Geochemistry of volcanic rocks from the Woodlark Basin

EGU2011-12864 The Woodlark Basin east of Papua New Guinea represents one of the few places on Earth where a spreading axis propagates into continental crust. This special tectonic setting allows insights into the evolution of magma composition as continental extension and break-up changes to the formation of ocean crust. We report here geochemical results on samples collected in 2009 from the four segments closest to the continental breakup, from segment 1 which abuts the detachment fault responsible for continental extension on Moresby Seamount in the West, to segment 4, representing mature oceanic crust in the East. A total of 208 glass samples have been analyzed for their major (EMPA) and trace element (LA-ICPMS) compositions. The data show strong E-W variations. Samples ranging from tholeiitic basalt and basaltic andesite to andesite and rhyolite are found on Segment 1. They have generally high alkali values and a wide range of trace element contents and ratios. Segments 2 to 4 magmas in contrast only comprise tholeiitic basalt with lower alkali contents and a more restricted range of trace element chemistry. The geochemical differences between the segments cannot be attributed to differentiation processes alone, and different sources are required. High Ba/La, (La/Sm)N, Rb/Sr, and Th/La on Segment 1 suggest a derivation from an enriched mantle source, while low Nd/Pb and Nb/U suggest that some of the enrichment may also reflect the influence of continental crust during magma genesis. Whether this continental signature is present in the form of recycled material in the mantle or as rafted continental blocks in the axial region is at present unclear. In contrast to rocks from segment 1, trace element compositions of volcanic glasses from segments 2 to 4 show a stronger MORB signature, presumably reflecting more mature spreading in this part of the basin. The influence of continental material appears to be minimal, suggesting that uncontaminated asthenosphere quickly flows into the rift and/or that continental blocks are not retained in the axial region for long time periods following the rifting-spreading transition

Temporal variations in Galápagos plume-ridge interaction at the Cocos-Nazca spreading center

The major goals of cruise SO208 with the German research vessel Sonne were to investigate 1) plume-ridge interaction through time at the Cocos-Nazca spreading center (CNS) north of the Galápagos Islands by sampling across axis profiles of the seafloor and 2) off axis volcanism at the East Pacific Rise (EPR) versus far field effects of the Galápagos hotspot documented in seamounts off the coast of N Costa Rica and Nicaragua. Overall the nature of material transfer from the plume to the ridge and its large scale distribution throughout the Eastern Pacific is being investigated by means of major and trace element and Sr-Nd-Pb (double spike) isotope data. The seamounts on the EPR generated part of the Cocos plate appear to originate on one hand from a depleted MORBlike source consistent with their formation near the EPR axis, while other seamounts formed through lower degrees melting of an enriched OIB source either more distant from the EPR or by intraplate volcanism. Geochemical profiles across the Western and Eastern CNS indicate the participation of two different Galápagos plume components with a change in the amount this material entering the CNS with time. While at the western profile element ratios of more to less incompatible elements show an overall decrease of a plume component, Wolf-Darwin or Northern domain [1], with increasing age, the opposite is observed at the eastern profile. The Central domain component [1] increases with increasing age of the crust in this area. These observations indicate variable flux of specific Galápagos plume components to the CNS over the past 800 000 years. Sr-Nd-Pb isotope data to verify these observations are currently being generated and will be presented at the conference. [1] Hoernle et al. (2000) Geology 28, 435–43

Evidence for Miocene subduction beneath the Alboran Sea (Western Mediterranean) from 40Ar/39Ar age dating and the geochemistry of volcanic rocks from holes 977A and 978A

