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We present new constraints on the opening of the South Atlantic Ocean from a joint interpretation of marine magnetic anomaly grids and forward modelling of conjugate profiles. We use 45,000 km of recently collected commercial ship track data combined with 561,000 km of publically available data. The new data cover the critical ocean–continental transition zones and allow us to identify and downgrade some poorly navigated older ship tracks relied upon in earlier compilations. Within the final grids the mean cross-over error is 14 nT computed from 8,227 ship track intersections. The forward modelling used uniformly magnetised bodies whose shapes were constrained from coincident deep-seismic reflection data. We find the oldest magnetic anomalies to date from M10r (134.2 Ma, late Valanginian) north of the Falkland-Agulhas Fracture Zone and M3 (129.3 Ma, Barremian) south of the Rio Grande Fracture Zone. Hence, assuming the GPTS used is correct, continental breakup was contemporaneous with the Parana and Etendeka continental flood basalts. Many of the landward linear anomalies overlap seismically mapped Seaward Dipping Reflectors (SDRs). We interpret this to mean that a significant portion of the SDRs overlay crust formed by subaerial seafloor spreading. Here crustal accretion is envisaged to be similar to that at mid-ocean ridges, but sheet lava flows (that later form the SDRs) rather than pillow basalts form the extrusive component. Segmentation of the linear anomalies generated implies that this stage of continental breakup is organised and parallels the seafloor spreading centre that follows. Our results call into question the common assumption that at volcanic continental margins the first linear magnetic anomalies represent the start of conventional (submarine) oceanic crustal generation.

Earth's tectonic and climatic systems may have changed fundamentally before the Great Oxidation Event (GOE) at about 2.3 Ga. Sulfur Mass Independent Fractionation (S-MIF) has demonstrated that Earth's atmosphere was virtually oxygen-free before the GOE. During 3.0 to 2.4 Ga, the change in Δ33S and Δ36S signals may reflect the perturbation of atmospheric chemistry, though the mechanisms of the change are uncertain. Here, we reported multiple sulfur isotopic studies of Archean volcano-sedimentary sequences of the Dharwar Supergroup, distributed in the Chitradurga Schist Belt (CSB), Southern India. New field mapping and zircon U–Pb dating allows us to reconstruct detailed lithostratigraphy of the Dharwar Supergroup. The lower unit consists of post-3.0 Ga conglomerate, stromatolitic carbonate, siliciclastics with diamictite, chert/BIF and pillowed basalt in ascending order, all of which are older than the 2676 Ma dacite dyke that had intruded into the lower unit. The upper unit unconformably overlies the pillow basalts at the top of the lower unit, and consists of conglomerate/sandstone with ∼2600 Ma detrital zircons, komatiitic basalt, BIF and siliciclastic sequence with mafic volcanics. Sulfur isotope analysis of extracted sulfides shows MIF signals ( Δ S 33 > + 1 ‰ ) with clear Δ S 33 – Δ S 36 correlations. The lower group of the Dharwar Supergroup shows a Δ S 36 / Δ S 33 slope of −1.48, the middle group shows −1.16 and −1.07, and the upper group shows −0.94. Reassessment of all the Archean S-MIF records from sedimentary rocks indicates that the Δ S 36 / Δ S 33 slope systematically changed during the Archean period. The observed trend in the Indian section is similar to those of its Pilbara–Kaapvaal equivalents, thus it could reflect a global atmospheric signature. Moreover, the isotopic trend seems to correlate with mid-Archean glaciation. Thus, the Δ S 36 / Δ S 33 slope could be a useful tracer for atmospheric chemistry and its link with climate change before the GOE.

The changes that occur at the boundary between the Archean and Proterozoic eons are arguably the most fundamental to affect the evolution of Earth's continental crust. The principal component of Archean continental crust is Granite–Greenstone Terranes (GGTs), with granites always dominant. The greenstones consist of a lower sequence of submarine komatiites and basalts, which erupted onto a pre-existing Tonalite–Trondhjemite–Granodiorite (TTG) crust. These basaltic rocks pass upwards initially into evolved volcanic rocks, such as andesites and dacites and, subsequently, into reworked felsic pyroclastic material and immature sediments. This transition coincides with widespread emplacement of granitoids, which stabilised (cratonised) the continental crust. Proterozoic supra-crustal rocks, on the other hand, are dominated by extensive flat-lying platform sequences of mature sediments, which were deposited on stable cratonic basements, with basaltic rocks appreciably less abundant. The siliceous TTGs cannot be produced by direct melting of the mantle, with most hypotheses for their origin requiring them to be underlain by a complimentary dense amphibole–garnet–pyroxenite root, which we suggest acted as ballast to the early continents. Ubiquitous continental pillow basalts in Archean lower greenstone sequences require the early continental crust to have been sub-marine, whereas the appearance of abundant clastic sediments, at higher stratigraphic levels, shows that it had emerged above sea level by the time of sedimentation. We hypothesise that the production of komatiites and associated basalts, the rise of the continental crust, widespread melting of the continental crust, the onset of sedimentation and subsequent cratonisation form a continuum that is the direct result of removal of the continent's dense amphibole–garnet–pyroxenite roots, triggered at a regional scale by the arrival of a mantle plume at the base of the lithosphere. Our idealised calculations suggest that the removal of 40 km of the amphibole–garnet–pyroxenite root would have raised the average level of the continental crust by ∼3 km. The emergence of the continental crust was an essential precursor to the rise of oxygen, which started some 200 Myr later.

This study presents the first multiple sulfur isotope dataset on sulfides from the ca. 3.5–3.2 Ga Onverwacht Group in the Paleoarchean Barberton Greenstone Belt (BGB) of South Africa. In situ δ 34 SCDT and Δ 33 S values of pyrite ( n = 568 ) are reported from a wide range of hydrothermal, volcanic and sedimentary environments and are used to explore Mid-Archean biogeochemical sulfur cycling. Samples are from fresh drill core collected by the Barberton Scientific Drilling Project that intercepted cherts, metabasalts and sheared ultramafics of the ∼ 3.3 – 3.35 Ga Kromberg Formation; the sedimentary units of the ∼ 3.432 Ga Noisy formation; and the unconformably underlying metabasaltic pillow lavas of the ∼ 3.472 Ga Hooggenoeg Formation. Pyrite in quartz-carbonate-veins in the lower diamictite of the Noisy sequence records the largest range and most negative δ 34 SCDT values so far reported from an Archean terrain ( δ 34 S CDT = − 55.04 to + 27.46 ‰ ). The Noisy sediments also contain detrital and diagenetic pyrites with a significant variation in Δ 33 S of between −0.62 to + 1.4 ‰ and δ 34 SCDT of between −7.00 and + 12.6 ‰ , interpreted to reflect tectonic exposure of these early sediments to atmospheric – shallow marine conditions. Early marine pyrites from the Kromberg Cherts also display strong positive Δ 33 S values up to + 2.50 ‰ with narrow range in δ 34 SCDT values (−6.00 to + 1.50 ‰ ), whereas hydrothermal veins in the basal ultramafic shear zones preserve magmatic values ( ∼ 0 ‰ ). This study reveals a potential proto-tectonic control on atmospheric, geodynamic and hydrothermal environments available for early sulfate reducing and/or methanogenic microbes in the Paleoarchean. No evidence for microbial sulfate reduction or disproportionation was identified in the Kromberg Cherts, despite previous morphological claims for microbial life. Highly variable and negative δ 34 SCDT values were found in the Noisy turbidites and Hooggenoeg pillow lava breccia supporting the presence of microbial sulfate reduction in early tectono-sedimentary basins and in the Paleoarchean sub-seafloor, respectively. In light of current controversies surrounding sulfur isotope studies in similar-aged rocks of the Pilbara Craton (West Australia), we argue that microbial elemental sulfur disproportionation was not a preferred metabolic pathway on the Paleoarchean earth.

