The footprints of mafic melts travelling from the depths to the surface are abundant in the mantle section of ophiolites. They constitute an important source of information about the melt migration mechanisms and related petrological processes in the shallowest part of the mantle beneath former oceanic spreading centres. In the field, these so-called ‘melt migration structures’ attract attention when they consist of mineral assemblages contrasting with that of their host peridotite. They therefore record a particular moment in the migration history: when the melt becomes out of equilibrium with the peridotite and causes a reaction impacting its modal composition, and/or when a temperature drop initiates the crystallization of the melt. The existence of cryptic effects of migration revealed by geochemical data shows that melts do not always leave a trail visible in the field. Although incomplete and patchy, the melt migration structures preserved in ophiolites are witnesses of processes that do actually occur in nature, which constitutes an invaluable support to the interpretation of geophysical data and inescapable constraints for numerical simulations and models of chemical geodynamics. Here we show how field observations and related petrological and geochemical studies allow us to propose answers to fundamental questions such as these: At which temperature is porous flow superseded by dyking? What are the factors governing melt trajectories? What is the nature of the ‘universal solvent’ initiating infiltration melting and making channelized porous flow the most common mode of transport of magmas through a peridotite matrix regardless the tectonic setting? A fundamental message delivered by ophiolites is that the shallow mantle behaves as a particularly efficient reactive filter between the depths and the surface of the Earth. Unexpectedly, the reactions occurring there are enhanced by the hybridization between mafic melts and a hydrous component, whatever its origin ( i.e. magmatic vs. hydrothermal). This hybridization triggers out of equilibrium reactions, leading to the formation of exotic lithologies, including metallic ores, and impacting the global geochemical cycle of a whole range of chemical elements.

ABSTRACT The petrology of anhydrous peridotite xenoliths hosted in the Cenozoic alkaline volcanic rocks from Handler Ridge (northern Victoria Land, Antarctica) provides new constraints on the characterization of the subcontinental lithospheric mantle beneath the West Antarctic Rift. For most samples, the temperature of equilibration was calculated on the basis of Fe/Mg partitioning among olivine, orthopyroxene, and spinel, at a pressure of 15 kbar. These results revealed a temperature of ~1030 °C and fO 2 ranging from –0.26 to +0.39 with respect to fayalite-magnetite-quartz buffer (ΔFMQ). Compared with other anhydrous and hydrated mantle xenolith suites occurring in northern and southern Victoria Land, these xenoliths represent the highest-temperature and most-oxidized conditions. On the basis of major-element modeling, we suggest that this portion of the mantle represents a residuum after 7%–18% partial melting. Geochemical and isotopic compositional evidence is indicative of significant metasomatism caused by an alkaline melt almost entirely overprinting the residual peridotite composition within a period of 10 2 –10 3 yr.

Abstract The tholeiitic basalt intrusives as sills and dykes from the Kutch region have been classified into high Ti and low Ti categories. The high-Ti basalts display characters comparable to the shield lavas of the Reunion Island with OIB type signature. The incompatible trace element patterns and ratios as well as the Sr-Nd composition of the high Ti-basalts support their generation from mixing of melts derived from both the convecting asthenospheric mantle (plume?) as the dominant source and the SCLM with little contribution from the continental crust. The petrographic and chemical data indicate dominant clinopyroxene and plagioclase fractionation and small olivine fractionation. In contrast, the low-Ti basalts exhibit trace element characters and Sr and Nd isotopic systematics similar to MORB with relatively more contribution from the SCLM, and less from Reunion like source that has been modified by variable contribution from the crustal contaminants. The variable amount of crustal interaction further divide the low Ti basalts into two subtypes viz. low-Ti-high Pb and low Ti-low Pb. The geochemical characters of both the high Ti and Low Ti basalts as well as the lava flows from Kutch do not correlate well with any of their southwestern counterparts identified in the main Deccan province and most probably represent lower levels. It is essential to treat these basalts therefore as separate stratigraphic unit as “Kutch Group” in the Deccan stratigraphy.

New major-element, trace-element, and isotopic (Nd, Sr) analyses of undersaturated alkaline lavas from the Kilimanjaro volcano (north Tanzania) are presented. These data concern 54 samples, ranging from basanites to phonolites, collected during a 1 mo field trip in March 2005. The three main cones of Kilimanjaro were sampled, Shira, Mawenzi, and Kibo, together with numerous parasitic cones located on a SE lineament on the main edifice. On the basis of both spatial distribution and major- and trace-element characteristics of analyzed samples, the previous classification of Kilimanjaro lavas is simplified into five groups: Shira, Mawenzi, Kibo 1, Kibo 2, and parasitic activity, each of which has distinct petrological and geochemical features. The major- and trace-element characteristics of the rare primitive lavas erupted on the volcano yield the ubiquitous signature of amphibole within the magma source. We propose that Kilimanjaro melts originated from the partial melting of lithospheric mantle. Combined modeling of trace-element behavior during partial melting + fractional crystallization and isotopic constraints allow us to propose a schematic model of melt genesis under the Kilimanjaro area. Thermal heating of the ancient continental lithosphere by an upwelling plume triggered partial melting in parts of the lithosphere where amphibole was present and led to the Shira volcanic episode. Then, during a time span of ~1 m.y., the depleted lithosphere was progressively infiltrated by plume melts that resulted in crystallization of a new generation of metasomatic amphibole. Finally, this rejuvenated lithosphere underwent partial melting, leading to the magmas that formed the main edifice (Mawenzi and Kibo), and leading to a progressive depletion of the source with time. An active contribution of true asthenospheric melts during the last magmatic events of the volcano cannot be excluded, but further detailed isotopic investigations are needed to test the model.