Normal-type Mid-ocean Ridge Basalts Research Articles

Geochemical constraints on source nature and recycled oceanic crust in the mantle of the Celebes Sea

Indian-type mantle is extensively distributed in the marginal basins of the western Pacific; however, the origin of this mantle remains unclear. The Celebes Sea is located between the Pacific and Indian oceans, and its igneous crustal rocks provide crucial constraints on if and on how this anomalous mantle developed in the region. However, limited knowledge has been obtained on the origin of mantle source of the Celebes Sea because only normal-type mid-ocean ridge basalts (N-MORBs) at ODP Site U770 in this basin have been examined. In this study, both N-MORBs and enriched (E)-MORBs were identified at Site U770, which were analyzed for whole-rock major and trace elements, Sr–Nd–Pb–Hf isotopes, and in situ olivine geochemistry. Site U770 basalt samples are Indian-type (i.e., plot above the Northern Hemisphere reference line [NHRL] on plots of 208Pb/204Pb and 207Pb/204Pb versus 206Pb/204Pb) MORBs, based on their Sr–Nd–Pb–Hf isotopic compositions. An enriched mantle end-member exists in the MORB-type depleted mantle source, as evidenced by correlations between ɛNd and SmN/YbN, Sr/Sr*, and Eu/Eu*. Recycled igneous oceanic crust that experienced plagioclase fractionation can account for the geochemically enriched end-member in the mantle source, based on the positive correlation of ɛNd vs. Sr/Sr* and Eu/Eu*. The recycled igneous oceanic crust plus minor amounts of pelagic sediments can also explain the origins of Indian-type mantle in the Celebes Sea. The E-MORBs have higher Zn/Mn and Zn/Fe ratios, and similar olivine Ni, Ca, and Mn contents as compared to global MORBs, which is also consistent with a recycled oceanic crust-derived, eclogite-rich, mantle source. This suggests recycled components contribute to the lithological and geochemical heterogeneity in the mantle beneath the Celebes Sea.

Mosaic zircon petrochronology and implications for the ultra-slow spreading process of Southwest Indian Ridge

Here we report the discovery of mosaic zircons in Hole U1473A on the Atlantis Bank in the Southwest Indian Ridge. Oxygen isotope, UPb dating, and geochemical analyses of the zircons were carried out directly in petrographic thin-sections. The mosaic zircons are from diorite and oxide gabbro at shallow depths, whereas the nonmosaic zircons occur in oxide gabbronorite and tonalite at greater depths. The majority of zircons have δ18O of 5.5 ± 0.1‰, which are mantle-like values, reflecting formation by simple fractional crystallization. No correlation exist between UPb ages, δ18O values, and the mosaic microstructures. The mosaic subdomains in a single zircon are well correlated with some trace elements and fractures. Therefore, the variable trace element contents were produced during crystallization, and associated with secondary brittle deformation due to movement on mid-ocean ridge detachment faults. The pervasive brittle deformation weakened the originally rigid zircon and triggered fracturing. The fractures became highly permeable pathways, allowing rapid grain-boundary diffusive loss of Pb, which resulted in the relatively young age of 11.42 ± 0.41 Ma for the mosaic zircons as compared with 12.16 ± 0.14 Ma for the non-mosaic zircons. The zircon trace element patterns are all indistinguishable from global oceanic zircons. However, the trace element abundances and ratios require a significantly depleted normal-type mid-ocean ridge basalt mantle source.