No young, off-axis, mid-ocean ridge lavas have yet been directly linked to underlying off-axis melt bodies. In this study, we present new measurements of 238 U– 230 Th– 226 Ra– 210 Pb isotope compositions for a suite of lavas from the overlapping spreading center (OSC) at 9°03′N on the East Pacific Rise (EPR). These lavas span a large range of compositions, from basalt to dacite, and include both axial and off-axis samples recovered from a prominent, axis-parallel pillow ridge and a flat-topped seamount that overlie the westernmost extent of a 4-km-wide melt lens ( Kent et al., 2000 ). We report 210 Pb excesses in axial basalts and basaltic andesites, which we suggest results from gas-magma fractionation of 222 Rn from 226 Ra beneath dacite magmas. In addition, our U-series ages agree with visual observations, indicating that while most recent volcanic activity occurs at the spreading axis, active volcanism also occurs away from the axis. Specifically, the off-axis pillow ridge and seamount samples overlying the off-axis subsurface melt body have eruption ages of less than 8 ka, and likely as young as 1 ka, despite being located on crust that has a spreading age of ~75 ka. The young ages of these lavas, combined with existing geological, geochemical and geophysical constraints, provide evidence for a genetic link between the pillow ridge and seamount lavas and the seismically imaged, underlying off-axis melt lens. This link demonstrates that off-axis volcanism does not necessarily come from a sub-axial magma body and can be sourced directly from off-axis magma bodies.

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A dipping low-velocity zone (LVZ) is a ubiquitous structural element of subduction zones worldwide. In this study we map seismic attributes characterizing the LVZ beneath the Cascadia subduction zone from northern Vancouver Island to northern California using receiver function waveform inversion. Throughout this region, the LVZ is characterized by high V P / V S ratios (mean=2.77), strong S-velocity contrasts ( ∼ 50%) and thicknesses averaging 3.37 km. The LVZ is immediately underlain by a second, weaker layer exhibiting moderate V P / V S ratios (mean=1.85) with mean thickness of 4.62 km. We interpret the combined structure in terms of subducting oceanic crust, based on classical structural/petrological descriptions and constraints from previous studies of ophiolites and ocean drill cores. The LVZ is identified with pervasively hydrated, high porosity pillow basalts and sheeted dikes of Layer 2 with possible contributions from sediments (Layer 1). Fluids released from metamorphic dehydration reactions are maintained near lithosphere fluid pressures through an impermeable plate boundary above, and a low porosity, gabbroic/mafic-cumulate dominated Layer 3 below.

The earliest subaerially exposed magmatic products of the Fiji–Tonga–Kermadec (FTK) arc are preserved in the Yavuna Group of Viti Levu, Fiji, and cobbles from ‘Eua, Tonga. They are similar in age and magma types to the earliest rocks of the Izu–Bonin–Mariana (IBM) arc. In Fiji they include typical island arc tholeiitic (IAT), boninitic (BON), and MORB-like early arc tholeiitic (EAT) pillow lavas that are interpreted as products of flux- and decompression-melting which occurred simultaneously during subduction initiation. Although the oldest rocks in the southwest Pacific (FTK) and the northwest Pacific (IBM) arcs are generally similar, they differ in two important respects. First, all magma types erupted simultaneously in the SW Pacific whereas a similar assemblage may have erupted sequentially in IBM. Second, the primary mantle wedge was “Pacific” in isotopic character in the SW Pacific, but “Indian” in the NW Pacific.

The Bonin Ridge and trench slope preserves the geological record of subduction initiation and subsequent evolution of the Izu–Bonin–Mariana (IBM) arc. Diving and dredging in this region has revealed a bottom to top stratigraphy of: 1) mantle peridotite, 2) gabbroic rocks, 3) a sheeted dyke complex, 4) basaltic pillow lavas, 5) boninites and magnesian andesites, 6) tholeiites and calcalkaline arc lavas. This forearc stratigraphy is remarkably similar to that found in other IBM forearc localities and many ophiolites. Zircon U–Pb ages obtained here for gabbros are 51.6–51.7 Ma. The overlying basalts have 40Ar/39Ar ages of 48–52 Ma. A forearc basalt from the Mariana forearc near Guam produced a similar 40Ar/39Ar age of 51.1 Ma. The collective geochronology of igneous rocks from throughout the IBM system now indicates that the first basaltic magmatism at subduction initiation was produced by decompression melting of the mantle and took place at 51–52 Ma. The change to flux melting and boninitic volcanism took 2–4 m.y., and the change to flux melting in counterflowing mantle and “Normal” arc magmatism took 7–8 m.y. This evolution from subduction initiation to arc normalcy occurred nearly simultaneously along the entire length of the IBM subduction system. Mesozoic rocks found in the deep Bonin forearc suggest that the overriding plate at subduction initiation consisted of Mesozoic terranes and subduction preceded the opening of most or all of the Philippine Sea basins. The contemporaneousness of IBM forearc magmatism with the major change in plate motion in Western Pacific at ca. 50 Ma suggests that the two events are intimately linked.

The Chivillas Formation is the easternmost record of Mesozoic marine volcanism in Mexico. It consists of thick intervals of pillow lavas interbedded with siliciclastic turbidites, and debrites, containing clasts derived from metamorphic, sedimentary and volcanic sources. Clast composition and detrital zircon geochronology indicate a continental provenance, with sources located south of the studied rocks. Detrital zircon ages range from 1573 ± 60 to 125 ± 1.6 Ma. The probability curves have peaks at 124 to 130 Ma (mean 126 Ma); 188 Ma and 921–1236 Ma (the latter with peaks at ∼ 1022 and ∼ 1157 Ma). Subordinate peaks at 277, 419 and 535 Ma are also present. We interpret the youngest zircon population ∼ 126 Ma (Barremian), as the maximum depositional age. Other peaks suggest Grenvillian-type basement and Permo-Triassic arc sources. Late Jurassic detrital zircons were probably derived from the Sierra de Juarez Mylonitic belt. Pillow lavas are mostly alkaline basalts, with SiO2 46%–53%, and alkali oxide (K2O + Na2O) 5–8 wt.%; all samples have low-TiO2 (

Ever since the discovery that the mid-ocean ridge axis deepens at most axial discontinuities, there has been vigorous debate about whether ridge axis depth and shape reflect magma supply. Here we report evidence, from one of the most thoroughly studied ridge discontinuities on a fast-spreading mid-ocean ridge, that volcanic eruption effusion rates have been low relative to nearby segment centers throughout its two million year history. This pattern of low volcanic effusion rates is inferred from the fields of constructional volcanic mounds on the seafloor which are much more abundant in the off-axis discordant zone of the East Pacific Rise (EPR) 9°N Overlapping Spreading Centers (OSC) than on typical EPR seafloor. Detailed seafloor mapping in 2007 shows that these mound fields consist of concentrations of large pillow mounds that formed within the current OSC basin and ridge tips, rather than off-axis. Pillow lava, erupted at approximately an order of magnitude lower effusion rate than lobate or sheet flows, primarily builds mounds on the seafloor. Crustal thickness, a proxy for long-term magma supply, does not decrease in spite of the decreased lava effusion rates implied by widespread pillow eruptions. To resolve the problem of achieving the same magma emplacement rate with lower effusion rates, without violating our observations by requiring drastically increased eruption frequency or unreasonably long eruption durations, we propose that eruptions occur across the entire OSC basin. The current configuration of mid-crustal melt sills within the OSC as resolved by multichannel seismic reflection surveys have neither the correct location nor appropriate size to deliver magma to these eruptive vents in the OSC basin via vertically propagating dikes. Instead, we propose that magma delivery to eruptions occurs via laterally propagating dikes from robust mid-crustal melt sills outside of the overlap zone.

