Primary Basaltic Magmas for the Pre-Caldera Volcanic Rocks of Santorini

These rocks have values of Mg/Mg + Fe ²⁺ (0.59 - 0.67) and contents of Ni (30 - 100 ppm) and Cr (20 - 350 ppm) intermediate between those of liquids produced by experimental melting of eclogite of oceanic tholeiite composition, and those of liquids derived from the melting of peridotitic compositions.

The most probable origin of the high-alumina basalts was by crystal fractionation from more mafic, mantle-derived primary magmas. This model is supported by calculation, on the basis of partition relationships of Fe ²⁺ and Mg, and Ni and Cr between basaltic liquids and coexisting crystalline phases, of liquid compositions capable of equilibrium with olivine-rich peridotitic mineral assemblages. Such liquids have Mg/Mg + Fe²⁺ values of 0.68 - 0.73, 9 - 12 % MgO, and about 200 ppm Ni and 600 ppm Cr. Primary magmas of similar composition could have given rise to the analysed Santorini high-alumina basalts by fractionation of 6 - 13 wt. % near-liquidus olivine, and small amounts of Cr-rich spinel and diopsidic pyroxene.

INTRODUCTION

Models of the origin and evolution of the pre-caldera rocks of Santorini volcano fall into two groups - those which favour primary basaltic magmas, derived from partial melting of upper mantle peridotite (Nicholls 1971a, b), and those which favour andesitic primary magmas, and propose a major role for the partial melting of subducted oceanic crust in the form of eclogite (Puchelt and Hoefs 1971; Pichler and Kussmaul 1972). Puchelt (1976) appears to favour primary basaltic magmas, and involves both partial melting of eclogite, and interaction of the resulting liquids with peridotitic mantle and lower crust in the origin of these magmas.

This paper attempts to evaluate the two groups of models, using geochemical tests based upon data from experimental melting studies of peridotitic and eclogitic materials. A similar approach has previously been used by Nicholls and Whitford (1976) in investigating the nature of primary magmas associated with Quaternary volcanism in the Sunda volcanic arc, Indonesia.

Major element geochemistry of the pre-caldera volcanic rocks.

Variation in the composition of the pre-caldera volcanic rocks of Santorini in terms of SiO₂ content and the atomic ratio Mg/Mg + Fe ²⁺ is summarised in Figs. 1 - 3. The analyses used were taken from Paraskevopoulos (1956), Nicholls (1971 a), Puchelt and Hoefs (1971) and Pichler and Kussmaul (1972).

Only analyses with Fe₂O₃ ≤ FeO, and H₂O ≤ 2.5 wt. % were included.

Before calculation of Mg/Mg + Fe ²⁺ values, ferrous iron in all analyses was adjusted to FeO = 0.85 Σ FeO. The basis of this procedure has been discussed by Nicholls and Whitford (1976). Following this recalculation of iron contents, all analyses were normalized to 100 wt. %, free of H₂O and CO₂.

SiO₂ content

The 76 analyses of pre-caldera volcanic rocks which met the criteria adopted for this paper span the range of SiO₂ content from 49 to 71 wt. %. The histogram of Fig. 1 shows an overall maximum at 51 - 52 % SiO₂, with a gradual decline toward more silicic compositions. According to the chemical classification of Taylor (1969), the most abundant analysed rock type is high-alumina basalt (< 53% SiO₂), with successively smaller proportions of basaltic andesites (or low-silica andesites), andesites, dacites and rhyodacites or rhyolites.

Puchelt and Hoefs (1971) and Pichler and Kussmaul (1972) have described the most mafic pre-caldera volcanics of Santorini as andesites on the basis of their relatively low colour index. While these rocks contain a relatively high proportion of calcic plagioclase, reflecting their richness in CaO and Al₂O₃, they are chemically high-alumina basalts, and normatively olivine tholeiites to quartz tholeiites (Nicholls 1971a; Puchelt 1976).