Using precious metal probes to quantify mid-ocean ridge magmatic processes

Basalts from the East Pacific Rise (EPR), Siqueiros transform zone and Mid-Atlantic Ridge (MAR) area have been analyzed for the platinum-group elements (PGE) and a wide range of incompatible elements. The low PGE content of the most primitive mid-ocean ridge basalts (MORB) suggests that they leave the mantle sulfide-saturated but become sulfide under-saturated, as a consequence of decompression, during their ascent from the mantle to the MOR magma chamber. Because the pressure drop is small, the ascending magma enters the magma chamber slightly sulfide under-saturated and requires only a small amount of fractional crystallization to return it to sulfide-saturation. Sulfide-saturation is marked by the Pd content of the melt falling by over an order of magnitude at ca. 9.5 wt.% MgO. However, once the magma has become sulfide-saturated, it shows no evidence of a further decline in Pd with decreasing MgO as the system evolves. This must result from regular replenishment of the underlying axial magma chamber by fresh batches of primitive magma. Most replenishments occur when the MgO content of the resident melt in the magma chamber is between ∼9 to 6.5 wt.% MgO. We combine the tight constraints imposed by highly compatible Pd abundances, with those imposed by strongly incompatible elements, to produce the first model that successfully accounts for the variations of both classes of elements in open system mid-ocean ridges magma chambers for normal-type MORB (N-MORB). We show that the lack of decline in the Pd content of sulfide-saturated mid-ocean ridge basalts, with decreasing MgO, requires frequent small replenishments (between 1% and 4%) of similar magnitude, rather than large initial inputs that systematically decline from near 100% to near zero during the life of an individual ridge system.Assuming the spreading rate of ∼110 mm/yr for the EPR at 9°N, and 14 yr between the 1991–1992 and 2005–2006 eruptions, the calculated volume of erupted and intruded magma is 2.3 × 103 m3/m of ridge length. If the expelled melt represents 0.5% of the accessible magma in the chamber, as suggested by the Pd modeling, the volume of the magma accessible during tapping is 4.6 × 105 m3/m of ridge length.

Carboniferous to Early Triassic magmatism and accretion in Alxa (NW China): implications for accretionary orogenesis of the southern Altaids

Alxa occupies a crucial position between the Tianshan–Beishan orogen to the west and the Solonker suture to the east and is important in our understanding of the accretionary orogenesis of the southern Altaids. To unravel the tectonic history of the Alxa region, we undertook an integrated study of the field geology, geochemistry and geochronology of magmatic rocks and an accretionary complex. Six granites and one rhyolite from the Zhusileng–Hangwula arc show a peraluminous, high-K calc-alkaline composition and one gabbro is tholeiitic. They show patterns rich in incompatible elements and negative Nb–Ta anomalies on primitive mantle-normalized trace element spider diagrams. Laser ablation inductively coupled plasma mass spectrometry zircon U–Pb dating reveals 348–329 and 267–250 Ma magmatic events. These data indicate that the Zhusileng–Hangwula arc was a Japan-type island arc from the late Paleozoic to Early Triassic. The Engger Us mélange comprises pillow basalts with a normal-type mid-ocean ridge basalt composition, tuffs, cherts, siliceous mudstones, limestones and turbidites, which are tectonically juxtaposed by a block-in-matrix structure. This mélange is interpreted as a Carboniferous–Permian accretionary complex. These new data, combined with previous studies, confirm that the Palaeo-Asian Ocean subducted bidirectionally in the Alxa region in the Carboniferous–Early Triassic. A consistent divergent subduction system existed from Alxa to Solonker before the terminal closure of the Palaeo-Asian Ocean in the Mid- to Late Triassic.Supplementary material: Major and trace element data and zircon U-Pb age results of all the samples are available at: https://doi.org/10.6084/m9.figshare.c.4962230

Geochemistry and geochronology of mafic rocks from the Jinghe ophiolitic mélange, northwest China: Implications for plume-related magmatism and accretionary processes within the North Tianshan Ocean

