Magma storage and ascent during the 1995 eruption of Fogo,Cape Verde Archipelago

The 1995 eruption of Fogo (Cape Verde Islands) differed from previous eruptions by the occurrence of evolved lavas, the SW-orientation of vents, and pre-eruptive seismicity between Fogo and the adjacent (~20 km) island of Brava. We have conducted a thermobarometric and chemical study of this eruption in order to reconstruct its magma plumbing system and to test for possible connections to Brava. The bimodal eruption produced basanites (5.2–6.7 wt% MgO) and phonotephrites (2.4–2.8 wt% MgO) that are related by fractional crystallization. Clinopyroxene-melt-barometry of phenocrysts yields pressure ranges of 460–680 MPa for the basanites and 460–520 MPa for the phonotephrites. Microthermometry of CO₂-dominated fluid inclusions in olivine and clinopyroxene phenocrysts yields systematically lower pressure ranges of 200–310 MPa for basanites and 270–470 MPa for phonotephrites. The combined data indicate pre-eruptive storage of the 1995 magmas within the lithospheric mantle between 16 and 24 km depth. During eruption, the ascending magmas stalled temporarily at 8–11 km depth, within the lower crust, before they ascended to the surface in a few hours as indicated by zonations of olivine phenocrysts. Our data provide no evidence for magma storage at shallow levels (<200 MPa) or lateral magma movements beneath the Fogo-Brava platform. Sr–Nd–Pb isotope ratios of samples from Brava differ significantly from those of the 1995 and older Fogo lavas, which rules out contamination of the 1995 magmas by Brava material and indicates different mantle sources and magma plumbing systems for both islands.     

Calcium Isotopes (δ44/40Ca) in MPI-DING Reference Glasses, USGS Rock Powders and Various Rocks: Evidence for Ca Isotope Fractionation in Terrestrial Silicates

We report δ^44/40Ca_{(SRM 915a)} values for eight fused MPI‐DING glasses and the respective original powders, six USGS igneous rock reference materials, the U‐Th disequilibria reference material TML, IAEA‐CO1 (Carrara marble) and several igneous rocks (komatiites and carbonatites). Sample selection was guided by three considerations: (1) to address the need for information values on reference materials that are widely available in support of interlaboratory comparison studies; (2) support the development of in situ laser ablation and ion microprobe techniques, which require isotopically homogenous reference samples for ablation; and (3) provide Ca isotope values on a wider range of igneous and metamorphic rock types than is currently available in the scientific literature. Calcium isotope ratios were measured by thermal ionisation mass spectrometry in two laboratories (IFM‐GEOMAR and Saskatchewan Isotope Laboratory) using ⁴³Ca/⁴⁸Ca‐ and ⁴²Ca/⁴³Ca‐double spike techniques and reported relative to the calcium carbonate reference material NIST SRM 915a. The measurement uncertainty in both laboratories was better than 0.2‰ at the 95% confidence level. The impact of different preparation methods on the δ^44/40Ca_{(SRM 915a)} values was found to be negligible. Except for ML3‐B, the original powders and the respective MPI‐DING glasses showed identical δ^44/40Ca_{(SRM 915a)} values; therefore, possible variations in the Ca isotope compositions resulting from the fusion process are excluded. Individual analyses of different glass fragments indicated that the glasses are well homogenised on the mm scale with respect to Ca. The range of δ^44/40Ca_{(SRM 915a)} values in the igneous rocks studied was larger than previously observed, mostly owing to the inclusion of ultramafic rocks from ophiolite sections. In particular, the dunite DTS‐1 (1.49 ± 0.06‰) and the peridotite PCC‐1 (1.14 ± 0.07‰) are enriched in ⁴⁴Ca relative to volcanic rocks (0.8 ± 0.1‰). The Carrara marble (1.32 ± 0.06‰) was also found to be enriched in ⁴⁴Ca relative to the values of assumed precursor carbonates (< 0.8‰). These findings suggest that the isotopes of Ca are susceptible to fractionation at high temperatures by, as yet, unidentified igneous and metamorphic processes.     