Transitions between effusive and explosive styles of lava dome eruptions are likely accompanied by changes in lava rheology. The common presence of crystals in dome lavas produces a complex non-Newtonian rheology. Thus models of such complex rheology are essential for volcanic eruption models. Here, we have measured the rheology of natural Unzen lavas with a compressive uniaxial press operating at stresses between 1 and 70 MPa and temperatures between 940 and 1010 °C. Crystal-rich Unzen lavas are characterised by two essential rheological features which produce non-Newtonian effects. The first is an instantaneous response of the apparent viscosity to applied stress which requires that the magma be described as a visco-elastic fluid that exhibits shear-thinning. The second effect takes the form of a time-dependence of the viscosity at moderate to high stress (≥ 10 MPa). In this regime, the apparent viscosity slowly decreases as increasing fracturing of the phenocrysts and the groundmass occurs. Fragmentation of crystals and alignment of crystal fragments are observed to produce flow banding-effects which in turn lower the apparent viscosity of natural dome lavas. Ultimately, deformation may lead to complete rupture of the lava if the stress is sufficient. Cracking thus stands as an important process in natural dome lava rheology. The ubiquitous non-Newtonian rheology of dome lavas, observed experimentally here, needs to be adequately treated in order to generate appropriate eruption models.

In Science May 11 2001 (p.1142–1145), Kusky et al. reported an ophiolite complex of late Archean age in the Dongwanzi area of the North China Craton, which has subsequently been considered a hallmark for Archean plate tectonics. However, this interpretation has been questioned because no convincing pillow lavas or sheeted dikes have been found in the Dongwanzi complex and, more importantly, as a major part of the ophiolite complex, the Dongwanzi ultramafic–mafic body has not previously been dated and it therefore remains unknown whether it is of Archean age. In this study, we carried out detailed field investigations and SHRIMP U–Pb zircon dating on the rocks of the Dongwanzi ultramafic–mafic body, as recognized by Kusky et al. Our field observations reveal that in several places along the boundaries of the Dongwanzi body, mafic dikes emanating from the main body intrude the country rocks of the unmetamorphosed Mesoproterozoic sedimentary successions of the Changcheng–Jixian ‘System’. One of these dikes yielded a SHRIMP U–Pb zircon age of 306 ± 6 Ma, similar to the SHRIMP U–Pb zircon age of 308 ± 4 Ma obtained from a plagioclase-bearing pyroxenite from the Dongwanzi ultramafic–mafic body. Four other samples collected from various parts of the ultramafic–mafic body (gabbro, leucogabbro or pyroxenite) record similar SHRIMP U–Pb zircon ages around ∼ 300 Ma. Therefore, both field relationships and SHRIMP U–Pb zircon dating results unequivocally demonstrate that the Dongwanzi ultramafic–mafic body is not part of an Archean ophiolite.

Exceptionally well-preserved pillow lavas and inter-pillow hyaloclastites from the Barberton Greenstone Belt in South Africa contain textural, geochemical, and isotopic biomarkers indicative of microbially mediated alteration of basaltic glass in the Archean. The textures are micrometer-scale tubular structures interpreted to have originally formed during microbial etching of glass along fractures. Textures of similar size, morphology, and distribution have been attributed to microbial activity and are commonly observed in the glassy margins of pillow lavas from in situ oceanic crust and young ophiolites. The tubes from the Barberton Greenstone Belt were preserved by precipitation of fine-grained titanite during greenschist facies metamorphism associated with seafloor hydrothermal alteration. The presence of organic carbon along the margins of the tubes and low d 13 C values of bulk-rock carbonate in formerly glassy samples support a biogenic origin for the tubes. Overprinting relationships of secondary minerals observed in thin section indicate the tubular structures are pre-metamorphic. Overlapping metamorphic and igneous crystallization ages thus imply the microbes colonized these rocks 3.4–3.5 Ga. Although, the search for traces of early life on Earth has recently intensified, research has largely been confined to sedimentary rocks. Subaqueous volcanic rocks represent a new geological setting in the search for early life that may preserve a largely unexplored Archean biomass. D 2005 Elsevier B.V. All rights reserved.

The Annieopsquotch ophiolite exposes a c. 5.5-km-thick section of tholeiitic gabbros, sheeted dykes and pillow basalts. Based on the along-strike consistency in thickness of the major crustal units, and lack of significant throw on spreading-related normal faults, the Annieopsquotch ophiolite is interpreted to have formed at an intermediate- to fast-spreading ridge. The upper c. 400 m of the gabbro zone is composed of massive, texturally heterogeneous gabbros with compositions that approach those of the sheeted dykes and basalts. Below this is c. 1.6 km of 10–30-m-thick planar intrusive sheets or sills. The lowermost part of the gabbro zone is composed of gabbroic rocks with relict troctolite and troctolitic gabbro enclaves, which are veined and partly replaced by gabbro and pyroxenite. Sill contacts within the central, sill-dominated part of the gabbro zone are sub-parallel to boundaries between the major ophiolite lithostratigraphic units. The upper and lower contacts of individual gabbro sills may have finer grain sizes, indicating that the intrusions cooled from the top and bottom. Locally well-preserved comb structures (crescumulates) indicate downward growth, supporting a sill interpretation. The sills are composed of weakly or un-deformed plagioclase+clinopyroxene ±olivine cumulates. Trace element modeling suggests that the parental magmas of these cumulates had compositions very similar to the overlying sheeted dykes and basaltic lavas, as do dykes emplaced within the gabbro zone. Model liquids calculated from the gabbroic sills generally become more evolved up-section, indicating that magma evolution in the Annieopsquotch ophiolite was dominated by fractionation in lower crustal conduits, below the level of a putative axial melt lens (AML). The model liquids, sheeted dykes and basalts preserve a similar, wide range of compositions, which may indicate that aggregation, homogenization and fractionation in an AML was inefficient. Similar intra-conduit fractionation of mantle-derived melts might also contribute to MORB evolution at mid-ocean ridges.

Self-reversal, the remarkable ability of some rocks to acquire a magnetization antiparallel to the Earth's magnetic field, was of great interest in the 1950s and 1960s, but it was subsequently found to be rare, limited to a few hemoilmenite-bearing igneous rocks. Here we show through thermal demagnetization experiments that pillow basalts from the northwestern Pacific Ocean lacking hemoilmenite contain two nearly antiparallel components of natural remanent magnetization. Partial thermoremanent magnetization experiments indicate that the component isolated at lower temperatures is a self-reversal of the primary magnetization. The self-reversal is carried by titanomaghemite formed by low temperature oxidation. Because maghemitization of oceanic crust is ubiquitous, self-reversals may be an important factor contributing to the decrease with age in marine magnetic anomaly amplitudes observed for crust formed during the last 30 million years.

We present the results of a detailed petrological study of a sparsely phyric basalt (MAPCO CH98-DR11) dredged along the Mid-Atlantic Ridge (30°41′N). The sample contains microphenocrysts of olivine that display four different rapid-growth morphologies. Comparison of these morphologies with those obtained in dynamic crystallization experiments allows us to constrain the thermal history of the sample. The dendritic morphology (swallowtail, chain and lattice olivine) is directly related to the final quenching during magma–seawater interaction. In contrast, the three other morphologies, namely the complex polyhedral crystal, the closed hopper and the complex swallowtail morphology result from several cycles of cooling–heating (corresponding to a maximum degree of undercooling of 20–25°C) during crystal growth. These thermal variations occurred before eruption and are interpreted to be the result of turbulent convection in a small magmatic body beneath the ridge. The results suggest that the Mid-Atlantic Ridge is underlain by a mush zone that releases batches of liquid during tectonic segregation. Aphyric basalts are emitted during eruptions controlled by the tectonic activity, whereas phyric basalts correspond to small fractions of magma from the mush zone mobilized by reinjections of primitive magmas.

Hysteresis measurements have been carried out on a suite of ocean-floor basalts with ages ranging from Quaternary to Cretaceous. Approximately linear, yet separate, relationships between coercivity (Bc) and the ratio of saturation remanence/saturation magnetization (Mrs/Ms) are observed for massive doleritic basalts with low-Ti magnetite and for pillow basalts with multi-domain titanomagnetites (with x=0.6). Even when the MORB has undergone low-temperature oxidation resulting in titanomaghemite, the parameters are still distinguishable, although offset from the trend for unoxidized multidomain titanomagnetite. The parameters for these iron oxides with different titanium content reveal contrasting trends that can be explained by the different saturation magnetizations of the mineral types. This plot provides a previously underutilized and non-destructive method to detect the presence of low-titanium magnetite in igneous rocks, notably MORB.

