1. Biotite Rb-Sr closure temperature and cation exchange (2001)
2. Post-Caledonian fluid events in West Connacht, Ireland (1998)
3. How rock mode affects cooling rate estimates from Rb-Sr mineral data (1997)
4. 200 Ma of fluid evolution in the Galway Granite, Ireland (1997)
5. Deformation, oxygen isotope exchange and temperatures of mylonites; Appalachians (1997)
6. Isotopic evolution of fluids, south Cornwall (1995)
7. Rb-Sr closure temperatures in bi-mineralic rocks (1995)
8. Meteoric fluid penetration into an active mountain belt; New Zealand (1994)
9. The origin of rapakivi texture; Finland (1994)
10. Oxygen isotope exchange and closure temperatures (1994)
11. Estimate of the diffusion rate of oxygen in diopside; Adirondacks (1994)
12. Fault related mineralisation at Tyndrum, Scotland (1993)
13. Studies of Mo mineralisation, Galway Granite, Ireland (1992)
14. A stable isotope study of retrograde alteration; Ireland (1992)
15. Fluid disturbed hornblende K-Ar ages; Ireland (1991)
16. Modeling mineral d¹⁸O values in an igneous aureole; Scotland (1991)
17. COOL: A FORTRAN-77 computer program for modeling stable isotopes (1991)
18. Textural evolution of Rapakivi granites: isotopic investigations; Greenland (1991)
19. First authored conference papers and abstracts

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The bottom line: Cation exchange during cooling may be as important as isotopic exchange in controlling the closure temperature. See also Jenkin (1997).

Abstract. Factors controlling closure in the biotite Rb-Sr system were investigated in a detailed study of an amphibolite-facies metacarbonate from the central Swiss Alps. Oxygen isotope data suggest that the rock cooled as a closed system. Calcite-dolomite thermometry temperatures of ~450-500°C and feldspar thermometry temperatures of ~300-400°C provide evidence of extensive Ca-Mg and Na-K exchange during cooling. Biotite in the sample is 90 mol% phlogopite and has high Rb (~900 ppm) compared to Sr (~0.3 ppm), giving precise Rb-Sr ages. Carefully separated and sized phlogopite shows a range of Rb-Sr ages that do not simply decrease with grain size as predicted by current models of closure temperature. Rb-Sr ages decrease from 18.1 Ma to 16.6 Ma with a decrease in mean grain diameter from 1.16 to 0.74 mm, but grains with mean diameter of 0.54 mm show an increase again to 17.6 Ma. This contrasts with Ar-Ar data for single phlogopites which do show a decrease in age with decreasing grain size. The Rb-Sr age pattern is due to Rb-loss during cooling, which is most pronounced in the finest fraction. The phlogopites are restricted to a 2 cm-thick layer in calcite marble; ⁸⁷Sr/⁸⁶Sr of the calcite decreases away from the phlogopite band over 4 cm, indicating that the calcite was moving towards Sr-isotope equilibration with the phlogopites over this distance and that the phlogopite was not equilibrating with an "infinite reservoir". Ion microprobe traverses across grains of different minerals reveal systematic core-rim variations in major and trace element concentrations. In particular, Sr decreases from calcite core to rim, but increases from core to rim in K-feldspar, whereas Rb decreases from core to rim in phlogopite but also increases from core to rim in K-feldspar. These gradients are interpreted as indicating the direction of transport of elements during cooling as a result of cation exchange reactions; calcite and phlogopite were respectively sources for Sr and Rb, whereas K-feldspar acted as a sink for both elements. This chemical equilibration was taking place at the same time as isotopic equilibration during cooling, and was equally important in controlling the apparent ages recorded by the mica grains. In contrast, closure temperature calculations for geochronological systems based on classic Dodson-type models assume parent and daughter element concentrations are homogeneous across grains and do not change with time, only isotopic exchange is modelled. Closure in mica Rb-Sr systems will depend both on the factors that control isotopic exchange (grain size, mode, ⁸⁷Sr diffusion coefficients) and those that control chemical exchange (grain size, mode, Rb and Sr diffusion coefficients, Rb and Sr contents of phases and their partition coefficients).

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The bottom line: Data can be interpreted to indicate a major fluid event affected the region in the Upper Triassic (~210 Ma), backing up the conclusions made by O'Reilly et al. (1997).

