana forearc have unradiogenic 187Os/188Os ratios (0.1193 to 0.1273), which give Proterozoic model ages of 820 to 1230 million years ago. If these peridotites are residues from magmatism during the initiation of subduction 40 to 48 million years ago, then the mantle that melted was much more depleted in incompatible elements than the source of mid-ocean ridge basalts (MORB). This result indicates that osmium isotopes record information about ancient melting events in the convecting upper mantle not recorded by incompatible lithophile isotope tracers. Subduction zones may be a graveyard for ancient depleted mantle material, and portions of the convecting upper mantle may be less radiogenic in osmium isotopes than previously recognized.

Two mantle sources, two plumbing systems : tholeiitic and alkaline magmatism of the maymecha river basin, siberian flood volcanic province

Abstract: Rocks of two distinctly different magma series are found in a ∼4000-m-thick sequence of lavas and tuffs in the Maymecha River basin which is part of the Siberian flood-volcanic province. The tholeiites are typical low-Ti continental flood basalts with remarkably restricted, petrologically evolved compositions. They have basaltic MgO contents, moderate concentrations of incompatible trace elements, moderate fractionation of incompatible from compatible elements, distinct negative Ta(Nb) anomalies, and Nd values of 0 to +2. The primary magmas were derived from a relatively shallow mantle source, and evolved in large crustal magma chambers where they acquired their relatively uniform compositions and became contaminated with continental crust. An alkaline series, in contrast, contains a wide range of rock types, from meymechite and picrite to trachytes, with a wide range of compositions (MgO from 0.7 to 38 wt%, SiO2 from 40 to 69 wt%, Ce from 14 to 320 ppm), high concentrations of incompatible elements and extreme fractionation of incompatible from compatible elements (Al2O3/TiO2∼1; Sm/Yb up to 11). These rocks lack Ta(Nb) anomalies and have a broad range of Nd values, from −2 to +5. The parental magmas are believed to have formed by low-degree melting at extreme mantle depths (>200 km). They bypassed the large crustal magma chambers and ascended rapidly to the surface, a consequence, perhaps, of high volatile contents in the primary magmas. The tholeiitic series dominates the lower part of the sequence and the alkaline series the upper part; at the interface, the two types are interlayered. The succession thus provides evidence of a radical change in the site of mantle melting, and the simultaneous operation of two very different crustal plumbing systems, during the evolution of this flood-volcanic province.

The Generation of Potassic Lavas from the Eastern Virunga Province, Rwanda

Abstract: Lavas from the eastern Virunga province, Rwanda, are dominated to lie within the continental mantle lithosphere (e.g. Fraser et al., 1986; Nelson et al., 1986; Dudas, 1991; by K-hawaiites, K-basanites and latites. All lavas are shoshonitic with 1 < K2O/Na2O < 2 and strongly enriched in incompatible Rogers et al., 1992; Gibson et al., 1995) of which they are a complementary sample to that provided by peridotite elements. Sr/Sr varies from 0·70586 in the K-basanites to 0·70990 in the latites, Nd/Nd from 0·51254 to 0·51206, xenoliths (e.g. Menzies et al., 1987; Jochum et al., 1989). In addition, their trace element and isotopic charand Pb isotopes define sub-vertical trends on isotope diagrams ( Pb/Pb 19·30–19·51, Pb/Pb 15·69–15·93 and acteristics can be used to infer the temporal evolution of the mantle lithosphere and the processes involved in Pb/Pb 40·28–41·5). Ar/Ar ages of leucite and phlogopite separates suggest that the latites are between 100 and 200 ka and lithosphere stabilization. Potassic magmas are also among the most compositionally extreme products of processes the K-basanites <100 ka. The latites are hybrid magmas produced by mixing between a K-basanite melt with a silicic melt from the that scavenge and fractionate incompatible elements in the upper mantle. Hence their geochemical variations deep crust. The low-silica K-basanites reflect interaction between a mafic K-basanitic melt with Nd/Nd ~0·51204, Sr/Sr can also be used to investigate the time-integrated effects of these processes on the radiogenic isotope evolution of ~0·707, and a nephelinite with Nd/Nd ~0·51267 and Sr/Sr ~0·7045. Both are derived from the mantle lithosphere the mantle lithosphere and to explore links between their lithospheric source regions and those of ocean-island with source ages of 1 Ga and 0·5 Ga, respectively, and the youngest ages correspond to the deepest magma sources. The magma production basalt (OIB) (McKenzie & O’Nions, 1983, 1995; Turner et al., 1996). rate in the Virunga is low (~0·04 km/yr), and reflects prolonged (10–15 My) heating of the lithosphere by the East African mantle The Virunga province of the western branch of the African Rift is a classic example of intra-plate potassic plume. magmatism, and the Ugandan part of the province was first described in remarkable detail by Holmes & Harwood (1937). They presented both excellent field and petrographic descriptions of mineralogically and

