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G. C. Smith*, M. B. Holness & J. M. Bunbury
Department of Earth Sciences, Downing Street, Cambridge, CB2 3EQ
Running title: Interstitial magmatic scapolite
*corresponding author email address: email@example.com
Keywords: magmatic scapolite, interstitial, nodules, Kula
Scapolite oikocrysts, up to 3 cm diameter, occur in cognate, cumulate nodules from the 2 Ma rift-related, alkali-basaltic, Kula Volcanic Province in Western Turkey. Three scapolite-bearing nodules were found within deposits originating from two adjacent cones, and contain primocrysts of clinopyroxene, biotite, apatite, and sphene, together with interstitial kaersutite, plagioclase and scapolite.
Kula scapolites fall mid-way between the meionite (Ca4[Al6Si6O24]CO3) and marialite (Na4[Al3Si9O24]Cl) end-members (~ Me45-67).
We believe that this is the first report of interstitial magmatic scapolite.
Scapolite is largely restricted to metamorphic and metasomatic environments. Shaw (1960 a, b) provides an extensive summary of the scapolite literature, including many examples from the Precambrian Grenville Province of Canada and occurrences in altered gabbro, marble, skarn, nepheline syenite, corundum syenite and iron ore deposits. Magmatic scapolite is rare and only previously reported as phenocrysts. These include megacrysts in alkalic tephra from the Massif Central (France) (Boivin & Camus, 1981), phenocrysts from a latite dome in Arizona (Goff et al., 1982) and megacrysts from a monchiquitic dyke from the West Greenland Tertiary igneous province (Larsen, 1981). Here we report the first known occurrence of scapolite as an intercumulus primary igneous phase.
P-T conditions of scapolite stability
The composition of scapolite may be expressed by the general formula W4Z12O24.R (Shaw, 1960a) where W = Ca, Na, K; Z = Si, Al; R = Cl, CO3, SO4 (and minor OH). Marialite (Na4[Al3Si9O24]Cl) and meionite (Ca4[Al6Si6O24]CO3) are the principal end-members, though naturally-occurring end-member compositions are unknown: the majority of reported compositions range from Me20 to Me85. Scapolites from the granulite facies tend towards the meionitic (CaCO3-rich) end-member (Rebbert & Rice, 1997), as do those associated with carbonate metasomatism (such as the skarn xenoliths of Vesuvius (Shaw, 1960a)). Marialite is more common in hydrothermal environments, where hot brines cause pseudomorphism of pre-existing plagioclase (Vanko & Bishop, 1982).
The pure end-member marialite has been synthesized (with seeding) dry at 1 atm between 700 and 850 oC (Deer et al., 1992). Meionitic s
capolite crystallises under a high CO2 partial pressure, at temperatures above ~ 850 oC (Goff, 1982; Goldsmith & Newton, 1977; Millhollen, 1974) and pressures above 3 kbar (Millhollen, 1974). Another end-member, sulphur-meionite (Ca4[Al6Si6O24]SO4), also referred to as silvialite (Teerstra et al., 1999), is associated with magmatic environments ( e.g. Boivin & Camus, 1981; Goff et al., 1982). This sulphur-rich scapolite also forms at elevated temperatures [Newton & Goldsmith, 1976], and cannot form at pressures below 5 kbar (Goff et al., 1982). Although most magmatic scapolite is sulphur-rich (Boivin & Camus, 1981; Goff et al., 1982), a sulphur-poor example has been described from Greenland (Larsen, 1981).
In their 1977 paper, Goldsmith & Newton noted that “scapolite .. can probably precipitate as a primary igneous mineral from a carbonate- or sulfate-bearing basic or intermediate magma if the pressure is high enough” and “although scapolite is generally a metamorphic mineral, the highly Ca-rich scapolites require such extreme conditions that if compositions approaching the pure end-members were to be produced in crustal rocks, even in the deep crust, they would probably be formed as primary magmatic minerals. All that would be required for direct crystallization from the melt would be an adequate amount of C02 or oxidizable sulfur.”
Two monogenetic, alkali-basaltic cinder cones in the Kula Volcanic Province of western Turkey (Fig. 1), have erupted glassy, crystalline nodules that contain interstitial scapolite which we believe to be magmatic in origin on the basis of textural observations.
Geology of the Kula Volcanic Province
Volcanism and basement lithology
The Kula Volcanic Province of western Turkey (Fig. 1) comprises ~ 80 small (up to ~ 100 m high) monogenetic cinder cones, within an area of approximately 300 km2 (Richardson-Bunbury, 1992; 1996). The erupted magmas (~ 2.3 km3 total volume, (Richardson-Bunbury, 1992; 1996)) sit on a regional metamorphic basement of cherts, serpentinites, garnet-mica schists, gneisses and marbles of the Menderes Massif (Fig. 1c). The marble bands vary in thickness between 0.5 m and 30 m, and the village of Incesu is built on one such band (Richardson-Bunbury, 1992; 1996). Xenoliths from cinder cones commonly comprise these rock types. Marble xenoliths show extreme reaction with the host magma, and schist fragments contain abundant partial melt (Bayhan et al., 2006).
