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in Peralkaline Oversaturated Systems and Volcanic Rocks. Jacques ... Abstract. A critical review of both experimental data and natural rock sequences allows a.
Contrib. Mineral Petrol. 49, 67--81 (1975) 9 by Springer-Verlag 1975

Alkali Feldspar Liquid Equilibrium Relationships in Peralkaline Oversaturated Systems and Volcanic Rocks Jacques Roux Centre de Synth~se et Chimie des Min6raux, 45045 Orl6ans-Cedex J a c q u e s Varet Laboratoire de P6trographie-Volcanologie, Universit6 Paris-Sud, 91405 Orsay Received July 17, 1974 / Accepted November 8, 1974

Abstract. A critical review of both experimental data and natural rock sequences allows a comparison of liquid lines of descent in peralkaline oversaturated systems. Available experimental data have been used to draw the liquidus surfaces and the fractionation curves in various planes including the alkali feldspar join. The thermal valley becomes steeper, the fractionation curves being channeled faster towards the thermal valley with increasing alkalisilica ratio. The slope of the thermal valley varies so that the higher the peralkalinity, the more sodic the most fractionated liquids. Rock series from Easter Island, Gran Canaria, Pantellaria Island, Boina (Afar), Fantale (Ethiopia) and Menengai (Kenya) in which Ithere is good evidence for alkali feldspar fractionation, show that such variations do occur in nature. Factors other than the peralkalinity of the series, such as additional components and changes in the confining pressure are shown to affect the orientation and location of the thermal valley.

Introduction PerMkaline rhyolites (sensu lato) form a complex group of rocks whose origin has been recently discussed. Several mechanisms have been proposed such as fractional crystallization from a more basic parent m a g m a , partial melting within a continental crust, or/and contamination b y an alkali bearing vapour. This large n u m b e r of genetic processes invoked reflects the complexity of the group which comprises strongly oversaturated terms with a more or less pronounced peralkaline nature (pantellerites and commendites) along with slightly oversaturated rocks of trachytic affinity. I n some cases, t h e y are associated in the field with either meta-aluminous rhyolites or/and undersaturated trachytes and phonolites as in Iceland or in K e n y a . A further complexity arises from the occurence of these rocks in a rather wide set of geological environments: besides the typical occurrence in areas of continental rifting or extension peralkaline rhyolites are also found in oceanic islands as well as near converging plate margins where eMcalkMine magm a t i s m prevails e.g. Basin and R a n g e (U.S.A.) or Mayor Island (New Zealand) (MacDonald, 1974). A m o n g the m a n y mechanisms which m a y account for the chemical variations a m o n g peralkaline rocks, the simplest is the fractional crystallisation. Peralkaline rhyolites can be the final products of a series derived from a basaltic p a r e n t m a g m a b y an early separation of Al-rich phases especially plagioclases (Bowen, 1937, 1945) producing a slightly peralkaline liquid whose evolution will t h e n be controlled b y the fractional crystallisation of an alkali feldspar alone. Evidences for 5*

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J. Roux and J. Varet

the occurenee of such rock series have been provided in the East African Rift (Weaver et al., 1972) and the Afar region on both a petrological and geochemical basis (Treuil and Yaret, 1973; Barberi et al., 1974a, b). The fractional crystallization of alkali feldspars, as the most important mechanism controlling the evolution of the liquids within the peralkaline field has been largely discussed in the early works by Carmichael (1962), Carmichael and MacKenzie (1963), Bailey and Schairer (1964) and Thompson and Mac Kenzie (1967). Special emphasis leading to controversy was placed on the particular liquid feldspar relationships in these rocks compared to those in meta-aluminous rhyolites or in the granite system experimentally investigated by Turtle and Bowen (1958). The aim of this paper is to review available data on alkali feldspar--liquid equilibrium relationships: (1) from the experimental data of Carmichael and Mac Kenzie (1963) and Thompson and Mac Kenzie (1967) in the system SiO2, AlsO3 (~Fe203), Na20, K20 (P~,o = 1 kbar) and (2) in several natural series of volcanic rocks selected from the literature which show convincing evidence of alkalifeldspar fractional crystallization and illustrate a wide range of peralkaline rhyolitic rock suites.

