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Day J.M.D.,University of Maryland University College | Day J.M.D.,Durham University | Pearson D.G.,Durham University | Macpherson C.G.,Durham University | And 2 more authors.
Geochimica et Cosmochimica Acta | Year: 2010

Shield-stage high-MgO alkalic lavas from La Palma and El Hierro (Canary Islands) have been characterized for their O-Sr-Nd-Os-Pb isotope compositions and major-, trace-, and highly siderophile-element (HSE: Os, Ir, Ru, Pt, Pd, Re) abundances. New data are also reported for associated evolved rocks, and entrained xenoliths. Clear differences in Pd/Ir and isotopic ratios for high Os (>50ppt) lavas from El Hierro (δ18Oolivine=5.17±0.08‰; 87Sr/86Sr=0.7029 to 0.7031; ε{lunate}Nd=+5.7 to +7.1; 187Os/188Os=0.1481 to 0.1750; 206Pb/204Pb=19.1 to 19.7; Pd/Ir=6±3) versus those from La Palma (δ18Oolivine=4.87±0.18‰; 87Sr/86Sr=0.7031 to 0.7032; ε{lunate}Nd=+5.0 to +6.4; 187Os/188Os=0.1421 to 0.1460; 206Pb/204Pb=19.5 to 20.2; Pd/Ir=11±4) are revealed from the dataset. Crustal or lithospheric assimilation during magma transport cannot explain variations in isotopic ratios or element abundances of the lavas. Shallow-level crystal-liquid fractionation of olivine, clinopyroxene and associated early-crystallizing minerals (e.g., spinel and HSE-rich phases) controlled compatible element and HSE abundances; there is also evidence for sub-aerial degassing of rhenium. High-MgO lavas are enriched in light rare earth elements, Nb, Ta, U, Th, and depleted in K and Pb, relative to primitive mantle abundance estimates, typical of HIMU-type oceanic island basalts. Trace element abundances and ratios are consistent with low degrees (2-6%) of partial melting of an enriched mantle source, commencing in the garnet stability field (≥110. km). Western Canary Island lavas were sulphur undersaturated with estimated parental melt HSE abundances (in ppb) of 0.07 ± 0.05 Os, 0.17 ± 0.16 Ir, 0.34 ± 0.32 Ru, 2.6 ± 2.5 Pt, 1.4 ± 1.2 Pd, 0.39 ± 0.30 Re. These estimates indicate that Canary Island alkali basalts have lower Os, Ir and Ru, but similar Pt, Pd and Re contents to Hawai'ian tholeiites. The HIMU affinities of the lavas, in conjunction with the low δ18Oolivine and high 206Pb/204Pb for La Palma, and elevated 187Os/188Os for El Hierro implies melting of different proportions of recycled oceanic crust and lithosphere. Our preferred model to explain isotopic differences between the islands is generation from peridotitic mantle metasomatised by <10% pyroxenite/eclogite made from variable portions of similar aged recycled oceanic crust and lithosphere. The correspondence of radiogenic 206Pb/204Pb, 187Os/188Os, elevated Re/Os and Pt/Os, and low-δ18O in western Canary Island lavas provides powerful support for recycled oceanic crust and lithosphere to generate the spectrum of HIMU-type ocean island basalt signatures. Persistence of geochemical heterogeneities throughout the stratigraphies of El Hierro and La Palma demonstrate long-term preservation of these recycled components in their mantle sources over relatively short-length scales (∼50km). © 2010 Elsevier Ltd.

Guillou H.,CEA Saclay Nuclear Research Center | Nomade S.,CEA Saclay Nuclear Research Center | Carracedo J.C.,Estacion Volcanologica de Canarias | Kissel C.,CEA Saclay Nuclear Research Center | And 3 more authors.
Quaternary Geochronology | Year: 2011

In this study, the effectiveness of combined unspiked K-Ar and 40Ar/39Ar dating methods as currently applied now at LSCE in the 14C age range was evaluated by studying two 30,000 years old phonolites from Tenerife (Canary Islands). New 40Ar/39Ar ages obtained in this work are comparable with the K-Ar ages, and of similar precision. This remarkable agreement obtained between the two dating methods in this young age range, validates the new LSCE 40Ar/39Ar facility and provides a very promising approach to calibrate the Quaternary timescale and to date key events such as geomagnetic field instabilities and climatic changes.The approach combining the 40Ar/39Ar and the unspiked K-Ar methods has the advantage that all the basic assumptions, which rule the K-Ar clock, are checked via the isochron approach. In the event of positive check, the 40Ar/39Ar and unspiked K-Ar ages can be pooled to produce very precise ages. © 2011 Elsevier B.V.

