High-MgO ultrapotassic rocks are found in four different areas of the Western Mediterranean basin associated in space and time with shoshonitic and calc-alkaline rocks. They represent different magmatic events at the active continental plate margin from Oligocene to Pleistocene. These rocks are found within the Western Alps (Northern Italy), in Corsica (France), in Murcia-Almeria (South-Eastern Spain), and in Southern Tuscany (Central Italy). Ultrapotassic terms are mostly lamprophyres, but olivine latitic lavas with a clear lamproitic affinity are also found. Lamproite-like rocks range from slightly silica under-saturated to silica over-saturated, and they are characterised by low Al 2 O 3 , CaO, and Na 2 O contents. They are plagioclase-free rocks, but K-feldspar is abundant beside other K-bearing phases. Shoshonitic and calc-alkaline rocks are invariably space associated to lamproites, and they either precede or follow them. High-Mg ultrapotassic rocks are characterised by strong enrichment of incompatible elements, which prevent further enrichment due to shallow level crustal contamination. K 2 O and incompatible element contents decrease passing from high-Mg ultrapotassic to high-Mg shoshonitic and calc-alkaline rocks suggesting that K and incompatible trace elements enrichments are a primary characteristic. Ultrapotassic to calc-alkaline rocks from Western Mediterranean regions, in spite of their different age of emplacement, are characterised by similar incompatible trace elements distribution. Depletion of High Field Strength elements with respect to Large Ion Lithophile elements is observed. Positive spikes at Th, U, and Pb, with negative spikes at Ba, Nb, Ta, Sr, P, and Ti, are common characteristics of ultrapotassic (lamproitic) to high-K calc-alkaline rocks. Ultrapotassic rocks are extremely enriched in radiogenic Sr and unradiogenic Nd with respect to the associated shoshonitic and calc-alkaline rocks. Different isotopic values are distinctive of the different magmatic provinces irrespective of magmatic affinities. 87 Sr/ 86 Sr i ranges between 0.71645 and 0.71759 for Western Alps lamproites, between 0.71226 and 0.71230 for Corsica lamproite, between 0.71642 and 0.72259 for Murcia-Almeria lamproites, and between 0.71578 and 0.71672 for Tuscany lamproites. Radiogenic Sr decreases along with K 2 O through shoshonitic to calc-alkaline rocks. Conversely 143 Nd/ 144 Nd i values increase with decreasing K 2 O, with the highest value of 0.51243 found for the one samples from Murcia-Almeria. Contrasting trends are observed among initial values of lead isotopes, but all falling well within the field of upper crustal rocks. Different trends of 207 Pb/ 204 Pb i and 208 Pb/ 204 Pb i vs. 206 Pb/ 204 Pb i for samples from the different provinces are observed. Several evidences indicate that most of the magmas of the different provinces have been generated in a depleted upper mantle (i.e., lithospheric) modified by metasomatism, but an asthenospheric component is also recognised in Corsica. At least two different s...

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The concentrations of incompatible trace elements in ocean island basalts (OIB) from the central Atlantic extend to relatively enriched and fractionated compositions in regions of older oceanic lithosphere. Certain trace element ratios normally considered to be uniform in the mantle, such as Ce/Pb, are particularly variable. However, other trace element ratios that are expected to be variable because of differences in bulk distribution coefficient, such as Cc/U, are relatively uniform. The Ce/Pb ratios in enriched OIB are correlated with unusually high U/PI, and low K/U. These U/Pb ratios would have generated excessively radiogenic Pb if they were long-term (lo9 yr) features of the source such as might result from core formation or recycling of h&rothermally altered ocean floor basalts. However, volcanic centers with high U/W do have high 206Pb/ 204Pb for their Pb/ 204Pb, a feature that is most easily modelled by enrichment in U relative to Pb about 10' yr prior to melting, a time similar to the age of the lithosphere. We propose that the source regions of these magmas are enriched by the introduction of small degree partial melts soon after the formation of the oceanic lithosphere. Metasomatism of the uppermost mantle by small degree partial melts produced in equilibrium with a combination of residual upper mantle major silicate phases, together with minor amphibole ( 5 2%), sulfide (< 0.2%) and phlogopite (< 0.2%) at about the time of formation of the lithosphere, would generate a 'near-surface fractionated' (NSF) source with low K/U and high U/Pb, AzosPb/*Pb and Ce/Pb, while maintaining Cc/U, Nb/U, Ba/C!e and Ba/Nb that are only slightly fractionated relative to other OIB. An important feature of the modelling of NSF mantle is that U is more incompatible than Ba or Rb. This is confirmed by the variability in incompatible trace element ratios with U concentration for enriched OIB. However, this contrasts with the relative incompatibility deduced from U-Th-Ra disequilibrium data for MORB and OIB, endorsing the view that the variability in highly incompatible trace element ratios in enriched OIB is dominated by source enrichment effects that are distinct from the fractionation that takes place during the production of the erupted magmas.The Cc/U, Ba/Ce and U/Pb ratios of all OIB, including enriched OIB from regions of old lithosphere, are uniform relative to data for MORB. This appears inconsistent with the degree of isotopic variability in OIB relative to MORB and is difficult to explain unless the variations in incompatible trace element ratios in MORB are dominated by effects other than melting. Ratio-element plots provide evidence that the incompatible element ratios of MORB are affected by OIB-component contamination in the source or in transit to the surface and this is consistent with covariation between trace element ratios and some isotopic compositions in MORB. The ratios and concentrations of highly incompatible trace elements in MORB vary as a consequence of this contamination, as well as deg...

Stromboli is the type locality of continuous and moderately explosive volcanic activity. Monitoring temporal variations in the composition of the material erupted allows constraints to be made on the magma chamber dynamics and volcanic hazard. Here we present an Sr isotope survey of scoriae and lavas erupted from Stromboli volcano during this century. The material erupted is transitional between shoshonite and High-K basalt, with relatively constant major and trace element composition. This implies no substantial physical separation of minerals during crystallization (ca. 50 vol% minerals in both lavas and scoriae) and hence a relatively homogeneous reservoir. 87 Sr= 86 Sr values are constant from 1900 to ca. 1980, then, beginning prior to the major lava flow eruption of December 1985, there is a smooth decrease. The Sr isotope decrease records the arrival of a new feeding magma, and allows estimation of the magma residence time and the volume of the reservoir beneath Stromboli volcano. Our results, along with a critical assessment of magma flux estimates, are best reconciled with steady state conditions, and establish the existence of a relatively small, ¾0.3 to 0.04 km 3 , reservoir with a magma residence time .− / of ca. 19 years. Monitoring the temporal variation of isotopic ratios in active volcanoes appears a successful tool in forecasting some major volcanic eruptions.