Abstract. The causes of hydrothermal alteration in dolerite dykes intruding Caledonian rocks of W Connacht are investigated using stable isotope, water content and K-Ar data for whole rocks and mineral separates. Using an isochron approach the Logmór dyke in the north is re-dated to 308±4 Ma; previously determined older whole rock ages reflect excess ⁴⁰Ar. The ~305 Ma age previously proposed for the Teach Dóite suite in the south is reinforced by a 305 Ma age on a pyroxene separate, although the severe resetting of most samples is emphasised by other pyroxene and plagioclase ages of ~210 Ma. Pyroxene ¹⁸O values for these Upper Carboniferous dykes are mostly 5.5 to 6.1, indicating negligible crustal contamination. Logmór whole rock samples have water contents of 1.7-2.1 wt.%, D = -59 to -47 and ¹⁸O = 9.4 to 9.6; plagioclase shows little mineralogical alteration but its ¹⁸O is 9.7. Hydrothermal alteration involving a local formation/metamorphic water took place at high fluid/rock ratios and high temperature during cooling after intrusion, most probably in a thermally-driven convection system. Teach Dóite dykes have water contents of 2.0-4.2 wt.%, D = -58 to -38 and ¹⁸O = 3.6 to 9.2, and were mostly altered in two stages; hydration upon intrusion to ~2 wt.% water by contemporaneous meteoric water at low fluid/rock ratios was followed by extensive chemical and isotopic alteration at ~210 Ma (Upper Triassic) by surface waters. This latter event could also have caused the extensive alteration observed in the host rocks.

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The bottom line: Mica Rb-Sr ages may give erroneous cooling paths if rock mode is not taken into consideration. See also Jenkin et al. (1995).

Abstract. A closed-system Sr isotope exchange model involving isotopic equilibration among minerals by volume diffusion within grains and grain boundary diffusion between grains is used to predict mica Rb-Sr ages for slowly cooled rocks containing biotite, muscovite, and feldspar. Mica ages depend on the rock mode and usually differ from ages predicted by Dodson's model. Cooling-rates derived from model ages by using the empirical difference in closure temperature between muscovite and biotite of 200°C are similar to the cooling-rate input to the model for feldspar-rich rocks, but are higher for feldspar-poor rocks. In biotite-rich rocks, biotite ages older than muscovite ages are predicted. Mica Rb-Sr ages may give erroneous cooling paths, and variations in cooling path between adjacent areas could be an effect of sampling rocks with different modes.

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The bottom line: Some mineralisation which is spatially related to a granite may be much later than the age of intrusion, and unrelated to magmatic heat.

Abstract. Fluid inclusions in granite quartz and three generations of veins indicate that three fluids have affected the Caledonian Galway Granite. These fluids were examined by petrography, microthermometry, chlorite thermometry, fluid chemistry and stable isotope studies.

The earliest fluid was a H₂O-CO₂-NaCl fluid of moderate salinity (4-10 wt.% NaCl eq.) that deposited late-magmatic molybdenite mineralised quartz veins (V₁) and formed the earliest secondary inclusions in granite quartz. This fluid is more abundant in the west of the batholith, corresponding to a decrease in emplacement depth. Within veins, and to the east, this fluid was trapped homogeneously, but in granite quartz in the west it unmixed at 305-390°C and 0.7-1.8 kb. Homogeneous quartz ¹⁸O across the batholith (9.5±0.4 n=12) suggests V₁ precipitation at high temperatures (perhaps 600°C) and pressures (1-3 kb) from magmatic fluids. Microthermometric data for V₁ indicate lower temperatures, suggesting inclusion volumes re-equilibrated during cooling.

The second fluid was a H₂O-NaCl-KCl, low-moderate salinity (0-10 wt.% NaCl eq.), moderate temperature (270-340°C), high D (-18±2), low ¹⁸O (0.5 to 2.0) fluid of meteoric origin. This fluid penetrated the batholith via quartz veins (V₂) which infill faults active during post-consolidation uplift of the batholith. It forms the most common inclusion type in granite quartz throughout the batholith and is responsible for widespread retrograde alteration involving chloritization of biotite and hornblende, sericitization and saussuritisation of plagioclase, and reddening of K-feldspar. The salinity was generated by fluid-rock interactions within the Granite. Within granite quartz this fluid was trapped at 0.5-2.3 kb, having become overpressured. This fluid probably infiltrated the Granite in a meteoric-convection system during cooling after intrusion, but a later age cannot be ruled out.