The petrogenesis of felsic calc-alkaline magmas from the southernmost Cascades, California: origin by partial melting of basaltic lower crust

Abstract: The majority of felsic rocks from composite centers in the southernmost INTRODUCTION Cascades have geochemical and Sr, Nd and Pb isotopic ratios Despite the importance of felsic magmas in calc-alkaline that suggest derivation by partial melting of lower crust that is rock suites, as both the end of the compositional spectrum compositionally similar to calc-alkaline basalts observed in the region. of lavas and as a potential assimilant or mixing endOnly a few felsic rocks have d 18 O and Pb isotopic compositions member, most petrogenetic studies have focused on bathat indicate interaction with the upper crust. Mineralogical and saltic magmas. This stems from the fact that sparsely geochemical diVerences among the felsic magmas result primarily crystalline basaltic lavas are usually the most primitive from melting under variable f(H2O) and temperature conditions. lavas within a given suite of rocks, and therefore have Partial melting under low f(H2O) and high temperature conditions compositions that are most indicative of magma sources. leaves an amphibole-poor residuum, and produces magmas that In contrast, felsic rocks have high incompatible element have orthopyroxene as the most abundant ferromagnesian phenocryst, abundances and are often highly crystalline, and may relatively low silica contents, and straight rare earth element patterns. therefore have been extensively modified by fractional Partial melting under higher f(H2O) and lower temperature con- crystallization and assimilation fractional crystallization ditions leaves an amphibole-rich residuum, and produces magmas (AFC). This study focuses on sparsely crystalline felsic that have amphibole ‐ biotite phenocrysts, relatively high silica (SiO2 > 68 wt %) volcanic rocks erupted in the southcontents, and pronounced middle rare earth element depletions. These ernmost Cascades that demonstrate evidence for only a conclusions are consistent with published thermal models that suggest minor amount of diVerentiation. We use mineralogy, that reasonable volumes of basaltic magma emplaced beneath large major and trace element geochemistry, and isotopic compositions of felsic rocks erupted from composite cencomposite centers in the southernmost Cascades can serve as the ters to constrain their petrogenesis. These felsic rocks heat source for melting of the lower crust. Melting of the lower provide new insights into petrogenetic processes that crust under variable f(H2O) conditions is likely to result from generate felsic arc magmas.

Trace Element Composition of Mantle-derived Carbonates and Coexisting Phasesin Peridotite Xenoliths from Alkali Basalts