Experimental Data In order to compare liquid lines of descent in both natural and experimental systems, a graphical projection must be chosen. Although this has been thoroughly discussed by Bailey and Schairer (1964), Thompson and MacKenzie (1967), Nicholls and Carmichael (1969) and Bailey and MacDonald (1969), no actual projection permitting such a comparison is yet available. The choice of a suitable projection arises naturally from the fact, already noticed by Bailey and MacDonald (1969), that any liquid, however complex its composition, fractionating a binary solid solution obviously remains in the plane defined by itself and the two end-members of the solid solution. In other words, if a liquid separates only alkali-feldspars, its composition can be considered as a linear combination of only three components, two of which are NaA1Si~Os (Ab) and KA1Si~Os (Or). Let us introduce the projection used. The granite system (Turtle and Bowen, 1958) and sections A, C, and D from Thompson and Mac Kenzie (1967) (granitic compositions ~ N a or K disilicate) belong to a four component water saturated system ( P H , o - 1 kb) namely: Na20, K20, Al~O3, SiO 2. Therefore any composition in this system can be located within a 3.dimensional diagram. Data from Carmichael and Mac Kenzie (1963) have been reported in the same system, grouping F%O 8 with Al~O~. Three rectangular axes corresponding to the following variables (atomic proportions) are used: x = K / ( N a + K)

Y = (Si/3A1) -- 1 Z ----((K ~-Na)/A1)--I

X is the alkali ratio, Y is the index of silica excess with respect to an alkali feldspar composition, and Z the agpaitic index modified to be positive for peralkaline compositions, 0 for alkali feldspars and negative for peraluminous compositions.

Equilibrium Relationships in Peralkaline Rocks

69

Z

Ab

• Fig. 1. Schematic diagram illustrating the graphical representation of the experimental and natural data, the three rectangular axes correspond to the following set of variables: X = K/ (Na-4-K); Y = (Si/3AI)--1 ; Z = ((K+Na)/A1)--1 (arm. proportions). The sections investigated by Carmichael and Mac Kenzie (1963), and Thompson and Mac Kenzie (1967) are in a position such as the dotted plane. The cross-hatched plane issuing from the alkali feldspar join (0X) is a feldspar fractionation plane. The granite system is the plane (OX, OY)

In this plot shown in Fig. 1, the alkali feldspar join is the 0 X axis ( Y = 0 , Z--0). The loci of liquids which fractionate alkali feldspars belong to planes of equation Z~Y =-constant, so that a given series of liquids produced by alkali feldspar ffaetionation is characterized by a given Z~Y value (alkali silica ratio of Bailey and MacDonald, 1969). We will refer hereafter to such planes as "feldspar fractionation planes". The cross-hached plane of Fig. 1 belongs to this family, as also does the granite system (Turtle and Bowen, 1958) defined by Z/Y--0. The sections experimentally investigated by Carmiehael and Mac Kenzie (1963) and Thompson and Mac Kenzie (1967) do not include the alkali feldspar join and are located in a position such as the dotted plane on Fig. 1. A tentative estimation of the liquids isotherms and feldspar-liquid equilibrium relationships was made b y interpolation of the experimental data on some arbitrarily selected feldspar fractionation planes, which are located on Fig. 2 along with the experimental sections, by their intercepts with planes (X, Y) and (Y, Z). Their equations are given in figure caption. An explanation of the interpolation procedure used is necessary. Two sets of data have been considered: (1) the liqnidus temperatures, (2) the alkali feldspar-liquid equilibrium relationships. (1) Liquidus temperatures within a quaternary system are usually represented as isothermal surfaces. Intercepts of these surfaces by various sections considered can be drawn on two dimensional diagrams. (2) The alkali feldspar and liquid equilibrium relationships may be visualized in any feldspar fractionation plane such as the "granite system" by the con-

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l~ig. 2A and B. The positions of the experimental sections and the feldspar fraetionation planes chosen as examples are given by their intercepts with the planes (0X, 0Z) and (0Y, 0Z). (A) Fig. 2A is for the iron bearing systems; C.M. 1, and C.M. 2 refer to Carmichael and MacKenzie (1963). (B) Fig. 2 B is for the iron free systems; T.M. A, B, and D refer to Thompson and MacKenzie (1967). The equations of these planes are: T.M. A Z=0.220+0.018 X+0.152 Y T.M. C Z = 0.173 + 0.014 X +}-0.120 Y T.M. D Z=0.193+0.016 X+0.134 Y C.M. 1 Z=0.199+0.016 X+0.137 Y C.M. 2 Z = 0.375 + 0.037 X + 0.260 Y f.f.p. I Z = 0.405 Y f.f.p. I I Z = 0.748 r f.f.p. I I I ,~ = 0.405 :Y

ventional feldspar-liquid tie lines. However, this is not feasible in a n y section which does n o t include the alkali feldspar join, a n d a convenient alternative is to draw w h a t e v e r the section is, the loci of the liquids in equilibrium at the liquidus temperature with feldspars of a given composition such as Abg0Orl0, Abs0Or20 . . . . AbloOrgo (feldspar iso-composition curves). This has been done from the alkali feldspar compositions experimentally determined at the liquidus for each of the 5 experimental sections located in Fig. 2 A a n d B. Fig. 3 A and B give two examples for the granite system and

