S. R. Hart

Nd and Sr isotope ratios and rare earth element abundances in Reykjanes Peninsula basalts evidence for mantle heterogeneity beneath Iceland

Post-glacial tholeiitic basalts from the western Reykjanes Peninsula range from picrite basalts (oldest) to olivine tholeiites to tholeiites (youngest). In this sequence there are large systematic variations in rare earth element (REE) abundances (La/Sm normalized to chondrites ranges from 0.33 in the picrite basalts to 1.25 in the fissure tholeiites) and corresponding variations in 143Nd/144Nd (0.51317 in the picrite basalts to 0.51299 in the fissure tholeiites). The large viaration in 143Nd/144Nd, more than one-third the total range observed in most ocean islands and mid-ocean ridge basalts (MORB), is accompanied by only a small variation in 87Sr/86Sr (0.7031–0.7032). These 87Sr/86Sr ratios are within the range of other Icelandic tholeiites, and distinct from those of MORB. n nWe conclude that the mantle beneath the Reykjanes Peninsula is heterogeneous with respect to relative REE abundances and 143Nd/144Nd ratios. On a time-averaged basis all parts of this mantle show evidence of relative depletion in light REE. Though parts of this mantle have REE abundances and Nd isotope ratios similar to the mantle source of “normal” MORB, 87Sr/86Sr is distinctly higher. Unlike previous studies we find no evidence for chondritic relative REE abundances in the mantle beneath the Reykjanes Peninsula; in fact, the data require significant chemical heterogeneity in the hypothesized mantle plume beneath Iceland, as well as lateral mantle heterogeneity from the Reykjanes Ridge to the Reykjanes Peninsula. The compositional range of the Reykjanes Peninsula basalts is consistent with mixing of magmas produced by different degrees of melting in different parts of the heterogeneous mantle source beneath the Reykjanes Peninsula.

The isotope systematics of a juvenile intraplate volcano" Pb, Nd, and Sr isotope ratios of basalts from Loihi Seamount, Hawaii

Sr, Nd, and Pb isotope ratios for a representative suite of 15 basanites, alkali basalts, transitional basalts and tholeiites from Loihi Seamount, Hawaii, display unusually large variations for a single volcano, but lie within known ranges for Hawaiian basalts. Nd isotope ratios in alkali basalts show the largest relative variation (0.51291–0.51305), and include the nearly constant tholeiite value ( ∼ 0.51297). Pb isotope ratios show similarly large ranges for tholeiites and alkali basalts and continue Tatsumotos [31] “Loa” trend towards higher 206Pb/204Pb ratios, resulting in a substantial overlap with the “Kea” trend. 206Pb/204Pb ratios for Loihi and other volcanoes along the Loa and Kea trends [31] are observed to correlate with the age of the underlying lithosphere suggesting lithosphere involvement in the formation of Hawaiian tholeiites. Loihi lavas display no correlation of Nd, Sr, or Pb isotope ratios with major element compositions or eruptive age, in contrast with observations of some other Hawaiian volcanoes [38]. Isotope data for Loihi, as well as average values for Hawaiian volcanoes, are not adequately explained by previously proposed two-end-member models; new models for the origin and the development of Hawaiian volcanoes must include mixing of at least three geochemically distinct source regions and allow for the involvement of heterogeneous oceanic lithosphere.

Helium isotopic variations in the mantle beneath the central North Atlantic Ocean

