Normal Mid-ocean Ridge Basalt Research Articles

Melt inclusions in olivine and plagioclase phenocrysts from Antarctic–Phoenix Ridge basalts: Implications for origins of N- and E-type MORB parent magmas

The Antarctic–Phoenix Ridge (APR) is a fossil spreading center in the Drake Passage, Antarctic Ocean. Spreading ceased in chron C2A (ca. 3.3Ma). Although the APR is a normal ridge that is not influenced by a hotspot, enriched (E-type) mid-ocean ridge basalt (MORB) coexists with normal (N-type) MORB in the ridge's axial region. The E-type APR basalt is relatively young (<3.1Ma) compared to the N-type basalt (>3.5Ma). The E-type basalt is characterized by elevated K2O/TiO2 (=0.4–0.8) and (La/Sm)N (=2.2–3.4) ratios, relative to the N-type basalt (K2O/TiO2=0.1–0.3; (La/Sm)N=0.7–0.8). To better understand the compositional variation in the APR basalts and their mantle source regions through time, silicate melt inclusions in primitive olivine (Fo87–89) and plagioclase (An85–89) phenocrysts from the N-type APR basalt were studied. Rehomogenized melt inclusions were analyzed by electron microprobe and LA-ICPMS for major and trace elements. The melt inclusions are more primitive than the host basalt, with Mg#s from 67.5 to 74.1. All inclusions exhibit patterns that are depleted in the light rare earth elements. The inclusions have K2O/TiO2 from 0.1 to 0.3 and (La/Sm)N ratios from 0.4 to 0.9; these values overlap with those of the N-type APR basalt. Furthermore, the melt inclusions have elevated (Lu/Hf)N and (Sm/Nd)N ratios compared to the E-type basalts. The N-type APR basalts do not contain any melt inclusions that are enriched in incompatible elements. The E-type basalt was generated by a low degree of partial melting of a relatively incompatible-element-enriched mantle source. In contrast, chemistries of melt inclusions and N-type basalts are compatible with high degrees of partial melting of an increasingly depleted mantle source. Assuming a veined or otherwise heterogeneous mantle, the absence of E-type inclusions from the N-type host has implications for cyclic magmatic activity beneath the APR. Multi-stage mantle melting and melt extraction from a composite source with sequential extraction of melt fluids might give rise to the primary melt diversity documented in the APR axis. The mantle source of the N-type melts may have been the residue from an earlier phase of melting that removed the easily melted, enriched components. The N-type APR basalt studied represents melt at the end of single cycle, whereas the E-type basalt may represent the early stage of a new pulse that was dominated by highly enriched components.

Heterogeneous upper mantle Ne, Ar and Xe isotopic compositions and a possible Dupal noble gas signature recorded in basalts from the Southwest Indian Ridge

Variations in heavy noble gas (Ne, Ar, Xe) isotopic compositions provide unique insights into the nature of heterogeneities in the mantle. However, few precise constraints on mantle source heavy noble gas isotopic compositions are available due to ubiquitous shallow-level atmospheric contamination. As a result, the extent of heterogeneity in mid-ocean ridge basalt (MORB) mantle source Ne, Ar and Xe isotopic compositions is unknown. Basalts from the ultra-slow spreading Southwest Indian Ridge (SWIR) between 7°E and 25°E exhibit remarkable variability in He isotopic composition: SWIR 4He/3He spans half the total range observed in all mantle-derived basalts. Therefore, basalts from the SWIR provide a unique window into upper mantle heterogeneity and present an ideal opportunity to characterize variations in upper mantle heavy noble gas isotopic composition. Here we present new high-precision Ne, Ar and Xe isotopic compositions as well as He, CO2, Ne, Ar and Xe abundances measured in basalt glasses from the SWIR. After correcting the measured values for shallow-level atmospheric contamination, significant and systematic variations in mantle source Ne, Ar and Xe compositions are observed. We note that large variations in source 40Ar/36Ar and 129Xe/130Xe are observed in basalts removed from the influence of known hotspots, indicating a heterogeneous mid-ocean ridge basalt source. Thus, SWIR heavy noble gas data reveal a greater degree of source heterogeneity than is evident in the 4He/3He systematics alone. The observed heavy noble gas isotopic heterogeneities imply that the average MORB source 40Ar/36Ar and 129Xe/130Xe ratios are not yet well-determined.Variation in MORB source 40Ar/36Ar and 129Xe/130Xe at a given 4He/3He and 21Ne/22Ne may reflect heterogeneous recycling of atmospheric Ar and Xe. In particular, we find low mantle source 40Ar/36Ar and 129Xe/130Xe ratios in the eastern region of the study area, which may reflect the noble gas signature of the Dupal mantle domain. Our observations require that the sampled mantle domain either is very ancient (>4.45Ga) or has been metasomatized by subduction zone fluids carrying recycled atmospheric Ar and Xe. However, our Xe isotopic measurements indicate that differences between MORB and ocean island basalt (OIB) source noble gas compositions cannot be explained by recycling of atmospheric noble gases alone. Instead, a relatively undegassed mantle reservoir is required to account for OIB noble gases. The SWIR data demonstrate that the reservoir supplying primordial noble gases to mantle plumes differentiated from the MORB source early in Earth history, and the two reservoirs have not been homogenized over 4.45Ga of mantle convection.