Equilibrium Relationships in Peralkaline Rocks

71

Carmiehael and MacKenzie section 1. Finally the liquidus isotherms and the feldspar iso-eomposition lines can be graphically interpolated onto any feldspar fractionation plane chosen within the compositional range covered by the experiments. Three of them whose positions are given in Fig. 2 have been drawn to illustrate the discussion. Plane I and I I are for the iron bearing system from Carmiehael and MacKenzie's experiments (Fig. 2A) whereas plane I I I (Fig. 2B) is for the iron free sections of Thompson and MacKenzie. The results are given in Fig. 3B, C, D, E, which shows the projections of the liquidus isotherms and feldspar iso-eomposition curves, from the feldspar fraetionation planes onto (X, Y) plane. This plot has been preferred to the very similar one of agpaitic index versus alkali ratio proposed for natural rocks byThompson and MacKenzie (1967), and adopted by !qieholls and Carmichael (1969) because it permits plotting on the same type of diagram series of various degree of peralkalinity including the non peralkaline i.e. the granite system. Fig. 4 derived from Fig. 3 shows alkali feldspar fraetionation curves. I t must be noticed that the portion of the diagram especially relevant to the natural liquids considered below (X in the range 0.2 to 0.5) has been enlarged on Fig. 4. On each feldspar fractionation plane are two sets of fractionation curves. One issues from the Ab corner and the other from the Or corner. The "unique fractionation curve" (MacKenzie personal communication) separates these two families. The unique fractionation curve and the thermal valley issue from the feldspar minimum. Both curves were found to coincide within experimental error in the system investigated, but we have not been able to show theoretically that they should/or not coincide perfectly. These diagrams show that the shape of the liquidus surface and the alkali feldspar-liquid equilibrium relationships vary significantly from one system to another. The slope of the sides of the thermal valley becomes steeper close to the Ab-Or join, so that liquids fractionating alkali feldspar are expected to be channeled much faster towards the thermal valley in peralkaline systems than in metaahiminous systems. This is shown in Fig. 4 where one notices that the curves of feldspar iso-composition (dashed lines) are much closer to one another in the peralkaline systems than in the granite system. Another feature of importance is the position and orientation of the unique fraetionation curve shown in the projection used: in the granite system, the unique fractionation curve crosses the lines of constant alkali ratio towards the potassic side line, whereas the reverse is true in most peralkaline systems (of large Z/Y) as noticed by Thompson and MacKenzie (1967). Between these two extremes, a case exists in which the general trend of the unique fraetionation curve will be nearly parallel to the lines of constant K/(Na-kK) ; Z/Y ratio for this particular case will vary with the various components of the system; this case occurs near to the feldspar fractionation plane I I I in the iron free system under PH,o ~ 1 kb (Fig. 4). We should emphasize that the proposed thermal valley for the planes I, I I and I I I depends very much on the position of the binary minimum, taken from Bowen and Turtle (1950) for Pg,o = 1 kb. The position of this minimum was determined many years ago in experiments of relatively short duration and Bowen and Turtle comment on the difficulty of fixing the position of this minimum accurately.

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Equilibrium Relationships in Peralkaline Rocks z

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Fig. 4. The plot is the same as Fig. 3 from which it derives, but the abscissa has been expanded to show more clearly the feldspar liquid relationships. The set of fraetionation curves issuing from the albite composition--thin solid lines--, the feldspar iso-composition curves-dashed lines--the thermal valley and the quartz saturation line--thick solid lines--are shown in 4 feldspar ffactionation planes including the granite system (B.T.). Notice that Y and Z are linearly correlated and indicated respectively on the left and right hand side of each diagram.