We present geological, whole-rock geochemical and Sr–Nd isotopic, and zircon U–Pb geochronological data for mafic rocks found as angular fragments in the Jinghe ophiolitic mélange of the Northern Tianshan orogenic belt, northwest China. The investigated exotic blocks comprise layered and massive gabbros and pillow basalts that are embedded in a serpentinite-rich matrix. The 380.5 ± 4.9 Ma layered gabbros have chondrite-normalized rare earth element (REE) patterns characterized by significant depletions in the light rare earth elements (LREE) and positive Eu anomalies, suggesting that the petrogenesis of these rocks was controlled by cumulus processes. The 381.5 ± 3.7 Ma massive gabbros have near-flat normal-type mid-ocean ridge basalt (MORB)-normalized multi-element patterns with moderate to strong negative Nb–Ti anomalies. These rocks have initial 87Sr/86Sr ratios (87Sr/86Sr(i)) of 0.703825–0.704758 and high εNd(t) values (+6.7 to +9.0). Two compositional groups of basalts can be recognized: ocean island basalt (OIB)- and enriched MORB-type rocks. Both groups display variable enrichments in large-ion lithophile elements (LILE) in N-MORB-normalized trace element diagrams. Semi-quantitative modelling demonstrates that the parental magmas of the massive gabbros were derived by 5%–10% partial melting of a spinel-bearing lherzolite source influenced by minor inputs of subducted crustal components. The parental magmas of the subalkaline and alkaline basalts were derived by 10%–20% and 1%–5% polybaric melting, respectively, of a lherzolite source containing spinel and garnet in equal amounts. The petrogenesis of the Jinghe basalts can be linked to continental rifting followed by seafloor spreading caused by upwelling of a mantle plume. The petrogenesis of the gabbros is linked to plate re-organization that provided the geodynamic impetus for southwestward subduction in the North Tianshan Ocean, and the onset of incorporation of oceanic materials with different origins into a growing accretionary prism during the Late Devonian. Accretion resulted in chaotic mixing of dismembered pieces of variably deformed oceanic rocks, which are now preserved as an extensive block-in-matrix ophiolitic mélange.

Geochronology, geochemistry and isotope systematics of a mafic–intermediate dyke complex in the İstanbul Zone. New constraints on the evolution of the Black Sea in NW Turkey

AbstractWe report new U–Pb zircon ages, major and trace element data, mineral chemistry, and Sr–Nd isotopic analyses of the mafic–intermediate dykes and intrusions in the İstanbul Zone. Mafic dykes are represented by calc-alkaline to alkaline lamprophyre and diabase. Intermediate dykes and subvolcanics are andesitic to dacitic in composition and calc-alkaline in character, while intrusive rocks (stocks and small plutons) are granodioritic and dioritic in composition. New zircon U–Pb laser ablation inductively coupled plasma mass spectrometry (LA-ICP-MS) dating yielded ages from 72.49 ± 0.79 (Upper Cretaceous–Campanian) to 65.44 ± 0.93 Ma (Lower Paleocene–Danian) for the intermediate dykes, and 58.9 ± 1.8 Ma (Upper Paleocene–Thanetian) for a small granodiorite stock. 87Sr/86Sr(i) values of the mafic and intermediate dykes and small stocks span a range from 0.703508 to 0.706311, while their 143Nd/144Nd(i) values vary from 0.512614 to 0.512812 and eNd(i) values from 5.09 to 1.24. NdTDM model ages range between 0.46 and 0.77 Ga.Dykes are enriched in large ion lithophile elements (LILEs) and light rare earth elements (LREEs) relative to high field strength elements (HFSEs). Normal-type mid-ocean ridge basalt (N-MORB)-normalized multi-element spidergrams of the majority of the mafic and intermediate dykes display a clear subduction signature, except a subset, which cut the Palaeozoic of İstanbul and the upper part of the Upper Cretaceous volcanics in the north of İstanbul (i.e. feeder dykes of the Kısırkaya Formation) and show a clear ocean island basalt (OIB) signature indicating that the melts feeding the dyke system during the Upper Cretaceous–Paleocene period were derived from two contrasting mantle sources: (1) initially a lithospheric mantle modified by subducted slab-derived melts which sourced the magmas with a clear subduction signature; and (2) followed by an asthenospheric mantle from which basic magmas with OIB signature. Petrological models indicate the interaction of these two discrete magma series via magma-mixing processes.Geothermometric calculations based on the composition of amphiboles are in the range of 769–953 and 938–994°C. Geobarometric calculations indicate crystallization depths ranging over an interval between 3.0 and 20.2 km, implying a polybaric crystallization. The oxygen fugacity (logƒO2) values vary between −10.10 and −13.07 bar in the dykes cutting the Upper Cretaceous volcanics, and from −8.71 to −10.33 bar in intermediate dykes cutting the İstanbul Palaeozoic unit. H2Omelt contents change between 4.91–6.89 and 4.82–7.51%, respectively implying that the dykes were emplaced at mid to shallow crustal levels. Dyke complexes of the İstanbul zone are interpreted to have been emplaced in a rifted volcanic arc margin related to the opening of the Black Sea during the Late Cretaceous–Paleocene period.

