Geochemical Dispersion of Metals in and around the Caldera of Thera

Below the "normal" sediments are extensive "pumice" horizons. The abundance of elements in both facies is controlled by the relative proportions of three phases, namely pumice (Al, Fe, Si, Ti, V, Zn, Cu, Mg, Mn), carbonates (Ca, Mg, Sr), and an unspecified (probably terrigenous mica or clay) component (Ni, Cr, Zn, Ba, Cu, Ga). Caldera sediments also consist of a "pumice" facies intercalated with a "normal" facies but, unlike the shelf sediments, the latter includes abundant iron oxides. Superimposed on a general pumice control of the geochemistry of caldera zone sediments are a variety of factors. These include the relative proportions of rhyolitic glass, feldspars, and pyroxenes (Al, Si, Ti); the dispersion of particulate phases from the submarine fumeroles (Fe, Mn); the remobilisation of Mn in the reducing horizons of the older "normal" unit; the scavenging of metals by particulate iron oxides (V, Pb, Ni, Cr, Cu, Li, Co); and the abundance of carbonates (Ca, Mg, Sr).

INTRODUCTION

The submerged portion of Santorini has been divided into five geochemical zones by Smith & Cronan (1975). Two of these are the shelf and caldera zones which comprise most of the sea floor within the caldera and all of the shelf outside. The geochemistry of sediments in each of these two zones is the subject of this paper. Sample locations are shown in Fig. 1.

Most of the sediments in and around Santorini are mixtures of redeposited pyroclastic material with biogenic and terrigenous components (Petersen & Müller, 1974) with intercalated tephra layers. Certain aspects of the work of previous investigators are relevant to the geochemistry of the sediments and include :

a)      that most sediments are pumiceous (rhyolitic glass)

b)      that most fine grained sediments are silt rich with only minor quantities of clay sized material. 

c)      that individual minerals are dominantly feldspars and pyroxenes although heavy minerals such as magnetite are important locally.

SHELF ZONE SEDIMENTS

Butuzova (1969) has made a comprehensive study of the geochemistry of the elements Fe, Mn, Ti, V, Cr, Ni, Co and Cu in the shelf sediments around Santorini. Average abundances of these elements are presented in Table 1 to enable comparison with average abundances determined in this investigation.

There is close agreement in all elements between the two investigations.

According to Butuzova, the hydrothermal influence on the shelf sediments is so low as to be undetectable. The volcanic influence is limited to the solid products of volcanism, particularly close the archipelago where the presence of pyroxene and magnetite result in the enrichment of Fe and Ti. Sedimentological investigations, particularly those of Raab & Stoffers (1975), support this conclusion.

Butuzova (op. cit.) notes that minor element abundances in the pyroclastic material become lower with increasing distance from Santorini.

In general, two distinct facies are present in most cores examined in the present work (Fig. 2). The upper horizon represents conditions of "normal" sedimentation and includes, from the shoreline seawards, volcanic sands, through coraline algae, to beige muds consisting of abundant calcium carbonate, pumice silt eroded from the Santorini pumice strata, and minor quantities of terrigenously derived clay material. Underlying the superficial deposits are extensive pumice horizons, probably derived from the 1400 B.C. eruption of Santorini, either by slumping from shallower water or by the subaqueous deposition of pyroclastic falls. In most cases the following discussion will refer simply to the "normal" and "pumice" horizons.

Table 2 summarises the range of element abundance found within the two horizons. It can be seen that the data for "normal" sediments straddle the average bulk composition for shelf sediments given in Table 1. In contrast, "pumice" sediments have a composition very similar to that of a sample of pumice from the Upper Pumice Strata on Santorini (Puchelt et al., 1973). This provides additional support for the suggestion that they have a common origin. Only the Ba appears to be depleted compared to the upper pumice strata.

When compared, the "pumice" sediments are enriched in Al, Si, Fe and Ti, whilst the "normal" sediments have higher concentrations of Ca and Mg, reflecting their different sedimentological characteristics. Of the minor elements, Zn, V, Sc, and possibly Co are enriched in the "pumice" sediments whereas Ba, Li, Cr, Ni and Sr are enriched in the "normal" sediments. Cr, Mn, Ga, K, Pb and Mo appear to be similar in both groups.