The final fluid to enter the Granite and its host-rocks was a H₂O-NaCl-CaCl₂-KCl fluid with variable salinity (8-28 wt.% NaCl eq.), temperature (125-205°C), D (-17 to -45), ¹⁸O (-3 to +1.2), ¹³C_CO2 (-19 to 0) and ³⁴S_sulphate (13 to 23) that deposited veins containing quartz, fluorite, calcite, barite, galena, chalcopyrite sphalerite and pyrite (V₃). Correlations of salinity, temperature, D and ¹⁸O are interpreted as the result of mixing of two fluid end-members, one a high-D (-17 to -8), moderate-¹⁸O (1.2 to 2.5), high ¹³C_CO2 (>-4), low-³⁴S_sulphate (13), high-temperature (205-230°C), moderate-salinity (8-12 wt.% NaCl eq.) fluid, the other a low-D (-61 to -45), low-¹⁸O (-5.4 to -3), low-¹³C (<-10), high-³⁴S_sulphate (20 to 23) low-temperature (80-125°C), high-salinity (21-28 wt.% NaCl eq.) fluid. Geochronological evidence suggests V₃ veins are late Triassic; the high-D end member is interpreted as a contemporaneous surface fluid, probably mixed meteoric water and evaporated seawater and/or dissolved evaporites, whereas the low-D end member is interpreted as a basinal brine derived from the adjacent Carboniferous sequence.

This study demonstrates that the Galway Granite was a locus for repeated fluid events for a variety of reasons; from expulsion of magmatic fluids during the final stages of crystallisation, through a meteoric convection system, probably driven by waning magmatic heat, to much later mineralisation, concentrated in its vicinity due to thermal, tectonic and compositional properties of granite batholiths which encourage mineralisation long after magmatic heat has abated.

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Abstract. Oxygen isotopic compositions of quartz and feldspar in greenschist-grade mylonites from the Blue Ridge thrust and the Brevard zone in the southern Appalachians were analysed by laser microprobe to examine the effect of deformation on isotopic behaviour. In mylonites, texturally homogeneous polycrystalline quartz ribbons have a constant isotopic composition (¹⁸O = 12.9 ± 0.0, n = 3), whereas monocrystalline quartz ribbons, which display heterogeneous intercrystalline strain and only minor recrystallisation, have variable ¹⁸O values (11.6 ± 0.5, n=5). Alkali feldspars in samples that contain fluid inclusion-decorated microcracks, reflecting heterogeneous deformation, show a range in isotopic composition (8.8 to 10.2; mean = 9.4 ± 0.7, n = 3). In contrast, recrystallised myrmekite rims surrounding alkali feldspar augen in Brevard zone mylonites are isotopically heavier by about 1 (9.2 ± 0.1, n = 5) compared to the cores (8.3 ± 0.3, n = 4), reflecting isotopic homogenisation during neocrystallisation. Deformation mechanisms that result in heterogeneous strain on the grain scale (either crystal plastic or brittle) are associated with only partial isotopic homogenisation, whereas deformation mechanisms that result in homogeneous strain (e.g., recrystallisation, neocrystallisation) are associated with isotopic homogenisation on the grain scale. Agreement between measured quartz-feldspar isotopic temperatures and calculated temperatures using a finite difference model indicates diffusional exchange occurred between phases during closed-system cooling, and that the measured temperatures in the mylonites are maximum temperatures for the deformation. The approximate agreement between measured temperatures in some mylonites and the the calculated Dodson quartz closure temperatures indicates that isotopic exchange below T_c quartz was not substantial. The necessary conditions under which isotopic temperatures in mylonites correspond to the deformation temperature are outlined. On the basis of this study and reconsideration of older data, the onset of total dynamic recrystallisation in quartz is estimated to be about 350°C in natural shear zones. Together with reaction weakening of feldspar observed in the mylonites, the temperature interval 350-400°C is likely to be important for weakening of both quartz and feldspar in the continental crust.

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The bottom line: Analysis of carefully characterised samples documents the evolution of fluid compositions in a metamorphic belt.

