Michael Bizimis

Determination of Mn, Fe, Co, Ni, Cu, Zn, Cd and Pb in seawater using high resolution magnetic sector inductively coupled mass spectrometry (HR-ICP-MS)

A novel method, combining isotope dilution with standard additions, was developed for the analysis of eight elements (Mn, Fe, Co, Ni, Cu, Zn, Cd and Pb) in seawater. The method requires just 12 mL of sample and employs an off-line pre-concentration step using the commercially available chelating resin Toyopearl AF-Chelate-650M prior to determination by high resolution inductively coupled plasma magnetic sector mass spectrometry (ICP-MS). Acidified samples were spiked with a multi-element standard of six isotopes ((57)Fe, (62)Ni, (65)Cu, (68)Zn, (111)Cd and (207)Pb) enriched over natural abundance. In addition, standard additions of a mixed Co and Mn standard were performed on sub-sets of the same sample. All samples were irradiated using a low power (119 mW cm(-2); 254 nm) UV system, to destroy organic ligands, before pre-concentration and extraction from the seawater matrix. Ammonium acetate was used to raise the pH of the 12 mL sub-samples (off-line) to pH 6.4+/-0.2 prior to loading onto the chelating resin. The extracted metals were eluted using 1.0 M Q-HNO(3) and determined using ICP-MS. The method was verified through the analysis of certified reference material (NASS-5) and the SAFe inter-comparison samples (S1 and D2), the results of which are in good agreement with the certified and reported consensus values. We also present vertical profiles of the eight metals taken from the Bermuda Atlantic Time Series (BATS) station collected during the GEOTRACES inter-comparison cruise in June 2008.

Trace and REE content of clinopyroxenes from supra-subduction zone peridotites. Implications for melting and enrichment processes in island arcs

We measured trace element and Rare Earth Element (REE) contents of clinopyroxenes (cpx) in peridotites from ophiolite complexes from the Hellenic Peninsula: Vourinos, Pindos, Othris (Greece), and Bulqiza (Albania). Compared to abyssal peridotites the Hellenic peridotites have a highly depleted mineralogy (<1% modal cpx) and the cpxs have extremely low concentrations in Ti (30–150 ppm) and heavy REE. The light and middle REE, Zr and Sr contents of the cpxs show enrichments compared to cpx from abyssal peridotites. Our cpx data falls in the field of previous studies on modern arc peridotites ([Parkinson, I.J., Pearce, J.A., Thirwall, M.F., Johnson, K.T.M., Ingram, G., 1992. Trace element geochemistry of peridotites from the Izu–Bonin–Mariana forearc, Leg 125. In: Fryer, P., Pearce, J.A., Stokking, L.B. (Eds.), Proc. Ocean Drilling Program. pp. 487–506; Bonatti, unpubl.]) suggesting that these complexes originated above a subduction zone. Dry melting of an upper mantle source similar to a mid-ocean ridge basalt (MORB) source (e.g., [Johnson, K.T.M., Dick, H.J.B., Shimizu, N., 1990. Melting in the oceanic upper mantle: an ion microprobe study of diopsides in abyssal peridotites. J Geophys. Res. 95, 2661–2678.]) cannot generate the extreme depletions in Ti and heavy rare earth element (HREE) seen in the cpxs from these supra-subduction zone (SSZ) peridotites. We propose that these high degrees of depletion can be achieved during hydrous melting of a MORB-depleted peridotite above a subduction zone. The melting rate of cpx during hydrous melting is expected to be less than in dry melting, while the rate of orthopyroxene (opx) consumption increases. These changes in melting modes allow for larger degrees of total melting and larger depletions in Ti and HREE in the cpx before cpx is exhausted from the residual peridotite. The enrichments in light rare earth element (LREE), MREE, Zr and Sr are best modeled by a constant flux of a slab-derived fluid component to the mantle wedge during melting. Ti and the HREE are not mobile in this fluid and are not enriched in the mantle peridotite. The continuous flux is necessary to sustain hydrous melting in the sub-arc mantle during the complete melting range. The calculated fluid composition matches well previous estimates based on the arc basalt chemistry ([McCulloch, M.T., Gamble, J.A., 1991. Geochemical and geodynamical constraints on subduction zone magmatism. Earth Planet. Sci. Lett. 102, 358–374; Stolper, E., Newman, S., 1994. The role of water in the petrogenesis of Mariana trough magmas. Earth Planet. Sci. Lett. 121, 293–325.]) and experiments on fluids ([Ayers, J.C., Dittmer, S.K., Graham, D.L., 1997. Partitioning of elements between peridotite and H2O at 2.0–3.0 GPa and 900–1100°C, and application to models of subduction zone processes. Earth Planet. Sci. Lett. 150, 381–398; Ayers, J.C., 1998. Trace element modeling of aqueous fluid–peridotite interaction in the mantle wedge of subduction zones. Contrib. Mineral. Petrol. 132, 390–404.]). The resulting melts from our model closely resemble melts from an arc environment (boninites) further arguing for the plausibility of this model.