Geochemical zonation of the Miocene Alborán Basin volcanism (westernmost Mediterranean): geodynamic implications

We present new major and trace element and O–Sr–Nd-isotope data for igneous rocks from the western Mediterranean Alborán Sea, collected during the METEOR 51/1 cruise, and for high-grade schists and gneisses from the continental Alborán basement, drilled during the Ocean Drilling Programme (ODP Leg 161, Site 976). The geochemical data allow a detailed examination of crustal and mantle processes involved in the petrogenesis of the lavas and for the first time reveal a zonation of the Miocene Alborán Sea volcanism: (1) a keel-shaped area of LREE-depleted (mainly tholeiitic series) lavas in the central Alborán Sea, generated by high degrees of partial melting of a depleted mantle source and involving hydrous fluids from subducted marine sediments, that is surrounded by (2) a horseshoe-shaped zone with LREE-enriched (mainly calc-alkaline series) lavas subparallel to the arcuate Betic-Gibraltar-Rif mountain belt. We propose that the geochemical zonation of the Miocene Alborán Basin volcanism results from eastward subduction of Tethys oceanic lithosphere coupled with increasing lithospheric thickness between the central Alborán Sea and the continental margins of Iberia and Africa. Electronic supplementary material  The online version of this article (doi:) contains supplementary material, which is available to authorized users.     

Enriched,HIMU-type peridotite and depleted recycled pyroxenite in the Canary plume: A mixed-up mantle

The Earth's mantle is chemically and isotopically heterogeneous, and a component of recycled oceanic crust is generally suspected in the convecting mantle [Hofmann and White, 1982. Mantle plumes from ancient oceanic crust. Earth Planet. Sci. Lett. 57, 421–436]. Indeed, the HIMU component (high µ = ²³⁸U/²⁰⁴Pb), one of four isotopically distinct end-members in the Earth's mantle, is generally attributed to relatively old (≥ 1–2 Ga) recycled oceanic crust in the form of eclogite/pyroxenite, e.g. [Zindler and Hart, 1986. Chemical geodynamics. Ann. Rev. Earth Planet. Sci. 14, 493–571]. Although the presence of the recycled component is generally supported by element and isotopic data, little is known about its physical state at mantle depths. Here we show that the concentrations of Ni, Mn and Ca in olivine from the Canarian shield stage lavas, which can be used to assess the physical nature of the source material (peridotite versus olivine-free pyroxenite) [Sobolev et al., 2007. The amount of recycled crust in sources of mantle-derived melts. Science 316, 412–417], correlate strongly with bulk rock Sr, Nd and Pb isotopic ratios. The most important result following from our data is that the enriched, HIMU-type (having higher ²⁰⁶Pb/²⁰⁴Pb than generally found in the other mantle end-members) signature of the Canarian hotspot magmas was not caused by a pyroxenite/eclogite constituent of the plume but appears to have been primarily hosted by peridotite. This implies that the old (older than ~ 1 Ga) ocean crust, which has more evolved radiogenic isotope compositions, was stirred into/reacted with the mantle so that there is not significant eclogite left, whereas younger recycled oceanic crust with depleted MORB isotopic signature (< 1 Ga) can be preserved as eclogite, which when melted can generate reaction pyroxenite.     

Geochemical variations in the Cocos Plate subducting beneath Central America: implications for the composition of arc volcanism and the extent of the Galápagos Hotspot influence on the Cocos oceanic crust

New geochemical data from the Cocos Plate constrain the composition of the input into the Central American subduction zone and demonstrate the extent of influence of the Galápagos Hotspot on the Cocos Plate. Samples include sediments and basalts from Ocean Drilling Program (ODP) Site 1256 outboard of Nicaragua, gabbroic sills from ODP Sites 1039 and 1040, tholeiitic glasses from the Fisher Ridge off northwest Costa Rica, and basalts from the Galápagos Hotspot Track outboard of Central Costa Rica. Site 1256 basalts range from normal to enriched MORB in incompatible elements and have Pb and Nd isotopic compositions within the East Pacific Rise MORB field. The sediments have similar ²⁰⁶Pb/²⁰⁴Pb and only slightly more radiogenic ²⁰⁷Pb/²⁰⁴Pb and ²⁰⁸Pb/²⁰⁴Pb isotope ratios than the basalts. Altered samples from the subducting Galápagos Hotspot Track have similar Nd and Pb isotopic compositions to fresh Galápagos samples but have significantly higher Sr isotopic composition, indicating that the subduction input will have a distinct geochemical signature from Galápagos-type mantle material that may be present in the wedge beneath Costa Rica. Gabbroic sills from Sites 1039 and 1040 in East Pacific Rise (EPR) crust show evidence for influence of the Galápagos Hotspot ∼100 km beyond the morphological hotspot track.     