From the lithological point of view both the "normal" and "pumice" sediments have certain characteristics in common. The most abundant component in both is pumice. This pumice probably has a common origin in each in that the lower pumice horizons are derived from some form of subaqueous deposition from the 1400 B.C. erupted material, and the pumice component of the "normal" sediment is derived from the erosion of pumice on land from the same eruption.

During the differing processes of erosion and sedimentation, the same mineral assemblages would be expected to develop but differing in the relative proportion of the three main pumice components - rhyolitic glass, feldspars, and pyroxenes. In addition to pumice in the "normal" sediments we have a varying carbonate and a minor clay component. An organic matter component might also be present.

It might be expected, that although sedimentologically distinct, compositionally the sediments may be considered as occuring at some point on a carbonate-pumice end member system. Only in the sediments with dominant pumice and only a minor diluting carbonate component, would variation in pumice mineralogy play a significant role in influencing composition. This feature (the carbonate-pumice relationship) is well illustrated by the relationship between Ca and Al (Fig. 3), representative of the carbonate and pumice components respectively. The carbonate-pumice line is constructed such that at the origin for Al, Ca is 40 %, representing approximately 100 % CaCO₃. This point is joined to four upper pumice strata analyses taken from Puchelt & Schock (1972).

Data points for both grab and core samples straddle the carbonate-pumice line indicating that varying proportions of the pumice and carbonate end members consitute the shelf sediments. Elements, therefore, with a strong pumice control would be expected to have a direct relationship with Al, and elements in phases which serve to dilute the pumice will have an inverse relationship to Al.

A positive relationship is observed between both Fe and Al (Fig. 4) and Ti and Al. Fe and Ti in shelf sediments would thus seem to be pumice controlled. Fe/Al and Ti/Al ratios are larger than those of the upper pumice strata on Santorini (see Puchelt & Schock 1972). This is thought to either reflect the greater abundance of ferromagnesian minerals (pyroxenes) in the "pumice" sediments, possibly due to the relative enrichment of these minerals during subaqueous deposition. Minor elements enriched in the "pumice" sediments include V, Zn and Sc. All show a more or less direct relationship with Al.

Although Mn also shows a direct relationship with Al, data points are scattered (Fig. 5). To a certain extent this could reflect a similar variation found by Puchelt & Schock (1972) in the Mn abundance in samples from the upper pumice strata on Santorini. However, there are certain features which indicate an additional control superimposed on the pumice Mn. Certain grab samples and the surface layers from cores 1103, 686 and 663 have the highest Mn/Al ratios. In contrast Mn/Al ratios in lower horizons of the "normal" sediments (686/2, 663/2) are significantly less even though lithologically they are similar to the upper layers. This similarity is reflected in the equal abundances of Al.

Remobilisation of Mn from the lower to the upper layers of the "normal" sediments could cause this increase in the Mn/Al ratio in a manner to be described subsequently.

Copper also shows a positive relationship with Al but with a great scatter of points. Pumice is thought to be the dominant control on Cu abundance but it seems that for a given Al abundance "normal" sediments (e.g. 1103/1, 663/1) are enriched in Cu compared to the "pumice" sediments (e.g. 663/3). An additional contributor of Cu is indicated by this apparent enrichment, but it is not possible to be more specific from the evidence available except to suggest that it may be related to an unidentified terrigenous component which appears to cause the enrichment of several other minor elements in the "normal" sediments.

Lithium, Ba, Cr and Ni are a group of elements, with an apparently inverse relationship to Al (Fig. 6), and which are enriched in "normal" sediment.

This indicates that the pumice component acts as a dilutent on the phases containing these elements. There is also limited evidence (sample 687/2 -high in carbonate) that the carbonate component also dilutes these phases, although this is not so apparent for Li. Analyses of a phyllite from Santorini (Puchelt et al. 1973), suggests that it could be a possible source for these elements.