Hf^Nd isotope decoupling in the oceanic lithosphere: constraints from spinel peridotites from Oahu, Hawaii §

Abstract We present a detailed geochemical investigation on the Hf, Nd and Sr isotope compositions and trace and major element contents of clinopyroxene mineral separates from spinel lherzolite xenoliths from the island of Oahu, Hawaii. These peridotites are believed to represent the depleted oceanic lithosphere beneath Oahu, which is a residue of a MORB-related melting event some 80–100 Ma ago at a mid-ocean ridge. Clinopyroxenes from peridotites from the Salt Lake Crater (SLC) show a large range of Hf isotopic compositions, from ϵHf=12.2 (similar to the Honolulu volcanics series) to extremely radiogenic, ϵHf=65, at nearly constant 143Nd/144Nd ratios (ϵNd=7–8). None of these samples show any isotopic evidence for interaction with Koolau-type melts. A single xenolith from the Pali vent is the only sample with Hf and Nd isotopic compositions that falls within the MORB field. The Hf isotopes correlate positively with the degree of depletion in the clinopyroxene (e.g. increasing Mg#, Cr#, decreasing Ti and heavy REE contents), but also with increasing Zr and Hf depletions relative to the adjacent REE in a compatibility diagram. The Lu/Hf isotope systematics of the SLC clinopyroxenes define apparent ages of 500 Ma or older and these compositions cannot be explained by mixing between any type of Hawaiian melts and the depleted Pacific lithosphere. Metasomatism of an ancient (e.g. 1 Ga or older) depleted peridotite protolith can, in principle, explain these apparent ages and the Nd–Hf isotope decoupling, but requires that the most depleted samples were subject to the least amount of metasomatism. Alternatively, the combined isotope, trace and major element compositions of these clinopyroxenes are best described by metasomatism of the 80–100 Ma depleted oceanic lithosphere by melts products of extensive mantle–melt interaction between Honolulu Volcanics-type melts and the depleted lithosphere.

Hawaiian mantle xenoliths and magmas: Composition and thermal character of the lithosphere

Abstract Recent seismological investigations into the Hawaiian mantle and geophysical studies of the Hawaiian Swell have led to divergent models regarding the thermal structure and role of the lithosphere in Hawaiian magmatism. Whereas some models require that the Hawaiian plume erodes the lower lithosphere and replaces it with plume-derived residues and cumulates, others do not require any thinning of the lithosphere. In this paper, we develop a model for the physical and thermal characteristics of the oceanic lithosphere beneath Oahu, Hawaii, from the mantle xenoliths included in the rejuvenated stage lavas (Honolulu Volcanics, HV) that erupted through the Salt Lake Crater (SLC) and Kaau-Pali-Kalihi (KPK) vents. Three main xenolith suites are found at Oahu: spinel peridotite, garnet pyroxenite, and dunite. Dunites are generally shallow cumulates, whereas the peridotite and pyroxenite xenoliths are from the mantle. Plagioclase peridotite (harzburgite) is rare, and represents a ~15 km thick lithospheric layer below the ~15 km thick crust (MOR-generated oceanic crust + thickened crust formed by Koolau shield volcano activity). The harzburgite layer is underlain by a spinel peridotite layer. Spinel peridotites from SLC and KPK differ in chemical and isotopic composition. SLC spinel peridotites have unusually enriched 176Hf/177Hf but highly nonradiogenic 187Os/188Os isotopic compositions, likely indicating that they are metasomatized, recycled (previously subducted) oceanic lithospheric fragments. On the other hand, KPK spinel peridotites exhibit compositional characteristics of the 90 m.y. old lithosphere beneath Oahu that has been variably metasomatized by passing HV magmas. We suggest that the lower lithosphere beneath SLC and KPK are very different in their petrologic composition. The lower lithosphere (60–90 km) beneath SLC is extensively veined and fragmented by garnet clinopyroxenite intrusives and mixed with spinel peridotite blobs that are recycled subducted lithospheric fragments. For the KPK lithosphere, we favor a model in which the lower part is peridotitic. The thickness of our model lithosphere beneath Oahu is ~90 km. The seismic low-velocity zone beneath Oahu is likely due to the presence of small amounts of hydrous-alkalic and carbonatitic (+kimberlitic) melts. The isotopic data suggest little or no interaction between the basal lithosphere and Koolau shield magmas (mostly plume-derived, particularly those with near bulk-earth Nd- and Sr-isotope ratios), suggesting that during shield-stage volcanism, magmas used well-insulated (i.e., a reaction zone of <2 km radius) narrow conduits that reached down to the base of the lithosphere. All of the deeper (>60 km) xenoliths are isotopically depleted and lack the typical Koolau-like signature. Therefore, the lower lithosphere was not significantly eroded by Koolau magmatism. KPK spinel peridotites give temperatures of 900–1040 °C for the intermediate lithosphere, which is about 200–400 °C hotter than what would be expected of a normal 90 Ma lithosphere. We suggest that such an anomalously high temperature is due to heating of the wall rocks (lithosphere) by ascending HV magmas and not due to simple basal heating of the lithosphere by the hot spot/plume followed by gradual (conductive) upward transport of heat. Thermobarometry of garnet-bearing xenoliths suggests that these rocks equilibrated at ~2−3 GPa and 1200−1350 °C. This temperature range is 50–200° less than that predicted by a recent model (Ribe and Christensen 1999). The plume that generated the Koolau shield magmas was a complex mixture composed predominantly of Koolau source materials at its center and blocks of old recycled lithosphere in its outer fringes that were later rafted into the lithosphere beneath SLC.