Mg/Mg + Fe ²⁺ ratio

The atomic ratio Mg/Mg + Fe ²⁺ (or 100 Mg/Mg + Fe ²⁺) has been used as a differentiation index, and to express compositional relationships between coexisting liquids and mafic minerals (Green and Ringwood 1967). A histogram of Mg/Mg + Fe ²⁺ values for pre-caldera volcanic rocks of Santorini is given in Fig. 2. The range covered is 0.68 - 0.24, with an overall decrease in frequency from higher to lower values. All but one of the high-alumina basalts have Mg/Mg + Fe ²⁺ within the range 0.59 - 0.67.

In Fig. 3, the parameters SiO₂ and Mg/Mg + Fe ²⁺ are plotted against each other, using both points for individual analyses and the average compositions and trends for the two complete lava series distinguished by Nicholls (1971a).

The initial more rapid decrease in Mg/Mg + Fe ²⁺ with increasing SiO₂ for the lavas of Nicholls' Main Series is well illustrated, corresponding to absolute iron enrichment in andesitic compositions.

Nicholls (1971a) interpreted the apparent serial relationships of lavas within his Akrotiri - Thira and Main Series (Fig. 3) as indicating a close genetic relationship, with the primary magmas of the two series being of high-alumina basalt composition. Puchelt (1976) has also recognised well defined compositional series from high-alumina basalt to rhyodacite or rhyolite.

Experimental SiO₂ - Mg/Mg + Fe ²⁺ trends for partial melts of eclogitic and peridotitic materials

When partial melting of eclogitic or peridotitic materials occurs, the compositions of liquids produced are constrained by the residual crystalline phases present - mainly garnet and clinopyroxene in the case of eclogite, and olivine, orthopyroxene, clinopyroxene, amphibole and spinel or garnet in the case of peridotitic compositions. From experimental investigations, we have data on the compositions of liquids and crystals which can coexist at equilibrium in eclogitic and peridotitic bulk compositions, and we have the theoretical basis for studies of element partitioning between crystals and liquids under conditions of both bulk equilibrium and surface equilibrium (e.g. Gast 1968; Shaw 1970; Albarede 1976). Using this information, we may compare experimentally or theoretically derived liquid compositions with those of natural igneous rocks and inferred primary magmas in an attempt to deduce the source materials for these magmas, and the nature of partial melting processes.

Estimates of compositional trends for liquids produced by experimental melting of eclogitic and peridotitic compositions are given in Fig. 3. Those for approximately 20 to 100 % melting of eclogites of olivine tholeiite (A) and average oceanic basalt (B) bulk compositions at 30 Kbar and with 5 % water were estimated by Nicholls and Whitford (1976) from variation diagrams of Stern (1974). That for up to approximately 20 % melting of peridotitic compositions at 10 - 15 Kbar in the presence of a water-rich vapour phase (C) is based mainly on the data of Nicholls (1974), while that for up to 25 % melting at 10 Kbar, and conditions of water-undersaturation (D), is based on the work of Green (1973) on the pyrolite model upper mantle composition.

The major features of the experimental trends are the relatively silicic nature of liquids produced by small degrees of melting of eclogites, and the more magnesian compositions of liquids derived by peridotite melting compared with eclogite-derived liquids of similar SiO₂ content. Comparison of these trends with the two pre-caldera lava series average trends for Santorini indicates that the most mafic lavas of these series are too magnesian to have been produced by melting of eclogite of oceanic basalt composition unless almost total melting occurred. On the other hand, aphyric pre-caldera lavas of Santorini are slightly less magnesian than probable primary magma compositions derived from the partial melting of peridotitic mantle.

Nickel and chromium contents

Nickel and chromium contents of pre-caldera volcanic rocks are plotted in Figs. 4 and 5, using data from Nicholls (1968, unpublished), Puchelt and Hoefs (1971) and Pichler and Kussmaul (1972). The maximum values, from a Mavrorachidi dome basalt, are 105 ppm Ni and 353 ppm Cr (Puchelt and Hoefs 1971), but few samples contain more than 75 ppm Ni or 150 ppm Cr.  