Origin of Late Oligocene to Middle Miocene Adakitic Andesites, High Magnesian Andesites and Basalts from the Back-arc Margin of the SW and NE Japan Arcs

Late Oligocene to Middle Miocene adakitic andesites are found in the southern part of Okushiri Island, the northern Noto Peninsula and in the Toyama region in the present-day back-arc margin of the SW and NE Japan arcs. On Okushiri Island, adakitic andesite is accompanied by moderately alkaline basalt, whereas on the Noto Peninsula, adakitic andesite has been erupted along with high magnesian andesite (HMA), bronzite andesite and tholeiitic basalt. Adakitic andesites from all three locations are characterized by high Sr/Y and low Y, and have higher MgO contents than adakitic melts generated by experimental melting of metabasalt and amphibolite. They also have higher Ni and Cr contents than either Archaean tonalite–trondhjemite–granodiorite (TTG) suites or Early Cretaceous adakitic granites, which have been attributed to partial melting of subducted oceanic crust. The Noto Peninsula adakitic andesite has Sr and Nd isotopic compositions identical to normal mid-ocean ridge basalt (N-MORB), whereas the Okushiri Island and Toyama adakitic andesites are more isotopically primitive than N-MORB. The Noto Peninsula primary adakitic melt was derived from subducted oceanic N-MORB crust, whereas the Okushiri Island and Toyama primary adakites are interpreted as melts of subducted N-MORB and sediment that have subsequently interacted with the overlying mantle wedge peridotite. To explain the comagmatism of adakite, HMA and basalt, the following model is proposed. A hydrated adakitic diapir ascends from the subducting slab and is heated because it enters the overlying hot mantle wedge. The subsequent establishment of thermal and H2O gradients in the adakitic diapir and surrounding mantle wedge peridotite results in concurrent generation of adakitic andesite magma in the inner adakitic diapir region (low temperature and high H2O content), HMA and bronzite andesite magmas in the intermediate peridotite region (intermediate temperature and H2O content), and tholeiitic basalt magma in the outer peridotite region (high temperature and lower H2O content). Comagmatic adakite and mildly alkaline basalt are found in cooler and wetter adakitic diapirs and hotter and drier peridotite regions respectively. The most likely tectono-magmatic situation for the genesis of adakitic magmas in this example of a cool subduction zone involves upwelling of hot asthenosphere into the subcontinental lithosphere beneath the back-arc side of the NE Japan arc and northern end of the SW Japan arc, during the period spanning the pre-Japan Sea opening to syn-opening stages. The unusually high temperature conditions established in the mantle wedge owing to upwelling of hot asthenosphere caused partial melting of the relatively cool subducting Pacific plate, resulting in the generation of adakitic magmas.

Back-arc and post-collisional volcanism in the Palaeoproterozoic Granites-Tanami Orogen, Australia

Pillow basalt, dolerite sills, and doleritic dykes are the major products of volcanism in the Granites-Tanami Orogen, and provide a window for understanding the magmatism and tectonic evolution of the orogen. The pillow basalt and dolerite sills from the Stubbins Formation have similar geochemical features, suggesting that they are genetically related. The pillow basalt and dolerite sills (ca. 1864Ma) have tholeiitic affinity, with high TiO2 contents (0.9–1.88wt.%), low Mg# (40–55), and positive ɛNd(t) values (+0.6 to +2.8). Enriched mid-ocean ridge basalt (E-MORB)-normalized REE and primitive mantle-normalized trace element patterns for these rocks show relatively flat REE distribution, slight enrichment of LILE (Rb, Th, U, and K) and weak negative anomalies of HFSE (Nb and P), which are the characteristics similar to those of the enriched back-arc basin basalt (E-BABB), but different from those of modern normal-MORB (N-MORB), E-MORB, oceanic island basalt (OIB), depleted back-arc basin basalt (D-BABB), and island arc basalt (IAB). The pillow basalt and dolerite sills are interpreted to be the results of a high-degree decompressional melting (5–15%) of the asthenosphere source in the spinel stability field. The mantle source of these rocks have primitive mantle signature with input of 3–4% subduction material. Convective flow operating during subduction, circulating the enriched material from wedge corner into the asthenospheric mantle beneath the Granites-Tanami back-arc basin and generated the mild enriched mafic magma that was emplaced as dolerite sills or erupted as basalt.“Late” (ca. 1800Ma) doleritic dykes in the region have high contents of MgO (7.15–14.0wt.%), Cr (262–757ppm), K2O (2.25–4.85, wt.%), with high Mg# (57–72), K2O/Na2O>2, and negative ɛNd(t) values range from −4.5 to 0.3. Their REE and trace element characteristics, such as enrichment of Rb, Ba, Th, U, K, LREE, and negative Nb–Ta anomalies, are similar to those of the upper (UCC) and middle continental crust (MCC), but more enriched than those of the lower continental crust (LCC). The interpreted continental–continental collision between the Granites-Tanami and Arunta orogens during the Tanami Orogeny (DGTO2; ca. 1800Ma) resulted in the thickening of the lower crust and lithospheric mantle root along the Willowra Lineament. The thickened lithospheric mantle root has higher density than the upper asthenospheric mantle leading to gravitational instability and resulting in convective removal. The detached denser lithospheric mantle root sank into asthenosphere triggered upwelling of the hot asthenospheric mantle material and extension. It is proposed here that melts derived from partial melting of the previously subduction-metasomatized lithospheric mantle with crustal assimilation generated the doleritic dykes, and modelling of geophysical data supports this interpretation. This post-collisional magmatism is approximately synchronously with the ca. 1800Ma gold mineralization and extensive granitic magmatism in the Granites-Tanami Orogen.

