ALBERT

Description: The magmatic evolution of the Alboran region (westernmost Mediterranean) contains important clues for improving our understanding of the origin of Mediterranean-style back-arc basins and the desiccation of the Mediterranean Sea in the Messinian. We use new laser 40Ar/39Ar age and geochemical (major and trace element and O–Sr–Nd–Pb isotope) data from igneous rocks from southern Spain, the Alboran Sea and northern Morocco to reconstruct the magmatic evolution of the westernmost Mediterranean since the Eocene. Lower Oligocene dikes near Malaga (33.6±0.6 Ma) and Middle to Upper Miocene volcanic rocks from the Alboran Sea area (6.57±0.04 to 11.8±0.4 Ma) can be subdivided into two groups: (1) LREE-depleted (relative to N-MORB), primarily tholeiitic series, and (2) LREE-enriched, primarily calc-alkaline series volcanic rocks. Both groups are generally enriched in fluid-mobile elements (e.g. Rb, Th, U, K and Pb) relative to fluid-immobile elements (e.g. Nb, Ta, LREE). The LREE-depleted group has 143Nd/144Nd (0.5128–0.5130) isotope ratios similar to Atlantic MORB but higher 87Sr/86Sr (0.7046–0.7100). In contrast, the LREE-enriched group has less radiogenic Nd (0.5121–0.5126) and tend to more radiogenic Sr (0.7066–0.7205) isotopic composition. Pb isotope ratios are surprisingly uniform and have compositions similar to marine sediments. Analyses of mineral separates show that mafic melts with relatively low δ18O (5.6–7.2‰) had high 87Sr/86Sr (0.7048–0.7088), Δ7/4 (10.6–14.1) and Δ8/4 (40.0–49.3). Modeling of the trace elements and Sr–Nd–Pb–O isotopic compositions provides compelling evidence for the contamination of the mantle source with hydrous fluids/melts, which can be explained through subduction of oceanic lithosphere beneath the Alboran Basin but not through detachment/delamination of lithospheric mantle. We present a geodynamic model that reconstructs the Late Eocene to Quaternary evolution of the western Mediterranean through westward roll-back of subducted Tethys oceanic lithosphere. Slab roll-back resulted in a large-scale reorganization of the western Mediterranean paleogeography, causing the closure of marine gateways linking the Atlantic Ocean to the Mediterranean Sea. Isolation of the Mediterranean Sea led to its desiccation, causing the Messinian Salinity Crisis.

Description: U-series disequilibria are presented for Holocene samples from the Canary Islands and interpreted with special emphasis on the separate roles of plume vs. lithospheric melting processes. We report Th and U concentrations and (238U)/(232Th), (230Th)/(232Th), (230Th)/(238U) and (234U)/(238U) for 43 samples, most of which are minimally differentiated, along with (226Ra)/(230Th) and (231Pa)/(235U) for a subset of these samples, measured by thermal ionization mass spectrometry (TIMS). Th and U concentrations range between 2 and 20 ppm and 0.5 and 6 ppm, respectively. Initial (230Th)/(238U) ranges from 1.1 to 1.6. (226Ra)/(230Th)o ranges between 0.9 and 1.8 while (231Pa)/(235U)o ranges between 1.0 and 2.0. Our interpretation of results is based on a three-fold division of samples as a function of incompatible element ratio, such as Nb/U. The majority of samples have Nb/U = 47 ± 10, similar to most MORB and OIB. Higher ratios are found exclusively in alkali basalts and tholeiites from the eastern Canary Islands whereas lower ratios are exclusively found in differentiated rocks from the western Canary Islands. Those with ordinary Nb/U ratios are attributed to melting within the slowly ascending HIMU-dominated Canary plume. Higher Nb/U, generally found in more silica rich basalts from the eastern islands, is attributed to lithospheric contamination. Based on their trace element characteristics, two possible contaminants are amphibole veins (± other minerals) crystallized in the mantle from previous plume-derived basanite or re-melted plume-derived intrusive rocks. The high Nb/U signature of these materials is imparted on a melt of the lithosphere created either by the diffusive infiltration of alkalis or by direct reaction between basanites and peridotite. Mixing between plume-derived basanite and lithospheric melt accounts for the U-series systematics of most eastern island magmas including the well-known Timanfaya eruption. Lower Nb/U ratios in differentiated rocks from the western islands are attributed to fractional crystallization of amphibole ± phlogopite ± sphene from basanite during its ascent through the lithosphere. Based on changes in disequilibria, phonolites and tephrites are interpreted to result from rapid differentiation of primitive parents within millennia.

