ALBERT

Description: We present a comprehensive major and trace element and Sr–Nd–Pb isotope dataset from the major volcanic units exposed on La Palma and show how geochemical and volcanic evolution can be linked to asthenospheric and lithospheric processes. Lavas from the northern shield (from Basal Complex to Taburiente to Bejenado volcanism, 3–4 Ma to 400 ka) become more alkalic and SiO2-undersaturated with decreasing age, but show little change in MgO-normalized trace element compositions. Their high (Nb,Ta)/U and Ba/Th but low La/Nb ratios suggest assimilation of amphibole, probably in the lithospheric mantle that was metasomatized by earlier melts. Lavas from the Cumbre Vieja unit (〈125 ka) in the southern half of La Palma are more incompatible-element enriched and probably formed through lower degrees of melting than those from the northern shield, which are nearly identical isotopically. Their Nb/U ratios are mostly within the range 47 ± 10, significantly below those of the earlier lavas. In 206Pb/204Pb versus 143Nd/144Nd, 208Pb/204Pb and 208Pb/206Pb isotope diagrams, the Basal Complex rocks and lavas from the adjacent El Hierro island form a separate trend compared with the younger subaerial La Palma lavas. Both groups share a common depleted end-member but require separate, enriched HIMU-like end-members, believed to be located within the asthenosphere. The temporal and spatial variations in the composition of La Palma and El Hierro lavas could be explained within the context of NE-directed plate motion over a zoned Canary plume. After La Palma moved away from the asthenospheric source domain of the Basal Complex, El Hierro formed above the same domain, whereas the younger units on La Palma tapped a distinct asthenospheric domain located further north. The short-lived Bejenado volcano that formed directly after the giant Cumbre Nueva sector collapse at c. 560 ka produced the isotopically most depleted lavas reported from La Palma thus far. Their compositions suggest incorporation of a depleted pyroxenitic component. The Bejenado lavas also extend to the highest Nb/U and Ba/Th and lowest La/Nb ratios of all La Palma lavas, consistent with increased melting of amphibole within the lithospheric mantle or lower crust. We propose that the collapse is related to the migration of magmatism to the south of La Palma, and led to short-term enhanced decompression melting of amphibole and pyroxenite within the lithosphere.

Description: We present major and trace element as well as Sr, Nd, and Hf isotope data on a suite of 87 plutonic rock samples from 27 felsic crustal intrusions in seven blocks of the Oman ophiolite. The rock compositions of the sample suite including associated more mafic rocks range from 48 to 79 wt% SiO2, i.e. from gabbros to tonalites. The samples are grouped into a Ti-rich and relatively light rare earth element (LREE)-enriched P1 group [(Ce/Yb)N 〉 0.7] resembling the early V1 lavas, and a Ti-poor and LREE-depleted P2 group [(Ce/Yb)N 〈 0.7] resembling the late-stage V2 lavas. Based on the geochemical differences and in agreement with previous structural and petrographic models, we define phase 1 (P1) and phase 2 (P2) plutonic rocks. Felsic magmas in both groups formed by extensive fractional crystallization of olivine, clinopyroxene, plagioclase, apatite, and Ti-magnetite from mafic melts. The incompatible element compositions of P1 rocks overlap with those from mid-ocean ridges but have higher Ba/Nb and Th/Nb trending towards the P2 rock compositions and indicating an influence of a subducting slab. The P2 rocks formed from a more depleted mantle source but show a more pronounced slab signature. These rocks also occur in the southern blocks (with the exception of the Tayin block) of the Oman ophiolite implying that the entire ophiolite formed above a subducting slab. Initial Nd and Hf isotope compositions suggest an Indian-MORB-type mantle source for the Oman ophiolite magmas. Isotope compositions and high Th/Nb in some P2 rocks indicate mixing of a melt from subducted sediment into this mantle.

Description: A suite of 48 samples, including both historical and prehistoric lavas and some plutonic rocks, have been analysed from the Cumbre Vieja rift, La Palma, Canary Islands. Additionally, mineral–melt partition coefficients have been measured for clinopyroxene, plagioclase, amphibole, titanite and apatite in selected rocks. The lavas range from basanite to phonolite (SiO2 = 41·2–57·5 wt % and MgO = 10–0·8 wt %) in composition and form coherent, curvilinear major and trace element arrays in variation diagrams, irrespective of eruption age. The mafic lavas have typical ocean island incompatible trace element patterns and Sr, Nd and Pb isotope compositions show little variation but have a HIMU-type character. Generation of the parental magmas is inferred to have involved ∼4% dynamic melting of a garnet lherzolite source that may have previously been metasomatized by melts derived from a recycled mafic component containing residual phlogopite. The major process of differentiation to phonotephrite involved fractional crystallization of basanitic magmas that evolved along the same liquid line of descent under similar pressure–temperature conditions. Numerical simulations using the MELTS algorithm suggest that this occurred across a temperature interval from c. 1320 to 950°C at 400 MPa and an oxygen fugacity equivalent to quartz–fayalite–magnetite (QFM), with an initial H2O content of 0·3 wt %. The later stages of differentiation (〈5 wt % MgO) were dominated by mixing with partial melts of young syenites formed from earlier magma batches. All of the lavas are characterized by 230Th and 226Ra excesses and (230Th/238U) decreases with decreasing Nb/U and increasing SiO2, with no accompanying change in (226Ra/230Th). To explain the observations, we propose a model in which there was a significant role for amphibole, and more importantly accessory titanite, in decre'asing Nb/U, Ce/Pb and Th/U ratios and increasing or buffering (226Ra/230Th) ratios during the later stages of differentiation and magma mixing. These processes all occurred over a few millennia in small magma batches that were repeatedly emplaced within the mid-crust of the Cumbre Vieja rift system prior to rapid transport to the surface.