Geochemistry and tectonic significance of ophiolites along the İzmir–Ankara–Erzincan Suture Zone in northeastern Anatolia

Abstract The Ankara–Erzincan suture zone includes large bodies of ophiolite and ophiolitic melange in northeastern Anatolia. The ophiolitic bodies are (1) Refahiye (Erzincan), (2) Şahvelet (Erzurum), (3) Karadağ (Erzurum) and (4) Kırdağ (Erzurum). The ophiolite-related units include well-preserved sections of oceanic lithospheric and accretionary melanges with local blueschist assemblages. The ophiolite-related units in NE Anatolia are unconformably overlain by Campanian–Maastrichtian-aged sediments that were later imbricated with the ophiolitic rocks. Geochemical data for the individual ophiolite sections indicate a tholeiitic composition, depletion in Nb, enrichment in large ion lithophile elements, parallel to slightly depleted high field strength element patterns (compared with normal-mid ocean ridge basalt), and slightly light rare earth element-depleted to parallel rare earth element trends. These features suggest progressive source depletion towards island arc tholeiites and finally boninites. A fore-arc setting is proposed for the generation of the ophiolites. In contrast, the volcanic rocks from the mélange units exhibit tholeiitic to alkaline compositions and either depletion or enrichment of rare earth element and high field strength elements. Seamount-type alkaline and subduction-related tholeiitic basaltic rocks were apparently juxtaposed during subduction/accretion. Models involving either a single north-dipping subduction zone or two north-dipping subduction zones may be applicable. Both models involve the generation of supra-subduction zone-type ophiolites in a forearc setting, an accretionary prism (with blueschists) and a volcanic arc during the Late Cretaceous. The ophiolites, ophiolitic melange and related blueschists were emplaced either northwards onto the Pontide margin or southwards over the passive margin of the Tauride platform.

Enriched and depleted characters of the Amnay Ophiolite upper crustal section and the regionally heterogeneous nature of the South China Sea mantle

The volcanic section of the Middle Oligocene Amnay Ophiolite in Mindoro, Philippines has previously been shown to be of normalmid-oceanic ridge basalt (NMORB) composition. Here we report for the first time an enriched mantle component that is additionally recorded in this crustal section. New whole rock major and trace element data are presented for nine mafic volcanic rocks from a section of the ophiolite that has not been previously examined. These moderately evolved tholeiitic basalts were found to have resulted from the bulk mixing of ∼10% ocean island basalt components with depleted mantle. Drawing together various geochemical characteristics reported for different rock suites taken as representatives of the South China Sea crust, including the enriched MORB (EMORB) and NMORB of the East Taiwan Ophiolite, the NMORB from previous studies of the Amnay Ophiolite and the younger ocean floor eruptives of the Scarborough Seamount–Reed Bank region, a veined mantle model is proposed for the South China Sea mantle. The NMORB magmatic products are suggested to have been derived from the more depleted portions of the mantle whereas the ocean island basalt (OIB) and EMORB-type materials from the mixing of depleted and veined/enriched mantle regions.

Downhole variation of lithium and oxygen isotopic compositions of oceanic crust at East Pacific Rise, ODP Site 1256

Bulk rock lithium and oxygen isotope compositions from ODP Site 1256 were analyzed to investigate the seawater circulation in the upper oceanic crust formed at the East Pacific Rise (EPR). The upper extrusive basalts have δ18O values from +6.1‰ to +9.2‰, reflecting alteration of oceanic crust by seawater at low temperatures (&lt;200–250°C). Bulk rocks from the sheeted dike complex and plutonic section have overall lower δ18O values (+3.0‰–+5.5‰). In the sheeted dike complex bulk rock δ18O values gradually decrease with depth, and then increase toward the fresh MORB δ18O value after reaching a minimum of +3.0‰ at ∼1350 m below seafloor (mbsf). The entire sampled crust is dominated by rocks with low lithium contents relative to fresh MORBs except for a few localized Li enrichment. The upper volcanic zone is characterized by a spread of δ7Li from low to high values relative to average unaltered MORB values (δ7Li = +3.4 ± 1.4‰). The presence of rocks with low δ7Li values in the upper crust most likely indicates zones of upwelling of relatively hot (∼200–250°C) hydrothermal fluids. In the sheeted dike complex, bulk rock δ7Li values show wide range of variation, but exhibit a general trend from enriched to depleted values at ∼1280 mbsf and then return to that for fresh MORB within the upper tens of meters of the plutonic section at the bottom of the after reaching a minimum at ∼1350 mbsf (δ7Li = −1.6‰). The downhole pattern of δ7Li principally reflects variations in water‐rock ratio (w/r) together with a downhole increase of temperature. Seawater flow in the upper volcanic zone is likely to be channeled with generally small but variable w/r ratios. The w/r ratios increase rapidly with depth in the lower volcanic section into the sheeted dike complex indicating water dominated pervasive hydrothermal flow due to intensive upwelling of hydrothermal fluids.