T h e shape of t h e isobaric fractional c r y s t a l l i z a t i o n curves is d i r e c t l y c o r r e l a t e d w i t h t h e p o s i t i o n a n d o r i e n t a t i o n of t h e t h e r m a l v a l l e y which varies w i t h Z/Y in t h e f e l d s p a r f r a e t i o n a t i o n planes s t u d i e d as a l r e a d y m e n t i o n e d . I n t h e a l b i t e rich side of t h e g r a n i t e system, (Fig. 4, BT) t h e f r a c t i o n a t i o n curves h a v e a f a i r l y s m o o t h c u r v a t u r e a n d are a l w a y s t r e n d i n g t o w a r d s t h e p o t a s s i e side. I n t h e m o s t p e r a l k a l i n e s y s t e m s such as p l a n e I I (Fig. 4) these curves d i s p l a y a m u c h m o r e i m p o r t a n t c u r v a t u r e , therefore t h e f e l d s p a r composition reaches v e r y q u i c k l y a f a i r l y potassie composition, w i t h i n t h e " a n o r t h o c l a s e " field. D u r i n g t h e final stages w h e n t h e liquid becomes v e r y close t o t h e u n i q u e f r a c t i o n a t i o n curve, t h e a l k a l i f e l d s p a r - l i q u i d relationships are r e v e r s e d : t h e f e l d s p a r becomes m o r e

Fig. 3 A---E. The liquidus isotherms and the feldspar iso-composition curves (the loci of liquids in equilibrium at the liquidus temperature with a feldspar of a given composition) are reported in various sections: 1) Fig. 3A, B--two among the experimental sections taken as examples1: --B.T. is the granite system from Tattle and Bowen (1958) revised by Carmichael and Mac Kenzie (1963). a b e d - - C.M. 1 ; coordinates of a, b, e, and d are:

Y

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1.00 1.33 0.40

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2) Fig. 3 C, D, E--The feldspar fractionation planes: ffp I, II, III. Symbols: Black filled circles:experimental points; Thin solid lines: liquidus isotherms (the temperatures are in ~ Dashed lines: feldspar iso-composition lines (the compositions of the feldspar are in mole To of KA1SisOs); Thick solid line: intercept of the quartz saturation surface. 1 Analogous figures have been drawn for the other experimental sections to allow the graphical interpolation of the liquidus isotherms, and feldspar iso-eomposition lines onto the three feldspar fractionation planes ehoosen.

74

J. Roux and J. Vare/~

potassic than the liquid. Bailey and Schairer (1964) introduced the term "orthoclase effect" to caracterise this last property of pcralkaline systems. However such a reversal of the tie-line slope is merely a general feature of any similar ternary system in which the thermal valley is not following lines of iso K/(Na~-K) ratio (or their equivalent). The crystallisation of an alkali feldspar more potassic (higher K/(Na-~-K)) than the magma, on the Ab-rieh side of the system is only possible in those strongly peralkMine systems where the thermal valley trends towards the sadie side line and is not possible in a good number of peralkaline systems. :Natural R o c k Series

The preceeding discussion reviews the main changes observed in the fraetionation curves between the granite system and several peralkaline systems experimentally investigated. Natural systems have been searched which show good evidence of alkMi-feldspar fractionation in order to provide natural examples of the corresponding fractionation curves, and to allow a comparison of natural and experimental alkali feldspar-liquid equilibrium relationships. Various natural series lead from basalts to peralkMine rhyolitic rocks, the composition of which ranges from rhyolites to trachytes and depends on the

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Equilibrium Relationships in Peralkaline Rocks

75

composition of the parent basalts (Barberi et al., 1974a) and on the phases controlling their evolution towards the peralkaline system such as plagioelase (Bowen, 1945) or Al-rich pyroxenes (Bryan, 1970). Peralkahne liquids r e m a r k a b l y depleted in calcium m a y erystallise alkali feldspars alone. If this is the only process occuring, their compositions should display a linear correlation on a Y versus Z plot (compositions on a straight line passing t h r o u g h the origin) i.e. belong to a single feldspar fraetionation plane. This is a simple criterion to check if natural rock series are produced b y alkah feldspar fraetionation. These series in turn, have to be selected on geological and petrological basis. The rocks should strictly belong to the same volcano. I t is well established now t h a t the character of the series m a y v a r y from one volcano to

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rocks from Pantelleria and Fantale bear some quartz phenocrysts. Dash-dotted lines join glass and bulk rock compositions from Pantellcria. Data sources: Nasaken and Eburru: Weaver et al. (1972) ; Mayor Island: Ewart et al. (1968) ; Nicholls and Carmichael (1969); Erta Ale: Barberi and Varet (1970); Iceland: Carmichael (1964) ; MacDonald and Bailey (1973); Jebcl Khariz: Gass and Mallick (1968); Easter Island: Baker et aL (1974); Grail canaria: Arafia et al. (1973) ; Boina: Barberi et al. (1974 a); Fantale: Gibson (1972) ; Pantelleria: Carmichael (1962); Nicholls and Carmichael (1969); Menengai: MacDonald et al. (1970)