New insights into the geodynamics of Neo-Tethys in the Makran area: Evidence from age and petrology of ophiolites from the Coloured Mélange Complex (SE Iran)

The Coloured Mélange Complex is part of the North Makran domain (SE Iran) and consists of an assemblage of metric- to decametric-thick slices mainly represented by volcanic rocks, locally stratigraphically associated with radiolarian cherts. In this paper, we present new geochemical data on volcanic rocks and biochronological data on the associated cherts. Our data indicate the occurrence of a wide range of volcanic rocks-types, which are: 1) normal-type mid-ocean ridge basalts (N-MORB); 2) oceanic plateau basalts (OPB); 3) alkaline basalts; 4) calc-alkaline basalts, basaltic andesites, and andesites; 5) volcanic arc tholeiitic basalts and andesites, and high pressure - low temperature metabasalts formed in deep levels of an accretionary wedge. The volcanic arc tholeiites range from Early (late Hauterivian - early Aptian) to Late (latest Cenomanian - lower late Campanian) Cretaceous, whereas the calc-alkaline rocks and OPBs are Late Cretaceous in age (early Coniacian - Santonian and early Turonian - early Campanian, respectively). Alkaline basalts, OPBs, and N-MORBs represent remnants of the Mesozoic Neo-Tethys oceanic branch located between the Arabian plate and the Lut block. In this paper we document that this oceanic sector was characterized by the development of an oceanic plateau in the Late Cretaceous. In contrast, calc-alkaline and volcanic arc tholeiitic rocks represent remnants of a continental volcanic arc and forearc, respectively, developed onto the southernmost realm of the Lut block. The petrogenesis and age of volcanic rocks allow us to propose a new tectono-magmatic model for the evolution of the convergent margin developed in the northern sector of the Neo-Tethys from Early to Late Cretaceous. This model is basically constrained by the collision of the oceanic plateau with the continental arc, which led to the jump of the subduction toward the south, as well as to the formation of the imbricate pile of different units today observed in the North Makran area.

Preservation of a fragmented late Neoproterozoic–earliest Cambrian hyper-extended continental-margin sequence in the Australian Delamerian Orogen

AbstractMafic and ultramafic rocks intercalated with metamorphosed deep-marine sediments in the Glenelg River Complex of SE Australia comprise variably tectonized fragments of an interpreted late Neoproterozoic–earliest Cambrian hyper-extended continental margin that was dismembered and thrust westwards over the adjacent continental margin during the Cambro-Ordovician Delamerian Orogeny. Ultramafic rocks include serpentinized harzburgite of inferred subcontinental lithospheric origin that had already been exhumed at the seafloor before sedimentation commenced, whereas mafic rocks exhibit mainly enriched- and normal-type mid-ocean ridge basalt (E- and N-MORB) compositions consistent with emplacement in an oceanic setting. These lithologies and their metasedimentary host rocks predate deposition of the Cambrian Kanmantoo Group and are more likely to represent temporal equivalents of the older Normanville Group or underlying Neoproterozoic Adelaide Supergroup. The Kanmantoo Group is host to basaltic rocks with higher degrees of crustal contamination and yields detrital zircon populations dominated by 600–500 Ma ages. Except for quartz greywacke confined to the uppermost part of the sequence, metasedimentary rocks in the Glenelg River Complex are devoid of detrital zircon, and are interstratified with subordinate amounts of metachert and carbonaceous dolomitic slate suggestive of deposition in a deep-marine environment far removed from any continental margin. Seismic reflection data support the idea that the Glenelg River Complex is underlain by mafic and ultramafic rocks, and preclude earlier interpretations based on aeromagnetic data that the continental margin incorporates a thick pile of seawards-dipping basaltic flows analogous to those of volcanic margins in the North Atlantic. Correlative hyper-extended continental rift margins to the Glenelg River Complex occur along strike in formerly contiguous parts of Antarctica.Supplementary material:Geochemical data for mafic and ultramafic rocks in the Glenelg River Complex and correlative terranes, and U–Th–Pb data for western Victoria gabbros are available at http://www.geolsoc.org.uk/SUP18821