Kissel C.,CEA Saclay Nuclear Research Center | Guillou H.,CEA Saclay Nuclear Research Center | Laj C.,CEA Saclay Nuclear Research Center | Carracedo J.C.,Estacion Volcanologica de Canarias | And 3 more authors.
Physics of the Earth and Planetary Interiors | Year: 2011

We present a coupled paleomagnetic/dating investigation conducted on three different lava flows from the island of Tenerife (Canary Islands; Spain) erupted during the Mono Lake excursion (MLE). Paleomagnetic analyses consist in zero field demagnetizations (AF and/or thermal) and of Thellier and Thellier experiments using the PICRIT-03 set of criteria to select reliable intensity determinations. One of the flows is characterized by a direction largely deviated from the one expected from an axial geocentric dipole (GAD) field. Its paleointensity value is very low (7.8μT). The two other sites are characterized by inclinations slightly shallower than the GAD value and by low intensity values (about 12 and 21μT; present value: 38μT). The three K/Ar ages combined with two 40Ar/39Ar ages range from 32.0 to 33.2ka and they are not statistically distinguishable from one another. It therefore appears that these lavas have recorded the MLE (the only excursion in this time interval) confirming its brief duration (shorter than the minimum age uncertainties available). The mean age is younger but, within the uncertainties, consistent with the age of the 10Be peak and of the marine intensity low when reported in the most recent ice age model. These new results are the first ones with radiometric dating produced from the northern hemisphere. Combined with existing cosmogenic, marine and volcanic paleomagnetic data, these results are discussed in terms of dating, and geometry of the earth magnetic field during the excursion. © 2011 Elsevier B.V.

Rodriguez-Gonzalez A.,University of Las Palmas de Gran Canaria | Fernandez-Turiel J.L.,CSIC - Institute of Earth Sciences Jaume Almera | Perez-Torrado F.J.,University of Las Palmas de Gran Canaria | Aulinas M.,University of Barcelona | And 4 more authors.
Geomorphology | Year: 2011

Modeling of volcanic morphometry provides reliable measurements of parameters that assist in the determination of volcanic landform degradation. Variations of the original morphology enable the understanding of patterns affecting erosion and their development, facilitating the assessment of associated hazards. A total of 24 volcanic Holocene eruptions were identified in the island of Gran Canaria (Canary Islands, Spain). 87% of these eruptions occurred in a wet environment while the rest happened in a dry environment. 45% of Holocene eruptions are located along short barrancos (S-type, less than 10. km in length), 20% along large barrancos (L-type, 10-17. km in length) and 35% along extra-large barrancos (XL-type, more than 17. km in length). The erosional history of Holocene volcanic edifices is in the first stage of degradation, with a geomorphic signature characterized by a fresh, young cone with a sharp profile and a pristine lava flow. After intensive field work, a careful palaeo-geomorphological reconstruction of the 24 Holocene eruptions of Gran Canaria was conducted in order to obtain the Digital Terrain Models (DTMs) of the pre- and post-eruption terrains. From the difference between these DTMs, the degradation volume and the incision rate were obtained. The denudation of volcanic cones and lava flows is relatively independent both their geographical location and the climatic environment. However, local factors, such as pre-eruption topography and ravine type, have the greatest influence on the erosion of Holocene volcanic materials in Gran Canaria. Although age is a key factor to help understand the morphological evolution of monogenetic volcanic fields, the Gran Canaria Holocene volcanism presented in this paper demonstrates that local and regional factors may determine the lack of correlation between morphometric parameters and age. Consequently, the degree of transformation of the volcanic edifices evolves, in many cases, independently of their age. © 2011 Elsevier B.V.

Caccavari A.,National Autonomous University of Mexico | Calvo-Rathert M.,University of Burgos | Goguitchaichvili A.,National Autonomous University of Mexico | Soler V.,Estacion Volcanologica de Canarias | And 2 more authors.
Geophysical Journal International | Year: 2014

A detailed palaeomagnetic, rock-magnetic and palaeointensity study has been carried out on a Miocene volcanic sequence which consists of 39 consecutive lava flows recording a polarity transition in La Gomera (Canary Islands, Spain). In addition, new 40Ar/39Ar ages were obtained in two flows, yielding 9.63 ± 0.06 Ma in the lower and 9.72 ± 0.08 Ma in the upper part of the sequence. Palaeomagnetic results allowed determining a ChRM direction in all studied lavas: The 25 lowermost flows of the sequence display normal polarity directions and above, a sequence of 14 flows correspond to a transitional geomagnetic regime. If considered together with palaeomagnetic results from a previous study, which were obtained on the flows immediately overlying the upper part of the sequence analysed in this work, these results indicate that the reversal recorded in the Hermigua sequence corresponds to the normal to reverse C4Ar2n to C4Ar3r polarity transition. The lower-lying 25 normal-polarity flows yield a mean direction D = 359.6°, I = 42.4° (α95 = 5.1° k = 33) which agrees well with the expected values. Above, a sequence of 14 flows displays a more irregular directional behaviour, including several transitional directions, suggesting the occurrence of a precursor to the transition. The presence of a virtual geomagnetic pole (VGP) cluster in the western Atlantic Ocean observed in this study coincides with previous records of Miocene transitions. Interestingly, this region corresponds to a near-radial flux centre of the present-day non axial dipole field. Angular dispersion of VGPs calculated for the 25 lowermost normal polarity flows of the sequence shows a lower than expected result. Palaeointensity determinations were carried out using a Thellier type double heating method. 27 of the 48 analysed samples measured yielded successful results. Mean VDMs mean values range from 1.1 ± 0.5 to 8.8 ± 0.9 × 1022 Am2. The intensity values decrease significantly on approaching the directional transitional zone, suggesting an earlier start of the polarity transition in the intensity record, typical of a decreasing axial dipole. © The Authors 2015. Published by Oxford University Press on behalf of The Royal Astronomical Society.

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