We have determined the helium concentrations and helium isotopic ratios for a set of 35 basaltic glass samples dredged from the Mid-Atlantic Ridge between 28° and 53°N latitude. Total helium concentrations vary between< 1 × 10−8cm3STP/g and1.4 × 10−5cm3STP/g. Most of this variation can be accounted for by gas loss on the sea floor via vesiculation, since the samples with the lowest concentrations have extremely high vesicularities. Analysis of the plagioclase phenocrysts from one of the samples suggests that helium behaves as an incompatible element (KD ≪ 0.01). Results of in vacuo crushing and melting experiments confirm that there is no helium isotopic fractionation during melt-vesicle or melt-phenocryst partitioning. The variations in3He/4He ratio along the ridge therefore must be interpreted to be a result of heterogeneity within the oceanic mantle. The3He/4He ratios for this set of samples range from 6.5 to 11.1 times the atmospheric ratio. These data, coupled with strontium isotopic analyses performed on the same samples by White and Schilling (1978), show the existence of two distinct geochemical provinces: the regions between 50–52.5°N and 27–33°N, which are characterized by a trend toward high3He/4He ratios and high87Sr/86Sr ratios, and the region between 33° and 50°N, characterized by a trend toward low3He/4He ratios and high87Sr/86Sr ratios. Since primitive mantle should retain more primordial3He, the low3He/4He trend requires the presence of a mantle reservoir with lower3He/(Th + U) and higher Rb/Sr. The generation of this component requires separation between He and Th + U, either by selective loss of He (degassing), or the addition of Th + U. We believe that the most reasonable source for this component is subducted oceanic crust that is recycled back into the mantle.

Petrology and geochemistry of basalts from the American-Antarctic Ridge, Southern Ocean: implications for the westward influence of the Bouvet mantle plume

Ridge segments and fracture zones from the American-Antarctic Ridge have been systematically dredge sampled from ∼4° W to ∼18° W. Petrographic studies of the dredged basalts show that the dominant basalt variety is olivine-plagioclase basalt, although olivine-plagioclase-clinopyroxene basalt is relatively common at some localities. Selected samples have been analysed for major and trace elements, rare earth elements and Sr and Nd isotopes. These data show that the majority of samples are slightly evolved (Mg#=69-35) N-type MORB, although a small group of samples from a number of localities have ‘enriched’ geochemical characteristics (T- and P-type MORB).These different types of MORB are readily distinguished in terms of their incompatible trace element and isotopic characteristics: N-type MORB have high Zr/Nb (17–78), Y/Nb (4.6–23) and 143Nd/144Nd (0.51303–0.51308) ratios, low Zr/Y (2.2–4.2) and 87Sr/86Sr (0.70263–0.70295) ratios and have (La/Sm)N<1.0; T-type MORB have lower than chondritic Zr/Nb ratios (8.8–15.5), relatively low Y/Nb (1.9–4.3) and 143Nd/144Nd (0.51296–0.51288) ratios and relatively high Zr/Y (3.1–4.7), 87Sr/86Sr (0.70307–0.70334) and (La/Sm)N (1.1–1.5) ratios; the single sample of P-type MORB has low Zr/Nb (6.3), Y/Nb (0.9) and 143Nd/144Nd (0.51287) ratios and high Zr/Y (7.1), 87Sr/86Sr (0.70351) and (La/Sm)N (2.4) ratios. The geochemical characteristics of this sample are essentially identical to those of the Bouvet Island lavas.Geochemically ‘enriched’ MORB are less abundant on the American-Antarctic Ridge than on the Southwest Indian Ridge but their geochemical characteristics are identical. The compositions of T- and P-type MORB are consistent with a regional mixing model involving normal depleted mantle and Bouvet plume type magma. On a local scale the composition of T-type MORB is consistent with derivation from depleted mantle which contains ∼4% veins of P-type melt.We propose a model for the evolution of the American-Antarctic Ridge lavas in which N-type MORB is derived from mantle with negligible to low vein/mantle ratios, T-type MORB is derived from domains with moderate and variable vein/mantle ratios and P-type MORB from regions with very high vein/mantle ratios where vein material comprises the major portion of the melt. The sparse occurrence of ‘enriched’ lavas and by implication ‘enriched’ mantle beneath the American-Antarctic Ridge, some distance (500–1,200 km) from the Bouvet plume location, is interpreted to be the result of lateral dispersion of enriched mantle domains by asthenospheric flow away from the Bouvet mantle plume towards the American-Antarctic Ridge.