Geochemistry and trace element behaviors of eclogite during its exhumation in the Xitieshan terrane, North Qaidam UHP belt, NW China

The Xitieshan terrane, located in the central part of the North Qaidam ultrahigh pressure (UHP) metamorphic belt, China, is mainly composed of orthogneiss and paragneiss and a few intercalated eclogite layers and boudins. Based on their bulk-rock TiO2-contents, the eclogites can be subdivided into a high-Ti group (TiO2>2%) and a low-Ti group (TiO2<2%). Whole-rock major and trace element analyses revealed that the protoliths of the low-Ti eclogites are normal-type mid-ocean ridge basalts (N-MORB), whereas those of the high-Ti eclogites are either enriched-type mid-ocean ridge basalts (E-MORB) or near ridge seamount basalts, respectively. The Sr–Nd isotopes of eclogites of both groups are similar to those of MORB. Those of the low-Ti eclogites are characterized by positive εNd(T) and restricted ISr values and therefore provide further evidence for the formation of the protoliths of the eclogites in an oceanic environment. On the other hand, the Sr–Nd isotopes of high-Ti eclogites show mainly positive but also some negative εNd(T) values and relatively broadly distributed ISr values, indicating minor crustal contamination of the ocean floor basalts. Considering available 750–877Ma protolith ages preserved in zircon cores, it is inferred that some of the eclogites derived from Neoproterozoic protoliths were emplaced onto the crust far ahead of the Paleozoic deep subduction, while the other eclogites originate from a different oceanic crust, e.g., the Paleo-Qilian ocean, indicating multiple orogenies in the geological history of the Xitieshan terrane, China.Whole-rock and in-situ LA-ICP-MS mineral trace element analyses of eclogites revealed two stages of fluid behavior during retrogression that correspond to the two exhumation stages uncovered by phase equilibrium calculations. The mineral scale trace element distributions and trace element inheritance of newly formed amphibole from its precursors indicate that, at the peak metamorphic stage (M1) and at the earlier (eclogite facies) overprint (M2), the fluid was internally controlled by the rock itself. Within a mafic lens, the amount of water-soluble elements (e.g., Rb, Sr, Ba, U, Pb and LREE), observed in the whole-rock compositions as well as in amphiboles, increases from the core (phengite-bearing eclogite) to the rim (amphibolite) and implies an external fluid source for the amphibolite facies retrogression (M3) which should be helpful for the final exhumation of UHP eclogite.

Geochemistry of mid-ocean ridge mafic intrusives from the Manipur Ophiolitic Complex, Indo-Myanmar Orogenic Belt, NE India

Mafic intrusives emplaced within the melange zone of the Manipur Ophiolitic Complex are subalkaline- tholeiitic affinity with Fe-enrichment. Based on the field occurrences, textures-mineralogy and whole-rock compositions, these mafic intrusives can be identified as type-I (gabbro intrusives) and type-II (basalt-dolerite dykes). The type-I resembling enriched-type mid-ocean ridge basalt (E-MORB) shows moderate LREE enrichment (La N /Sm N = 2.5-2.6), slightly enriched MORB normalized HFSE patterns possibly represent melts derived from enriched MORB sub-oceanic mantle sources by small degree of partial melting. The other type-II has normal-type mid-ocean ridge basalt (N-MORB) geochemical features, as it exhibits nearly flat to depleted LREE (La N /Sm N = 1.0-0.6), flat MORB normalized HFSE patterns with slight LREE/HREE depletion (Ce N /Yb N = 1.37-0.46). It might have been derived from depleted MORB type sub-oceanic mantle source. The MORB signature displayed by these mafic intrusives indicates that they are dismembered fragments of oceanic crust generated at mid-ocean spreading ridge system and support the hypothesis that the Manipur ophiolites was initially formed in the divergent plate margin.