Kula lies to the north-east of the Gediz graben, part of a series of approximately east-west trending grabens in western Turkey formed during regional north-south extension. The degree of extension at the Gediz graben is small, with a β-value of 1.2-1.3 (Paton, 1992). The bounding faults extend to a depth of approximately 10 km (Paton, 1992). Saunders et al. (1998) determined a crustal thickness of 30 km and a total lithosphere thickness of 80 km in the Kula area. A low velocity (3.25 km s-1) zone, 4-5 km thick, is also inferred to exist between 15 and 20 km below the surface (Saunders et al., 1998), and is likely to be related to either a batch of older, andesitic magma (Şengör & Yilmaz, 1981) or to a sill complex (Holness & Bunbury, 2006).
The alkali-basalt volcanic rocks of Kula are the youngest rift-related lavas in western Turkey, and are generally well exposed and fresh. K-Ar dates for the three main groups of cones in the region (Erinç, 1970; Ercan, 1981) show that the volcanism has migrated south-westward towards the Gediz graben over the last 2 million years. The two most recent phases of activity, β3 and β4, are dated at ~ 300 ka and 30 ka respectively (Richardson-Bunbury, 1992). All the lavas are volatile-rich; bread-crust bombs are common on the scoriaceous flanks of cinder cones, and a number of flows in the province contain hornitos. The basalts contain clinopyroxene (~ Wo45En45Fs10), olivine (~ Fo80) and kaersutitic amphibole phenocrysts (with variable extents of low pressure breakdown) in a fine-grained groundmass of labradoritic plagioclase, euhedral clinopyroxene and Ti-magnetite (Alıcı et al., 2002).
crystalline nodules, ranging in size from 5 mm to 24 cm in diameter are common in the Kula Volcanic Province (Holness & Bunbury, 2006). Nodules are abundant toward the base of the cinder cones and appear to have been ejected during the gas-rich, cinder-forming phase of eruption. Larger nodules (longest axis > 12 cm) are restricted to the lava flows emanating from the cinder cones. The nodules are rounded to highly angular. Some have ‘jackets’ of vesicular lava, and where observed, the contact between nodule and lava is commonly sharp. The principal phases in the nodules have been described by Holness & Bunbury (2006), and are acicular kaersutitic amphibole (0.05 – 20 mm length) and clinopyroxene (< 2 mm) (~ Wo49En40Fs11), with rare interstitial plagioclase (An17), acicular apatite and olivine (< 5 mm) (Fo83-85). The modal proportions (Fig. 2) of these minerals are variable, from highly porous nodules containing framework-forming primocrystic amphibole with abundant intercumulus vesicular glass, to glass-poor nodules richer in clinopyroxene with intercumulus plagioclase or scapolite. The nodules are believed to represent fragments of a partially solidified crystal mush from the margins of the subvolcanic conduits and/or chambers at pressures of 2-15 kbar, with the low porosity, clinopyroxene-rich samples interpreted to have come from deeper in the crystal pile (Holness & Bunbury, 2006). Scapolite is associated only with clinopyroxene-rich examples (Fig. 2).
A suite of over 350 cognate nodules from the Kula Volcanic Province has been collected with each nodule linked to a specific cone or flow by location. A cluster of seven cones 3 km to the north-east of the town of Kula, here called the “Incesu Group” (Fig. 1) after the nearest village, has a much greater population of nodules than across the remainder of the volcanic field. These cones form part of the 300 ka β3 phase of activity (Richardson-Bunbury, 1992). Three of the nodules from the Incesu Group contain scapolite. Sample K96-056 was found on cone 51B (cones numbered according to the scheme of Richardson-Bunbury, 1992), which may be a parasitic vent on the flanks of cone 51, and samples GS1-51C-01 and K96-053B were found in the lava flow from cone 51C, a larger cone with a basal diameter of ~ 1 km. These two cones are adjacent, with summits less than 1 km apart (Fig. 1).
All electron micro-probe analyses were performed on a Cameca SX-100 instrument in the Department of Earth Sciences, University of Cambridge. Standard analytical conditions for scapolite were 15kV accelerating voltage and a 10 μm beam diameter. Dual beam conditions allowed two different currents (4 nA and 40 nA) to be used to minimise errors induced by the current when analyzing certain elements (e.g. Na). Operating conditions for other mineral phases were a 5.5 μm beam diameter and currents of 10 nA and 100 nA. Scanning electron microscope photographs were taken on a JEOL JSM at the Department of Earth Sciences, University of Cambridge, using a 15 kV accelerating voltage and working distance of 15 mm.