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J. Roux and J. Varet

another, even within a given area such as the East African Rift, or the Afar, or Iceland ere . . . . Moreover, the rocks should have been emplaeed during the same volcanic event, as it has been shown that important variations may occur in the nature of the products erupted from a single volcano according to their position in the eruptive cycle. This has been observed in volcanic centers such as Fantale, Ma'alalta (P. Pruvost massif), Pantelleria (Gibson, 1972; Barberi et al., 1974a; Villari, 1974) where pre and post-caldera sequences have been distinguished. The sampling must be, of course representative of the various occuring rock types including the non peralkaline. When selecting samples special emphasis should be placed on the nature of the groundmass (crystalline or glassy), owing to possible modification in the composition of a peralkaline rock during devitrification as suggested by Noble (1965), Noble et al. (1967) and strenghthen by MacDonald and Bailey (1973). However restricting the samples to obsidians only will artificially reduce the range of natural rock varieties. Not all the series considered were found to display the linear correlation on the Y/Z plot. Among those which do not fullfill this condition are Mayor Island (New Zealand) (Ewart et al., 1968; Nieholls and Carmiehael, 1969) Jebel Khariz and other rocks from Arabia (Gass and Malliek, 1968; Cox et al., 1969). Some of the rhyolitic and comenditic series such as those from Iceland (Carmichael, 1964) or Erta Ale (Barberi and Varet, 1970) are only slightly peralkaline and have not undergone any significant alkali feldspar fractionation, as separation of plagioclases takes place in these series of tholeitie affinity until they almost reach a composition close to the quartz boundary surface, near the minimum of the granite system. All these series appear on Fig. 5A. Fig. 5 B shows the rocks suites which were found to fullfillthe conditions previously discussed. These series illustrate several evolutionary trends of magma controlled by alkali feldspar fractionation whose final products range from pantelleritie traehytes to pantellerites, comendites and slightly peralkaline rhyolites ; Z~ Yvaries in the range 1.1 to 0.02). For each series it is possible to propose aliquid line of descent i n X - Y plot, consistent with perfect fractionation, fitting the points of the bulk rock composition and their feldspar plienocrysts. This was done in Fig. 6. We noticed in the discussion of the experimental systems that the position and orientation of the thermal valley is a typical feature which broadly indicates the aspect of the fractionation curves of a given system (see Fig. 4). Respectively, to locate accurately the thermal valley it would be necessary to get several fraetionation curves on both of its sides. Let us emphasize that the rocks from a single volcanic series produced by alkali feldspar fractionation necessarily belong to a single fraetionation curve. Reciprocally rocks belonging to several rock series (viz. fractionation curves) will not generally belong to the same system (variations of Y/Z ratio, pressure, etc.), hence cannot be used to locate a unique fraetionation curve, but only a so called "low temperature zone". The same applies to undersaturated systems fraetionating alkali feldspars (Nash et al., 1969). However we reckon that the knowledge of one fractionation curve constrains fairly tightly the position of the thermal valley of the system to which it belongs. Particularly in those rock suites which turn back towards the sodie side line, i.e. whose feldspar liquid tielines have negative slopes, the thermal valley must have a negative slope too (see Fig. 4). Fig. 6 shows a tentative location of the thermal valley for each natural

Fig. 6. This figure presents the natural series selected as shown in Fig. 5B and discussed in the text. The plot is similar to Fig. 4. Feldspar liquid relationships are shown when the feldspar compositions are available, as well as the inferred fractionation curve and thermal valley for each serie. Filled squares represent non peralkaline rhyolitic trachytes in the Boina serie. I n Fantale post-caldera figure, the fractionation curve of the pre-caldera serie has been added for comparison. Data sources are in Fig. 5 caption; unpublished data on Fantale feldspar phenocrysts are from Gibson (personal communication)