Abstract. Structural analysis of quartz veins systems and fluid inclusion criteria were used to distinguish five different fluid types which flowed through a segment of Palaeozoic crust in southwest England during the Variscan orogeny. Mineralogical constraints in combination with fluid inclusion thermobarometry enabled the temperature of vein formation to be estimated, and isotopic compositions of fluids were determined by analysis of vein material and direct measurement of fluid extracted from inclusions. Peak, low-grade (pumpellyite-actinolite facies) metamorphic fluids had a high D and ¹⁸O signature (D= -18 to -10, ¹⁸O= +10.6 to +11.9) which evolved to compositions in the range D= -28 to -13, ¹⁸O= +7.9 to +11.4 during later retrogression and uplift. Fluids in the contact aureole of the Cornubian batholith had D-values intermediate between typical magmatic compositions and regional metamorphic fluids (-23 to -43), and a similar range of ¹⁸O-values to both magmatic and regional metamorphic fluids (between +5.6 and +14.0). These compositions are comparable with those of fluids responsible for Sn-W mineralisation in the province. Post-orogenic fluid chemical and isotopic compositions were exotic and indicate significant infiltration of externally-derived fluids during late- to post-orogenic brittle faulting. Low-temperature, low-salinity fluids which circulated in ENE-WSW-trending brittle normal faults had low ¹⁸O values (-0.3 to +7.4) suggestive of a significant meteoric component. Low-temperature, high-salinity fluids, which flowed through N-S- to NNW-SSE-trending strike-slip faults and fractures and were responsible for Pb-Zn mineralisation, had significantly D- and ¹⁸O-depleted compositions (D= -80 to -49, ¹⁸O= -0.1 to +4.7), typical of basinal brines. These data document the isotopic evolution of fluids in an external (Rhenohercynian) part of the Variscan orogen, through the complete cycle of foreland thrust-belt development and low-grade regional metamorphism, S-type granite emplacement and associated hydrothermal systems, post-orogenic collapse and low-temperature fluid flow in regional fractures. There is limited overlap in isotopic composition between the different fluid types, indicating that fluids flowing through the same host rocks at each stage of orogenesis may be distinguished on the basis of their oxygen and hydrogen isotopic compositions. These data provide a framework for future studies involving fluids of unknown origin in the Variscan and are a reference for comparison with the isotopic evolution of fluids in other orogenic belts.

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The bottom line: The Rb-Sr age given by a mica grain is predicted to be dependent on the mode of the rock.

Abstract. Various diffusion models can be applied to Rb-Sr systems in cooling rocks. It is argued that a closed-system diffusion-controlled model, involving local equilibrium along grain boundaries, is most appropriate for describing the exchange of Sr-isotopes between minerals in a rock sample during cooling. A finite-difference numerical method is used to solve the diffusion and mass balance equations. Closure temperatures of mineral-pair Rb-Sr isochrons are predicted to depend on the factors involved in Dodson's infinite reservoir closure temperature formulation (cooling and diffusion rate, grain size and shape) of both the minerals. In addition the closure temperature for mineral pairs is also dependent on the proportion of Sr in each mineral, which is dominantly a function of rock mode. This implies that Rb-Sr cooling ages from interbedded rocks having the same cooling history, but distinct modes, should differ: at slow cooling rates age differences could be more than 100 Ma. Such effects, if unrecognised could result in erroneous cooling curves, but if recognised could be utilised in estimating true cooling rates. Our closed-system model may also apply to other isotope decay schemes, such as the Sm-Nd system. A simple test is proposed which would allow the relevance of different diffusion models to be assessed.

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The bottom line: ¹⁸O-shifted meteoric waters can lie in Sheppard's magmatic and metamorphic water box and be lithostatically pressured, hence appearing to be of deep origin.