The three nodules contain clinopyroxene, scapolite, amphibole, biotite, glass, plagioclase, apatite, and sphene, with 1-2 vol. % vesicles and voids (Fig. 2).
Representative chemical analyses of scapolite and other phases from cones 51B and 51C are given in Table 1. Scapolites from cone 51B are more meionitic (typically around Me70) than those from cone 51C (~ Me50) (Table 2).
Primocrystic clinopyroxene, up to 5 mm in length, has a green straw pleochroism, commonly with pale-coloured rims. It displays slightly anomalous extinction colours in sample K96-053. Isolated grains show planar growth faces and may be euhedral, but sintered clumps have 120o angles at pyroxene-pyroxene-pyroxene triple junctions. There is ubiquitous patchy, topotactic replacement of the clinopyroxene by amphibole (Figs. 3c, d), predominantly in the grain centres. Clinopyroxene-clinopyroxene-apatite triple junctions show a wide dihedral angle (approaching 160o) associated with constant mean curvature. Clinopyroxene-clinopyroxene-scapolite junctions also show high dihedral angles (Fig. 4b), although on closer inspection the actual junction contains a void, with a low pyroxene-pyroxene-void angle. A second clinopyroxene generation forms topotactic overgrowths (up to 300 μm thick) on primocrystic clinopyroxene, sphene, biotite and amphibole (Fig. 4f) where in contact with glass. This later clinopyroxene overgrowth is commonly rich in opaque inclusions. Clinopyroxene overgrowths are common in samples K96-056 and GS1-51C-01, but extremely rare in K96-053.
The apatite primocrysts are up to 1 mm long in sample K96-056 and up to 3 mm in the other samples. They are rarely enclosed by pyroxene grains or scapolite. Apatite-apatite-scapolite dihedral angles are high.
Amphibole forms dark brown, turbid, highly lobate grains interstitial to the clinopyroxene primocrysts. It may also form oikocrysts up to 9 mm in diameter, enclosing clinopyroxene, biotite and in one instance, plagioclase. Amphibole also forms overgrowths on biotite grains.
Scapolite is an abundant interstitial phase in these samples, forming oikocrysts up to 12 mm across (e.g. Figs. 3c, d, e, 4b), enclosing clinopyroxene and apatite, and forming between 9 and 18 % of the rock. Scapolite grains commonly have a film of glass at their margins (e.g. Figs. 3a, b), 5-500 μm thick. The contact between scapolite and primocrystic clinopyroxene is sharp (Figs. 3a, e, 4b, c), and commonly marked by a thin film of vesicular glass or voids. The interior of the scapolite oikocrysts is mostly turbid due to trails of minute fluid inclusions (<<1 μm). Edges of scapolite oikocrysts, where in contact with glass, commonly have small-scale vermicular intergrowths with the glass or a complex network of voids (Fig. 4a). Scapolite is intergrown with sieve-textured plagioclase toward the edge of oikocrysts, particularly near triple-junctions with two primocrysts (Fig. 4e).
Glass is ubiquitous (Figs. 3a, b) and forms thin (< 20 μm) films on most grain boundaries, and larger, highly vesicular pockets. The large pockets (particularly in K96-053) may contain abundant microcrysts of plagioclase (Fig. 4d). Glass-lined cracks link most of the glassy pockets in K96-053. Small, euhedral crystals of clinopyroxene, apatite and amphibole (< 100 μm) are present in larger pockets of glass.
Biotite occurs in samples K96-053 and GS1-51C-01. Both samples contain biotite grains interstitial to clinopyroxene, but K96-053 also has biotite oikocrysts up to 4 mm in diameter, enclosing clinopyroxene and apatite. Biotite internally replaces clinopyroxene in sample K96-056.
Partially melted plagioclase is intergrown with interstitial scapolite (Fig. 4e) . There is no evidence for any reaction between the plagioclase and scapolite. There are several examples of plagioclase growing with well-defined, planar faces (Fig. 3d). Very fine plagioclase needles are also found between grain boundaries in association with glass (Fig. 4d).
Voids are present at triple junctions and along many grain boundaries, particularly clinopyroxene-scapolite boundaries. The edge of clinopyroxene grains is commonly planar (Fig. 4c), while the scapolite margin is undulose. Gaps between grain boundaries are commonly up to 50 μm wide, and in some cases, both sides of the gap have a complementary, “jigsaw-fit” morphology. Highly vesicular melt containing elongate plagioclase crystals up to 50 μm long is present in many of these areas (Fig. 4d).
Table 1 shows average compositions of the main phases present in the scapolite-bearing nodules from Kula. Mineral compositions from samples GS1-51C-01 and K96-053B (i.e. those that came from cone 51C) are similar. Sample K96-056, which came from cone 51B, has different mineral chemistry from the cone 51C samples.
Structural formulae for scapolite have been calculated (Table 2) to highlight the differences between the two cones. Cone 51B scapolite is more meionitic and also more sulphur-rich than cone 51C.