Lithium Isotope Variations in Ocean Island Basalts—Implications for the Development of Mantle Heterogeneity

Lithium elemental and isotopic compositions of 33 glass and whole-rock samples from nine oceanic island regions were determined to characterize the Li inventory of the deep mantle. The Li contents of the investigated lavas range from 1·5 to 13·3 μg g−1, whereas δ7Li ranges from 2·4 to 4·8‰. There are weak co-variations between the Li/Y, δ7Li, and Sr–Nd–Pb isotope compositions of the lavas, indicating that the Li elemental and isotopic characteristics of ocean island basalt to some extent reflect mantle source heterogeneity. In detail, HIMU-type lavas are characterized by δ7Li values (up to 4·8‰) slightly heavier than those for average normal mid-ocean ridge basalt (3·4 ± 1·4‰) and by comparatively low Li contents; EM1-type lavas are characterized by isotopically light Li (average 3·2‰) and relative Li enrichment, whereas EM2-type lavas tend to heavier δ7Li values (up to 4·4‰) with high Li concentrations. The Li contents and isotope characteristics of HIMU-type lavas are consistent with recycling of altered and dehydrated oceanic crust, whereas those of the EM1-type lavas can be attributed to sediment recycling. The Li characteristics of EM2-type lavas may reflect reworking of mantle wedge material that has been infiltrated by fluids derived from the subducting plate.

Variety and origin of magmas on Shatsky Rise, northwest Pacific Ocean

Shatsky Rise consists of thick (∼30 km maximum) basaltic crust with various geochemical compositions. Geochemistry data indicate that four magma types exist on the plateau; namely normal, low‐Ti, high‐Nb, and U1349 types. The normal type is the most abundant in volume and appears on all three large edifices of the plateau: Tamu, Ori, and Shirshov massifs. Composition of the normal type is similar to normal mid‐ocean ridge basalt (N‐MORB) composition, but with slight relative enrichment of the more incompatible elements. The low‐Ti type is distinguished from the normal type basalt by slightly lower Ti content at a given MgO. Composition of the high‐Nb type is characterized by distinctively high contents of incompatible trace elements. U1349 type basalts are composed of more primitive and depleted compositions compared with the others. The normal type basalts constitute ∼94% of the lava units of the oldest Tamu Massif and non‐normal types (i.e., the other three types) basalts comprise ∼57% on the younger Ori Massif, implying that geochemical compositions may have become heterogeneous with time. Petrological examination demonstrates that compositions of the normal‐, low‐Ti‐, and high‐Nb‐type basalts evolved through fractional crystallization of olivine, plagioclase, and augite in shallow magma chambers (&lt;200 MPa). Model calculations of immobile trace elements estimate that the normal type basalt can be formed by ∼15% melting of a depleted mantle source in the presence of residual garnet. This degree of melting is similar to N‐MORB, but the larger effect of residual garnet during petrogenesis implies that a greater depth of melting.

Structure, geochemistry, and tectonic evolution of trench-distal backarc oceanic crust in the western Norwegian Caledonides, Solund-Stavfjord ophiolite (Norway)