Helium: problematic primordial signals

Various lines of evidence suggest that pre-eruptive degassing of basalts is not only important but may dominate the He flux from the mantle to the oceans and atmosphere. These include: (1) correlations between U/4He ratios and Mg# in volcanic suites, which suggest that U/4He increases as differentiation proceeds; (2) the observation that U/4He ratios in MORB average about a factor of 400 lower than those from Loihi Seamount, although3He/4He ratios suggest that Loihi is derived from a source with a lower U/4He ratio than is appropriate for the MORB source; and (3) the3He budget of the oceans which, as defined by Craig and Lupton [6], requires that MORB is on average 70–90% outgassed in He. These observations suggest, as noted in [1], that mantle3He/4He ratios may be subject to perturbation due to radiogenic accumulation of4He in systems where U/4He has been increased by pre-eruptive degassing. n nModels involving continuous diffusive loss (CDL) of He from a magma chamber and exhalative loss (EL) via solution of He in a CO2-rich gas or fluid phase are investigated. Low U/4He ratios in MORBs preclude significant pre-eruptive reduction of3He/4He ratios; however, OIBs with U/4He > 106 may be subject to significant reduction of3He/4He (⩾ 10%) with pre-eruptive aging over time periods ranging from 1 × 103 to 5 × 105 years.

Osmium–oxygen isotopic evidence for a recycled and strongly depleted component in the Iceland mantle plume

Abstract Highly magnesian lavas characterised by strong light rare earth element depletion are a feature of Theistareykir and the Reykjanes Peninsula of Iceland, which are marginal to the proposed axis of the mantle plume. These lavas define positive covariations between whole rock osmium and olivine oxygen isotope ratios ( 187 Os/ 188 Os=0.1269–0.1369; δ 18 O olivine =4.2–5.7‰) that extend the array defined by Hawaiian samples to more unradiogenic Os isotope ratios and lower δ 18 O. The Os–O variation is difficult to explain in terms of high level crustal assimilation of Icelandic crust, with the possible exception of a subset of large volume lava flows from Theistareykir. The strong coupling of Os and O isotopic compositions of the lavas in addition to large excesses in large ion lithophile elements (Rb, Ba, Sr), positive Eu anomalies, and deficiencies in Hf and Zr relative to the rare earth elements clearly distinguishes these recent picrites from mid-ocean ridge basalts. The Reykjanes and Theistareykir lavas appear to represent melting of a very ancient (Archaean) mantle source which has isotopic and elemental characteristics suggestive of recycled oceanic lithosphere. We suggest that tapping of the refractory and depleted part of such a mantle plume (i.e. low 187 Os/ 188 Os and δ 18 O) is only possible due to the fortuitous location of the Iceland plume beneath a spreading ridge, which permits more extensive melting than would occur in an intraplate setting (e.g. Hawaii).

Geochemical characteristics of basaltic glasses from theamar andfamous axial valleys, Mid-Atlantic Ridge (36°–37°N): Petrogenetic implications

The axial valley of the Mid-Atlantic Ridge from 36° to 37°N was intensively sampled by submersible during thefamous andamar projects. Our research focussed on the compositional and isotopic characteristics of basaltic glasses from theamar valley and thenarrowgate region of thefamous valley. These basaltic glasses are characterized by: (1) major element abundance trends that are consistent with control by multiphase fractionation (olivine, plagioclase and clinopyroxene) and magma mixing, (2) near isotopic homogeneityδ18O= 5.2to6.4,87Sr/86Sr= 0.70288to0.70299 and206Pb/204Pb= 18.57to18.84, and (3) a wide range of incompatible element abundance ratios; e.g., within theamar valley chondrite-normalizedLa/Sm ranges from 0.7 to 1.5 andLa/Yb from 0.6 to 1.6. These ratios increase with decreasing MgO content. Because of the limited variations in isotopic ratios of Sr, Nd and Pb, it is plausible that these compositional variations reflect post-melting magmatic processes. However, it is not possible to explain the correlated variation in MgO content and incompatible element abundance ratios, such asLa/Sm andZr/Nb, by fractional crystallization or more complex processes such as boundary layer fractionation. These geochemical trends can be explained by mixing of parental magmas that formed by very different extents of melting. In particular, the factor of three variation in Ce content in samples with ∼ 2.1% Na2O and 8% MgO requires a component derived by < 1% melting. If the large variations in abundance ratios of incompatible elements reflect the melting process, a large, long-lived magma chamber was not present during eruption of theseamar lavas. The geological characteristics of theamar valley and the compositions ofamar lavas are consistent with episodic volcanism; i.e., periods of magma eruption were followed by extensive periods of tectonism with little or no magmatism.