Data for Ni in olivine and Cr in calcic clinopyroxene phenocrysts in these rocks (Nicholls 1968, unpublished) are also plotted in Figs. 4 and 5, and illustrate the rapid depletion of these minor elements in both rocks and constituent minerals as major element compositions become less magnesian. The indicated partition coefficient,

D = concentration of element in crystals / concentration of elements in bulk rocks

for Ni in olivine ranges from 12 to 22 (Fig. 4). Values of D for Cr in two calcic clinopyroxene - bulk rock pairs are 30 and 13 (Fig. 5).

The Ni and Cr contents of peridotitic mantle and basaltic oceanic crust are very different, peridotitic compositions being at least an order of magnitude richer in these elements. Recent estimates cited for Ni and Cr in peridotitic mantle are 2000 ppm Ni and 3000 ppm Cr (Nesbitt and Sun 1976) and 1500 ppm Ni and 2200 ppm Cr (Lopez-Escobar et al. 1977). Estimates of average Ni and Cr in oceanic crust by both groups of authors are 100 ppm and 300 ppm respectively.

Magmas produced by partial melting of peridotite and eclogite will also differ greatly in Ni and Cr. Preferred values of partition coefficients for Ni and Cr applicable to partial melting of eclogitic and peridotitic mineral assemblages have recently been tabulated by Lopez-Escobar et al. (1977). For the case of partial melting of lherzolite, the relevant partition coefficients are all greater than 1 (olivine - 10, orthopyroxene and clinopyroxene - 3, amphibole - 1.7).

Liquids produced by 10 - 25 % fractional melting of a spinel lherzolite with 1500 ppm Ni will contain about 210 - 220 ppm Ni (for details of calculations, see Appendix). In the case of Cr, partition coefficients are 1 for olivine and orthopyroxene, 30 for clinopyroxene and 5 for amphibole (Lopez-Escobar et al. 1977), while that for Cr-spinel is of the order of 100. Chromian spinel, if present, will control the Cr content of liquids produced by up to about 30 % melting of lherzolite (Nesbitt and Sun 1976). For lherzolite with 2200 ppm Cr, and 1% modal Cr-spinel 10 - 25 % fractional melting will produce liquids with 520 - 830 ppm Cr. Olivine tholeiites and quartz tholeiites containing high pressure phenocrysts, described from the Saar - Nahe area of West Germany by Nicholls and Lorenz (1973) and interpreted as almost unfractionated primary magma compositions, contain 170 - 330 ppm Ni and 360 - 700 ppm Cr, values similar to those estimated for liquids derived by up to 20 % partial melting of spinel lherzolite.

For the melting of eclogite, the relevant partition coefficients for Ni and Cr are 3 and 30 for clinopyroxene and 0.7 and 20 for garnet (Lopez-Escobar et al. 1977). Moderate degrees of melting of an eclogite with 67 % clinopyroxene and 33 % garnet, 100 ppm Ni and 300 ppm Cr produce liquids with very low Ni and Cr contents (e.g. 60 ppm Ni and 17 ppm Cr at 60 % fractional melting) and even 90 % melting produces a liquid, with only about 90 ppm Ni and 30 ppm Cr.

Nickel and chromium contents of the pre-caldera basaltic rocks of Santorini (Figs. 4 and 5) are higher than those of liquids produced by fractional melting of eclogite (cf. Puchelt 1976). On the other hand, they are lower than those of liquids derived from peridotitic compositions. A similar observation may be made for many basaltic rocks in orogenic environments (Taylor et al. 1969; Lopez-Escobar et al. 1977).

Mantle -derived primary magDlas for the mafic volcanic rocks of Santorini

Nicholls and Whitford (1976) calculated possible primary magma compositions for lavas of the western Sunda arc, Indonesia, using a simple model which assumed that the main crystalline phases removed from primary magmas to produce the observed range of basalts and basaltic andesites were olivine and chromian spinel. This model was based on experimental investigations into the stability of liquidus olivine in olivine-poor tholeiitic basalts and basaltic andesites by Nicholls and Ringwood (1973) and Nicholls (1974), which showed that in the presence of water, olivine is the major near-liquidus phase for such compositions over a wide range of pressures, and is therefore likely to play a major role in fractionation processes.