Geochemical characteristics of Carboniferous greenstones in the Inner Zone of Southwest Japan

Abstract Greenstones, representing remnants of paleo‐oceanic crust, occur in Permian and Jurassic accretionary complexes of the Inner Zone in the Southwestern Japan arc. The formation age of most of the greenstones is early Carboniferous, based on fossil ages for overlying limestones and Sm‐Nd isotope ages of the greenstones themselves. The geochemistry of such greenstones is similar to those of present‐day oceanic islands. Greenstones of the Permian accretionary complex (Akiyoshi belt) are alkalic and tholeiitic in composition. Some alkali basalts show peculiar features from an EM‐1 mantle source, such as the Gough Island and Tristan da Chunha basalts in the South Atlantic. Greenstones of the Jurassic accretionary complex (Tamba belt) are also alkali and tholeiitic basalts with both basalt types in the northern part of the Tamba belt coming from strongly depleted characters similar to a mid‐ocean ridge basalt source mantle. The variable geochemistry of the oceanic basalts is explained by hypothesis on existence of a Carboniferous mantle plume below the spreading ridge which divides the Farallon and Izanagi plates. The Akiyoshi belt seamounts and/or oceanic islands of the Farallon plate and Tamba belt seamounts and/or oceanic islands of the Izanagi plate formed simultaneously by the upwelling of the thermal plume. Some part of the Akiyoshi belt basalts originated locally from an EM‐1 mantle source, while basalts from the northern parts of the Tamba belt have a normal‐type mid‐ocean ridge basalt (N‐MORB) source component. Existence of an N‐MORB signature is consistent with the presence of a spreading center in a Carboniferous ‘Pacific Ocean’ that caused separation of the Farallon and Izanagi plates. Disparity in accretion ages of the basaltic rocks in the Permian and Jurassic may have been caused by differences in the relative motion of the two plates.

Sr–Nd isotope ratios of gabbroic and dioritic rocks in a Cretaceous‐Paleogene granite terrain, Southwest Japan

Abstract Whole‐rock chemical and Sr and Nd isotope data are presented for gabbroic and dioritic rocks from a Cretaceous‐Paleogene granitic terrain in Southwest Japan. Age data indicate that they were emplaced in the late Cretaceous during the early stages of a voluminous intermediate‐felsic magmatic episode in Southwest Japan. Although these gabbroic and dioritic rocks have similar major and trace element chemistry, they show regional variations in terms of initial Sr and Nd isotope ratios. Samples from the South Zone have high initial 87Sr/86Sr (0.7063–0.7076) and low initial Nd isotope ratios (ɛNd, −2.5 to −5.3); whereas those from the North Zone have lower initial 87Sr/86Sr (usually less than 0.7060) and higher Nd isotope ratios (ɛNd, −0.8 to + 3.3). Regional variations in Sr and Nd isotope ratios are similar to those observed in granitic rocks, although gabbroic and dioritic rocks tend to have slightly lower Sr and higher Nd isotope ratios than granitic rocks in the respective zones. Limited variations in Sr and Nd isotope ratios among samples from individual zones may be attributed partly to a combination of upper crustal contamination and heterogeneity of the magma source. Contamination of magmas by upper crustal material cannot, however, explain the observed Sr and Nd isotope variations between samples from the North and South Zones. Between‐zone variations would reflect geochemical difference in magma sources. The gabbroic and dioritic rocks are enriched in large ion lithophile elements (LILE) and depleted in high field strength elements (HFSE), showing similar normal‐type mid‐ocean ridge basalt (N‐MORB) normalized patterns to arc magmas. Geochronological and isotopic data may suggest that some gabbroic and dioritic rocks are genetically related to high magnesian andesite. Alternatively, mantle‐derived mafic or intermediate rocks which were underplated beneath the crust may be also plausible sources for gabbroic and dioritic rocks. The magma sources (the mantle wedge and lower crust) were isotopically more enriched beneath the South Zone than the North Zone during the Cretaceous‐Paleogene. Sr and Nd isotope ratios of the lower crustal source of the granitic rocks was isotopically affected by mantle‐derived magmas, resulting in similar initial Sr and Nd isotope ratios for gabbroic, dioritic and granitic rocks in each zone.