Palaeomagnetic results from 27 sites at five localities within the dismembered Bae «r^Bassit ophiolite of northern Syria are presented. The ophiolite forms part of a series of thrust sheets emplaced over Mesozoic carbonates of the Arabian platform in the middle Maastrichtian. A positive inclination-only area-wide tilt test applied to four locality mean remanences and positive fold and reversal tests from palaeohorizontal units (pillow lavas, lava flows) within one of these localities indicate that the ophiolite preserves pre-deformation magnetisations. Variable directions of remanence between localities demonstrate that the ophiolite has experienced extreme relative anticlockwise rotations on a kilometric scale. Within the most extensively sampled ophiolite massif (Bassit sheet) there is a progressive increase in rotation from north to south. The southernmost units at the lowest structural level in the imbricate thrust stack record the highest rotation (exceeding 200‡). Although tectonic rotation during imbricate thrusting has been reported in a number of orogenic belts, the pattern of rotations in the Bassit sheet is difficult to explain by differential thrust sheet rotation. Instead, regional comparisons with the Hatay ophiolite of southern Turkey and the Troodos ophiolite of Cyprus suggest that a significant component of rotation may be ascribed to intraoceanic deformation of a coherent region of oceanic crust within the southern Neotethyan basin prior to ophiolite emplacement. The partially rotated Bae «r^Bassit ophiolite was then emplaced and structurally dismembered by thrust faulting. During the Late Tertiary the ophiolitic units were further rotated during the initiation and development of a major sinistral strike-slip fault zone, linking the Cyprus subduction zone to the Dead Sea Transform system. The extreme rotations observed in the study are therefore of composite origin, and reflect the complex development of structural fabrics within the ophiolite. < 2002 Elsevier Science B.V. All rights reserved.

Water contents have been measured in basaltic glasses from submarine and subglacial eruption sites along the Reykjanes Ridge and Iceland, respectively, in order to evaluate the hypothesis of Schilling et al. [Phil. Trans. R. Soc. London A 56 (1980) 147–178] that hot spots are also wet spots. Having erupted under pressure the water contents measured in these samples are potentially unaffected by degassing. After correcting these water contents for the effects of crystallisation (to give H2O(8) values) they indicate that the concentration of water in the source regions increases from 165 ppm at the southern end of the Reykjanes Ridge to between 620 and 920 ppm beneath Iceland. This suggests that Iceland is a wet spot and the H2O(8) values indicate that its influence on basalt compositions increases northwards along the Reykjanes Ridge from ∼61°N (650 km from the plume centre) towards Iceland. The existence of wetter Icelandic source regions have important implications for mantle melting, as enrichments of this magnitude depress the mantle solidus, increasing the degree of melting at a given temperature. Therefore the enhanced rates of volcanism on Iceland may be a result of wetter sources in addition to a thermal anomaly beneath Iceland.

The lava section in the Troodos ophiolite, Cyprus, is chemically stratified and divided into a shallow lava sequence with low TiO 2 content and a deeper lava sequence with high TiO 2 content. We calculate the viscosity at magmatic temperature based on major element chemistry of lavas in Cyprus Crustal Study Project (CCSP) Holes CY-1 and 1A. We find that typical shallow low-Ti lavas have a magmatic viscosity that is two to three orders of magnitude lower than that of the deeper high-Ti lavas. This implies that, after eruption on-axis, Troodos low-Ti lavas would have been able to flow down the same slope faster and farther than high-Ti lavas. The calculated lava viscosity increases systematically from the lava–sediment interface to the bottom of the composite Hole CY-1/1A. This suggests that an efficient process of lava segregation by viscosity on the upper flanks of the paleo Troodos rise may have been responsible for the chemical stratification in the Troodos lava pile. Calculated magmatic temperature and molar Mg/(Mg+Fe), or Mg#, decrease systematically down-section, while SiO 2 content increases. Correlation of Mg# in the lavas with Mg# in the underlying, lower crustal plutonic rocks sampled by CCSP Hole CY-4 shows that the shallow lavas came from a high-temperature, lower crustal magma reservoir which is now represented by high-Mg# pyroxenite cumulates, while the deeper lavas were erupted from a lower-temperature, mid-crustal reservoir which is now represented by gabbroic cumulates with lower Mg#.

Ophiolitic rocks with a zircon age of ∼1020 Ma occur in the Dunzhugur complex of East Sayan, Siberia, and are part of a Neoproterozoic to early Palaeozoic segment of the Central Asian fold belt. The most spectacular suite is exposed along the Oka and Bokson rivers, where a complete ophiolite sequence with mantle tectonites, a layered sequence composed of dunite, wehrlite, and pyroxenite, a gabbro section, a sheeted diabase dyke complex and basaltic pillow lavas are exposed. Petrologic and geochemical data suggest that all members of the ophiolite originally belonged to the same cogenetic mafic–ultramafic crustal section and support a supra-subduction zone setting in a fore-arc rifting environment for its origin. Two multigrain zircon size fractions from a plagiogranite are both slightly discordant but yielded a combined mean 207Pb/206Pb age of 1020±10 Ma. Evaporation of three additional fractions of three to four grains each from the same sample produced a mean 207Pb/207Pb age of 1019.9±0.7 Ma that we consider to most closely reflect the time of igneous crystallization of the plagiogranite. This is the oldest ophiolite so far dated from the Central Asian fold belt. The southern margin of the Siberian craton and the palaeo-Asian ocean were established at the end of the Mesoproterozoic, at least 1000 Ma ago. During the time interval 1000–570 Ma, one or several large ocean basins existed between Baltica, Siberia, Kazakhstan, Tarim and northern China, and these blocks are therefore unlikely to have been part of the supercontinent Rodinia. Rifting, initiation of subduction, and marginal basin formation began prior to 1000 Ma and continued through 570 Ma. The Dunzhugur ophiolite of Eastern Sayan provides evidence for the early opening of the palaeo-Asian ocean not later than 1000 Ma ago.

A paleomagnetic study of Late Paleozoic exotic units exposed in the Madre de Dios Archipelago and Diego de Almagro island, southern Chile, is reported. These units correspond to the Tarlton Limestone and the metabasaltic pillow lavas of the Denaro Complex. Nineteen sites were drilled (14 in limestone and five in lavas). Standard demagnetization procedures show that the primary remanence is not present in the investigated rocks, preventing the determination of the original latitudinal position of these rocks. However, a post-tectonic secondary magnetic component was isolated in eight sites at the Madre de Dios Archipelago, in both limestone and basaltic pillow lavas. Rock magnetic experiments and microscopic observations indicated that the carrier of this remanence is secondary titanomagnetite in the limestone and titanomagnetite–titanomaghemite in the lavas. The most likely event to produce this secondary magnetization is the nearby intrusion of the south Patagonian batholith in the Early Cretaceous. Mean direction from eight sites shows a very large (117.7±29.9°) counterclockwise rotation of the study area with negligible paleolatitude anomaly. This rotation is only constrained as post-Early Cretaceous. Restoration of the structures to their pre-rotation position indicates that accretion of the exotic block took place from the northwest and not from the south as previously proposed. This is more consistent with a low latitude origin for the limestone and the general paleogeographic framework. Different tectonic mechanisms and ages for this large rotation are possible.

High-Mg basaltic pillow lavas, important for an understanding of the petrogenetic evolution of the Tertiary volcanism of Sardinia, have recently been recognized in the lower part of the Early-Burdigalian volcano-sedimentary succession of the Villanovaforru (VF) area (Southern Sardinian Trough). Their textures vary from porphyritic to highly phyric types (doleritic) and the paragenesis is represented by Pl+Ol+Cpx±Opx±Opq. These basalts are sub-alkaline with a tholeiitic tendency; they display high MgO (mg-number 64–68), high Ni and Cr contents and their geochemical features are related to subduction-related magmas, with large-ion lithophile element enrichment and negative Nb, Zr and Ce spikes in mantle-normalized diagrams. The mineralogical and geochemical characteristics indicate that the VF basalts crystallized from primitive magmas within an extension-dominated tectonic setting. The presence of primitive basalts suggests crustal thinning in association with the Burdigalian extensional tectonics affecting the Sardinian Trough.