Description: Major element, trace element and Sr–Nd–Pb isotope data, combined with 40Ar/39Ar age determinations, of volcanic rocks from the Selvagen Islands and neighboring seamounts in the eastern North Atlantic reveal the earlier history of the ≥70 Ma old Canary hotspot. A basanitic to phonolitic late shield stage intrusive complex (29 Ma) is exposed on Selvagem Pequena. The evolution of Selvagem Grande can be divided into three magmatic phases: a tephritic to phonolitic late shield stage intrusive complex (24–26 Ma) and two rejuvenated or post-erosional stages (8–12 and 3.4 Ma) consisting of alkali basalt, basanite and rare phonolite. During the early to mid-Miocene volcanic hiatus (12–24 Ma), the top of the volcano was beneath sea level as evidenced by marine carbonate sediments (13–24 Ma, dated through correlation of 87Sr/86Sr with the seawater Sr isotope curve). The geochemistry of the shield stage lavas indicates that they derive from plume sources, whereas the post-erosional lavas are derived from metasomatized lithospheric sources. Five sampled seamounts to the east and northeast of the islands range in composition from alkali basalt and basanite to phonolite. Samples from Dacia, Conception Bank and Lars were dated at 9, 17 and 68 Ma, respectively. Geochemical data suggest that the dredged samples come from the post-erosional stage of volcanism, and therefore, the dates represent minimum ages for the seamount volcanoes. The elevation of erosional platforms formed at wave base decrease from Selvagen Grande (∼100 m above sea level) to Lars seamount (∼900 m below sea level), suggesting a southwest to northwest age progression and that all of these seamounts are older than the Selvagen Islands. Trace element and Sr–Nd–Pb isotopic composition of the Selvagen Islands and neighboring seamounts are consistent with their origin from the Canary plume. Interaction of the weak Canary mantle plume with a slow moving plate appears to be responsible for generating a 450-km-wide, irregular hotspot track extending 800 km from the youngest Canary Island of Hierro in the southwest to Lars seamount in the northeast.

Description: Oceanic flood basalts are poorly understood, short-term expressions of highly increased heat flux and mass flow within the convecting mantle. The uniqueness of the Caribbean Large Igneous Province (CLIP, 92–74 Ma) with respect to other Cretaceous oceanic plateaus is its extensive sub-aerial exposures, providing an excellent basis to investigate the temporal and compositional relationships within a starting plume head. We present major element, trace element and initial Sr–Nd–Pb isotope composition of 40 extrusive rocks from the Caribbean Plateau, including onland sections in Costa Rica, Colombia and Curaçao as well as DSDP Sites in the Central Caribbean. Even though the lavas were erupted over an area of ∼3×106 km2, the majority have strikingly uniform incompatible element patterns (La/Yb=0.96±0.16, n=64 out of 79 samples, 2σ) and initial Nd–Pb isotopic compositions (e.g. 143Nd/144Ndin=0.51291±3, ϵNdi=7.3±0.6, 206Pb/204Pbin=18.86±0.12, n=54 out of 66, 2σ). Lavas with endmember compositions have only been sampled at the DSDP Sites, Gorgona Island (Colombia) and the 65–60 Ma accreted Quepos and Osa igneous complexes (Costa Rica) of the subsequent hotspot track. Despite the relatively uniform composition of most lavas, linear correlations exist between isotope ratios and between isotope and highly incompatible trace element ratios. The Sr–Nd–Pb isotope and trace element signatures of the chemically enriched lavas are compatible with derivation from recycled oceanic crust, while the depleted lavas are derived from a highly residual source. This source could represent either oceanic lithospheric mantle left after ocean crust formation or gabbros with interlayered ultramafic cumulates of the lower oceanic crust. High 3He/4He in olivines of enriched picrites at Quepos are ∼12 times higher than the atmospheric ratio suggesting that the enriched component may have once resided in the lower mantle. Evaluation of the Sm–Nd and U–Pb isotope systematics on isochron diagrams suggests that the age of separation of enriched and depleted components from the depleted MORB source mantle could have been ≤500 Ma before CLIP formation and interpreted to reflect the recycling time of the CLIP source. Mantle plume heads may provide a mechanism for transporting large volumes of possibly young recycled oceanic lithosphere residing in the lower mantle back into the shallow MORB source mantle.

Description: Major element, trace element and Sr–Nd–Pb isotopic data of volcanic rocks from the Madeira Archipelago (eastern North Atlantic) and seamounts of the Madeira hotspot track (Ampère, Coral Patch and Ormonde) are presented in this study. Although the Sr and Nd isotopic ratios are similar to those in normal mid ocean ridge basalt, the incompatible element signatures and Pb isotopic compositions (206Pb/204Pb=18.7–19.8) show similarities to the high time integrated 238U/204Pb mantle component. On the 206Pb/204Pb versus 207Pb/204Pb isotope diagram, all samples plot below the Northern Hemisphere reference line (Δ 7/4=−1.0 to −7.2) and form a 430 Ma isochron. The Pb, as well as Sm–Nd, isotope data are consistent with the presence of recycled Paleozoic (≤500 Ma) oceanic crust in the Madeira source. Variations in major element and isotopic geochemistry (e.g. positive correlation between SiO2 and FeOT with Pb isotope ratios in primitive samples) point to a heterogeneous plume source containing upper (primarily hydrothermally altered basaltic) and lower (primarily unaltered gabbroic) ocean crust and lithospheric mantle. The more fertile basaltic crustal component is preferentially sampled during the shield stage of volcanism, whereas the more depleted lower crust and lithospheric mantle components are preferentially sampled during the post-erosional stage. We propose that plume material becomes progressively depleted through melt extraction as it spreads out along the base of the lithosphere in the direction of plate motion. A systematic decrease in 143Nd/144Nd and increase in 207Pb/204Pb isotopic ratios with increasing age along the hotspot track and proximity to the Iberian peninsula are attributed to increased contamination by continental lithosphere.