CALDERA ZONE SEDIMENTS

Butuzova (1969) believed there to be four main components in Caldera zone sediments - a direct volcanic hydrothermal influence on the abundance of Fe and Mn; an indirect influence on the abundance of V and Cu due to the scavenging action of hydrothermally derived particulate ferric hydroxides; the solid products of volcanic activity influencing the concentration of Ti, V, Cr, Ni, Co and Cu, and a biogenic influence providing CaCO₃, organic carbon, and possibly silica.

Based on their general lithological character, and, in particular, the variation in carbonate with depth in the sediment, two major facies have been identified in Caldera zone sediments (Fig. 2). A "normal" facies which, as with the shelf zone, represents sedimentation in relatively quiescent periods. Characteristic of the "normal" facies are fine-grained sediments (dominantly silt) enriched in calcium carbonate, iron oxides, and minor amounts of clay minerals, including montmorillonite (Bonatti et al. 1972, Petersen & Müller 1974). Organic matter is locally abundant producing mildly reducing conditions at depth in some cores, particularly in the northern basin.

Intercalated with the "normal" facies is the "pumice" facies deposited at times of volcanic disturbance and characterised by more or less extensive pumice horizons, with a low carbonate content. Unlike the "normal" facies from both the caldera and the shelf, and the "pumice" facies from the shelf, the pumice component of the "pumice" facies in the caldera may be derived from eruptions post-dating the 1400 B.C. eruption of Santorini.

Variation in the range of elements within each major facies is presented in table 3 for cores 651 and 645. Locations of core and grab samples used are indicated in Fig. 1. 

Calcium and magnesium co-vary with depth in the cores, with both being highest in the "normal" facies, and lowest in the "pumice" facies. As with the shelf facies, this is not a simple relationship (Fig. 7). The Mg/Ca ratio for the "pumice" field is higher than that for the "normal" field and represents the variation in Ca and Mg within the pumice itself. In a similar fashion to the shelf facies, the addition of carbonate Ca and Mg results in a decreased slope of the Mg vs Ca relationship. Mg in the "pumice" facies varies between 0.6 % and 1.3 %  , and Ca between 1.5 % and 4.5 %. In contrast, in the "normal" samples Mg varies between 1 % and 1.5 % and Ca between 3 % and 6 %.

From the core profiles it is difficult to recognise any significant relationship between Al and the various facies. Instead, Al abundance covers a narrow range and with an apparently erratic distribution. The "pumice" facies sediments have an Al concentration ranging between 5.7 % and 7.5 % and a Si concentration between 32 % and 40 %. Generally, the Al content of the "normal" facies is higher (between 6.7 % and 7.8 %) and the Si abundance lower (between 22 % and 35 %) and fall in a similar field to that of the shelf samples with a similar Al abundance, close to the Si/Al field of pumice samples analysed by Puchelt & Schock (1972), (Fig. 8). Variation in the Si/Al ratio in caldera sediments is considered to reflect the variation in mineralogy of the pumice component, particularly the antipathetic variation in the silica (quartz) and plagioclase feldspars.

Titanium abundance varies between 0.38 % and 0.50 % in cores 651 and 645, but there is little distinction in abundance between "normal" and "pumice" sediments (Table 3). Undoubtedly the variation in pumice composition controls the absolute abundance of Ti.

Iron is enriched in the "normal" facies of the caldera, varying between 4.0 % and 5.4%, compared to a range between 2.8 % and 4.0% in the "pumice" facies. Fe profiles with depth in the sediment (Fig. 9) illustrate the enrichment in the "normal" horizons, particularly for cores 651, 1106 and 645. To provide an assessment of the excess Fe due to hydrothermal activity, Butuzova (1969) compared the Fe and Ti relationships for the shelf and caldera. Scatter plots for Fe against Ti for this investigation are similar to those from the data of Butuzova, and an excess Fe over the detrital component is apparent in the "normal" sediments of the caldera. Compared to the average Fe /Ti ratio for detrital sediments, namely the "pumice" facies (from the caldera and shelf) and the "normal" facies from the shelf, an "excess Fe" of probable hydrothermal origin between 0 and 2 % is indicated for the "normal" caldera facies. This may not seem particularly large but Butuzova (1969) reports Fe concentrations up to 17 % in the northern basin. This level has not been found in the present investigation but sample density is low compared to that of Butuzova (1969).