Volcanoes of the passive margin: The youngest magmatic event in eastern North America

The rifted eastern North American margin (ENAM) provides important clues to the long-term evolution of continental margins. An Eocene volcanic swarm exposed in the Appalachian Valley and Ridge Province of Virginia and West Virginia (USA) contains the youngest known igneous rocks in the ENAM. These magmas provide the only window into the most recent deep processes contributing to the postrift evolution of this margin. Here we present new 40 Ar/ 39 Ar ages, geochemical data, and radiogenic isotopes that constrain the melting conditions and the timing of emplacement. Modeling of the melting conditions on primitive basalts yielded an average temperature and pressure of 1412 ± 25 °C and 2.32 ± 0.31 GPa, corresponding to a mantle potential temperature of ∼1410 °C, suggesting melting conditions slightly higher than average mantle temperatures beneath mid-ocean ridges. When compared with magmas from Atlantic hotspots, the Eocene ENAM samples share isotopic signatures with the Azores and Cape Verde. This similarity suggests the possibility of a large-scale dissemination of similar sources in the upper mantle left over from the opening of the Atlantic Ocean. Asthenosphere upwelling related to localized lithospheric delamination is a possible process that can explain the intraplate signature of these magmas that lack evidence of a thermal anomaly. This process can also explain the Cenozoic dynamic topography and evidence of rejuvenation of the central Appalachians.

Water in Hawaiian peridotite minerals: A case for a dry metasomatized oceanic mantle lithosphere

The distribution of water concentrations in the oceanic upper mantle has drastic influence on its melting, rheology, and electrical and thermal conductivities and yet is primarily known indirectly from analyses of OIB and MORB. Here, actual mantle samples, eight peridotite xenoliths from Salt Lake Crater (SLC) and one from Pali in Oahu in Hawaii were analyzed by FTIR. Water contents of orthopyroxene, clinopyroxene, and the highest measured in olivine are 116–222, 246–442, and 10–26 ppm weight H2O, respectively. Although pyroxene water contents correlate with indices of partial melting, they are too high to be explained by simple melting modeling. Mantle-melt interaction modeling reproduces best the SLC data. These peridotites represent depleted oceanic mantle older than the Pacific lithosphere that has been refertilized by nephelinite melts containing <5 weight % H2O. Metasomatism in the Hawaiian peridotites resulted in an apparent decoupling of water and LREE that can be reconciled via assimilation and fractional crystallization. Calculated bulk-rock water contents for SLC (50–96 ppm H2O) are on the low side of that of the MORB source (50–200 ppm H2O). Preceding metasomatism, the SLC peridotites must have been even drier, with a water content similar to that of the Pali peridotite (45 ppm H2O), a relatively unmetasomatized fragment of the Pacific lithosphere. Moreover, our data show that the oceanic mantle lithosphere above plumes is not necessarily enriched in water. Calculated viscosities using olivine water contents allow to estimate the depth of the lithosphere-asthenosphere boundary beneath Hawaii at ∼90 km.

