Hydrothermal alteration of Tertiary igneous rocks from the Isle of Skye, northwest Scotland

Ferry, John M.

doi:10.1007/bf00413352

Cited by 47 publications

(9 citation statements)

References 45 publications

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“…in gabbros from Skye (Ferry 1985). In those rocks, Ferry (1985) suggests that fluid flow and oxygen isotope exchange began at temperatures as high as 500-1000 C, but that mineralogical alteration only occurred after cooling, dominantly at 450-550 C. We propose that the Logmor dyke was altered in a similar manner.…”

Section: Temperature Of Alteration Of Logmor Dykementioning

confidence: 85%

Carboniferous dykes as monitors of post-Caledonian fluid events in West Connacht, Ireland

Jenkin

Mohr

Mitchell

et al. 1997

Transactions of the Royal Society of Edinburgh: Earth Sciences

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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 δ18O 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.%, δ5D= 59 to –47‰ and δ18O = 9·4 to 9·6‰; plagioclase shows little mineralogical alteration but its δ18O is 9·7‰. Hydrothermal alteration involving a local formation or 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.%. δD= –58 to –38‰ and δ18O = 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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Section: Temperature Of Alteration Of Logmor Dykementioning

confidence: 85%

Carboniferous dykes as monitors of post-Caledonian fluid events in West Connacht, Ireland

Jenkin

Mohr

Mitchell

et al. 1997

Transactions of the Royal Society of Edinburgh: Earth Sciences

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“…Fe-Ti oxides were excluded because magnetite and ilmenite may have formed during both prograde and retrograde metamorphism (Table 1). Ferry (1985) used a similar measure in his study of gabbro alteration. Values of AI acz are plotted in Figure 8B as a function of distance from the contact.…”

Section: Extent Of Metamorphismmentioning

confidence: 97%

Porosity, permeability, and basalt metamorphism

Manning¹,

Bird²

1995

Low-Grade Metamorphism of Mafic Rocks

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The effects of primary porosity on fluid flow during contact metamorphism were studied in basalts from central East Greenland. The gabbroic Skaergaard magma intruded interbedded massive and aa basalts with mean macroscopic primary porosities of 4% and 11%, respectively. Heat transport from the cooling gabbros led to three metamorphic mineral zones within 1 km of the contact: the actinolite + chlorite zone beyond 250 m, where the mineral assemblage records peak temperatures (7) of <550 °C; the pyroxene zone (T = 700-850 °C); and the olivine zone, within 10 m (T > 850 °C). In the actinolite + chlorite zone, aa clasts record more extensive mineralogic alteration of igneous minerals than do massive samples. Extents of prograde recrystallization in the olivine and pyroxene zones are 100% in both flow morphologies, but modal volumes of retrograde minerals in the pyroxene and olivine zones are higher in aa units. Extents of prograde reactions do not correlate with primary porosity because they were solid-solid reactions that occurred at high temperatures, whereas retrograde alteration involved low-temperature hydration reactions in which the availability of H 2 0 as a reactant, as controlled by porosity, probably influenced reaction extent. In the pyroxene zone, where mineralogic and textural evidence suggests oxygen isotope exchange equilibrium, whole-rock 5 18 0 compositions are 1.7%e to 3.0%c and are similar or lower in aa units than in massive units at any given distance from the contact. The isotopic ratios suggest average time-integrated fluid fluxes of 3.6 and 4.0 x 10 3 mol cm -2 in massive and aa units, respectively, if fluid infiltration occurred during prograde metamorphism. Similar values were computed assuming that part of the isotopic exchange was retrograde. These differences imply that time-averaged matrix permeability was ~10% higher in aa flow breccias.

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“…However, the existence of porosity in analcime replacing leucite cannot easily be explained in this way. Another approach to the role of molar volume difference between parent and product in pseudomorphic reactions is taken by Ferry (2000) who makes the observation that pseudomorphs are relatively uncommon in prograde metamorphic rocks but are distinctive textural features of retrograde rocks. Figure 2 shows a typical example À a partial pseudomorphic replacement of andalusite by muscovite.…”

Section: Analcimization Of Leucitementioning

confidence: 99%

Mineral replacement reactions: from macroscopic observations to microscopic mechanisms

2002

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Mineral replacement reactions take place primarily by dissolution-reprecipitation processes. Processes such as cation exchange, chemical weathering, deuteric alteration, leaching, pseudomorphism, metasomatism, diagenesis and metamorphism are all linked by common features in which one mineral or mineral assemblage is replaced by a more stable assemblage. The aim of this paper is to review some of these aspects of mineral replacement and to demonstrate the textural features they have in common, in order to emphasize the similarities in the underlying microscopic mechanisms. The role of volume change and evolution of porosity is explored both from natural microtextures and new experiments on model replacement reactions in simple salts. It is shown that the development of porosity is often a consequence of mineral replacement processes, irrespective of the relative molar volumes of parent and product solid phases. The key issue is the relative solubility of the phases in the fluid phase. Concepts such as coupled dissolution-precipitation, and autocatalysis are important in understanding these processes. Some consequences of porosity generation for metamorphic fluid flow as well as subsequent crystal growth are also discussed.

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Hydrothermal alteration of Tertiary igneous rocks from the Isle of Skye, northwest Scotland

Cited by 47 publications

References 45 publications

Carboniferous dykes as monitors of post-Caledonian fluid events in West Connacht, Ireland

Carboniferous dykes as monitors of post-Caledonian fluid events in West Connacht, Ireland

Porosity, permeability, and basalt metamorphism

Mineral replacement reactions: from macroscopic observations to microscopic mechanisms

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