Results from a palaeomagnetic study of pillow basalts and associated pelagic sediments of the Late Cretaceous–Palaeocene Poya Terrane of New Caledonia are presented. The Poya Terrane forms a nappe that was thrust SW over the island, i.e. the Pacific side, in the middle Eocene. Data from four outcrops (nine sites), spread over 250 km, yield a normal polarity tilt-corrected mean inclination of −57.2°, where α95=11.2° and k=67.4, which suggests formation at 37.8° (±12.1°) S. The between-outcrop declinations are, unfortunately, too scattered to be used in regional tectonic discussions; they indicate rotations that are localised and sometimes very large. Following reference to SW Pacific Late Cretaceous–Palaeogene plate tectonic reconstructions, the inclination data suggest that the Poya Terrane formed close to the New Caledonian portion of the Indo–Australia plate. This interpretation is consistent with tectonic models where the Poya Terrane is generated in a marginal basin immediately to the northeast of New Caledonia during the break-up of east Gondwana. The terrane was thrust over New Caledonia in the middle Eocene, possibly in response to a change in relative motion of the Australian and Pacific plates at about this time. A short time later it was overthrust by a supra-subduction zone ultramafic nappe and together they have since drifted northwards as passengers on the Australia Plate.

The validity of magnetic granulometric estimates relies heavily on the ability to distinguish ultrafine particles from coarser grains. For example, populations with dominantly superparamagnetic (SP) or multidomain (MD) grains both are characterized by low remanence and coercivity, and distinguishing these endmembers may provide valuable clues to the origin of magnetization in the intervening stable single domain (SD) size range. The natural grain size variations associated with variable cooling rates in submarine lavas provide a rare opportunity for examining progressive changes in average magnetic grain size, from SP–SD mixtures in submarine basaltic glass to SD–MD mixtures in flow interiors. Based on microanalysis and rock magnetic measurements on pillow basalt samples dredged from the flanks of the Mid-Atlantic Ridge (ages

Biological mediation has been suggested as a control of the chemical exchange between the oceanic crust and seawater, but very little is known about its distribution within the oceanic crust and the relative importance of biotic and abiotic processes. Alteration textures in glassy pillow lava margins record the proportions of biotic and abiotic alteration, and the fraction of biotic alteration may be determined by point counting methods. We used this method at DSDP/ODP Sites 417D and 418A (110 Ma crust south of Bermuda Rise) and Holes 504B and 896A (5.9 Ma Costa Rica Rift). Biotic alteration dominates glass alteration in the upper 250 m of the oceanic crust (60–85% of the total glass alteration) and steadily declines in importance down to 10–20% at 500 m. The consistency of data between two crustal sections of very different age and tectonic setting suggest that microbially mediated glass alteration may be largely confined to the upper oceanic crust. However, both sites studied are sealed by thick sedimentary layers and, thus, are typical for ocean crust underlying the oceanic basins, rather than crust at mid-ocean ridges with possibly deep and rapid hydrothermal circulation. Down-hole temperature measurements at the Costa Rica Rift suggest that glass-altering microbes are hyperthermophilic and thrive at least up to temperatures of about 90°C. Microbial activity does occur at higher temperatures (up to about 110°C) but with reduced apparent abundance.

It is widely accepted that oceanic lithosphere is generated by partial melting of fertile mantle peridotites producing basaltic melts and residual peridotites. This theory implies a cogenetic relationship between peridotites and associated crustal rocks, but the actual existence of such a genetic link has been tested in very few places. In this paper, we use Sr and Nd isotopes to test the relationship between mafic and ultramafic mantle rocks from a MORB-type ophiolite in the Internal Liguride Units of the Northern Apennines. This ophiolite is a remnant of the oceanic lithosphere of the Jurassic Ligurian Tethys, and consists of depleted mantle peridotites intruded by a gabbroic complex and covered by pillow lavas and ophiolitic breccias. Whole rocks and mineral separates from the gabbroic rocks yield a Sm–Nd isochron with an age of 164±14 Ma. The whole rock data for pillow lavas are also consistent with this isochron, yielding an initial value of eNd (164)=8.6±0.3 (1σ). The mantle peridotites, by contrast, have eNd(164) values ranging from 11.9 to 14.8, indicating an extreme depletion unlike that seen in modern oceanic mafic and ultramafic rocks. These results demonstrate that some ophiolites consist of mantle and crustal sections that are not genetically linked by a simple melt–residue relationship, and consequently do not represent mature oceanic lithosphere. Similar evidence has previously been available only from the Xigaze and Trinity ophiolites. The Internal Liguride peridotites yield Sm–Nd model ages of about 270 Ma, assuming a normally depleted mantle source with eNd = 9, indicating a Permian time of `extra' depletion. The ophiolite thus consists of mantle peridotites which were depleted during Permian time, and were later intruded and covered by MORB-type magmas during Jurassic time. On a regional scale, this interpretation is consistent with widespread evidence that extensional processes leading to asthenospheric upwelling and magma production were active during the Permian.

A detailed microbiological and geochemical study related to the alteration of basaltic glass of pillow lavas from the oceanic crust recovered from Hole 896A on the Costa Rica Rift (penetrating 290 m into the volcanic basement) has been carried out. A number of independent observations, pointing to the influence of microbes, may be summarized as follows: (1) Alteration textures are reminiscent of microbes in terms of form and shape. (2) Altered material contains appreciable amounts of C, N and K, and the N=C ratios are comparable to those of nitrogen-starved bacteria. (3) Samples stained with a dye (DAPI) that binds specifically to nucleic acids show the presence of DNA in the altered glass. Further, staining with fluorescent labeled oligonucleotide probes that hybridize specifically to 16S-ribosomal RNA of bacteria and archaea demonstrate their presence in the altered part of the glass. (4) Disseminated carbonate in the glassy margin of the majority of pillows shows d 13 C values, significantly lower than that of fresh basalt, also suggests biological activity. The majority of the samples have d 18 O values indicating temperatures of 20‐100oC, which is in the range of mesophilic and thermophilic micro-organisms. © 1998 Elsevier Science B.V. All rights reserved.

New oxygen isotope data are presented for submarine lavas erupted close to the transition between the oceanic Kermadec island arc and the continental Taupo Volcanic Zone, New Zealand. Volcanic glasses display δ18O values ranging from +5.65‰ to +5.83‰, clinopyroxenes range from +5.23‰ to +5.78‰ and olivines range from +4.83‰ to +5.47‰. Coexisting glass and phenocrysts in the lavas are in isotopic equilibrium, with one exception. Oxygen isotope ratios of back-arc lavas erupted through oceanic crust are indistinguishable from mid-ocean ridge basalts or lavas erupted in nearby back-arc settings. Although lavas from the arc front display elevated oxygen isotope ratios, the magnitude of 18O-enrichment is too great to result from recycling of subducted material alone. A single back-arc lava erupted through continental crust is also relatively 18O-rich suggesting that the most likely origin for the high δ18O signature is limited amounts of interaction between continental crust and melts derived from a mantle wedge that has been variably fluxed by recycled oxygen. The results of modelling open system behaviour in this volcanic system highlight the need for strong controls on the composition of local contaminants. Application of `average' crustal lithologies, as in other volcanic provinces, may lead to erroneous conclusions regarding the involvement of local basement.

Scanning and transmission electron microscopy (STEM) of ocean-floor pillow basalts with ages between 0 and 70 Ma reveals progressive alteration of submicrometer titanomagnetite to phases such as goethite and clays. In contrast, larger titanomagnetite grains (> 1 μm) oxidize to titanomaghemite without apparent change in crystal morphology. Rock magnetic experiments are consistent with a selective removal of the submicrometer grains as the basalts age, and show good correlations between the ranges of grain sizes observed by STEM and those indicated by hysteresis properties. Remanence contributions from the larger (pseudo-single domain and multi-domain) titanium-iron oxides are inferred to decrease only slightly as the ocean floor becomes older, whereas the overall remanence decays with age as the substantial contribution from stable, single-domain titanomagnetite grains diminishes greatly due to their alteration to other phases. Although more work on many more samples is required to verify our conclusions, the current data imply that this alteration is one of the reasons that amplitudes of marine magnetic anomalies diminish with age over time scales of tens of millions of years.