Magmatic evolution of a dying spreading axis: Evidence for the interaction of tectonics and mantle heterogeneity from the fossil Phoenix Ridge,Drake Passage

New ⁴⁰Ar–³⁹Ar ages of 5.6 to 1.3 Ma for lavas from the fossil Phoenix Ridge in the Drake Passage show that magmatism continued for at least 2 Ma after the cessation of spreading at 3.3 ± 0.2 Ma. The Phoenix Ridge lavas are incompatible element-enriched relative to average MORB and show an increasing enrichment with decreasing age, corresponding to progressively decreasing degrees of partial melting of spinel peridotite after spreading stopped. The low-degree partial melts increasingly tap a mantle source with radiogenic Sr and Pb but unradiogenic Nd isotope ratios implying an ancient enrichment. The post-spreading magmas apparently form by buoyant ascent of enriched and easily fusible portions of the upper mantle. Only segments of fossil spreading ridges underlain by such enriched and fertile mantle show post-spreading volcanism frequently forming bathymetric highs. The Phoenix Ridge lavas belong to the Pacific, rather than the Atlantic, mantle domain in regional Sr–Nd–Pb space. Our new data show that the southern Pacific Ocean mantle is heterogeneous containing significant enriched portions that are preferentially tapped at low melt fractions. Isotopic mapping reveals that Pacific-type upper mantle flows eastward through Drake Passage and surrounds the subducting Phoenix Plate beneath the Bransfield Basin.     

Mineralogy, geochemistry and stratigraphy of the Maslovsky Pt–Cu–Ni sulfide deposit, Noril’sk Region, Russia

We report new data on the stratigraphy, mineralogy and geochemistry of the rocks and ores of the Maslovsky Pt–Cu–Ni sulfide deposit which is thought to be the southwestern extension of the Noril’sk 1 intrusion. Variations in the Ta/Nb ratio of the gabbro-dolerites hosting the sulfide mineralization and the compositions of their pyroxene and olivine indicate that these rocks were produced by two discrete magmatic pulses, which gave rise to the Northern and Southern Maslovsky intrusions that together host the Maslovsky deposit. The Northern intrusion is located inside the Tungusska sandstones and basalt of the Ivakinsky Formation. The Southern intrusion cuts through all of the lower units of the Siberian Trap tuff-lavas, including the Lower Nadezhdinsky Formation; demonstrating that the ore-bearing intrusions of the Noril’sk Complex post-date that unit. Rocks in both intrusions have low TiO₂ and elevated MgO contents (average mean TiO₂ <1 and MgO?=?12?wt.%) that are more primitive than the lavas of the Upper Formations of the Siberian Traps which suggests that the ore-bearing intrusions result from a separate magmatic event. Unusually high concentrations of both HREE (Dy+Yb+Er+Lu) and Y (up to 1.2 and 2.1?ppm, respectively) occur in olivines (Fo_79.5 and 0.25% NiO) from picritic and taxitic gabbro-dolerites with disseminated sulfide mineralization. Thus accumulation of HREE, Y and Ni in the melts is correlated with the mineral potential of the intrusions. The TiO₂ concentration in pyroxene has a strong negative correlation with the Mg# of both host mineral and Mg# of host rock. Sulfides from the Northern Maslovsky intrusion are predominantly chalcopyrite–pyrrhotite–pentlandite with subordinate and minor amounts of cubanite, bornite and millerite and a diverse assemblage of rare precious metal minerals including native metals (Au, Ag and Pd), Sn–Pd–Pt–Bi–Pb compounds and Fe–Pt alloys. Sulfides from the Southern Maslovsky intrusion have δ ³⁴S?=?5–6‰ up to 10.8‰ in two samples whereas the country rock basalt have δ ³⁴S?=?3–4‰, implying there was no in situ assimilation of surrounding rocks by magmas.     

Silicification of peridotites at the stalemate fracture zone (Northwestern Pacific): Reconstruction of the conditions of low-temperature weathering and tectonic interpretation

During cruise SO201-1b of the joint Russian-German expedition on the R/V Sonne in 2009, mantle peridotites affected by varying secondary alteration were dredged on the eastern slope of the northwestern segment of the Stalemate transverse ridge adjacent to the eponymous fracture zone. The collection discussed in this paper included four samples of silicified serpentinites after dunites and 11 lherzolite samples serpentinized to a varying degree. The abundance of amorphous silica and quartz, very high SiO₂ content (up to 88.7 wt %), and unusually low MgO (up to 1.4 wt %) in the serpentinized dunites strongly distinguish these rocks from the known products of hydrothermal alteration and low-temperature (seafloor) weathering of peridotites in the oceanic crust. In order to determine the conditions and processes resulting in the silicification of peridotites at the Stalemate Fracture Zone, thermodynamic modeling accounting for the kinetics of mineral dissolution implemented in the GEOCHEQ program package was used in this study. The results of modeling allowed us to suppose that the geochemical and mineralogical effects observed in the silicified serpentinized dunites of the Stalemate Fracture Zone are consequences of low-temperature deserpentinization of oceanic materials under subaerial conditions.