Abstract. Calcite and quartz veins have formed, and are forming, in steeply dipping fissures in the actively rising Alpine Schist metamorphic belt. The fluids that deposited these minerals were mostly under hydrostatic pressure almost down to the brittle-ductile transition, which has been raised to 5-6 km depth by rapid uplift. Some fluids were trapped under lithostatic pressures. Fluids in the fissure veins were immiscible H₂O+NaCl-CO₂ mixtures at 200-350°C. Bulk fluid composition is 15-20 mol% CO₂ and <4.3 total mol CH₄+N₂+Ar/100 mol H₂O. Water hydrogen isotopic ratio DH₂O in the fissure veins spans -29 to -68, ¹⁸OH₂O -0.7 to 8.5, and bulk carbon isotopic ratio ¹³C ranges from -3.7 to -11.7. The oxygen and hydrogen isotopic data suggest that the water has a predominantly meteoric source, and has undergone an oxygen isotope shift as a result of interaction with the host metamorphic rock. Similar fluids were present during cooling and uplift. Dissolved carbon is not wholly derived from residual metamorphic fluids; part may be generated by oxidation of graphite.

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The bottom line: Isotopic measurements demonstrate that development of rapakivi texture continues over a prolonged period, perhaps up to 200 Ma.

Abstract. The mantling of large ovoids of K-feldspar by a rim of plagioclase has been investigated in the rapakivi granites from the Mid-Proterozoic Wiborg batholith of SE Finland. The formation of rapakivi texture, in this type area, was examined using a variety of techniques including isotopic analyses of mineral separates from specific textural sites. Cathodoluminescence combined with microprobe analyses points to the pulsed development of the mantles involving growth of successive plagioclases of composition An30, An25, and An3, the last being in optical continuity with perthitic plagioclase exsolved from the K-feldspar. Plagioclase mantles have high ¹⁸O and ⁸⁷Sr/⁸⁶Sr signatures relative to K-feldspar, which indicate the presence of a late, low-temperature component thought to represent albite exsolved from the K-feldspar and redistributed onto the ovoid margin. Oligoclase components of the mantles are formed by a similar, although higher-temperature magmatic process. This involves the subsolidus re-equilibration of the alkali feldspar compositions with evolving melt conditions. Redistribution of the exsolved plagioclase from the alkali feldspar phenocrysts is linked to high fluorine contents of rapakivi-type magmas, and this major reconstruction of the feldspar phenocrysts generates their distinctive ovoidal shape.

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The bottom line: Closure temperatures in oxygen isotope systems depend on the mode of the rock.

Abstract. Retrograde exchange of oxygen isotopes between minerals in igneous and metamorphic rocks by means of diffusion is explored using a finite difference computer model, which predicts both the zonation profile of ¹⁸O within grains, and the bulk ¹⁸O value of each mineral in the rock. Apparent oxygen isotope equilibrium temperatures that would be observed in these rocks are calculated from the ¹⁸O values of each mineral pair within the rock. In systems which cool linearly from a sufficiently high temperature or at a low enough cooling rate, such that the final oxygen isotope values are not dependent upon the initial oxygen isotope values ("slow cooling"), the apparent oxygen isotope temperature derived for a rock composed of a single mineral pair can be shown to be simply related to the Dodson (1973) closure temperatures (Tc) for the two phases and the mode of the rock. Adding a third phase into a system which undergoes "slow" cooling will cause the apparent temperature derived for the two minerals already present to differ from the simple relationship for a two-phase system. In some systems oxygen isotope reversals can be developed. If cooling is not "slow", then the mineral ¹⁸O values resulting from cooling will be partly dependent upon the initial temperature of the system concerned. The model successfully simulates the mineral ¹⁸O values that are often observed in granitic rocks. Application of the model will help in assessing the validity of oxygen isotope thermometry in different geological settings, and allow quantitative prediction of the oxygen isotope fractionations that are developed in cooling closed systems.

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The bottom line: A field test confirms that the diffusion of oxygen is extremely slow under anhydrous conditions.

Abstract. The intracrystalline diffusion rate of oxygen in diopside was constrained based on natural isotopic variations from a granulite facies marble from Cascade Slide, Adirondacks (New York, USA). The oxygen isotope compositions of the diopsides, measured as a function of grain size, are nearly constant (20.9±0.3 vs. SMOW) over the entire measured size range (0.3-3.2 mm diameter). The ¹⁸O values of the cores of the calcite grains are 23.0. Temperature estimates based on the ¹⁸O(calcite-diopside) are 800°C, in agreement with the highest previous thermometric estimates for these rocks.