U-Pb zircon dating, Sr-Nd isotope tracing and major/trace/RE element analyses were performed to constrain the age, origin and geodynamic significance of plagiogranites that intrude lherzolites and gabbros in the Ligurian Alps and the Northern Apennines. In addition, a host Fe-diorite was investigated. Samples from the Ligurian Alps were collected from the Voltri Group and the Sestri-Voltaggio Zone, whereas the plagiogranites from the Northern Apennines were taken in the Bracco unit. All these units have been affected by Alpine metamorphism reaching eclogite facies in the Voltri Group, blueschist degree in the Sestri Voltaggio samples, and prehnite-pumpellyite facies in the Bracco Unit, which has additionally been affected by rodingitization. U-Pb zircon ages of 150 ± 1, 153 ± 1 and ≈ 156 Ma were obtained, respectively, for two plagiogranites and the host Fe-diorite in the Ligurian Alps, and an age of 153 ± 1 Ma was determined for the plagiogranite in Northern Apennines. Inherited components in zircon and initial Pb in plagioclase indicate mixing of variously differentiated basaltic magmas with small amounts of roughly 1.7–2.1 Ga old continental crust material. REE patterns in both the plagiogranites and the host diorite are characterized by high REE abundance, and moderate LREE enrichment. Nd isotopic compositions lie in the range of N-MORB sources, yielding initial epsilon Nd values between + 8.8 and + 9.7, whereas Sr is isotopically heterogeneous. The geochemical pattern of the plagiogranites and the host Fe-diorite requires melting of a MORB-type mantle source that experienced LREE enrichment shortly before melting. The most likely explanation for such enrichment is the injection of melts derived by small degrees of melting from an adjacent mantle region. The basaltic, LREE-enriched parent magmas generated from this enriched domain have probably undergone up to about 72% of low-pressure fractional crystallization prior to their emplacement into the gabbro-peridotite complex. The 156–150 Ma magmatism occurred in close relation to normal faulting, sedimentation of breccias, and detachment of the mantle complex from its overlying continental crust, followed by exposure on the ocean floor. This tectono-magmatic event in the Ligurian Alps and the Northern Apennines reflects rifting of the Adriatic-Iberian continental plate segment, preceding wider opening of the Piedmont-Ligurian ocean basin and pillow basalt deposition.

Studies of seafloor magnetic anomaly patterns suggest the presence of Jurassic oceanic crust in a large area in the western Pacific that includes the East Mariana, Nauru and Pigafetta Basins. Sampling of the igneous crust in this area by the Deep Sea Drilling Program (DSDP) and the Ocean Drilling Program (ODP) allows direct evaluation of the age and petrogenesis of this crust. ODP Leg 129 drilled a 51 m sequence of basalt pillows and massive flows in the central East Mariana Basin. 40 Ar 39 Ar ages determined in this study for two Leg 129 basalts average 114.6 ± 3.2 Ma. This age is in agreement with the Albian-late Aptian paleontologic age of the overlying sediments, but is distinctively younger than the Jurassic age predicted by magnetic anomaly patterns in the basin. Compositionally, the East Mariana Basin basalts are uniformly low-K tholeiites that are depleted in highly incompatible elements compared to moderately incompatible ones, which is typical of mid-ocean ridge basalts (MORB) erupted near hotspots. The Sr, Nd and Pb isotopic compositions of the tholeiites ( 87 Sr 86 Sr init = 0.70360–0.70374 ; 143 Nd 144 Nd init = 0.512769–0.512790 ; 206 Pb 204 Pb meas = 18.355–18.386 ) also overlap with some Indian Ocean Ridge MORB, although they are distinct from the isotopic compositions of Jurassic basalts drilled in the Pigafetta Basin, the oldest Pacific MORB. The isotopic compositions of the East Mariana Basin tholeiites are also similar to those of intraplate basalts, and in particular, to the isotopic signature of basalts from the nearby Ontong Java and Manihiki Plateaus. The East Mariana Basin tholeiites also share many petrologic and isotopic characteristics with the oceanic basement drilled in the Nauru Basin at DSDP Site 462. In addition, the new 110.8 ± 1.0 Ma 40 Ar 39 Ar age for two flows from the bottom of Site 462 in the Nauru Basin is indistinguishable from the age of the East Mariana Basin flows. Thus, while magnetic anomaly patterns predict that the igneous basement in the Nauru and East Mariana Basins is Jurassic in age, the geochemical and chronological results discussed here suggest that the basement formed during a Cretaceous rifting event within the Jurassic crust. This magmatic and tectonic event was created by the widespread volcanism responsible for the genesis of the large oceanic plateaus of the western Pacific.

Partially crystalline melt inclusions in anorthite phenocrysts from a Galapagos seamount have been rehomogenized in a series of heating experiments in order to accurately determine their initial compositions. They are primitive tholeiites, high in CaO and Al2O3, and low in TiO2 and alkalies. We infer that they were entrapped at a temperature close to 1270°C. At this temperature, the trapped melt composition is multiply saturated with olivine, plagioclase and spinel at 1 atmosphere pressure, but it is also near-primary requiring addition of less than 5% olivine plus or minus plagioclase and spinel in order to be in equilibrium with mantle olivine. The inclusions occur in association with, and are potentially parental to, a suite of MORB-like pillow lavas which range in MgO content from 10 to 8 wt% and which could have been derived by up to 40% fractionation of olivine plus plagioclase from the inclusion composition. Although the host anorthites are remarkably homogeneous and unzoned and the inclusions themselves are uniform in major element composition, minor element contents vary by a factor of two or more. Unlike many other plagioclase megacrysts, these anorthites must have crystallized under conditions in which the major element compositions were buffered but minor (and presumably trace) elements were free to vary. Such conditions could be achieved by liquid-solid interactions either in the melting regime or, perhaps more likely, in a liquid-crystal mush within the oceanic crust. These melt inclusions are not unique. They appear to belong to a class of primitive, high CaO and Al2O3 MORB that occur in association with high-An plagioclase in a variety of oceanic settings, usually where magma supply is low. We propose that such magmas are derived from the shallowest, most depleted part of a mantle melt column, and that they constitute one end member of a spectrum of primary magmas which gives rise to the global array of MORB compositions.

Detailed observations have been made of the extrusive and intrusive units of the uppermost crust emplaced at the axis of the East Pacific Rise during dives conducted with the submersible Nautile on the walls of Hess Deep. At four widely separated locations, the same sequence was found: an upper, thin (100–200 m) unit of extrusives (pillow lavas) underlain by a mixed zone of variable thickness (50–500 m, averaging 200 m) where extrusives are found and intrusives and massive subhorizontal layers which may represent sills. The mixed zone itself is underlain by a sheeted dyke unit, that may be up to 1200 m thick, where dykes are subvertical (i.e. untilted) and parallel to the East Pacific Rise fabric. This sequence may be a general feature of medium to fast-spreading crust because it is similar to that observed in Hole 504B of the Costa Rica Rift and is consistent with images of the upper oceanic crust derived from seismic experiments on the axis of the East Pacific Rise. Dyke thickness is a good measure of the depth to the roof of the axial magma chamber. The observed sections provide much needed “ground-truth” for East Pacific Rise crust. The thickness of the lava sequence and the overall thickness of Layer 2 (at most 1400 m) appear to have been overestimated in previous studies of the oceanic crust.

