Petrology of the GPK-1 Drill-hole Lavas, Palaea kameni Hot Springs, Santorini Volcano, Greece: Constraints on the Low-T Lava Fluid Interaction

Their striking characteristic is the lack of significant alteration despite their prolonged interaction with Fe-rich hydrothermal fluids. It is possible that in this case alteration is kinetically inhibited due to the physicochemical and chemical particularities of the hydrothermal solutions.

INTRODUCTION

It is now well established that processes related to ridge-crest hydrothermal convection of sea water are responsible for the metamorphism of the oceanic crust and the formation of metal-rich deposits on the ocean floor (Andrews and Fyfe 1976; Ballard et al. 1981; Cann 1979; Edmond et al. 1979; Rona 1985). Analogous processes occur in island arc systems and geothermal fields on land, albeit with important differences regarding their metallogenic consequences. In fact, hot springs in geothermal systems on land are metal-poor, and deposition of Fe-rich deposits with very high Fe/Mn ratios appears to be mainly restricted to island arcs, e.g. Santorini. Kuriles, Banu Wuhu (Indonesia), Matupi Harbour, Deception Island (Boström et al. 1990a). Drilling of the Palaea Kameni hot springs on Santorini could therefore offer a unique opportunity to study a cross section of a currently active ore-forming shallow submarine hydrothermal system in an attempt to understand the behaviour of hydrothermal systems at different tectonic settings.

CORE PETROLOGY

The GPK-1 bore hole, drilled under a joint IGME - University of Stockholm project, is located about 40 m away from the hydrothermal discharge zone of Fe-rich solutions at the Palaea Kameni islet (Arvanitides et al. 1988) (Fig. 1). Drilling reached a depth of 201.5 m and recovered 155 m of lavas and 46 m from an underlying well-sorted mature pyroclastic gravel horizon. Macroscopically, the lavas often appear to be 'bleached', 'corroded' and dissected by an abundance of ferric oxide-coated veins and cracks. The core was sampled at close intervals and 75 thin sections were made for petrographic study.

The lavas are dacitic in composition (Boström et al. 1990b) belonging to the 197 BC eruption (Fig. 1). They are remarkably uniform in their mineralogical composition and consist of plagioclase, orthopyroxene and clinopyroxene phenocrysts, as well as magnetite and ilmenite microphenocrysts set in a hyalopilitic to pilotaxitic groundmass (Fig. 3). Amphibole occurs only rarely, whereas xenoliths of plagioclase + clinopyroxene + opaque phases are quite common. The groundmass is predominantly crystalline, comprising plagioclase and orthopyroxene microlites, subordinate amounts of clinopyroxene and interstitial glass.

Plagioclase, andesite to labradorite in composition (Table 1), is by far the most abundant mineral in these rocks and the earliest phase to crystallize. Phenocrysts are often embayed, remarkably fresh and significantly more An-rich than the groundmass microlites. Orthopyroxene, hypersthene in composition, occurs both as blocky subhedral phenocrysts, occasionally intergrown with clinopyroxene and/or plagioclase, and as minute tabular grains in the groundmass. Compositions of analysed pyroxenes are plotted on the pyroxene quadrilateral in Fig. 2 together with the compositional fields of pyroxenes from other post-caldera lavas for comparison. Brown amphibole, cummingtonite, occurs as isolated blocky grains, overgrowths or reaction rims around orthopyroxene and as needles often associated with plagioclase. Application of the two pyroxene geothermometer of Lindsley (1983) to the GPK-1 dacites yielded pre-eruption temperatures in the order of 960-1060° C.

A striking characteristic of the GPK-1 dacites is the absence of secondary minerals. Their salient alteration feature is the replacement of glass by palagonite and their occasional staining by ferric oxides and hydroxides, indicating low-T, brownstone facies hydrothermal fluid-lava interaction. With the exception of the very rare occurrence of secondary quartz, filling veins and cracks (Fig. 4), the presence of other secondary phases has not been ascertained by XRD, microscopic and qualitative electron microprobe investigation. In fact, there is no evidence that suggests prolonged interaction with hot iron-rich solutions or any pervasive alteration which may be expected at the high water/rock ratios involved in such an extensively fractured volcanic carapace. These observations are also corroborated by the uniformity of the LIL element contents of the dacites (data in Boström et al. 1990b) which implies their relative immobility.

DISCUSSION

Considering the extensive hydrothermal alteration which characterizes rocks from the ocean floor (Alt and Honnorez 1984; Cann 1979; Staudigel and Hart 1983), island arcs (Anonymous 1977; Natland and Tarney 1981; Wood et al. 1981) and geothermal fields on land (McDowell and Elders 1980; Tomasson and Kristmannsdotir 1972; White et al. 1988) it is very difficult to reconcile their virtual lack of alteration with their residence within an ore-forming submarine hydrothermal cell. There is at present no forthcoming explanation for this seemingly paradoxical situation.

The physico-chemical properties and chemistries of the hydrothermal solutions constantly evolve during their convection in the crust and this is reflected by the great diversity of secondary mineral assemblages which develop in the rocks interacting with these solutions (Andrews and Fyfe 1976; Humphris and Thompson 1978; Stakes and O'Neal 1982). Moreover, there are no simple means of correlating such parameters with solution reactivities. It is possible, however, that hydrothermal fluid-GPK dacite interaction is kinetically inhibited by such factors as T, pH, fO₂ and chemistry of hydrothermal solutions discharging at Palaea Kameni. Data by Boström et al. (1990c) indicate that these solutions have low temperatures (28°-34° C), almost neutral pH (around 6) and low fO₂. These conditions may not be conducive to rock alteration under the prevailing flow rates and water/rock ratios. Such an explanation, however, is no more than a mere hypothesis and it is evident that further investigation is needed to elucidate this problem.

-----------------------------------------------------

------------------------------------------------------