Research Article| July 01, 2012 Structure, geochemistry, and tectonic evolution of trench-distal backarc oceanic crust in the western Norwegian Caledonides, Solund-Stavfjord ophiolite (Norway) Harald Furnes; Harald Furnes † 1Department of Earth Science & Centre for Geobiology, University of Bergen, 5007 Bergen, Norway †E-mail: harald.furnes@geo.uib.no Search for other works by this author on: GSW Google Scholar Yildirim Dilek; Yildirim Dilek 2Department of Geology & Environmental Earth Science, Miami University, Oxford, Ohio 45056, USA3School of Earth Science and Mineral Resources, China University of Geosciences, Beijing 100083, China Search for other works by this author on: GSW Google Scholar Rolf Birger Pedersen Rolf Birger Pedersen 1Department of Earth Science & Centre for Geobiology, University of Bergen, 5007 Bergen, Norway Search for other works by this author on: GSW Google Scholar GSA Bulletin (2012) 124 (7-8): 1027–1047. https://doi.org/10.1130/B30561.1 Article history received: 07 Jun 2011 rev-recd: 10 Feb 2012 accepted: 16 Feb 2012 first online: 08 Mar 2017 Cite View This Citation Add to Citation Manager Share Icon Share Facebook Twitter LinkedIn MailTo Tools Icon Tools Get Permissions Search Site Citation Harald Furnes, Yildirim Dilek, Rolf Birger Pedersen; Structure, geochemistry, and tectonic evolution of trench-distal backarc oceanic crust in the western Norwegian Caledonides, Solund-Stavfjord ophiolite (Norway). GSA Bulletin 2012;; 124 (7-8): 1027–1047. doi: https://doi.org/10.1130/B30561.1 Download citation file: Ris (Zotero) Refmanager EasyBib Bookends Mendeley Papers EndNote RefWorks BibTex toolbar search Search Dropdown Menu toolbar search search input Search input auto suggest filter your search All ContentBy SocietyGSA Bulletin Search Advanced Search Abstract The Late Ordovician (443 Ma) Solund-Stavfjord ophiolite complex in west Norway represents the youngest phase of oceanic crust formation in the western Norwegian Caledonides. It contains three structural domains with different crustal architecture that formed during two episodes of seafloor spreading evolution of a Late Ordovician marginal basin. The fossil oceanic crust of the younger episode contains pillow lavas, massive sheet flows, and hyaloclastites, NE-trending sheeted dikes, and high-level isotropic gabbros. The pillow lava versus massive sheet flow distribution and the occurrence of an extensive sheeted dike complex in the Solund-Stavfjord ophiolite complex are typical of in situ oceanic crust developed at modern intermediate-spreading mid-ocean ridges. The Solund-Stavfjord ophiolite complex lavas and dikes are composed predominantly of normal mid-ocean-ridge basalt (N-MORB) Fe-Ti basalts, and their trace-element patterns indicate a weak subduction influence. The Nd isotope data of these rocks suggest derivation of their magmas from an isotopically homogeneous melt source with no indication of continental crustal contamination. The Solund-Stavfjord ophiolite complex extrusive sequence contains phyllite interlayers and is conformably overlain by a continentally derived, quartz-rich metasandstone that is intercalated with sills of N-MORB basaltic lavas and shallow-level intrusions. The geochemical features of the upper-crustal rocks of the Solund-Stavfjord ophiolite complex indicate their formation from magmas in which the melt evolution involved only minor or no slab-derived fluids. The evolution of the Solund-Stavfjord ophiolite complex oceanic crust occurred in a short-lived (<20 m.y.), trench-distal, continent-proximal backarc basin, adjacent to the eastern margin of Greenland-Laurentia, during the closure of Iapetus. This inferred tectonic setting is reminiscent of the modern Andaman Sea at the eastern periphery of the Indian Ocean. You do not have access to this content, please speak to your institutional administrator if you feel you should have access.

Reactive Melt Flow as the Origin of Residual Mantle Lithologies and Basalt Chemistries in Mid-Ocean Ridges: Implications from the Red Hills Peridotite, New Zealand

A reactive flow geochemical model based on pMELTS thermodynamic calculations explains the observed modal, major, and trace element variations in the Red Hills peridotite, New Zealand. The model also reproduces the major and trace element chemical variations in mid-ocean ridge basalts (MORB) observed in present-day spreading ridges. The Red Hills peridotite is thought to originate from palaeo-MOR magmatic processes in the mantle–Moho transition zone. The peridotite body consists of a harzburgite matrix and dunite channels. The harzburgite forms the Lower Unit and is intruded by replacive dunite channels in the Upper Unit. This lithology gradually turns into a massive dunite zone in which disseminated to lenticular clinopyroxene aggregates are present. The rare earth element (REE) abundances in the peridotite samples vary greatly depending on their lithologies. In the Lower Unit, REE are extremely depleted, whereas in the Upper Unit they are relatively enriched, in contradiction to the depleted lithologies. Our model consists of two stages. The first stage assumes melting of depleted MORB source mantle in the garnet stability field, and the second assumes reactions between residual solids and the melts from the first stage in the spinel stability field in an open system. The model explains the formation of depleted harzburgite and the formation of dunite channels in the harzburgite matrix well. The major and trace element compositions of the melts calculated by the model vary from ultra-depleted MOR melts in harzburgite to normal MORB in dunite, suggesting that these lithologies are residues of a palaeo-MOR. The model also explains the origins of the local and global geochemical trends observed in MORB and the geochemical variation in abyssal peridotite samples. Our model confirms the important role of reactive flow in the mantle–Moho transition zone beneath MORs.

Hydrogen and oxygen isotope behaviors during variable degrees of upper mantle melting: Example from the basaltic glasses from Macquarie Island

We present measurements of hydrogen and oxygen isotopes in MORB glasses from Macquarie Island (SW. Pacific Ocean) coupled with determination of bulk H2O content by two independent techniques: total dehydration and FTIR. The incompatible trace elements in these glasses vary by a factor of 12 to 17, with K2O varying from 0.1 to 1.7wt.%; these ranges reflect a variable degree of closed-system mantle melting, estimated from 1 to 15%. Water concentrations determined by the two techniques match well, yielding a range from 0.25 to 1.49wt.% which correlates positively with all of the measured incompatible trace elements, suggesting that water is un-degassed, and behaves conservatively during mantle melting. Also, the agreement between the FTIR-determined and extracted water contents gives us confidence that the measured isotopic values of hydrogen reflect that of the mantle. Comparison of the range of water content with that of other incompatible trace elements allows estimation of the water partition coefficient in lherzolite, 0.0208 (ranging from 0.017 to 0.023), and the water content in the source, 386ppm (ranging from 370 to 440ppm). We observe a fairly narrow range in δD and δ18O values of −75.5±4.5‰ and 5.50±0.05‰ respectively, that can be explained by partial melting of normal lherzolitic mantle. The measured δD and δ18O values of Macquarie Island glasses that range from nepheline- to hypersthene-normative, and from MORB to EMORB in composition, are identical to those in average global MORB. The observed lack of variation of δD and δ18O with 1 to 15% degree of mantle melting is consistent with a bulk melting model of δD and δ18O fractionation, in which water is rapidly scavenged into the first partial melt. The narrow ranges of δD and δ18O in normal mantle are mostly due to the buffering effect of clino- and orthopyroxenes in the residual assemblage; additionally, fast “wet” diffusion of oxygen and hydrogen isotopes through the melting regions may further smooth isotopic differences.