A Sea Beam survey of the Pacific-Cocos-Galapagos triple junction at 2°40′N on the East Pacific Rise (EPR) establishes that it is a ridge-ridge-ridge junction, with the EPR axis making a small but sharp bend where it is intersected by a slow-spreading (15 mm/yr) fast-propagating rift. At the tip of the narrow triangle of crust accreted at this slow-spreading axis is volcanic ridge built on EPR crust. Tholeiitic lavas erupted along the slow-spreading axis, here and at the rather similar Pacific-Nazca-Galapagos junction, differ in minor element and rare earth element geochemistry from lavas on adjacent segments of the EPR axis, being slightly enriched in incompatible elements.

Multi-vent volcanic extrusions in the Sumisu back-arc rift have formed individual cones in the outer rift and short segment en echelon ridges in the inner rift. Alternating or coincident phases of constructional volcanism and mass wasting have produced a terrain consisting mainly of large basalt pillow lavas, lesser amounts of fresh-looking lobate, ropy, and sheet flows along the ridge crests, and volcanic debris ubiquitous throughout the rift. A rhyolite volcano and inactive hydrothermal chimneys occur at the step in the largest en echelon ridge. At present, constructional volcanism predominates over tectonism on the features studies. The development of en echelon basaltic ridges is greater in other western Pacific back-arc rifts with more extension. Continued extension in Sumisu Rift may cause a shift in the concentration of volcanism from the cross-rift trend to an along-axis orientation, eventually leading to the coalescing of the inner rift volcanic ridges into a back-arc spreading system.

Paleomagnetic research has been carried out on volcanic rocks from two formations exposed in Western Timor, Indonesia: i.e., the Metan Formation of Eocene age, and the Manamas Formation of Late Miocene age. Because of the chaotic tectonic setting of the rocks at the Metan Formation it was the inclination data from five lavas only that could be interpreted in the final analysis of the characteristic remanence directions (ChRM) with I = 33.6°, a 95 = 4.1°. The Metan volcanics most probably extruded originally in the northern hemisphere at a latitude of about 17°. The volcanics may have extruded on a crustal unit which includes a continental fragment that became detached from Gondwana in Mesozoic times and subsequently shifted to the southeast Asian continent. This crustal unit broke away from southeast Asia in Eocene times, moved towards the south and collided 3 Ma ago with the frontal part of the Australian continental margin. The Manamas Formation of Late Miocene age includes obducted volcanics consisting of both deep-sea pillow lavas and island arc volcanics with ChRM directions of D = 132.6°, I = 15.8°, no a 95 , because 2 sites only, and D = 295.9°, I = −26.3°, a 95 = 2.8°, respectively. The declinations of the ChRM directions, derived from the rocks from two localities, show counterclockwise deviations of approximately 45 and 60°, respectively, relative to the present earth's field direction. The deviating directions can be explained in terms of a counterclockwise rotation of Timor which may have begun as early as 3 Ma ago, and which could be a result of the obliquely northward protruding Australian continent, where the originally west-east directed Banda Arc rotated in a counterclockwise sense. It is also possible that during the obduction the lava piles performed counterclockwise rotations. Rock magnetic analyses show that magnetite or/and titanomagnetite is the main carrier of remanence.

The North Arch volcanic field is located north of Oahu on the Hawaiian Arch, a 200-m high flexural arch formed by loading of the Hawaiian Islands. These flood basalt flows cover an area of about 25, 000 km 2 ; the nearly flat-lying sheet-like flows extend about 100 km both north and south from the axis of the flexural arch. Samples from 26 locations in the volcanic field range in composition from nephelinite to alkalic basalt. Ages estimated from stratigraphy, thickness of sediment on top of the flows, and thickness of palagonite alteration rinds on the recovered lavas, range from about 0.75–0.9 Ma for the youngest lavas to somewhat older than 2.7 Ma for the oldest lavas. Most of the flow field consists of extensive sheetflows of dense basanite and alkalic basalt. Small hills consisting of pillow basalt and hyaloclastite of mainly nephelinite and alkalic basalt occur within the flow field but were not the source vents for the extensive flows. Many of the vent lavas are highly vesicular, apparently because of degassing of CO 2 . The lavas are geochemically similar to the rejuvenated-stage lavas of the Koloa and Honolulu Volcanics and were generated by partial melting of sources similar to those of the Koloa Volcanics. Prior to eruption, these magmas may have accumulated at or near the base of the lithosphere in a structural trap created by upbowing of the lithosphere.

Relative directions of magnetization have been measured within individual pillow basalts collected from the Atlantic Ocean and Caribbean Sea. The angle between the magnetic directions was determined and is referred to as the directional difference. Although one pillow contained a directional difference of 44°, the remaining ten pillows had differences less than 14°. The maximum orientation and measurement error was 7°. Dispersion on the scale found in these fine-grained pillow basalts would not appreciably affect the magnetic anomaly pattern on the sea floor. We detected no reversals of magnetization despite the sometimes large and variable low-temperature oxidation. Comparison of directions within homogeneous segments of the pillow, viscous remanent magnetization (VRM) acquisition experiments, and alternating field (AF) demagnetization indicate a large portion of the dispersion was due to the acquisition of a viscous component in the larger grained, less oxidized portion of the pillows. Evidence from one variably weathered pillow suggests that extreme low-temperature oxidation may lead to the acquisition of a secondary component with high coercivities (20–80 mT). We could not determine whether this was a chemical remanent magnetization (CRM) or a VRM acquired by single domain grains near the superparamagnetic threshold. Hysteresis properties confirmed by microscopic examination indicated that the magnetic grain size in all the pillows was at least as small as pseudo-single domain.

Characteristic geochemical features of the ophiolite suite from the Bay of Islands Complex have been determined by major and trace element analyses of 13 rocks. Based on elements, such as rare earth elements (REE), whose abundances are relatively immobile during alteration and metamorphism, we find that (1) the pillow lavas and diabases are relatively depleted in light REE similar to most tholeiites occurring along spreading oceanic ridges, in back-arc basins and comprising the early phases of volcanism in island arcs; (2) the gabbros, composed of cumulate plagioclase and olivine with poikilitic clinopyroxene, have REE contents consistent with formation as cumulates precipitated from magmas represented by the overlying pillow lavas and diabases; (3) as in most harzburgites from ophiolites, the Bay of Islands harzburgite and dunite have relative REE abundances inconsistent with a genetic relationship to the overlying basic rocks — this inconsistency may be primary or it may result from late-stage alteration, contamination and/or metamorphism; (4) some Bay of Islands lherzolites have major and trace element abundances expected in the mantle source of the overlying basic rocks. Overall, the geochemical features of this Bay of Islands ophiolite suite are similar to those from Troodos and Vourinos, but these data are not sufficient to distinguish between different tectonic environments such as deep ocean ridge, small ocean basin or young island arc.

Most abyssal basalt can be divided into two groups based on major and minor element compositions of the glassy rinds of pillow lavas. Crystal differentiation models cannot relate one group to the other. Instead, depth of partial melting may exert the primary control. The first group (MORB-1) is characterized by low TiO2 and Na2O and higher MgO, and is derived from a primary melt originating from the spinel peridotite facies. The higher TiO2-NaO2 group (MORB-2) appears to have been derived from the lower-pressure (shallower depth) plagioclase peridotite facies. Mossbauer spectra show that MORB-1 is from a mantle source region with lower oxidation states (between the QFM and NNO buffers) than for MORB-2 (around the NNO buffer).

Major element and trace element analyses of samples dredged from six sites along extensional plate boundaries around the Caroline Sea in the western equatorial Pacific define three separate igneous suites. Three dredge sites were located in the Sorol Trough along the Caroline-Pacific plate boundary and three sites were located in the Ayu Trough along the Philippine-Caroline plate boundary. Within the Sorol Trough, fresh transitional-to-alkalic pillow basalts were dredged from a seamount, weathered oceanic tholeiites were sampled at the base of the northern margin, and fresh, Ti-rich ferrobasalts and ultramafics were recovered from a deep valley in the western section. Pyroxenites, high-Mg basalt and a lherzolite, chemically related to the ferrobasalts in the western section of the trough, include schistose fragments and visibly tectonized ultramafics. High-grade metamorphic features observed in these specimens suggest that a significant component of shear accompanies crustal accretion in the Sorol Trough. Fresh pillow-lava fragments from the axial rift and weathered pillows from the eastern margin of the Ayu Trough have compositions similar to Mid-Ocean Ridge basalts (MORBs) and indicate that the Ayu Trough formed through crustal accretion processes. Additionally, an intriguing assemblage of meta-basalts, meta-gabbros and silicic plutonic rocks were recovered from a shallow scarp along the western margin of the Ayu Trough. The chemical characteristics of the meta-basalts and meta-gabbros suggest that they may be related through crystal fractionation. The chemistry of the silicic plutonics, however, indicates that they may not be derived by fractionation of primary magmas with chemical compositions similar to those of the metamorphosed oceanic crustal rocks. Alternatively, the silicic plutonics may represent an assemblage of intrusive island-arc tholeiitic rocks which formed during the initial stages of magma genesis in the island arc that was rifted apart by seafloor spreading in the nascent Ayu Trough.