He and Ne isotopes in oceanic crust: implications for noble gas recycling in the mantle

Abstract In an attempt to determine the helium and neon isotopic composition of the lower oceanic crust, we report new noble gas measurements on 11 million year old gabbros from Ocean Drilling Program site 735B in the Indian Ocean. The nine whole rock samples analyzed came from 20 to 500 m depth below the seafloor. Helium contents vary from 3.3×10−10 to 2.5×10−7 ccSTP/g by crushing and from 5.4×10−8 to 2.4×10−7 ccSTP/g by melting. 3He/4He ratios vary between 2.2 and 8.6 Ra by crushing and between 2.9 and 8.2 by melting. The highest R/Ra ratios are similar to the mean mid-ocean ridge basalt (MORB) ratio of 8±1. The lower values are attributed to radiogenic helium from in situ α-particle production during uranium and thorium decay. Neon isotopic ratios are similar to atmospheric ratios, reflecting a significant seawater circulation in the upper 500 m of exposed crust at this site. MORB-like neon, with elevated 20Ne/22Ne and 21Ne/22Ne ratios, was found in some high temperature steps of heating experiments, but with very small anomalies compared to air. These first results from the lower oceanic crust indicate that subducted lower oceanic crust has an atmospheric 20Ne/22Ne ratio. Most of this neon must be removed during the subduction process, if the ocean crust is to be recirculated in the upper mantle, otherwise this atmospheric neon will overwhelm the upper mantle neon budget. Similarly, the high (U+Th)/3He ratio of these crustal gabbros will generate very radiogenic 4He/3He ratios on a 100 Ma time scale, so lower oceanic crust cannot be recycled into either MORB or oceanic island basalt without some form of processing.

Tectonics, alteration and the fractal distribution of hydrothermal veins in the lower ocean crust

Abstract Gabbros from Hole 735B preserve the igneous, structural and metamorphic history beneath the Southwest Indian Ridge down to middle amphibolite facies conditions. The gabbro was isolated from the zone of lithospheric necking by detachment faulting, unroofing and block uplift at the inside-corner high of the Atlantis II Transform. Lower crustal accretion, as preserved in the core, is a complex integration of igneous, hydrothermal and tectonic processes. Alteration began at anhydrous granulite conditions as surface faulting penetrated the brittle-ductile transition to form brittle-ductile shear zones. In the amphibolite facies, hydrous alteration around these zones was extensive, while undeformed sections remained virtually unaltered. Amphibole veins formed during the brittle-ductile deformation have a high fractal dimension, reflecting an unclustered distribution, consistent with high strain and cracking rates in the zone of lithospheric necking beneath the ridge. However, the fractal dimensions of the two major gneissic amphibolite zones in Hole 735B are different and suggest that they represent discrete fault zones formed at different times. Below middle amphibolite facies a dramatic drop in the extent of alteration reflects cooling, cracking and alteration under static conditions, similar to layered intrusions. Initially, fracture and vein formation exploited undeformed sections, where elastic strain accumulated during cooling from high temperatures was unrelieved by recrystallization. Plagioclase-diopside veins formed at this time have the same orientation as higher temperature veins, but a mineralogy reflecting much lower rock permeabilities, and a low fractal dimension, reflecting a tight clustering of veins, as would be anticipated for crack formation at low strain rates. Late zeolite and carbonate veins and irregular, smectite-lined, near-vertical cracks also have a very low fractal dimension, reflecting clustering appropriate to low strain, near static conditions. These, however, show no preference for undeformed sections of the core and are oriented with subvertical and subhorizontal maxima. This indicates crack formation under the present day stress field. Thus, elastic strain, due to cooling from high temperatures and extension during lithospheric necking, dissipated after plagioclase-diopside vein formation.

