Petrology

ABSTRACT Petrogenetic conditions are evaluated for a poorly studied segment of the MAR axial zone. Mantle peridotites from MAR at 19A degrees 42'aEuro"19A degrees 59' N are determined to show low to moderately low degrees of melting. The geochemistry of the peridotites is close to that of typical abyssal peridotites hosting active hydrothermal fields in the Atlantic (such as the Ashadze-Semenov-Logachev), except that plagioclase lherzolite occurs among the peridotites. Some of our samples collected in the area represent magnesian metabasites which could have been produced by metasomatic transformation (rodingitization) of harzburgite at contact with gabbroids. Cr and Ni concentrations in serpentine were proved to be indicative of the precursor mineral of the serpentine. The distribution of the isotopic composition data points of the MAR abyssal peridotites in the Sr-87/Sr-86-Nd-143/Nd-144 diagram suggests that outcrops of mantle peridotites and related plutonic rocks in the crest zone of the ridge expose blocks of deep-sitting rocks that were not simultaneously brought to the surface. The MAR axial segment between 19A degrees 42' and 19A degrees 59' N can be viewed as prospective for searches for hydrothermal ore mineralization. Compositional parameters of basalt in our rock collection correspond to those of the most depleted MORB varieties in the Atlantic.

Petrogenetic conditions are evaluated for a poorly studied segment of the MAR axial zone. Mantle peridotites from MAR at 19A degrees 42'aEuro"19A degrees 59' N are determined to show low to moderately low degrees of melting. The geochemistry of the peridotites is close to that of typical abyssal peridotites hosting active hydrothermal fields in the Atlantic (such as the Ashadze-Semenov-Logachev), except that plagioclase lherzolite occurs among the peridotites. Some of our samples collected in the area represent magnesian metabasites which could have been produced by metasomatic transformation (rodingitization) of harzburgite at contact with gabbroids. Cr and Ni concentrations in serpentine were proved to be indicative of the precursor mineral of the serpentine. The distribution of the isotopic composition data points of the MAR abyssal peridotites in the Sr-87/Sr-86-Nd-143/Nd-144 diagram suggests that outcrops of mantle peridotites and related plutonic rocks in the cres...

Data presented in the paper suggest significant differences between the thermodynamic conditions under which magmatic complexes were formed in MAR at 29°–34° N and 12°–18° N. The melts occurring at 29°–34° N were derived by the melting of a mantle source with a homogeneous distribution of volatile components and arrived at the surface without significant fractionation, likely, due to their

A suite of basalt-hosted peridotites originating from the shallow lithospheric mantle beneath Marsabit volcanic field have been investigated in order to constrain the compositional characteristics and evolution of the mantle beneath this part of the Kenya Rift. Four types of peridotite xenoliths provide evidence of a cold, highly deformed, and heterogeneous upper mantle. Textures, thermobarometry and geochemical characteristics of minerals indicate that low temperatures in the spinel stability field (ca. 750-800°C at <1.5 GPa) were attained by decompression and cooling from initially high pressures and temperatures in the garnet stability field (970-1080°C at 2.3-2.9 GPa). Cooling, decompression and penetrative deformation are consistent with lithospheric thinning, probably related to the development of the Mesozoic to Paleogene Anza graben. Re-equilibrated and recrystallized peridotite xenoliths record heating up to 1100°C. Mineral trace element signatures indicate variable degrees of metasomatic overprint ranging from cryptic (LILE enrichment in cpx) up to strong modal metasomatism (formation of amphibole and phlogopite) by mafic silicic melts parental to the Quaternary host basanites and alkali basalts. Relationships between geochemistry, P-T conditions of equilibration and mineral texture can be explained by metasomatism and heating of the lithosphere related to the formation of the Kenya Rift above a zone of upwelling mantle (asthenospheric or plume-related).

ABSTRACT At Cima di Gagnone, garnet peridotite and chlorite harzburgite lenses within pelitic schists and gneisses correspond to eclogite-facies breakdown products of hydrated peridotites and are suitable for studying dehydration of serpentinized mantle. Thermobarometry and pseudosection modelling yield peak temperatures of 750-850 degrees C and pressures < 3 GPa. The minimum temperature recorded by the garnet peridotite corresponds to the maximum conditions experienced by the chlorite harzburgite, suggesting that these rocks recrystallized cofacially at similar to 800 degrees C. Alternatively, they might have decoupled during subduction, as achieved in tectonically active plate interface boundaries. The major and rare earth element (REE) variability of the peridotites was mostly acquired during pre-subduction mantle evolution as a result of partial melting and reactive melt flow. The ultramafic suite is also characterized by fluid-mobile element enrichments (B, Pb, As, Sb, Cs, Li, U, Be), which confirm derivation from variably serpentinized protoliths. Similarity in the U, Pb, B, Li and Sr contents of the Gagnone peridotites to present-day oceanic serpentinites suggests that these elements were partly taken up during initial serpentinization by seawater-derived fluids. Positive Be, As and Sb anomalies suggest involvement of fluids equilibrated with crustal (metasedimentary) reservoirs during subsequent subduction metamorphism and peridotite entrainment in (meta)sediments. Fluid-mobile element enrichment characterizes all peak eclogitic minerals, implying that multiple hydration events and element influx pre-dated the eclogite-facies dehydration. Peak anhydrous minerals retain B, Li, As and Sb concentrations exceeding primitive mantle values and may introduce geochemical anomalies into the Earth's mantle. The relatively low contents of large ion lithophile elements and light REE in the Gagnone peridotites with respect to much higher enrichments shown by metasomatized garnet peridotite pods hosted in migmatites (Ulten Zone, Eastern Alps) suggest that the crustal rocks at Gagnone did not experience partial melting. The Gagnone garnet peridotite, despite showing evidence for chlorite dehydration, retains significant amounts of fluid-mobile elements documenting that no partial melting occurred upon chlorite breakdown. We propose that the Gagnone ultramafic rocks represent a prime example of multi-stage peridotite hydration and subsequent dehydration in a plate interface setting.

A suite of basalt-hosted peridotites originating from the shallow lithospheric mantle beneath Marsabit volcanic field have been investigated in order to constrain the compositional characteristics and evolution of the mantle beneath this part of the Kenya Rift. Four types of peridotite xenoliths provide evidence of a cold, highly deformed, and heterogeneous upper mantle. Textures, thermobarometry and geochemical characteristics of minerals indicate that low temperatures in the spinel stability field (ca. 750-800°C at <1.5 GPa) were attained by decompression and cooling from initially high pressures and temperatures in the garnet stability field (970-1080°C at 2.3-2.9 GPa). Cooling, decompression and penetrative deformation are consistent with lithospheric thinning, probably related to the development of the Mesozoic to Paleogene Anza graben. Re-equilibrated and recrystallized peridotite xenoliths record heating up to 1100°C. Mineral trace element signatures indicate variable degr...