Similar calculations have been performed on the average high-alumina basalts of the two complete lava series of Santorini. The procedure used "reverses" the process of crystallization under conditions of surface equilibrium.

Using the Fe ²⁺/Mg ratio of each average basaltic composition, and the value of 0.30 for the distribution coefficient,

K_D = (Fe²⁺/Mg) olivine / (Fe²⁺/Mg) liquid,

determined by Roeder and Emslie (1970), corresponding equilibrium olivine compositions were calculated. Next 0.1 wt. % of these olivines was added to the basaltic compositions, and the new Fe²⁺/Mg ratios and equilibrium olivine compositions calculated. The calculation was then repeated until the composition of the last increment of olivine added was that of "mantle" olivine. Two cases - olivines with Mg/Mg + Fe ²⁺ of 0.88 and 0.90 - were studied. The resulting "primary magma" compositions for basalts of the Akrotiri - Thira and Main Series are given in Table 1. Addition of approximately 6 and 12 wt. % olivine was required to produce liquids in equilibrium with olivines with Mg/Mg + Fe ²⁺ of 0.88 and 0.90 respectively.

Ni and Cr contents of primary magmas

The model of calculation of possible primary magma compositions based on Fe²⁺/Mg systematics may be evaluated using Ni and Cr data. The Ni contents of the average Santorini high-alumina basalts are about 70 and 55 ppm (Table 1). Using the Ni partition coefficient value of 10 for coexisting olivine and basaltic liquid, addition of 13.6 and 16.8 wt. % olivine respectively produces liquids with Ni contents (220 ppm) similar to that calculated above for a 20 % partial melt of lherzolite. A nickel partition coefficient value of 13, derived from the data of Fig. 4, reduces these proportions to about 10.1 and 12.6 wt. % olivine.

Since partition coefficients for nickel in olivine and coexisting basaltic liquids depend on temperature and the compositions of both phases involved in the equilibrium (Mysen 1976; Irvine and Kushiro 1976), agreement between the proportions of olivine added to produce liquids capable of equilibrium with "mantle" olivine, calculated using Fe^2+/Mg and Ni systematics, is satisfactory.

Similar procedures for Cr are less well constrained, since both chromian spinel and Cr-bearing clinopyroxene (up to 3700 ppm Cr - Fig. 5) have crystallized from the pre-caldera high-alumina basalts of Santorini, and could have been fractionated from primary magmas. However, Cr contents of about 600 ppm, similar to that calculated for a liquid produced by 20 % melting of lherzolite are achieved by adding less than 0.5 wt. % Cr-spinel and 4 wt. % Cr-bearing diopsidic pyroxene to the average high-alumina basalts.

CONCLUSIONS

The results of calculations described in the preceding sections indicate that the pre-caldera high-alumina basalts of Santorini could have been derived from primary magmas slightly richer in Mg, Ni and Cr, which were produced by partial melting of peridotitic mantle. Removal of only small amounts of near-liquidus phases - olivine, Cr-spinel and perhaps diopsidic clinopyroxene - would have been necessary to produce the observed lava compositions. The common occurrence of olivine phenocrysts with Cr-spinel inclusions, and coarse phenocrysts of Cr-bearing diopsidic augite in the more mafic pre-caldera lavas of Santorini, strongly supports the operation of such processes.

These processes of minor crystal fractionation correspond to the final stage of a sequence described by Nicholls and Ringwood (1973), Ringwood (1974) and Nicholls (1974) for the origin of rocks of the calc-alkaline orogenic association (Fig. 6). In the first stage of this model, partial melting of subducted oceanic crust enriches the overlying wedge of peridotitic mantle in water and elements such as K, Rb, and light rare earth elements. Subsequently, partial melting of this "modified" mantle material produces primary basaltic magmas with the geochemical characteristics of the calc-alkaline association - i.e. relatively high K and related elements, and light-enriched REE patterns. Finally, differentiation within the crust produces the range of magmas, from basalt to rhyodacite, which erupt at the earth's surface.

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