SECUENCIA DIDÁCTICA:

From the preceeding discussion, the following two points emerge:

1. Randomly interstratified chlorite-smectite is present throughout the lavas and also occurs within metadomains. A discrete smectite phase such as saponite, which was reported by Hall et.al. (1989) within these lavas, was not found.

2. Within metadomains there are a variety of phases, both mixed layer varieties and discrete phases are found. The former includes mixed layer chlorite-smectite and chlorite-vermiculite.

1

t

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the latter includes chlorite, illite and occasionally kaolinite. These discrete phases are

not

There now exists a wide body of data (Thompson, 1983) indicating that during the initial stages of basalt alteration, shortly after eruption, smectite is the dominant phyllosilicate phase. Alt et al. (1986) report successive stages of alteration within sea floor basalts to a depth of 400 metres in which a range of secondary mineralogies develop. Phyllosilicate phases which are found include both saponite (Mg rich smectite) and nontronite (Fe rich

smectite). At depths greater than 500 metres there is sporadic chlorite. Alteration is attributed 4

to repeated interaction with seawater derived solutions which are progressively modified as interaction proceeds.

Modelling of basalt-seawater interaction by Bowers and Taylor (1987) indicated that both temperature and reaction path progress (a measure of the degree of interaction) are important controls on the alteration mineralogy. At temperatures well below 200 °C, in the early stages of interaction, abundant smectites (eg. saponite and nontronite) form. As both temperature and reaction progress increase, then chlorite develops. Typically, temperatures range from 200 to 250 ”C, whilst progiess, measured in terms of grams basalt dissolved in

1 kg seawater, equals approximately 220g kg'\

Mottl (1983), in a review of experimental results of basalt-seawater interaction notes

that a characteristic of the process is the complete removal of Mg^^ from seawater solutions |

at temperatures of approximately 150 ®C. The net result of this process is the precipitation of Mg-smectites such as saponite. Cann (1979) and Thompson (1983) label basalts which undergo alteration at temperatures below approximately 100 °C as belonging to the Brownstone facies. Such basalts are characterised by the palagonitization of glass and partial replacement of igneous pre-cursors by smectites.

As alteration proceeds, then there is a fundamental change in the observed alteration mineralogy. Kristmannsdottir (1979) notes that for Icelandic basalts interlayering of smectite commences at approximately 200 °C, gradual transformation of chlorite occuring between 200-240 ”C. Mixed-layer clays of chlorite and smectite are dominant within this temperature

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range. Bettison and Schiffman (1988) note that this tiansition within tlie Point Sal Ophiolites roughly parallels the transition from prehnite-pumpellyite to prehnite-actinolite facies defined by Liou et al. (1987) on the basis of calc-silicate mineral assemblages. However, Cann (1979) suggests that the top of the zeolite facies is marked by the disappearance of all mixed layer phases.

lijima (1978) noted the occurrence of a 14-15 Â mixed-layer phase, together with chlorite at temperatures as low as 100 ”C (at depths of 1 700-3 500 meties) within marine volcaniclastics in Japan. Furthermore, Viereck (1982) following a study of alteration minerals within a tuff-basalt sequence on Iceland, described the occunence of a Si-rich chlorite which was replaced by an Al-rich chlorite at temperatures significantly lower than 200 °C. When compared to basalt sequences these two studies indicate the importance of permeability on alteration mineralogy, supporting the evidence from experimental and modelling studies that reaction progress as well as temperature is also an important variable in determining obseved alteration mineralogy.

From the previous discussion it is now possible to consider the situation within the Clyde Lavas. It was eai'lier stated that mixed-layer clays are found throughout the lavas, both within and outside metadomains, whilst "pure" species occur only within. Furthermore, smectite, a product of early, low temperature alteration, documented in many studies, is absent. Therefore, it is unlikely that the phyllosilicate assemblages found within the Clyde Lavas were fonned immediately after emption of the lavas following interaction with cool surface waters. It is much more likely that they represent the products of a process of fluid- rock interaction which lead to the progressive transformation of an earlier smectite phase.

It is unlikely that the assemblages seen within the metadomains represent a very early metastable assemblage developed exclusively during burial. If this were the case the development of phyllosilicates with a uniform compositions might be expected both within and outside metasdomains owing to uniformity of fluid compositions during burial. This is not observed. Therefore, the inference is that metadomain development did not occur immediately following eruption but some time after, possibly contemporaneous with burial

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metamorphism; the presence of "pure" species representing the acme of a tr ansition through less ordered mixed-layer species ultimately leading to the formation of chlorite. Regular and random mixed-layer chlorite-smectites occur as intermediate products of the continuous transformation of smectite to chlorite. This change has been documented by several studies such as that by lijima and Matsumoto (1982) and Curtis et al. (1985) for phyllosilicates within the sedimentary environment undergoing deep burial diagenesis.

Given that fluid temperatures within metadomains (deduced from fluid inclusion studies; see Chapter 5) do not differ radically from those outside as concluded by Evans (1987) then it is suggested that temperature may assume secondaiy importance in controlling development of alteration phases. This issue is further addiessed in the following sections.

Kaolinite appears to have been tlie stable phyllosilicate phase within hydrothermal fluids present within the Campsie Glen metadomain. This suggests that the metadomain may represent a buried zone of acid alteration. Inflow of oxygenated water might oxidise any sulphide which was present leading to acidic conditions. However, there is abundant carbonate also present at this locality suggesting that low pH conditions did not exist. Alternatively, studies such as that by Browne and Ellis (1971) suggest that kaolinite is the stable phyllosilicate phase in situations where both Mg, Na and Ca concendations are low in the hydrothermal fluid, such as the Ohaki-Broadlands geothermal field.