U‐Pb zircon constraints on the age and provenance of the Rocas Verdes basin fill, Tierra del Fuego, Argentina

The Late Jurassic to Early Cretaceous Rocas Verdes basin constitutes one of the most poorly understood components of the southernmost Andes. As a result, accurate reconstructions and interpretations of deformation associated with the Andean orogeny and the kinematics of Scotia arc development also remain poorly constrained. In this data brief, we report U-Pb zircon ages from sandstones of the Rocas Verdes basin fill and from a crosscutting pluton in the southernmost Andes of Argentine Tierra del Fuego. Detrital samples contain predominant Early to early Middle Cretaceous (circa 130–105 Ma) U-Pb zircon age populations, with very small or single-grain middle Mesozoic and Proterozoic subpopulations. A very small subpopulation of Late Cretaceous ages in one sample raises the unlikely possibility that parts of the Rocas Verdes basin are younger than perceived. A sample from a crosscutting syenitic pegmatite yields a crystallization age of 74.7 +2.2/−2.0 Ma. The data presented herein encourage further geochronologic evaluation of the Rocas Verdes basin in order to better constrain the depositional ages and provenance of its contents.

Supercontinental inheritance and its influence on supercontinental breakup: The Central Atlantic Magmatic Province and the breakup of Pangea

The Central Atlantic Magmatic Province (CAMP) is the large igneous province (LIP) that coincides with the breakup of the supercontinent Pangea. Major and trace element data, Sr-Nd-Pb radiogenic isotopes, and high-precision olivine chemistry were collected on primitive CAMP dikes from Virginia (VA). These new samples were used in conjunction with a global CAMP data set to elucidate different mechanisms for supercontinent breakup and LIP formation. On the Eastern North American Margin, CAMP flows are found primarily in rift basins that can be divided into northern or southern groups based on differences in tectonic evolution, rifting history, and supercontinental inheritance. Geochemical signatures of CAMP suggest an upper mantle source modified by subduction processes. We propose that the greater number of accretionary events, or metasomatism by sediment melts as opposed to fluids on the northern versus the southern Laurentian margin during the formation of Pangea led to different subduction-related signatures in the mantle source of the northern versus southern CAMP lavas. CAMP samples have elevated Ni and low Ca in olivine phenocrysts indicating a significant pyroxenite component in the source, interpreted here as a result of subduction metasomatism. Different collisional styles during the Alleghanian orogeny in the North and South may have led to the diachroneity of the rifting of Pangea. Furthermore, due to a low angle of subduction, the Rheic Plate may have underplated the lithosphere then delaminated, triggering both the breakup of Pangea and the formation of CAMP.

Mesoproterozoic and Paleoproterozoic subcontinental lithospheric mantle domains beneath southern Patagonia: Isotopic evidence for its connection to Africa and Antarctica

New isotopic studies on mantle xenoliths from Santa Cruz Province, southern Patagonia, Argentina, reveal that at least three discrete subcontinental lithospheric mantle (SCLM) domains—the Deseado Massif, Tres Lagos, and Pali Aike—form the southernmost part of South America. Re-Os systematics yield early Paleoproterozoic (up to 2.5 Ga) SCLM formation ages (rhenium depletion ages, T RD ) for Pali Aike spinel peridotites, while samples from the Deseado Massif and Tres Lagos indicate a younger SCLM origin with Neoproterozoic to Mesoproterozoic (0.9–1.3 Ga) and Mesoproterozoic to late Paleoproterozoic (1.3–1.9 Ga) T RD ages, respectively. Hf-Sr-Nd isotopic compositions indicate metasomatic overprinting of the majority of the samples, which, however, has not affected the Os isotopic system. Based on similar formation ages, the geological evolution of the Deseado Massif is most likely connected to the evolution of the Namaqua-Natal belt of South Africa. T RD ages from SCLM domains underneath Tres Lagos and Pali Aike indicate a common origin with crustal sections from Shackleton Range, Antarctica, positioning the southern tip of South America closer to west Antarctica in the reconstructed Rodinia supercontinent than previously assumed.

Record of massive upwellings from the Pacific large low shear velocity province

Large igneous provinces, as the surface expression of deep mantle processes, play a key role in the evolution of the planet. Here we analyse the geochemical record and timing of the Pacific Ocean Large Igneous Provinces and preserved accreted terranes to reconstruct the history of pulses of mantle plume upwellings and their relation with a deep-rooted source like the Pacific large low-shear velocity Province during the Mid-Jurassic to Upper Cretaceous. Petrological modelling and geochemical data suggest the need of interaction between these deep-rooted upwellings and mid-ocean ridges in pulses separated by ∼10–20 Ma, to generate the massive volumes of melt preserved today as oceanic plateaus. These pulses impacted the marine biota resulting in episodes of anoxia and mass extinctions shortly after their eruption.