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J. Roux and J. Varet

rock series. Thermal valleys are drawn in order to be consistent with the fractionation curve, assumed to be on the albite rich side of the system. Let us describe and discuss the variations observed in Fig. 6 between the natural systems and compare them with the experimental fractionation curves of Fig. 4. 1) Easter Island located on the East Pacific rise (Baker et al., 1974) is the least peralkaline serie selected (Z/Y ~ 0.05). The liquid line of descent has been drawn from 4 whole rock analyses, unfortunately feldspar compositions are not available. This line trends slightly but distinctly towards increasing K/(Na-~-K) ratio. The thermal valley has been drawn to fit with such a fractionation curve. This valley is closer to the Na side line than any of those from the experimental systems previously discussed. 2) Gran Canaria located away from the mid-Atlantic ridge (Arafia et al., 1973) is but slightly peralkaline ( Z / Y - 0.26). The liquid line of descent shows a definite tendency to potassie enrichment of the liquids (compared to Na), consistent with the alkali feldspar analyses. The thermal valley seems to be at very high K/(Na + K ) ratio, and to have a positive slope as (1). 3) B o i n a ~ A f a r - - ( B a r b e r i et al., 1974a) provides a good example of a mildly peralkaline pantelleritie sequence, (Z/Y----0.46). The fractionation curve is well defined by both liquid and feldspar phenocrysts analyses. In this serie the feldspars progressively change from X-rich oligoclase in the non peralkaline trachytie rocks, Ca-bearing anorthoclase in the trachy-rhyolites and to Ca-free anorthoclase in the pantellerites. The feldspar becomes more potassic with increasing peralkalinity of the magma, to finally attain a constant composition (0r3~ mole-% ) while the liquid follows a straight line of descent, in this particular ease of constant K/(Nad-K) ratio. 4) Fantale--Ethiopia--is a volcanic center located towards the northern extremity of the East African l~ift. Two series have been distinguished by Gibson (1972) on the basis of petrological and geochemical data, with respect to the caldera collapse and concommitant ignimbritic eruption. Both series are found to be produced by alkali feldspar fractionation and display distinct correlation on the Y - Z plot (see Fig. 5B), the postcaldera suite beeing less peralkaline (Z/ Y --0.53) than the precaldera suite (Z/Y =0.60). The precaldera serie is very similar to the Boina one, although a slight enrichment in sodium is noticed in the most fractionated liquids. The thermal valley is therefore drawn within slight negative slope. The post-ealdera sequence is slightly more potassic than the precaldera sequence. 5) Pantelleria: The pantelleritic sequence of Villari (1974), is a strongly peralkaline serie (Z/Y =0.68). The data from Carmichael (1962), and ~qicholls and Carmiehael (1969), include several feldspar, glass and whole rock analyses. K/ (NECK) decreases along the fraetionation curve, so that the thermal valley must have a negative slope, as opposed to the less peralkalinc systems and rock series. 6) Mencngai--Kenya--provides a possible example of pantelleritic trachytes (Z/Y-~ 1.1) produced by alkali feldspar fractionation, in contrast with the interpretation proposed by MacDonald et al. (1970)3. The fraetionation curve trends 2 l~IaeDonald e~al., (1970) as well as MacDonald and Baitley (1973) may have used a biased method in treating rocks from various environments as a single unit and selecting only the most glassy samples.

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drastically towards more sodic compositions. This is consistent with the fairly potassium rich composition of the anorthoclase (Or40 mole-%). In this case it is not clear wether the thermal valley is approached from the sodic or potassic side of the system. If the first case applies then the thermal valley can be tentatively drawn as it is in Fig. 5. Yet in either case it will probably have a negative slope. Discussion

The preceeding description showed that the variations of the slope of the thermal valley are analogous in the natural series and the experimental systems, changing from positive to negative with increasing degree of peralkalinity (Z/Y). In particular cases such as Boina this valley is nearly parallel to lines of iso-K/(Na-kK) ratio. Strong Na enrichment with respect to K, controlled by a thermal valley of negative slope occurs only for the most peralkaline series. However we must point out the differences between natural and experimentally investigated systems. Whereas the prevailing confining pressure is unambiguously known in the experiments (PH,o ~ 1 kb), this is generally not the case in any of the natural rock series. Yet let us mention that in contrast with these experimental conditions, pantellerites are generally thought to be " d r y " rocks rich in hallogens (Nicholls and Carmichael, 1969; Barberi et aI., 1974a). The variations of the liqnidus surface and the feldspar-magma equilibrium relationships under variing confining pressures are to the best of our knowledge not accurately known. Preliminary experiments by J. T. Iiyama (personal communication) examplifie this point showing that the alkali feldspar minimum is shifted towards K-rich compositions with an alkali chloride bearing vapour if compared with the system studied by Bowen and Tuttle (1950) under PH,O ~ 1 kb. Beside it is realistic to assume that the confining pressure differed widely between the various rock series belonging to different types of geological environments (oceanic island, continental or oceanic rifts etc.). This factor may easily account for the differences in the location of the alkali feldspar minimum (which does not depend on the Y/Z ratio) between the experimental systems and the natural rock series. Conclusions