Abstract: The trace element compositions of carbonate, clinopyroxene, ampyroxenite and eclogite xenoliths as texturally equiphibole and silicate glass were determined in four mantle lherzolite librated or interstitial grains (Wass, 1979), ‘globules’ in xenoliths in alkali basalts from Spitsbergen and Mongolia by laser association with silicate glass (Amundsen, 1987; Ionov et ablation ICP-MS. Carbonates in the xenoliths occur in fine-grained al., 1993, 1996; Pyle & Haggerty, 1994; Kogarko et al., pockets that appear to have been produced by reaction of carbonate1995; Norman, 1998) and components of fluid microrich melts with the host peridotites. The carbonates are rich in Sr inclusions (Frezzotti et al., 1994; Schiano & Clocchiatti, and Ba, but have low contents of rare earth elements. (Na,Al)-rich 1994). There is unambiguous experimental evidence that silicate glass commonly associated with the carbonates is a major carbonates are stable in peridotitic assemblages at aphost for many incompatible lithophile elements in the carbonatepropriate temperatures and pressures, and that smallbearing pockets. The carbonates in the xenoliths do not appear to degree partial melting of carbonated peridotite at presrepresent quenched carbonatite liquids but probably are crystal sures [2 GPa produces carbonatitic liquids (Wyllie, cumulates from carbonate-rich melts. Trace element patterns es1987; Dalton & Wood, 1993a; Lee & Wyllie, 1998). timated for liquids that may have produced the carbonate-bearing Interaction of carbonate melts with the lithospheric pockets are consistent with general characteristics of carbonatemantle has been invoked to explain the formation and related metasomatism (enrichments in light rare earth elements, Th, compositional characteristics of some metasomatized U, Ba and negative anomalies for high field strength elements). mantle peridotites (Wallace & Green, 1988; Yaxley et al., However, the absolute incompatible element abundances estimated 1991; Dautria et al., 1992; Hauri et al., 1993; Ionov et al., for those liquids cannot provide the extremely strong enrichments 1993; Rudnick et al., 1993). invoked by some models of carbonate mantle metasomatism. ClinoLittle is known about trace element composition of pyroxene and amphibole outside the carbonate-bearing pockets in carbonates in mantle rocks and of primary carbonatethe xenoliths from Spitsbergen have high contents of incompatible rich liquids generated in the mantle. The models relating trace elements, indicating that the lherzolites also experienced mantle metasomatism to carbonate-rich melts usually metasomatic enrichment before the formation of the carbonates. assume that these melts are similar in trace element composition to carbonatites exposed in the crust and, in particular, that such melts should be strongly enriched in incompatible trace elements. Unfortunately, crustal

A study of melt inclusions at Vulcano (Aeolian Islands, Italy): insights on the primitive magmas and on the volcanic feeding system

Abstract: This work presents the results of a microthermometric and EPMA-SIMS study of melt inclusions in phenocrysts of rocks of the shoshonitic eruptive complex of Vulcano (Aeolian Islands, Italy). Different primitive magmas related to two different evolutionary series, an older one (50–25 ka) and a younger one (15 ka to 1890 A.D.), were identified as melt inclusions in olivine Fo88–91 crystals. Both are characterized by high Ca/Al ratio and present very similar Rb/Sr, B/Be and patterns of trace elements, with Nb and Ti anomalies typical of a subduction zone. The two basalts present the same temperature of crystallization (1180±20 °C) and similar volatile abundances. The H2O, S and Cl contents are relatively high, whereas magmatic CO2 concentrations are very low, probably due to CO2 loss before low-pressure crystallization and entrapment of melt inclusions. The mineral chemistry of the basaltic assemblages and the high Ca/Al ratio of melt inclusions indicate an origin from a depleted, metasomatized clinopyroxene-rich peridotitic mantle. The younger primitive melt is characterized with respect to the older one by higher K2O and incompatible element abundances, by lower Zr/Nb and La/Nb, and by higher Ba/Rb and LREE enrichment. A different degree of partial melting of the same source can explain the chemical differences between the two magmas. However, some anomalies in Sr, Rb and K contents suggest either a slightly different source for the two magmas or differing extents of crustal contamination. Low-pressure degassing and cooling of the basaltic magmas produce shoshonitic liquids. The melt inclusions indicate evolutionary paths via fractional crystallization, leading to trachytic compositions during the older activity and to rhyolitic compositions during the recent one. The bulk-rock compositions record a more complex history than do the melt inclusions, due to the syneruptive mixing processes commonly affecting the magmas erupted at Vulcano. The composition and temperature data on melt inclusions suggest that in the older period of activity several shallow magmatic reservoirs existed; in the younger one a relatively homogeneous feeding system is active. The shallow magmatic reservoir feeding the recent eruptive activity probably has a vertical configuration, with basaltic magma in the deeper zones and differentiated magmas in shallower, low-volume, dike-like reservoirs.