Geochemistry of basaltic lavas from the southern Lau Basin: input of compositionally variable subduction components

We present new major element, trace element, and Sr–Nd–Pb isotope data for 18 basaltic lavas and six glasses collected in situ from the Eastern Lau Spreading Centre (ELSC) and the Valu Fa Ridge (VFR). All lava samples are aphanitic and contain rare plagioclase and clinopyroxene microlites and microphenocrysts. The rocks are sub-alkaline and range from basalt and basaltic andesite to more differentiated andesite. In terms of trace element compositions, the samples are transitional between typical normal mid-ocean ridge basalt (MORB) and island arc basalt. Samples from the VFR have higher large ion lithophile element/high field strength element ratios (e.g. Ba/Nb) than the ELSC samples. VFR and ELSC Sr–Nd isotopic compositions plot between Indian MORB and Tonga arc lavas, but VFR samples have higher 87Sr/86Sr for a given 143Nd/144Nd ratio than ELSC analogues. The Pb isotopic composition of ELSC lavas is more Indian MORB-like, whereas that of VFR lavas is more Pacific MORB-like. Our new data, combined with literature data for the Central Lau Spreading Centre, indicate that the mantle beneath the ELSC and VFR spreading centres was originally of Pacific type in composition, but was displaced by Indian-type mantle as rifting propagated to the south. The mantle beneath the spreading centres also was variably affected by subduction-induced metasomatism, mainly by fluids released from the altered, subducting oceanic crust; the influence of these components is best seen in VFR lavas. To a first approximation, the effects of underflow on the composition and degree of partial melting of the mantle source of Lau spreading centre lavas inversely correlate with distance of the spreading centres from the Tonga arc. Superimposed on this general process, however, are the effects of the local geographic contrasts in the composition of subduction components. The latter have been transferred mainly by dehydration-generated fluids into the mantle beneath the Tonga supra-subduction zone.

Perovskite Phase Relations in the System CaO-MgO-TiO2-SiO2 and Implications for Deep Mantle Lithologies

Experiments at 20^97 GPa and 2000 K in the system CaO^MgO^ TiO2^SiO2 constrain phase relations involving Mg-rich and Ca-rich perovskite solid solutions at conditions relevant to the Earth’s deepTransition Zone and lower mantle. Bulk compositions were investigated with molar Ti/(Tiþ Si) up to 0·5 within the quasi-ternary ‘perovskite plane’ , which is defined by a reciprocal solution among the components MgSiO3, MgTiO3, CaSiO3, and CaTiO3. Multi-anvil experiments at 20 GPa and 2000 K on bulk compositions within the plane produce akimotoite coexisting with Ca-perovskites that lie close to the CaSiO3^CaTiO3 join. Higher-pressure experiments using a laser-heated diamond anvil cell constrain the position of a two-perovskite field that extends into the perovskite plane from the solvus along the MgSiO3^CaSiO3 binary join, where limited mutual solubility exists between MgSiO3 and CaSiO3 perovskites. On the join MgSiO3^MgTiO3, MgTiO3 solubility in MgSiO3 perovskite increases with pressure, with MgSi0·8Ti0·2O3 perovskite stable at 50 GPa. Limited reciprocal solution at 25 GPa results in an expansive two-perovskite field that occupies much of the Si-rich portion of the perovskite plane. Solution ofTi into Mg-rich and Ca-rich perovskites enhances the solubility of reciprocal Ca and Mg components, respectively. Increase in pressure promotes reciprocal solution, and the two-phase field collapses rapidly with pressure toward the MgSiO3^CaSiO3 join.We find that a single-phase, orthorhombic perovskite with near equimolar Ca and Mg is stable in a composition with Ti/ (Tiþ Si) of only 0·05 at 97 GPa, requiring that by this pressure the two-phase field occupies a small area close to the MgSiO3^CaSiO3 join. On the basis of experiments at 1500 K, temperature has only a mild effect on the position of the Ca-rich limb of the solvus. Ca(Ti,Si)O3 mineral inclusions in deep sublithospheric diamonds could not have formed in equilibrium with Mg-perovskite owing to their virtual lack of MgSiO3 component at pressures of Mg-perovskite stability, but may have equilibrated withTransition Zone MgSiO3-rich phases at lower pressures; this observation can be extended generally to near-endmember CaSiO3 inclusions. On an iron-free basis, the average bulk compositions of clinopyroxene^ilmenite and orthopyroxene^ilmenite megacrysts from kimberlites plot in single-perovskite fields at pressures greater than about 45 and 65 GPa, respectively, when projected onto the perovskite plane.We predict that the effect of iron will not be large, and estimate that single-phase perovskites may form at somewhat lower pressures than in the iron-free system. Thus, the origin of pyroxene^ilmenite megacrysts from single-phase perovskite solutions in the lower mantle is plausible on the basis of phase relations, although a lower pressure magmatic origin appears more likely. Deeply subducted Ti-rich lithologies such as ocean-island basalt will crystallize a single perovskite rather than a two-perovskite assemblage beginning at pressures of 80 GPa. Normal mid-ocean ridge basalt and primitive mantle peridotite are expected to remain within a two-phase perovskite field until Mg-perovskite transforms to post-perovskite.