The choice of a suitable projection plane enables us to draw fractionation curves in a set of peralkaline systems using the available experimental and natural data. Several natural rock series have been found whose evolution is consistent with the model of perfect fractional crystallisation of an alkali feldspar. The peralkalinity (Z/Y) plays a fairly similar role in both natural and experimental systems. I t changes the orientation and the shape of the thermal valley so that the final products are the more sodic, and the fractionation curves converge the quicker towards the unique fractionation curve, as the serie is the more peralkaline. The present study points to a great variety of natural magmatic liquid-feldspar equilibrium relationships within peralkaline oversaturated rocks. I t is the authors opinion that the variations of the position of the thermal valley estimated in the various series studied is not fortuitous but reflects variations in both the major

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o x y d e c o m p o n e n t s a n d confining pressure. T h e n e e d for i n v e s t i g a t i o n of o t h e r volcanic series l e a d i n g to p e r a l k a l i n e m a g m a s , w i t h a d d i t i o n a l f e l d s p a r liquid a n a l y s e s emerges. Because p a n t e l l e r i t e s are c e r t a i n l y n o t w a t e r s a t u r a t e d rocks as o p p o s e d to t h e e x p e r i m e n t a l s y s t e m s discussed, i t is desirable to achieve e x p e r i m e n t s u n d e r alkali h a l i d e p a r t i a l pressures, a n d first of t h e a l k a l i f e l d s p a r join. Moreover we t h i n k t h a t l o c a t i n g e x a c t l y t h e t h e r m a l v a l l e y in a given s y s t e m is o n l y a m e n a b l e t o e x p e r i m e n t s , because one c a n n o t e x p e c t to h a v e several n a t u r a l f r a c t i o n a t i o n curves belonging t o t h e s a m e f e l d s p a r f r a c t i o n a t i o n plane. Y e t a l t h o u g h t h i s p a p e r is d e d i c a t e d t o a l k a l i f e l d s p a r f r a c t i o n a t i o n , a n d b o t h e x p e r i m e n t a l a n d n a t u r a l d a t a h a v e been t r e a t e d in t h a t view i t is n o t our a i m t o defend t h e opinion t h a t this is t h e o n l y process leading to or a c t i n g w i t h i n p e r a l k a l i n e o v e r s a t u r a t e d rocks.

Acknowledgments. The authors are very much indebted to Professor W. S. Mac Kenzie and Professor 1~. Barberi for detailed and constructive discussion of the views expressed in this paper, Dr. J. T. Iiyama and Dr. I. L. Gibson for providing unpublished data. Professor I. S. E. Carmichael is gratefully acknowledged for reviewing the manuscript. This work was partly supported by C.N.R.S.R.C.P. 341.

References Arafia, V., Badiola, E. R., Hernan, F. : Peralkaline acid tendencies in Gran Canaria (Canary Islands). Contrib. Mineral. Petrol. 40, 53-62 (1973) Bailey, D.K., MacDonald, R.: Alkali-feldspar fractionation trends and the derivation of peralkaline liquids. Am. J. Sci. 267, 242-248 (1969) Bailey, D. K., MacDonald, R. : Petrochemical variations among mildly peralkaline (commendire) obsidians from the oceans and continents. Contrib. Mineral. Petrol. 28, 340-351 (1970) Bailey, D. K., Schairer, J. F. : Feldspar liquids equilibria in peralkaline liquids; the orthoclase effect. Am. J. Sci. 262, 1198-1206 (1064) Bailey, D. K., Sehairer, J. F. : The system NaaO--A1203--Fe~Oa--SiO2 at 1 atmosphere, and the petrogenesis of alkaline rocks. J. Petrol. 7, 114-170 (1966) Baker, P.E., Buckley, F., Holland, J. G.: Petrology and Geochemistry of Easter Island. Contrib. Mineral. Petrol. 44, 85-106 (1974) Barberi, F., Ferrara, G., Santaeroce, R., Treuil, M., Varet, J.: Geochemical and petrological evidences for a fractionation origin of pantellcrites: the Boina center (Northern Afar, Ethiopia). J. Petrol. (under press) (1974a) Barberi, F., Santacroce, R., Varet, J. : Silicic peralkaline volcanic rocks of the Afar depression (Ethiopia). In: Pantellerites. Bull. Volcanol., Special Issue, to be published (1974b) Barberi, 1~., Varet, J.: The Erta'Ale volcanic Range (Afar, Ethiopia). Bull. Volcanol. 84, 848-917 (1970) Bowen, N. L.: Recent high temperature research on silicates and its significance in igneous petrology. Am. J. Sci. 83, 1-21 (1937) Bowen, I~. L. : Phase equilibria bearing on the origin and differentiation of the alkaline rocks. Am. J. Sci. 243A, 75-89 (1945) Bowen, I~. L., Turtle, O. 1~.: The system NaA1Si3Os--KA1SiaOs--H~O. J. Geol. 69, 439-460 (1950) Bryan, W . B . : Alkaline and peralkaline rocks of Socorro Island, Mexico. Carnegie Inst. Year Book 68, 194-200 (1970) Carmichael, I. S. E.: Pantelleritic liquids and their phenoerysts. Mineral. Mag. 33, 86-113 (1962) Carmichael, I. S. E. : The petrology of Thingmuli, a tertiary volcano in Eastern Iceland. J. Petrol. 5, 95-131 (1964)

