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Intercumulus texture and glass association
Several lines of evidence point to the primary magmatic nature of the scapolite.
Large oikocrystic grains of scapolite in the Kula nodules are in optical continuity. The association with fresh magmatic glass precludes scapolite growth as a hydrothermal replacive phase (e.g. Vanko & Bishop, 1982).
The intimate mingling of glass and scapolite at the oikocryst margins suggests remelting of the scapolite as a result of entrainment in the Kula basalts. The sieve-textured plagioclase associated with the scapolite is also indicative of heating (Nakamura & Shimakita, 1998; Tsuchiyama, 1985; Tsuchiyama & Takahashi, 1983) or decompression (Nelson & Montana, 1992).
There is also strong evidence for significant subsequent evolution of the primary magmatic texture. The patchy transformation (uralitisation) of clinopyroxene to amphibole is interpreted to be the result of residual melt becoming sufficiently volatile-rich to stabilize primary amphibole rather than pyroxene. Other explanations for this phenomenon have been proposed, including formation as a result of heating and net hydration during prograde metamorphism (Fagan & Day, 1997) and solid solution between pyroxene and amphibole (Smith, 1977). The presence of scapolite, amphibole and biotite in these clinopyroxene-rich nodules, in addition to the bread-crust bombs and hornitos seen on the surface, may also be indicative of a large dissolved volatile component in the early melt. Chambers and Brown (1995) attribute uralitisation in the Lilloise Intrusion, Greenland, to an aqueous vapour phase present at a late stage in the magma’s evolution. The Lilloise Intrusion is chemically similar to the magmas seen in the Kula Volcanic Province, and can be considered to be a plutonic equivalent.
Dihedral angles formed by the infilling of primary porosity by interstitial phases in a crystal mush are generally low, as the interstitial phase inherits the shape of the pores formed by impingement of generally planar-sided framework-forming grains (Holness et al., 2005). The median value of such a population of inherited angles is ~ 60˚. However, if given opportunity, the grain boundaries in the vicinity of these junctions will change in the sub-solidus, driven by a minimization of internal energies, towards solid-solid textural equilibrium. The change from the initial magmatic texture towards sub-solidus textural equilibrium involves an increase in dihedral angle towards ~120 o (Vernon, 1968) which occurs by the rotation of large areas of the grain boundary like an opening book (Holness et al., 2007b). The establishment of the equilibrium sub-solidus dihedral angle creates a change in curvature of the grain boundaries near the junction, and this change then propagates outwards, establishing a new, constant mean curvature (Holness et al., 2005; Higgins, 2006).
Clinopyroxene-clinopyroxene-scapolite triple junctions display high dihedral angles, with propagation of the associated grain boundary curvature some tens of microns away from the junction. Similar features have been described for an olivine cumulate from the Eastern Layered Intrusion of the Isle of Rum (Holness et al., 2007b) (Fig. 3f). These features denote significant time spent at high temperatures in the absence of either reaction or deformation, consistent with a primary magmatic origin for the scapolite. Importantly they also suggest that the voids now present at many pyroxene-pyroxene-scapolite junctions and on pyroxene-scapolite grain boundaries are secondary.
Voids are present at both three-grain and two-grain boundaries.
The voids at clinopyroxene-clinopyroxene-scapolite triple junctions must be secondary, as the constant mean curvature of the clinopyroxene-scapolite grain boundaries requires that scapolite originally occupied the pore corner between the two clinopyroxene grains. The voids have therefore appeared after subsolidus textural equilibration.
At two-grain boundaries, scapolite-void contacts are generally not planar, and show highly undulose boundaries (Fig. 4c). The close association with glass suggests that the voids formed during decompression, when expansion of volatiles (due to coupled exsolution and gas expansion) drove remaining interstitial, reactive melt into opening grain boundaries.
Sample GS1-51C-01 contains fine, elongate plagioclase grains in vesicular glass along grain boundaries (Fig. 4d). We interpret these plagioclase crystals, which are on the order of 50 μm in length, to have grown from a devolatilising melt migrating along expanding grain boundaries during entrainment and ascent.
The average clinopyroxene composition (Wo50En37Fs13) of the scapolite-bearing nodules from cone 51C is similar to the mean composition of clinopyroxenes in non-scapolite-bearing nodules from other cones in the province (Wo49En40Fs11) (Fig. 5). Sample K96-056 (cone 51B) has a mean clinopyroxene composition of Wo51En29Fs20. Differences between the clinopyroxene compositions of the scapolite-bearing nodules from cones 51B and 51C suggest that the site of clinopyroxene crystallization is likely to have been different between the cones.
Previously described magmatic scapolite generally has a high proportion of SO42- in its structure (e.g. Boivin & Camus, 1982; Goff et al., 1982), although Larsen (1981) discovered a primary scapolite megacryst from West Greenland with a sulphur content of 0.04 wt %, showing that a high sulphur content is not a prerequisite for the crystallization of primary magmatic scapolite. The Kula scapolites have a maximum sulphur content of < 0.5 wt % (Table 2). Figure 7 shows the S and C contents of the other reported magmatic scapolites compared with those from this study.
The considerable difference in meionite content over a small surface distance (~ 1 km) suggests that the two scapolite-bearing nodules did not crystallise from the same melt. Assimilation of a small amount of the marble country rock by the magma which fed cone 51B could explain the more meionitic character (Table 2) of the scapolite compared to that from the neighbouring cone, 51C. Marble accounts for a significant proportion of the basement near cones 51B and 51C (Fig. 1b), but assimilation will only be a valid cause of chemical disparity if the marble extends to mid / lower crustal depths. We consider this improbable, as the graben bounding faults only extend to a depth of around 10 km (Paton, 1992). The chemical differences between the nodules from the two cones may simply be an artifact of low-volume melting in the heterogeneous upper mantle or due to slight variations in the fractionation path of the melts.
It is clear however that scapolite is rare and is stabilised under conditions which are not satisfied in most parts of the sub-volcanic domain.
The typically interstitial texture of scapolite oikocrysts in glass-bearing crystalline nodules from the Kula Volcanic Province provides strong evidence for a primary magmatic origin within intercumulus liquid. We believe this to be the first reported example of interstitial magmatic scapolite. High dihedral angles at triple-junctions point to significant sub-solidus modification of primary magmatic textures, consistent with the rocks remaining at high temperature for a sustained period. The oikocryst interiors are free of relict plagioclase, precluding formation by pseudomorphism of intercumulus plagioclase. The well-formed crystal faces of plagioclase adjacent to the scapolite also preclude pervasive alteration. Unusually for a magmatic scapolite, the Kula examples are sulphur-poor, with compositions similar to a scapolite phenocryst described by Larsen (1981).
GS is supported by a NERC studentship. We are grateful to Andy Buckley, Chris Hayward and Chiara Petrone for assistance with the electron microprobe and David Vanko for the initial identification of scapolite. Ian Marshall assisted in obtaining SEM images. We also thank Andy Buckley for introducing us to his Formula Recalculations program. Thanks to Murat Yilmaz in Kula for his help with the Turkish language, and Judy Baker, Victoria Martin, Francis Nimmo and Nigel Smith for assistance in the field. Earlier versions of this manuscript were greatly improved as a result of Madeleine Humphreys’ careful reviews.
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