An interesting difference exists, in the Mn profiles, between cores from the northern basin (651 and 1106) and core 645 from one of the smaller southern basins within the Caldera (Fig. 9). In all cores manganese appears to be enriched in the "normal" sediments but the abundances reached in the northern basin (up to ½ %) are much higher than in core 645 (less than 0.2 %). Close inspection of the older "normal" sediments in 651 and 1106 reveals that the high Mn contents are limited to the uppermost sample of the horizon, unlike the iron which is uniformly distributed within the "normal" horizons. A high manganese content at the oxidised surface layer which decreases with depth in the core is a characteristic of the post-depositional migration of Mn (Lynn & Bonatti 1965). It is a well-known feature in marine sediments in which mildly reducing conditions develop within the sediment column causing a reduction of the Mn to the soluble divalent state thereby enriching Mn in the pore waters of the reduced sediment. Due to the concentration gradient developed in the pore waters, there is an upward flux in Mn through ionic or molecular diffusion. Manganese subsequently reprecipitates in the oxidised surface layer.

It is important to note that none of the trace elements determined co-vary with manganese. Had the high Mn concentration precipitated from seawater under abnormally quiescent conditions, positive trace element -manganese relationships would be expected as "hydrogenous" manganese normally has a significant trace element signature (Bonatti et al. 1972b). This feature lends further support to diagenetic remobilisation of Mn, as "diagenetic" Mn has a low trace element signature (Bonatti et al. 1972b) because the post-depositional mobility of the transition elements such as Ni, Cu, Co, V and Cr is much less than that of the Mn.

Nickel, Cr, Ba and Li in the caldera sediments are much lower than observed in shelf samples, particularly Cr, Ni and Li. Either the unspecified shelf component mentioned previously is much less abundant in the caldera, or an alternative source is present.

Barium is enriched in the "normal" sediments of the caldera. Interestingly, samples high in Ba are also those enriched in Sr (Fig. 10). Both Ba and Sr can substitute for K in the feldspar lattice and this would explain the strong correlation between Ba and Sr and the pumice samples. An apparent carbonate association of Ba is not understood except to say that Ba association with biogenous components is known but generally through nonskeletal processes (Riley & Chester 1971).

For K, Ga and Sc the differences between strata are too small to assess the dominant control on these elements, but general levels indicate pumice to be the major phase contributing them to the sediments. Mo is close to the detection limit for all caldera samples.

SUMMARY AND CONCLUSIONS

The relationships of various elements to Al suggest that the abundances of elements in sediments from the shelf zone are controlled by the relative proportions of three end members - pumice, carbonate and an unidentified component.

To summarise, the dominant phase/element associations are as follows:

1. Al, Fe, Si and Ti are controlled by the pumice, Ca by the carbonate and Mg reflects variation in both these phases;
2. V, Zn, Cu and Ga are directly related to the pumice phase, possibly within ferromagnesian minerals;
3. Sr is directly related to the carbonate phase;
4. Ni, Cr, Zn, Ba, Cu and Ga are possibly related to one or more unidentified components (probably terrigenous mica or clay minerals) of the "normal" sediments; and
5. Mn has a direct relationship with the pumice phase but superimposed on this is the possible remobilisation of Mn in the "normal" sediments, resulting in enrichment in the surface layers.

The dominant phase/element association in Caldera Zone sediments are as follows:

1. Al, Si and Ti abundances are controlled by the relative proportions of rhyolitic glass, feldspars, and pyroxenes.
2. Dispersion of particulate Fe from the exhalative zones appears to be enriching the caldera sediments in this element.
3. Manganese is contributed to the caldera sediments from a variety of sources and is sometimes subsequently remobilised to provide horizons of high Mn content.
4. V, Pb, Ni, Cr, Cu, Li and Co abundances reflect the combined influence of scavenging by iron and manganese oxides and of the pumice. Both influences are of a similar order of magnitude.
5. Carbonate phases are the major control on the abundance of Ca, Mg and Sr, but the pumice contribution is also significant.

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