Description: 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.

Description: The Canary Island primitive basaltic magmas are thought to be derived from an HIMU-type upwelling mantle containing isotopically depleted (NMORB)-type component having interacted with an enriched (EM)-type component, the origin of which is still a subject of debate. We studied the relationships between Ni, Mn and Ca concentrations in olivine phenocrysts (85.6–90.0 mol.% Fo, 1,722–3,915 ppm Ni, 1,085–1,552 ppm Mn, 1,222–3,002 ppm Ca) from the most primitive subaerial and ODP Leg 157 high-silica (picritic to olivine basaltic) lavas with their bulk rock Sr–Nd–Pb isotope compositions (87Sr/86Sr = 0.70315–0.70331, 143Nd/144Nd = 0.51288–0.51292, 206Pb/204Pb = 19.55–19.93, 207Pb/204Pb = 15.60–15.63, 208Pb/204Pb = 39.31–39.69). Our data point toward the presence of both a peridotitic and a pyroxenitic component in the magma source. Using the model (Sobolev et al. in: Science 316:412–417, 2007) in which the reaction of Si-rich melts originated during partial melting of eclogite (a high pressure product of subducted oceanic crust) with ambient peridotitic mantle forms olivine-free reaction pyroxenite, we obtain an end member composition for peridotite with 87Sr/86Sr = 0.70337, 143Nd/144Nd = 0.51291, 206Pb/204Pb = 19.36, 207Pb/204Pb = 15.61 and 208Pb/204Pb = 39.07 (EM-type end member), and pyroxenite with 87Sr/86Sr = 0.70309, 143Nd/144Nd = 0.51289, 206Pb/204Pb = 20.03, 207Pb/204Pb = 15.62 and 208Pb/204Pb = 39.84 (HIMU-type end member). Mixing of melts from these end members in proportions ranging from 70% peridotite and 30% pyroxenite to 28% peridotite and 72% pyroxenite derived melt fractions can generate the compositions of the most primitive Gran Canaria shield stage lavas. Combining our results with those from the low-silica rocks from the western Canary Islands (Gurenko et al. EPSL 277:514–524, 2009), at least four distinct components are required. We propose that they are (1) HIMU-type pyroxenitic component (representing recycled ocean crust of intermediate age) from the plume center, (2) HIMU-type peridotitic component (ancient recycled ocean crust stirred into the ambient mantle) from the plume margin, (3) depleted, MORB-type pyroxenitic component (young recycled oceanic crust) in the upper mantle entrained by the plume, and (4) EM-type peridotitic component from the asthenosphere or lithosphere above the plume center.

Description: The broad belt of intraplate volcanism in the East Atlantic between 25° and 37° N is proposed to have formed by two adjacent hotspot tracks (the Madeira and Canary tracks) that possess systematically different isotopic signatures reflecting different mantle source compositions. To test this model, Hf isotope ratios from volcanic rocks from all individual islands and all major seamounts are presented in this study. In comparison with published Nd isotope variations (6 εNd units), 176Hf/177Hf ratios span a much larger range (14 εHf units). Samples from the proposed Madeira hotspot track have the most radiogenic Hf isotopic compositions (176Hf/177Hfm up to 0.283335), extending across the entire field for central Atlantic MORB. They form a relatively narrow, elongated trend on the Nd vs. Hf isotope diagram (stretching over 〉10 εHf units) between a depleted N-MORB-like endmember and a moderately enriched composition located on, or slightly below, the Nd–Hf mantle array, which overlaps the proposed "C" mantle component of Hanan and Graham (1996). In contrast, all samples from the Canary hotspot track plot below the mantle array (176Hf/177Hfm = 0.282943–0.283067) and form a much denser cluster with less compositional variation (~4 εHf units). The cluster falls between (1) a low Hf isotope HIMUlike endmember, (2) a more depleted composition, and (3) the moderately enriched end of the Madeira trend. The new Hf isotope data confirm the general geochemical distinction of the Canary and Madeira domains in the East Atlantic. Both domains, however, seem to share a common, moderately enriched endmember that has "C"-like isotope compositions and is believed to represent subducted, 〈1-Ga-old oceanic lithosphere (oceanic crust and possibly minor sediment addition). The lower 176Hf/177Hf ratio of the enriched, HIMU-like Canary domain endmember indicates the contribution of oceanic lithosphere with somewhat older recycling ages of 〉=1 Ga.