The lack of isotopic variation in the diopsides as a function of grain size requires that the oxygen intracrystalline diffusion rate in diopside from the Adirondack samples was very slow. The maximum diffusion rates (D800°C parallel to the c-axis) were calculated with an infinite reservoir model (IRM) and a finite reservoir model (FRM) that incorporates mineral modal abundances and initial isotopic variations. For an assumed activation energy (Q) = 100 kJ/mol, the IRM diffusion rate estimate of 1.6 X 10^-20 cm²/s is two orders of magnitude faster than the from the FRM; at Q = 500 kJ/mol, the D800°C estimate for both methods is c. 5.6 X 10^-20 cm²/s. The present results require that a hydrothermal fluid significantly enhances the diffusion rate of oxygen in diopside if previous data are correct.

The ¹⁸O(SMOW) and ¹³C(PDB) values of the calcite, measured in situ with a CO₂ laser, are 22.9±0.3, 0.1±0.3 in the grain cores, 22.1±0.3, 0.2±0.1 at the grain boundaries and 21.7±0.4, -0.6±0.1 abutting diopside grains. The ¹⁸O and ¹³C values measured conventionally are: crystal cores, 22.96, -0.95; abutting diopside grains, 22.38, -0.93; bulk, 22.79, -0.95. Use of the bulk ¹⁸O(calcite) values for thermometry yields unreasonably high temperatures. The lower ¹⁸O values at the calcite grain boundaries are not due to retrograde diffusional exchange with the diopside, they are thought to be a result of a late retrograde fluid infiltration.

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The bottom line: The fluid depositing the gold veins has stable isotope ratios consistent with a magmatic origin. However, see Jenkin, Craw and Fallick (1994).

Abstract. Gold bearing structures in the Tyndrum area, Scotland, formed from CO₂-bearing fluids that contained 6 equiv. wt% NaCl at temperatures in the range 290-350°C. Loss of CO₂ (phase separation) from the fluids was one cause of mineral precipitation. Oxygen and hydrogen isotopic data suggest that the fluids were magmatic in origin but included an additional component- probably Lower Devonian meteoric water. Sulphur isotopic ratios indicate two sources of sulphur, one magmatic in character and the other the country rocks. In contrast, historically exploited base-metal veins in the area formed from highly saline (~15 equiv. wt% NaCl), relatively low-temperature (140-200°C) fluids, which have a different stable isotopic signature from that of the gold veins.

Comparison with information from other mineralisation in the region suggests that the genesis of the Tyndrum gold veins is related to Caledonian magmatism; the mineralising fluids could have evolved from an underlying magma that was either granitic or appinitic.

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The bottom line: Mo-mineralisation is undoubtedly of magmatic origin, but what about the late overprint?

Abstract. Mo mineralization within the Galway Granite at Mace Head and Murvey, Connemara, western Ireland, has many features of classic porphyry Mo deposits including a chemically evolved I-type granite host, associated K- and Si-rich alteration, quartz vein- (Mace Head) and granite-hosted (Murvey) molybdenite, chalcopyrite, pyrite and magnetite mineralisation and a gangue assemblage which includes quartz, muscovite and K-feldspar. Most fluid inclusions in quartz veins homogenise in the range 100-350°C and have a salinity of 1-13 eq. wt.% NaCl. They display Th-salinity covariation consistent with a hypothesis of dilution of magmatic water by influx of meteoric water. CO₂-bearing inclusions in an intensely mineralised vein at Mace Head provide an estimated minimum trapping temperature and pressure for the mineralising fluid of 355°C and 1.2 kb and are interpreted to represent a H₂O-CO₂ fluid, weakly enriched in Mo, produced in a magma chamber by decompression-activated unmixing from a dense Mo-bearing NaCl-H₂O-CO₂ fluid. ³⁴S values of most sulphides range from c. 0 at Murvey to 3-4 at Mace Head and are consistent with a magmatic origin. Most quartz vein samples have ¹⁸O of 9-10.3 and were precipitated from a hydrothermal fluid with ¹⁸O of 4.6-6.7. Some have ¹⁸O of 6-7 and reflect introduction of meteoric water along vein margins. Quartz-muscovite oxygen isotope geothermometry combined with fluid inclusion data indicate precipitation of mineralised veins in the temperature range 360-450°C and between 1 and 2 kb. Whole rock granite samples display a clear ¹⁸O-D trend towards the composition of Connemara meteoric waters. The mineralization is interpreted as having been produced by highly fractionated granite magma; meteoric water interaction postdates the main mineralising event. The differences between the Mace Head and Murvey mineralizations reflect trapping of migrating mineralizing fluid in structural traps at Mace Head and precipitation of mineralization in the granite itself at Murvey.