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Carmichael, L S. E., MacKenzie, W. S. : Feldspar-liquid equilibria in pantellerites: an experimental study. Am. J. Sci. 261, 382-396 (1963) Cox, K. G., Gass, L. G., Mallick, D. I. J. : The evolution of the volcanoes of Aden and Little Aden, South Arabia. Quart. J. Geol. Soe., London 124, 283-308 (1969) Ewart, A., Taylor, S. R., Capp, A. C. : Geochemistry of the Pantellerites of Mayor Island, New Zealand. Contrib. Mineral. Petrol. 17, 116-140 (1968) Gass, I. C., Mallick, D. I. J.: Jebel Khariz: an upper-miocene stratovolcano of comenditie affinity on the South Arabian coast. Bull. Volcanol. 32, 33-88 (1968) Gibson, I. L. : The chemistry and petrogenesis of a suite of pantellerites from the Ethiopian Rift. J. Petrol. 13, 31-44 (1972) MacDonald, R.: Tectonic settings and magma associations. In "Pantellerites". Bull. Volcanol., Special Issue, to be published (1974) MacDonald, R., Bailey, D. K. : The chemistry of the peralkaline oversaturated obsidians. U.S. Geol. Surv. Profess. Papers 440-N, Part 1 (1973) MacDonald, R., Bailey, D. K., Sutherland, D. S. : Oversaturated peralkaline glassy trachytes from Kenya. J. Petrol. 1], 507-517 (1970) Nieholls, J., Carmichael, I. S. E.: Peralkaline acid liquids: a petrological study. Contrib. Mineral. Petrol. 28, 89-111 (1969) Nash, W. P., Carmiehael, I. S. E., Johnson, R. W. : The mineralogy and petrology of mount Suswa, Kenya. J. Petrol. 10, 409-439 (1969) Noble, D. C. : Gold flat member of Thirsty canyon tuff. A pantellerite ash flow sheet in Southern Nevada. U.S. Geol. Surv. Profess. Papers 525-B, 85-90 (1965) Noble, D. C., Smith, V. C., Peck, C. C. : Loss of halogens from crystallised and glassy silicic volcanic rocks. Geochim. Cosmochim. Acta 32, 215-224 (1967) Thompson, 1%.N., MacKenzie, W. S. : Feldspar-liquid equilibria in peralkaline acid liquids: an experimental study. Am. J. Sci. 265, 714-734 (1967) Treuil, M., Varet, J. : Crit~res p~trologiques, g~ochimiques et structuraux de la gen~se et de la differentiation des magmas basaltiques, exemple de l'Afar. Bull. Soc. Geol. France 15, 506-540 (1973) Turtle, O.F., Bowen, N.L.: Origin of granite in the light of experimental studies in the system NaA1SiaOs--SiO~--H~O. Geol. Soe. Am. Mem. 74, 153 (1958) Villari, L. : The island of Pantelleria. In "pantellerites ". Bull. Voleanol., Special Issue, to be published (1974) Weaver, S.D., Sceal, J. S. C., Gibson, L L.: Trace element data relevant to the origin of trachytic and pantelleritic lavas in the East African Rift System. Contrib. Mineral. Petrol. 36, 181-194 (1972) Dr. J. Roux Centre de Recherehes sur la Synth~se et Chimie Min6raux Rue de la Ferollerie F-45045 Orl6ans-Cddex France

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