Volcanic pebbles in gravels from Sites 977 and 978 in the Alboran Sea (western Mediterranean) were dated (using the 40Ar/ 39Ar single-crystal laser technique) and analyzed for their major- and trace-element compositions (determined by X-ray fluorescence and inductively coupled plasma-mass spectrometry). The samples range from basalts to rhyolites, and belong to the tholeiitic, calc-alkaline, and shoshonitic series. Single-crystal and step-heating laser 40Ar/39Ar analyses of plagioclase, sanidine, biotite, and amphibole phenocrysts from basaltic to rhyolitic samples indicate that eruptions occurred between 6.1 and 12.1 Ma. The age data conform to the stratigraphy and agree with microfossil ages, when available. The major-element and compatible trace-element data of samples with H2O < 4 wt% show systematic variations, consistent with fractionation of the observed phenocryst phases (plagioclase, olivine, clinopyroxene, magnetite, hornblende, quartz, and biotite). The incompatible-element patterns formed by normalizing to primitive mantle for all samples show spiked patterns with peaks generally at mobile elements and troughs at immobile elements, in particular Nb and Ta. The calc-alkaline affinities and the incompatible-element systematics are characteristic of subduction zone volcanism, which indicates that subduction occurred beneath the eastern Alboran from 6 to at least 12 Ma. We propose that the change in chemistry from calc-alkaline and potassic to sodic compositions between 5– 6 Ma reflects detachment of the subducting slab. Uplift of the Strait of Gibraltar, associated with this detachment, could have caused the Messinian Salinity Crises

Chemical Heterogeneities along the South Atlantic Mid-Ocean-Ridge (5-11°S): Shallow or Deep Recycling of Ocean Crust?

Between 5° and 11°S, the Mid-Atlantic Ridge displays anomalous crustal thickness and geochemical compositions, thought to be related to either small scale upper mantle heterogeneities or a weak, diffuse mantle plume. We report new high precision trace element and Sr, Nd and Pb (DS) isotope data for 72 ridge axis samples and 9 off-axis seamount samples along with U–Th–Ra disequilibria data for off axis seamounts at c. 9.7°S. At least four distinct components are needed to explain the geochemical variations along the ridge: 1) a common depleted (D-MORB-like) component near and north of 4.8–7.6°S, 2) an enriched component upwelling beneath Ascension Island and the northern A1 ridge segment (segment numbers ascend from north to south), 3) an enriched component upwelling beneath the A2 ridge segment, and 4) an enriched component upwelling beneath the line of seamounts east of the A3 segment and the A3 and A4 segments. The A1 and the A3+A4 segment lavas form well-defined mixing arrays from Ascension Island and the A3 seamounts respectively to the depleted D-MORB component. We propose that the enriched components represent different packages of subducted ocean crust and/or ocean island basalt (OIB) type volcanic islands and seamounts that have either been recycled through 1) the shallow mantle, upwelling passively beneath the ridge system or 2) the deep mantle via an actively upwelling heterogeneous mantle plume that interacts with the ridge system

E-MORB glasses from the Gakkel Ridge (Arctic Ocean) at 87°N: evidence for the Earth's most northerly volcanic activity

During the ARCTIC '91 expedition aboard RV Polarstern (ARK VIII/3) to the Central Arctic Ocean, a box corer sample on the Gakkel Ridge at 87 degrees N and 60 degrees E yielded a layer of sand-sized, dark brown volcanic glass shards at the surface of the sediment core. These shards have been investigated by petrographic, mineralogical, geochemical and radiogenic isotope methods. The nearly vesicle-free and aphyric glass shards bear only minute microphenocrysts of magnesiochromite and olivine (Fo(88-89)). Most glasses are fresh, although some show signs of incipient low-temperature alteration. From their shapes and sizes, the glass shards most likely formed by spalling of glassy rinds of a nearby volcanic outcrop. Geochemically, the glasses are relatively unfractionated tholeiites with E-MORB trace element compositions. Thus, they are quite similar to the previously investigated ARK IV/3-11-370-5 basalts from 86 degrees N. The Nd and Sr isotopic ratios of PS 2167-2 glasses are significantly lower than for ARK IV/3-11-370-5 basalts and suggest an isotopically heterogeneous mantle source of Gakkel Ridge MORE between 86 degrees and 87 degrees N. The positive Delta-8/4 Pb value (similar to 16) and high Sr-87/Sr-86 ratio (0.70270), found for PS 2167-2 glasses are similar to that of ARK IV/3-11-370-5 basalts and show the influence of the DUPAL isotopic anomaly in the high Arctic mantle. These results argue against the presence of an 'anti-DUPAL anomaly' in the mantle below the North Pole region and simple models of whole-mantle convection