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The bottom line: Widespread infiltration of meteoric fluid, but when??

Abstract. Dalradian metamorphic rocks, Lower Ordovician meta-igneous rocks (MGS) and Caledonian granites of the Connemara complex in S.W. Connemara all show intense retrograde alteration. Alteration primarily involves sericitization and saussuritization of plagioclase, the alteration of biotite and hornblende to chlorite and the formation of secondary epidote. The alteration is associated with sealed microcracks in all rocks and planes of secondary fluid inclusions in quartz where it occurs, and was the result of a phase of fluid influx into these rocks. In hand specimen K-feldspar becomes progressively reddened with increasing alteration.

Mineralogical alteration in the MGS and Caledonian granites took place at temperatures ~275±15°C and in the MGS Pfluid is estimated to be <=1.5kb during alteration. The D values of alteration phases are: -18 to -29 (fluid inclusions), -47 to -61 (chlorites) and -11 to -31 (epidotes). Chlorite ¹⁸O values are +0.2 to +4.3, while ¹⁸O values for quartz - K-feldspar pairs show both positively sloped (MGS) and highly unusual negatively sloped (Caledonian granites) arrays, diverging from the normal magmatic field on a - plot. The stable isotope data show that the fluid that caused retrogression continued to be present in most rocks until temperatures fell to 200-140°C. The retrograde fluid had D ~-20 to -30 in all lithologies, but the fluid ¹⁸O varied both spatially and temporally within the range -4 to +7. The fO₂ of the fluid that deposited the epidotes in the MGS varied with its ¹⁸O value, with the most ¹⁸O-depleted fluid being the most oxidizing.

The D values, together with low (<0) ¹⁸O values for the retrograde fluid in some lithologies indicate that this fluid was of meteoric origin. This meteoric fluid was probably responsible for the alteration in all lithologies during a single phase of fluid infiltration. The variation in retrograde fluid ¹⁸O values is attributed to the effects of variable oxygen isotope shifting of this meteoric fluid by fluid-rock interaction. Infiltration of meteoric fluid into this area was most likely accomplished by convection of pore fluids around the heat anomaly of the Galway granite soon after intrusion at ~400 Ma. However convective circulation of meteoric water and mineralogical alteration could possibly have occurred considerably later.

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The bottom line: Closure temperature affected by fluid, the origin of the fluid and precise mechanism still in doubt.

Abstract. The results of K-Ar age determinations and hydrogen and oxygen stable isotope measurements on hornblendes from the poly-metamorphic Dalradian schists of Connemara, western Ireland indicate that the hornblende K-Ar systems have been significantly disturbed by low temperature fluid reaction. Correlations between K-Ar ages, D values, structural water contents and the occupancy of the hornblende structural 'A' sites suggest that excess water as H₃O⁺ occupies vacant 'A' site positions during stable isotope exchange and displaces Ar* causing lowering of K-Ar ages.This process occurs below the normally accepted closure temperature for Ar in hornblende and is not detectable by optical or X-ray investigation of the mineral. This process may not be unique to Connemara hornblendes and so is of potential importance to geochronology in other metamorphic terranes.

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The bottom line: Uses COOL to demonstrate you do not always need fluid infiltration to produce reversed (negative) quartz-K-feldspar oxygen isotope fractionations.

Abstract. Differences in oxygen isotope compositions between quartz and alkali feldspar in partially melted pelitic and semipelitic rocks from the aureole of the Ballachulish igneous complex (Scotland) range from relatively normal values of 1 down to nonequilibrium reversed values of -0.7. Normally, an open system process (i.e., fluid infiltration after crystallization) would be invoked to explain such reversed values. However, application of a closed-system model to these rocks shows that the observed isotopic differences are actually consistent with such behavior. In this case, no fluid infiltration is required to explain the observed oxygen isotope compositions.

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The bottom line: The original and best O-isotope modelling program implements Giletti's (1986) model.