The mineralogy, major element composition, REE patterns and Sc, Co, Hf, Cr, Sr, and Cu contents of nine pillow lavas from the pre-Cenozoic basement complex of Maio reveal the presence of oceanic tholeiites on the Cape Verde archipelago. These uplifted tholeiites constitute the oldest rocks exposed on the islands and are related to an Early Cretaceous (or still somewhat older?) oceanic ridge crest. It is suggested that the formation of east-west trending fracture zones in the Atlantic Ocean induced the alkalic character of the Cape Verde magmatism from the middle of the Cretaceous up to historical times.

The Point Sal ophiolite, California, contains a volcanic pile of pillowed and massive basalts and keratophyres that exhibit a downward change in metamorphic facies, i.e., zeolite to greenschist. Analyses of 14 stratigraphically sequential lavas reveals the existence of three lava types: (1) highly fractionated plagioclase phyric lavas at the base of the lava pile; (2) clinopyroxene-plagioclase phyric lavas at different eruptive levels throughout the pile, and (3) olivine-clinopyroxene phyric lavas (low ΣREE) in the midst of the eruptive suite. The upper portion of the lava pile contains lavas with chondritic to slightly light-REE-depleted profiles (La= 3.5−13.0×chondrite) and negative Ce anomalies. The lower part of the lava pile contains lavas with light-REE-depleted profiles (La= 1.7−28.0×chondrite), negative Eu anomalies, but no negative Ce anomalies. Application of fractionation laws to the olivine-clinopyroxene phyric lavas (approximations of initial mantle melts) verifies that the fractionation process involved olivine-clinopyroxene and plagioclase. Consideration of this and the eruptive sequence allows semiquantitative verification of an origin for the Point Sal lavas by low-pressure fractionation of multiple chambers of olivine basalt.

The Urals contain a 2000 km belt of mafic-ultramafic bodies. The Sm-Nd and Rb-Sr systematics of two of these bodies, the Kempersai Massif in the South Ural Mountains and the Voykar-Syninsky Ophiolite Complex in the Polar Ural Mountains have been examined. These data confirm the hypothesis that these bodies represent fragments of pre-collision oceanic crust and establish constraints on the nature and timing of events in the Uralian Orogeny. Two Kempersai gabbros define Sm-Nd internal isochrons of 397 ± 20 My and 396 ± 33 My with e_(Nd)(T) = + 8.7 ∓ 0.6 and + 8.4 ∓ 1.3, respectively. Whole rock samples of pillow basalt, diabase, gabbros, troctolite, and a metasediment give Sm-Nd values which lie on this isochron indicating that these rocks are genetically related and have an igneous crystallization age of 397 My. Whole rock samples of Voykar-Syninsky diabase, gabbros, and clinopyroxenite give Sm-Nd values which lie on or within∼ 1 e-unit of this isochron indicating an age and e_(Nd)(T) virtually identical to those of Kempersai. e_(Nd)(T) for the Kempersai and Voykar-Syninsky mafic samples range from +7.3 to + 9.0 with an average value of + 8.4. This indicates that the Urals ophiolites are derived from an ancient depleted mantle source and are most plausibly pieces of the oceanic crust and lithosphere. The fact that a metasediment has the same e_(Nd)(397 My) as the other samples indicates derivation from an oceanic source with negligible continental input. e_(Nd)(T) for the massifs is∼ 1.5 e-units lower than the average for modern MORBs. This may be due to the differential evolution of the MORB source over the past 397 My and in conjunction with data for other ophiolites and Mesozoic MORB suggests that over the past 750 My the source for MORB has evolved at a rate less than or equal to its rate of evolution averaged over the age of the earth. Initial ^(87)Sr^(86)Sr ratios are highly variable ranging from e_(Sr)(T) = −25.2 for a gabbro to +70.3 for a highly serpentinized harzburgite. This reflects the effects of seawater alteration which is particularly strong on ultrabasic rocks. We conclude that the long belt of mafic-ultramafic rocks in the Urals, which includes the Kempersai and Voykar-Syninsky Massifs, represents segments of Siluro-Devonian oceanic crust. Our igneous age for Kempersai in conjunction with other age constraints suggest that these segments of oceanic crust formed at least 80 My before the collision that produced the Urals.

Indications of a narrow region of high magnetization within the central magnetic anomaly on some mid-ocean ridges are found on near-bottom and sea surface magnetic profiles. This zone, which probably represents the most recent extrusions onto the ocean floor, is similar to the narrow region of high magnetization found on the Mid-Atlantic Ridge at 45°N with a suite of dredge samples. This narrow region is probably the result of the initial high magnetization of pillow basalts when they are extruded onto the ocean floor and the subsequent rapid oxidation of the outer variolitic zone of the pillows. The large-amplitude, short-wavelength (

The Canyon Mountain ophiolite, Oregon, is exceptional in lacking sheeted dikes, basaltic pillow lavas, and sediments that are characteristic of many other ophiolites. Instead, the uppermost portion of the complex consists of a significant volume of plagiogranites, which, in addition to minor basalts, intrude a large section of keratophyres believed to be of volcanic origin. The trend of intrusive rocks and of bedding in the keratophyres is mostly parallel to layering in the underlying gabbroic cumulates and to contacts between units in the remainder of the ophiolite. It is suggested that the plagiogranites, basalts, and keratophyres comprise a sill complex. Both the plagiogranites and the keratophyres are similar, respectively, to low-K2O plutonic and extrusive rocks of island arcs. The mineralogy and penetrative deformation structures of the ultramafic and some of the gabbroic rocks of the ophiolite indicate greater depth of formation, related to magmatism and diapirism above a Benioff zone. Radiometric age dates of plagiogranites confine the minimum age of the complex to the Early Permian. The Canyon Mountain ophiolite may thus be correlative with other fragments of a Lower Permian arc terrane throughout northeastern Oregon which were chaotically mixed during renewed subduction in middle to late Triassic time.

The ferromanganoan umbers which overlie the uppermost pillow lavas of the Troodos Massif, Cyprus, are strongly enriched in REE relative to normal pelagic clays. Together, the umbers, related ferruginous sediments, and the ochres which are associated with massive cupriferous sulphides located within the lavas, all show light REE enrichment. There is a marked negative Ce anomaly which is characteristic of seawater. Field and chemical data suggest that the REE were incorporated into ferruginous precipitates derived from deep leaching of tholeiitic lavas by seawater. All the metalliferous sediments can be interpreted as related events in the evolution of the Troodos ocean ridge. In contrast, REE patterns of bentonitic clays above the umbers indicate a predominantly continental derivation.

The occurrence of excess 129 Xe and a higher 3 He/ 4 He ratio than that of the atmosphere has been confirmed in olivine phenocrysts of Kapuho lava, Kilauea, and Hualalai xenolithic dunites with CO 2 inclusions from Hawaii. The 3 He/ 4 He ratio observed in dunites is similar to those of submarine pillow basalts, whereas that of olivine phenocrysts is higher than the former. This may be related to the different characteristics of their sources with respect to rare gas isotopes. Normalized to 36 Ar, Ne is depleted in olivine phenocrysts and enriched in dunites, whereas Xe is more enriched in both samples than in the atmosphere.