A model to explain the various paradoxes associated with mantle noble gas geochemistry

Abstract: As a result of an energetic accretion, the Earth is a volatile-poor and strongly differentiated planet. The volatile elements can be accounted for by a late veneer (≈1% of total mass of the Earth). The incompatible elements are strongly concentrated into the exosphere (atmosphere, oceans, sediments, and crust) and upper mantle. Recent geochemical models invoke a large primordial undegassed reservoir with chondritic abundances of uranium and helium, which is clearly at odds with mass and energy balance calculations. The basic assumption behind these models is that excess “primordial”^3He is responsible for ^3He/^4He ratios higher than the average for midocean ridge basalts. The evidence however favors depletion of ^3He and excessive depletion of ^4He and, therefore, favors a refractory, residual (low U, Th) source Petrological processes such as melt-crystal and melt-gas separation fractionate helium from U and Th and, with time, generate inhomogeneities in the ^3He/^4He ratio. A self-consistent model for noble gases involves a gas-poor planet with trapping of CO_2 and noble gases in the shallow mantle. Such trapped gases are released by later tectonic and magmatic processes. Most of the mantle was depleted and degassed during the accretion process. High ^3He/^4He gases are viewed as products of ancient gas exsolution stored in low U environments, rather than products of primordial reservoirs.

Formation of juvenile continental crust in the Arabian-Nubian shield: Evidence from granitic rocks of the Nakasib suture, NE Sudan

Abstract: Major and trace element data, U–Pb zircon ages, and initial isotopic compositions of Sr, Nd, and Pb are reported for ten granitic and one rhyolitic rock sample from the neo-Proterozoic Nakasib suture in NE Sudan. Chemical data indicate that the samples are medium- to high-K, "I-type" granitic rocks that mostly plot as "volcanic arc granites" on discriminant diagrams. Geochronologic data indicate that rifting occurred 790±2 Ma and constrain the time of deformation associated with suturing of the Gebeit and Haya terranes to have ended by approximately 740 Ma. Isotopic data show a limited range, with initial 87Sr/86Sr=0.7021 to 0.7032 (mean=0.7025), eNd(t) =+5.5 to +7.0 (mean=+6.4), and 206Pb/204Pb = 17.50–17.62. Neodymium model ages (TDM; 0.69–0.85 Ga; mean = 0.76 Ga) are indistinguishable from crystallization ages (0.79–0.71 Ga; mean=0.76 Ga), and the isotopic data considered together indicate derivation from homogeneously depleted mantle. The geochronologic data indicate that the terrane accretion to form the Arabian–Nubian shield began just prior to 750 Ma. The isotopic data reinforces models for the generation of large volumes of juvenile continental crust during neo-Proterozoic time, probably at intra-oceanic convergent margins. The data also indicate that crust formation was associated with two cycles of incompatible element enrichment in granitic rocks, with an earlier cycle beginning approximately 870 Ma and culminating approximately 740 Ma, and the second cycle beginning after pervasive high-degree melts – possibly hot-spot related – were emplaced approximately 690–720 Ma.