Halogen systematics (Cl, Br, I) in Mid-Ocean Ridge Basalts: A Macquarie Island case study

The abundance of halogens (Cl, Br and I) in the Earth’s mantle has the potential to provide information about mantle metasomatism and volatile recycling in subduction zones. Basalt melts sample different parts of the Earth’s mantle, but few data are available for Br or I in basalt melts, and the relative partitioning behaviour of these elements has not been investigated rigorously. To address these issues, we determined the abundances of Cl, Br and I in enriched Mid-Ocean Ridge Basalt (E-MORB) glasses from Macquarie Island in the southwest Pacific. The Macquarie Island glasses are fairly typical of Pacific MORB with MgO of ∼5.5. to 9wt%, 87Sr/86Sr of 0.70257–0.70276, 143Nd/144Nd of 0.51300–0.51306 and 3He/4He of ∼8Ra. These glasses provide a unique opportunity to investigate halogen partitioning behaviour, because their variable MgO contents and trace element signatures (e.g. La/Sm ∼1.4–7.9) result from different degrees of partial melting and fractional crystallisation.The combined measurement of Br/Cl, I/Cl and K/Cl, together with correlations between Cl and other trace elements, demonstrate that the halogens in the Macquarie Island glasses had a mantle source and were not influenced by seawater contamination. Log–log correlation diagrams indicate that Cl, Br and K were not statistically fractionated during partial melting, crystallisation or degassing of CO2 from these melts. The behaviour of I is less well constrained and minor fractionation of I/Cl cannot be precluded during multi-stage melting and enrichment processes. The data indicate the mantle source of the Macquarie Island glasses was characterised by K/Cl of 13±4, Br/Cl of (3.7±0.5)×10−3 and I/Cl of (130±100)×10−6 (2σ uncertainties; weight ratios). The K/Cl ratio of the Macquarie Island glasses is equivalent to the median of all published MORB data. This suggests the Br/Cl and I/Cl values may also be representative of average MORB mantle values.

Geochemistry of mafic dykes from the Southeast Anatolian ophiolites, Turkey: Implications for an intra-oceanic arc–basin system

The Late Cretaceous–Tertiary accretionary prism in Eastern Turkey includes several ophiolitic megablocks and/or tectonic slivers (Mehmetalan, Mollatopuz and Alabayir) within a mélange complex, mainly comprising harzburgite, dunite and cumulate-textured gabbro. The diabases, which are the main focus of this study, cut across the ophiolites as parallel and variably thick dyke-swarms. Geochemistry of the diabases reveals three distinct groups, including a) supra-subduction zone (SSZ) type, which is characterized by marked Nb-anomaly and normal mid-ocean ridge basalt (N-MORB) like HFSE distribution, b) enriched MORB (E-MORB) type, showing some degree of enrichment relative to N-MORB, c) oceanic-island basalt (OIB) type with characteristic hump-backed trace element patterns, coupled with fractionated REE distribution. Among these groups, SSZ- and E-MORB-type signatures are acquired from the Mehmetalan and Mollatopuz suites, whereas OIB-type characteristics are found in the Alabayir suite. The melting models indicate involvement of both depleted and enriched sources for the genesis of the studied dykes. The close spatial relationship, similar ages (based on Ar–Ar dating) and the presence of variable subduction component displayed by Mehmetalan and Mollatopuz suites may indicate melt generation in an intra-oceanic SSZ within the southern branch of Neotethys. In spite of the solely OIB-like character of the Alabayir suite, the similar age obtained from these dykes may suggest their formation in a similar SSZ setting. Alternatively, the Alabayir suite may have represented an oceanic island or seamount formed in an intra-plate setting with or without plume influence. We suggest that decompression melting triggered by slab roll-back mechanism during the closure of the southern branch of the Neotethys during the Late Cretaceous may have been the main process that led to generation of magmas of both depleted and enriched characteristics.

Geochemical constraints on blueschist- and amphibolite-facies rocks of the Central Cordillera of Colombia: the Andean Barragán region

Subduction zones are one of the most characteristic features of planet Earth. Convergent plate junctions exert enormous influence on the formation and recycling of continental crust, and they are also responsible for major mineral resources and earthquakes, which are of crucial importance for society. A subduction-related geologic unit containing high-pressure rocks occurs in the Barragán area (Valle del Cauca Department) on the western flank of the Central Cordillera of the Colombian Andes. Blueschists and amphibolites, serpentinized meta-ultramafic rocks, graphite-chlorite-muscovite-quartz schists, protocataclasites, and graphite-chlorite-andalusite-andesine-garnet-muscovite ± titanite schists are exposed in this region. In spite of the petrotectonic importance of blueschists, the high-pressure metamorphism of the Central Cordillera of Colombia has been rarely studied. New geochemical data indicate that protoliths of the blueschist- and amphibolite-facies rocks possessed normal mid-ocean ridge basalt bulk compositions. 40Ar/39Ar geochronology for a metapelite rock associated with the blueschists shows a plateau age of ∼120 million years. We suggest that high-P/T conditions were present from ∼150 to 125 Ma, depending on the model of generation and exhumation considered.