Abstract. COOL is a FORTRAN-77 program for modelling stable isotope ratios of minerals in cooling closed systems. The closure temperatures of the minerals in a rock are used to define the temperature at which each mineral ceases stable isotope exchange with the other minerals in the rock. The mass balance relationship between the minerals is used together with the closure temperatures to calculate the values of the individual minerals as the rock cools. As well as calculating the mineral values in a cooling rock, COOL can also calculate apparent isotope equilibrium temperatures and estimate the rate at which a rock has cooled from experimental data. COOL can be used to model mineral values over a limited range of temperature, so that the input data can be obtained from, or the output data used in, programs which model stable isotope exchange in open systems under isothermal conditions.

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The bottom line: First paper to show importance of prolonged low temperature elemental redistribution in rapakivi texture.

Abstract. The development of rapakivi texture in feldspars from the Ketilidian granitoids of south Greenland has been investigated using Sr, O and H isotopes. A low temperature signature is found in the Sr and O data which seemingly contradicts some textural features that point to a magmatic origin of the plagioclase mantles around the K-feldspar ovoids. An origin for these mantles involving exsolution from an original alkali feldspar solid solution is proposed, which involves growth of mantles over a range of conditions determined by the mobility of the exsolving sodic feldspar. This mobility may be enhanced at high temperatures in the presence of melts or increased fluid pressures and at lower temperatures by the processes responsible for the transformation of K-feldspar to microcline. Rapakivi granites with both white and dark green feldspar occur in south Greenland but show no major isotopic differences, although the dark alkali feldspars contain significantly more fluid. Equivalent fluids in the white alkali feldspars may have escaped during plagioclase exsolution

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[19]. Jenkin, G.R.T. & Fallick, A.E. (1987) A stable isotopic study of retrograde hydration in a syn-metamorphic intrusion. Terra Cognita, 7, (2-3), 136.

[20]. Jenkin, G.R.T., Leake, B.E. & Fallick, A.E. (1988) A stable isotopic study of fluids associated with retrogression in S.W. Connemara, Ireland. J. Geol. Soc. Lond. 145, (1), 179.

[21]. Jenkin, G.R.T. (1988). Stable isotope studies of a hydrothermal system in Connemara. 1st Connemara Discussion meeting, Glasgow University, November 1988.

[22]. Jenkin, G.R.T. & Fallick, A.E. (1989) Hydrogen isotope kinetics during hydrothermal alteration in SW Connemara, Ireland. In: Miles, D.L. (ed) Proceedings of Water-Rock International 6, 331-335, Malvern, England. Balkema, Amsterdam.

[23]. Jenkin, G.R.T. (1990). Stable isotope studies of hydrothermal alteration in Carboniferous dolerites from West Connacht. 2nd Connemara Discussion meeting. Galway University, September 1990.

[24]. Jenkin, G.R.T., Fallick, A.E., Linklater, C., Farrow, C.M. & Bowes, G.E. (1991). Computer modelling of stable isotope ratios in slowly cooling closed systems using COOL. Terra abstracts 3, 7.

[25]. Jenkin, G.R.T., Fallick, A.E., Linklater, C., Farrow, C.M. & Bowes, G.E. (1991). COOL: Computer program for modelling stable isotopes in slowly cooling closed systems. Terra abstracts 3, (1), 497-498.

[26]. Jenkin, G.R.T., Fallick, A.E., Linklater, C., Farrow, C.M., Bowes, G.E. & McConville, P. (1991). Why qtz-fsp <0 may not always indicate open system behaviour: Computer modelling ¹⁸O values using COOL. EOS Trans. Am. Geophys. Union. 72, (17), 307.

[27]. Jenkin, G.R.T., Fallick, A.E., Linklater, C., Farrow, C.M., Parmentier, E.M. & Giletti, B.J. (1992). Computer modelling of oxygen isotope distributions in metamorphic rocks. Metamorphic Studies Group/IGCP meeting on Stable isotopes as tracers of metamorphic processes, Edinburgh.

[28]. Jenkin, G.R.T., Mohr, P. & Mitchell, J.G. (1993). Carboniferous dikes as monitors of post-400 Ma fluid circulation in Connemara, Western Ireland. Terra Nova 5, 460.

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[30]. Jenkin, G.R.T. (1994). The geology of the Leedstown area. pp 26-30. In A.T. Jenkin (ed.) Leedstown in our lifetime. The story of a Cornish village. Published by Leedstown W.I. ISBN 0 9524601 0 6.