U-Pb isotopic dating of fluid infiltration and metasomatism during Dalradian regional metamorphism in Connemara, western Ireland

A metasomatic diopside rock occurs at the top of the dolomitic Connemara Marble Formation of western Ireland and contains titanite and K-feldspar in addition to around 90% diopside (X(Mg) = 0.90-0.97). U-Pb isotopic measurements on this mineral assemblage show that the titanite is both unusually uranium-rich and isotopically concordant, with the result that a precise U-Pb age of 478 +/- 2.5 Ma can be determined. The age is identical within error to a less precise Rb-Sr age of diopside-K-feldspar of 483 +/- 6 Ma. Petrological evidence indicates that the assemblage crystallized at c. 620-degrees-C close to or below the closure temperature of titanite. The age thus provides a precise estimate of the time of metamorphism; this age is 11 +/- 3 Ma younger than the 490 Ma age for nearby gabbroic plutons which has previously been used to constrain the peak metamorphic age. This difference accords well with geological evidence that the gabbros were emplaced prior to the metamorphic peak. Analysis of minerals with high closure temperature from assemblages whose crystallization is unambiguously associated with a specific episode of fluid infiltration at the peak of metamorphism provides the basis for a new approach to dating metamorphism. The success of this approach is demonstrated by the results from Connemara

From basin to basement: the movement of surface fluids into the crust

Quartz veins from Precambrian, high grade, basement rocks in Southern Norway contain both aqueous and biogenically sourced hydrocarbon fluid inclusions. Aqueous fluids show a wide range in salinities (0-40 wt% NaCl eq.) and densities (0.80-1.16 g/cm(3)) but the ultimate source of this salinity is unclear. The oxygen isotopic composition of most of the veins is dominated by the host-rock compositions. Surface derived fluids migrated downwards as a result of extension of dry crystalline basement rocks with pore fluid pressures less than hydrostatic. Quartz precipitation occurred in the veins initially at near hydrostatic pressures but later quartz generations were precipitated at pressures closer to lithostatic

Mineralising brines in the South West Massif Central, France

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