Partial melting of a refractory subducted slab in a Paleoproterozoic island arc: Implications for global chemical cycles

This study focuses on the rare occurrence of high-Fe, low-Si tholeiitic basaltic rocks of a Paleoproterozoic (1.9 Ga) backarc-arc succession (Amisk Lake, Flin Flon Greenstone Belt, Canada) and their implications for subarc mantle processes and the global elemental budget of Ta, Nb, and Ti. Volcanism at Amisk Lake is characterized by interbedded tholeiitic and boninite-like backarc basalts, overlain by intermediate to felsic calc-alkalic arc volcanic rocks. The change from dominantly backarc to arc volcanism is marked by picritic flows overlain by the high-Fe tholeiites. The high-Fe tholeiites are characteristically high in Fe 2 O 3 T (15–21 wt%), low in SiO 2 (39–45 wt%), and slightly light rare earth element enriched ([La/Sm] n = 1.3–1.93). They are unusually high in Ta, Nb, and Ti (normally depleted in arc-related rocks), and low in Zr, Hf, and Y, relative to normal-type mid-ocean ridge basalt. The petrogenesis of these rocks requires high degrees of partial melting of a dehydrated subducting slab, at low pressure. The high-Fe tholeiites represent either direct partial melts of the slab, or partial melts of the overlying mantle wedge, which has been metasomatized by this slab melt. These rocks provide evidence for the retention of Ta, Nb, and Ti in subducting slabs, and the subsequent isolation of these elements in the deep mantle, thus explaining the lack of complementarity of Ta, Nb, and Ti between the continental crust and depleted mantle. Conditions of higher heat flow and rapid subduction of young oceanic crust during the Paleoproterozoic could have resulted in partial melting of the subducted slab at shallow depths.

Sr, Nd and Pb isotopic variation along the Pacific–Antarctic risecrest, 53–57°S: Implications for the composition and dynamics of the South Pacific upper mantle

Sr, Nd and Pb isotope data for basalts from spreading axes and off-axis volcanoes near the Pacific–Antarctic risecrest, from Vacquier transform to just south of Udintsev transform, reveal an isotopically heterogeneous upper mantle. The isotopic composition of the mantle is represented by three end-members: (1) the `depleted' source of the bulk of Pacific normal-type mid-ocean ridge basalts (N-MORB); (2) an `enriched' source that produces basalts of the Hollister Ridge; and (3) a source, restricted to two adjacent sample locales, similar to that of Indian MORB. The distribution of these isotopic heterogeneities along the Pacific–Antarctic risecrest suggests two alternative hypotheses on the nature and dynamics of the south Pacific upper mantle. The whole area could be a single N-MORB mantle domain that shows a weak but continuous increase in 143Nd/ 144Nd from northeast to southwest across more than 2000 km of sea floor. The gradient is unrelated to the Louisville hotspot because Louisville basalts have low 143Nd/ 144Nd and the hotspot's influence along the ridge is spatially limited and near the high 143Nd/ 144Nd southwestern end of the gradient. The gradient appears consistent with a southwestward flow of the Pacific N-MORB-type mantle that has been proposed mainly on the basis of ridge morphology. That the N-MORB mantle domain is continuous across Heezen suggests that large-scale magmatic segmentation is not related to the largest structural offsets of the Pacific ridges. Alternatively, the higher 87Sr/ 86Sr, ΔNd and Δ8/4 of samples from southwest of the Heezen transform relative to those from the northeast could result from southwestward pumping of both plume and Indian Ocean-type mantle material by the Louisville hotspot. The Heezen transform forms a prominent tectonic and mantle domain boundary that prohibits the Louisville- and Indian Ocean-type mantle from flowing towards and contaminating the depleted Pacific-type source in the northeast.

Major and trace element chemistry and D/H, 18O/16O, 87Sr/86Sr and 143Nd/144Nd ratios of rocks from the spreading center of the Okinawa Trough, a marginal back-arc basin.

