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,An absolute value has been obtained for the isotopic abundance ratio of a reference sample of rhenium, using surface emission mass spectrometry. Samples of known isotopic composition , prepared from nearly isotopically pure se parat ed rhenium isotopes , were used to calibrate the mass spectrometers. The resulting absolute '85 Re/'·7 Re ratio is 0.59738 ± 0.00039, which yields atom per· cents of ' 8s Re=37 .398±0.016 and '87Re=62.602±0.016. The atomi c weight calculated from this isotopi c c omposition is 186.20679 ± 0.00031. The indi cated uncertainties are overall limits of e rror based on 95 perce nt confidence limits for th e mean and allowances for the effe cts of known sources of possible systematic error.

Gravity gradients derived from an updated Bouguer anomaly map of Ireland reveal large-scale lineaments, many of which can be related to geological structures and tectonic development. First horizontal derivatives of the anomaly calculated at various azimuths accentuate linear trends in different directions, while the second derivative helps resolve near surface structure. The dominant NE-SW Caledonian-trending lineaments are interpreted as due to fault boundaries and folds in pre-Carboniferous basement and Caledonian fault reactivation during the Variscan Orogeny. Subsidiary NNE-SSWtrending lineaments are present, as well as weaker NW-SE-trending lineaments. The largest negative anomalies are produced by late Caledonian granites and widespread buried granitic rock in the middle to upper crust is proposed. Both exposed and inferred granites appear to be spatially related to intersections of NE-SW-with NNE-SSW-and NW-SE-trending lineaments. The orientation of the dominant gravity trend changes from NE-SW towards ENE-WSW going westwards across Ireland and forms part of a more regional change in the 'Caledonian' gravity fabric recognised throughout the northeast Atlantic region. 'Variscan' E-W gravity trends in the south of Ireland overlay the 'Caledonian' trends. Relationships between gravity lineaments and anomalies support a 'thin-skinned' model for the Variscan cover.

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