The Incompatible Element Characteristics of an Ancient Subducted Sedimentary Component in Ocean Island Basalts from French Polynesia

Abstract: marized by Woodhouse & Dziewonski, 1984; Castillo,1988; Zhang & Tanimoto, 1992; Nataf & VanDecar,1993; White & Duncan, 1996). The isotopic char-acteristics of the end-member mantle components areproposed to be derived by (1) ancient subduction ofoceanic crust, (2) delamination and subsequent sinkingof subcontinental lithosphere deep into the mantle, or (3)

Complications to Carbonate Melt Mobility due to the Presence of an Immiscible Silicate Melt

Abstract: The relative interfacial energies of immiscible carbonate and silicate number of incompatible elements, and can be very mobile. Their mobility is a result of their low viscosities melts were investigated in olivine and clinopyroxene matrices. Carbonate melt has a higher melt–solid interfacial energy than does the [0·01–0·1 Pa s at temperatures >500°C, for a range of compositions (Dawson et al., 1990; Wolff, 1994; Dobson coexisting silicate melt. The silicate melt therefore selectively wets the grain-edge channels between solid phases, excluding the carbonate et al., 1996)] and their ability to form an interconnected grain-edge melt at low melt fractions. Dunitic assemblages melt to the center of melt pockets, away from grain edges. This prevents the carbonate melt from migrating independently of the (and probably other mineralogies) with an interconnected carbonate melt should have a high permeability to flow. silicate melt and the carbonate melt is unable to separate from the silicate melt in a solid-dominated assemblage. The carbonate melt These melts may transport a variety of trace elements and can infiltrate long distances into the country rock will migrate effectively only after the silicate melt has solidified, or by separating from the silicate melt within liquid-dominated reservoirs surrounding melt channels, altering the chemistry and isotopic systems of the host rock. Diffusion is fast within (sills, dikes, or chambers), unrestricted by solid interfaces. This relative wetting behavior may help explain the close association of these melts, so even a stagnant interconnected melt can greatly enhance the transport of trace and major elements carbonate and silicate magmas in alkali complexes, and their relative timing of emplacement. These results also place constraints on the (Watson, 1991). generation and separation of derivative melts in carbonated silicate The relative interfacial energy between grain–grain melt systems and on the style and timing of alkali wall-rock contacts and grain–fluid contacts can be parameterized metasomatism. by the dihedral angle, the angle subtended at a fluid– grain–grain junction. Dihedral angles >60° indicate that system interfacial energy is minimized by reducing the contact of the fluid with the solid, whereas systems with dihedral angles <60° minimize interfacial energy by

31. mantle melting systematics: transition from continental to oceanic volcanism on the southeast greenland margin 1

Abstract: We present trace element analyses of basaltic and picritic lavas recovered from the Ocean Drilling Program Leg 152 transect on the southeast Greenland volcanic rifted margin. These lavas span the stratigraphic interval from continental to oce anic magmatism. Incompatible element patterns define two geochemical groups within the Site 917 upper series. Group 1 flows are characterized by (La/Sm) N = 0.3-0.8, whereas group 2 flows have (La/Sm) N = 0.7-1.2 and higher mantle-normalized Th, Ba, and Pb, and lower Nb and U concentrations. The transition upsection from group 1 to group 2 at Site 917 occurs between 114 and 65 meters below seafloor, and is represented by interfingering of flows belonging to the two groups. (La/Sm) N, Ba/Zr, and Th/Pb ratios and mantle-normalized incompatible element concentrations decrease systematically with stratigraphic height within each group, and the Site 915 and Site 918 units are generally continuous with group 2. These variations imply an increase in the extent of partial melting with time. Lu/Hf ratios vary from 0.14 to 0.25 through the upper series and into the Site 915 and Site 918 flows. This relationship suggests the importance of residual garnet in the mantle source melting decreased with time. We develop a quantitative model for mantle melting to investigate melting systematics responsible for these relation ships. Comparison between observed data and model results suggests a progressive increase in the extent of partial melting (from 4% to 12%) and decrease in mean pressure of melting with time. This temporal evolution of primary magma compositions is explicable by rapid thinning of the continental lithosphere during eruption of the upper series. We conclude that grou p 1 units were derived from mantle with normal mid-ocean-ridge basalt source characteristics, whereas units from group 2 and from Sites 915 and 918 were derived from a source similar to depleted Icelandic mantle. We infer that the thermal anomaly associated with the ancestral Iceland plume pre-dated the transition in mantle source compositions.