Basalts (OKTB) from the Okinawa Trough (OKT), which is a now opening marginal back-arc basin, are chemically quite similar to those from the intra-oceanic back-arc basins such as the Mariana Trough. Both olivine-normative and quartz-normative basalts occur, and major and trace element concentrations show a systematic variation, probably derived by differentiation of the magma. D/H, 87Sr/86Sr and 143Nd/144Nd ratios are almost identical in the dacite and basalts, and the 18O/16O ratio in the former is only slightly higher than in the latter, suggesting that the dacites are ultimately cognate with basalts. Concentrations of Al2O3, total iron, MgO, CaO, Na2O, Ni, Cr, and V in OKTB are within the range of variation in normal type mid-ocean ridge basalts (N-MORB), whereas OKTB have significantly higher K2O, Rb and Sr contents and D/H, 18O/16O and 87Sr/86Sr ratios and a lower 143Nd/144Nd ratio than N-MORB have. These chemical and isotopic features of OKTB can be well explained by the concept, which has been proposed by many authors for magma genesis in the intraoceanic back-arc basins, that the H2O-rich primary magma is generated from normal-type mantle peridotite modified by component from the subducted slab. Compared with those from intra-oceanic back-arc basins, the rocks from the OKT have distinctly higher 87Sr/86Sr and lower 143Nd/144Nd ratios, reflecting the higher contribution of continent-derived sedimentary components to the magma source. Oxygen, Sr and Nd isotopic characteristics of the rocks of the OKT are fitted well with paired two-component mixing models: one for the source-mixing and another for the crustal contamination.

Petrology and geochemistry of volcanic rocks dredged from the Okinawa Trough, an active back-arc basin.

Volcanic rocks dredged from the Okinawa Trough, an active back-arc basin behind the Ryukyu island arc (IA)-trench system associated with a subduction of the Philippine Sea plate, encompass a wide variety of petrology and geochemistry, ranging from basalts to rhyolites through andesites. The basalts, consisting of highly vesicular pillow lavas, are moderately porphyritic with phenocrysts of olivine, plagioclase and minor clinopyroxene. Their glass and bulk rock compositions, especially their large-ion lithophile element (LILE) abundances, are transitional between the Ryukyu IA basalts (LILE-enriched) and normal-type mid-ocean ridge basalts (N-MORBs, LILE-depleted). The rhyolites are moderately porphyritic, including phenocrysts of plagioclase and orthopyroxene with or without clinopyroxene and hornblende, and have the close geochemical affinity with the Ryukyu IA rhyolites. It follows that the rhyolites were derived from an IA-type source material similar to that for the Ryukyu IA volcanics, but the basalts were derived from a less LILE-enriched source material approaching that for N-MORBs. Available K-Ar ages indicate the rhyolitic volcanism prior to the basaltic one. It is, therefore, most likely that the magma source region beneath the Okinawa Trough has changed with time from an IA type to a N-MORB type during the back-arc rifting. On the other hand, the andesites are highly porphyritic with phenocrysts of plagioclase, orthopyroxene and clinopyroxene, and have the K-Ar age similar to that of the basalts. Most importantly, the andesites exhibit many lines of textural and compositional evidence for mixing between basaltic melt, and rhyolitic partial melt of pre-existing IA lower crust that was presumably heated by the former melt. This again suggests the structural transition of the magma source region beneath the Okinawa Trough, which may be most characteristic of the initial stage of back-arc basin volcanism in general.

Geochemical features of intraplate oceanic plateau basalts

Summary Coeval with the Cretaceous thermal uplift of a large region of the ‘old Pacific’ oceanic lithosphere was the widespread development of two expressions of intraplate volcanism: edifice building (ocean islands, seamounts) and submarine sheet-flows (oceanic plateaux). The geochemical features of the development of oceanic plateaux are reviewed using the Nauru Basin complex as a model. Ocean plateau basalts (OPBs) are mildly altered, poorly phyric, hy - or qz -normative tholeiites, rapidly extruded submarine sheet-flow lavas with a relatively uniform chemistry. Although their eruptive setting is apparently intraplate they have a chondrite-normalized incompatible-element-depleted pattern similar to normal-type mid-ocean ridge basalt (MORB) formed at ocean spreading centres. They are chemically distinguished from normal-type MORB by being enriched in selected incompatible elements and can be matched by various enriched or ‘transitional’ MORB types. Isotopically they have similarities to some types of ocean island basalt (OIB), such as Hawaii. They were probably derived by high degrees of melting from an upper mantle MORB-type source with an enriched OIB-type component; melts being mixtures derived from the two mantle components. In terms of crustal development they are characteristic of anomalously thickened oceanic crust and were formed in an off-axis (or near-axis) eruptive setting shortly after the development of axis-generated oceanic basement over which they flowed. OPB appears to be restricted to the widespread Cretaceous magmatic event.