The Izera metabasites, West Sudetes, Poland: Geologic and isotopic U–Pb zircon evidence of Devonian extension in the Saxothuringian Terrane

In the Lausitz–Izera Massif of the Saxothuringian terrane, part of the central European Variscides, 515–480 Ma metagranites have been intruded by a swarm of basic veins that have within-plate alkali basalt geochemistry, or, rarely, normal mid-ocean ridge basalt (N-MORB) geochemistry. Based on their spatial orientation, distribution in the region and structural relationships to the host metagranites, these basic veins could be grouped into three types that displayed six varieties of host–vein contacts. Zircons extracted from eight of the basic veins revealed that there were two populations: clear zircons and brown zircons. Each type were subjected to both single grain conventional U–Pb analysis and to sensitive high-resolution ion microprobe (SHRIMP) U–Pb analysis. The clear grains yielded ages > 480 Ma, clustering at ~ 500 Ma; the brown grains yielded ages between ~ 390 and 365 Ma, clustering at ~ 370 Ma. The colourless zircons possess an oscillatory zoned structure and rare earth element (REE) distribution pattern that suggests they come from the host granites and from deeper Neoproterozoic granitoid basement. However, the basic host rocks show almost no obvious contamination by felsic material. The turbid brown grains, which are dark under cathodoluminescence, also seem to be mainly magmatic because of their internal structures, high Th/U ratios and REE pattern. When the zircon results are integrated into the regional geology, an ~ 30 m.y. period of episodic, intra-plate, predominantly alkaline basic magmatism at the passive margin of the Saxothuringian Terrane is suggested, with maximum activity at ~ 370 Ma during an extensional regime. Combining the factors of the relatively high thermal gradient during early metamorphism of the metabasites (~ 450 °C, 2 kbar), the geology of Saxothuringia and the regional structural data, we propose a plume model of “hot fingers” type whereby the basic magmatism of the Izera region was due to repeated Devonian intrusions derived from garnet-bearing enriched (OIB type) and less commonly depleted asthenosphere sources. The waning stages of this activity were coeval with the early deformation of the Izera granite host–basic vein system and matched by rifting and extensional widening of the Saxothuringian Basin in its Izera–Kaczawa segment.

Oceanic lithospheric mantle beneath the continental crust of the Chinese Altai

Abstract: Although subduction–accretion is proposed as a major regime in making new continental crust, how the lithospheric mantle forms remains unclear. Formed after the closure of the Palaeo-Asian Ocean, the Ashele basalt shows normal mid-ocean ridge basalt (N-MORB)-like characteristics with light REE-depleted patterns and extremely low contents of high field strength elements. The low Zr/Y and Nb/Y ratios of the basalt are significantly different from those of asthenosphere-derived melts, and the excess Eu and Sr suggest that the basalt was probably derived from accreted oceanic lithospheric mantle. The presence of the N-MORB-like terrestrial basalt implies that subduction–accretion is an effective mechanism in building the refractory lithospheric mantle of Phanerozoic continents. Supplementary material: A photograph of the outcrop, and age and geochemical data are available at http://www.geolsoc.org.uk/SUP18464 .

Enriched basalts at segment centers: The Lucky Strike (37°17′N) and Menez Gwen (37°50′N) segments of the Mid-Atlantic Ridge

Basalts from the Mid-Atlantic Ridge change progressively in composition with increasing distance from the Azores platform. Study of the Lucky Strike and Menez Gwen segments reveals much complexity in the gradient. Both segments contain only basalts enriched relative to normal mid-oceanic ridge basalt, but in two distinct groups. Moderately enriched basalts occur throughout the segments, with proximal Menez Gwen enriched relative to Lucky Strike. Highly enriched basalts occur at segment centers. Incompatible element ratios of the highly enriched basalts exceed those of the Azores platform, while isotopic compositions are less enriched. These observations can be explained by a low-degree melt of garnet-bearing Azores mantle added to mantle depleted by previous melt extraction. Melting this “metasomatized” mantle produces lavas that match the enriched samples. The Azores gradient cannot be explained by simple two-component mixing; rather, it reflects recent melt extraction and addition processes related to southward flow of the Azores plume. The Azores gradient also permits tests of segmentation models. Central supply models predict step functions in chemical compositions between segments. Within-segment gradients require vertical supply. Central supply is supported by robust central volcanoes, thicker crust at segment centers, and a step function in isotopes between the segments. The lava diversity at segment centers, however, requires batches of distinct magma that are preserved through melting and melt delivery. Within-segment gradients in moderately incompatible element ratios support a component of multiple supply. The data suggest partial homogenization of magma within a segment and preferential melt focusing to segment centers with some vertical transport.