Melt Origin across a Rifted Continental Margin: a Case for Subduction-related Metasomatic Agents in the Lithospheric Source of Alkaline Basalt, NW Ross Sea, Antarctica

&NA; Alkaline magmatism associated with the West Antarctic rift system in the NW Ross Sea (NWRS) includes a north‐south chain of shield volcano complexes extending 260 km along the coast of Northern Victoria Land (NVL), numerous small volcanic seamounts located on the continental shelf and hundreds more within an ˜35 000 km2 area of the oceanic Adare Basin. New 40Ar/39Ar age dating and geochemistry confirm that the seamounts are of Pliocene‐Pleistocene age and petrogenetically akin to the mostly middle to late Miocene volcanism on the continent, as well as to a much broader region of diffuse alkaline volcanism that encompasses areas of West Antarctica, Zealandia and eastern Australia. All of these continental regions were contiguous prior to the late‐stage breakup of Gondwana at ˜100 Ma, suggesting that the magmatism is interrelated, yet the mantle source and cause of melting remain controversial. The NWRS provides a rare opportunity to study cogenetic volcanism across the transition from continent to ocean and consequently offers a unique perspective from which to evaluate mantle processes and the roles of lithospheric and sub‐lithospheric sources for mafic alkaline magmas. Mafic alkaline magmas with > 6 wt % MgO (alkali basalt, basanite, hawaiite, and tephrite) erupted across the transition from continent to ocean in the NWRS show a remarkable systematic increase in silica‐undersaturation, P2O5, Sr, Zr, Nb and light rare earth element (LREE) concentrations, as well as LREE/HREE (heavy REE) and Nb/Y ratios. Radiogenic isotopes also vary, with Nd and Pb isotopic compositions increasing and Sr isotopic compositions decreasing oceanward. These variations cannot be explained by shallow‐level crustal contamination or by changes in the degree of mantle partial melting, but are considered to be a function of the thickness and age of the mantle lithosphere. We propose that the isotopic signature of the most silica‐undersaturated and incompatible element enriched basalts best represent the composition of the sub‐lithospheric magma source with low 87Sr/86Sr (≤0·7030) and &dgr;18Oolivine (≤5·0‰), and high 143Nd/144Nd (˜0·5130) and 206Pb/204Pb (≥20). The isotopic ‘endmember’ signature of the sub‐lithospheric source is derived from recycled subducted materials and was transferred to the lithospheric mantle by small‐degree melts (carbonate‐rich silicate liquids) to form amphibole‐rich metasomes. Later melting of the metasomes produced silica‐undersaturated liquids that reacted with the surrounding peridotite. This reaction occurred to a greater extent as the melt traversed through thicker and older lithosphere continentward. Ancient and/or more recent (˜550–100 Ma) subduction along the Pan‐Pacific margin of Gondwana supplied the recycled subduction‐related material to the asthenosphere. Melting and carbonate metasomatism were triggered during major episodes of extension beginning in the Late Cretaceous, but alkaline magmatism was very limited in its extent. A significant delay of ˜30 to 20 Myr between extension and magmatism was probably controlled by conductive heating and the rate of thermal migration at the base of the lithosphere. Heating was facilitated by regional mantle upwelling, possibly driven by slab detachment and sinking into the lower mantle and/or by edge‐driven mantle flow established at the boundary between the thinned lithosphere of the West Antarctic rift and the thick East Antarctic craton.