RESULTS

The dredged spinifex-textured xenolith from Satanic Mills (CSIRO sample 142421) (Fig. F1A) is enclosed within perfectly glassy, aphyric dacite with numerous vesicles of several size generations. It is disk shaped, ~3 cm thick, and 20 cm in diameter. Its margins are irregular as a consequence of wedging apart by the dacite host. Although there is no fracturing apparent within the xenolith, it was evidently comparatively fluid at the time of incorporation. The quench structure described below persists across the whole xenolith and is not just a marginal feature. Numerous tiny miarolitic cavities and some larger, spherical vesicles are prominent on the hand specimen.

The xenolith fabric (Fig. F2) is dominated by randomly oriented, thin blades of olivine (Fo_72–80) up to 1 cm in length, with hollow cores filled by pyroxene and plagioclase and locally with skeletal margins (Fig. F3). These constitute only ~5% of the volume. Polygonal spaces between the olivine blades are filled by radiating aggregates of subparallel pyroxene blades (Figs. F2, F4), between which, in turn, are small, simply twinned laths of plagioclase (An_61–67). Calcic clinopyroxene is predominant, with compositions varying little from Ca₃₉Mg₅₀Fe₁₁ and containing 0.6 wt% TiO₂, 1.8 wt% Al₂O₃, and 0.2 wt% Cr₂O₃. It is accompanied by rarer, more prismatic crystallites of orthopyroxene (Ca₃Mg₇₈Fe₁₉) containing 0.3 wt% TiO₂, 0.9 wt% Al₂O₃, and 0.1 wt% Cr₂O₃. The interstitial plagioclase laths commonly project into miarolitic microcavities (Fig. F4) or extend as circumferential linings on vesicles (Fig. F5) in a manner suggesting that rapid crystallization of the volatile-rich mafic melt has progressed into a supercritical hydrothermal stage. Because the monomineralic plagioclase outgrowths and linings contain no glass or mafic phases, an origin by gas filter-pressing or vapor differentiation (Goff, 1996) is less likely. Tiny crystals of chrome spinel (Al₂₉Cr₅₉Fe³⁺₁₂; Mg/[Mg + Fe²⁺] = 35%) and ilmenite are scattered through the interstices (Figs. F2, F4), and hercynite spinel is a rare accompaniment of plagioclase. Subhedral crystals of pyrite (Fig. F2) are a rare and unexplained constituent.

At the xenolith margins, skeletal laths of plagioclase project out into the dacite glass (Fig. F6). Although crystallized from the dacite melt, many of these appear to be epitaxial outgrowths from finer plagioclase between pyroxenes in the adjacent xenolith.

The altered spinifex-textured xenolith revealed by sawing ODP Sample 193-1188A-7R-2 (Piece 1, 17–21 cm; CSIRO 142652) is also discoidal, 1.2 cm across, and at least 4 cm in diameter (Fig. F1B). It is contained within clay-altered, formerly glassy dacite, dissected by numerous cracks adjacent to which the host rock is greenish gray in color for several millimeters. The kernels of larger host remnants between these cracks are very pale green to white in color and possess relic perlitic microfabrics (Fig. F1B). Microscopically, the color changes are associated with Liesegang-like zoning inward from the cracks (Fig. F7). Many but not all of the cracks have been reopened and filled with a succession of cristobalite then cristobalite-anhydrite veins with minor pyrite (Fig. F7), the latter also cutting across the xenolith (Fig. F1B).

The altered xenolith in Sample 193-1188A-7R-2 (Piece 1, 17–21 cm; CSIRO 142652) contains conspicuous dark, randomly oriented blades rich in pyrite (Fig. F1B), not quite so abundant as the olivine blades of the fresh spinifex-textured xenolith (CSIRO dredge sample 142421) but clearly their equivalent. In detail, the fine-grained pyrite replacing the blades is accompanied by clay-chlorite and some fine-grained anhydrite, whereas clay-chlorite in the spaces between blades exhibits a pseudomorphous structure resembling the radiating pyroxene aggregates of the fresh xenolith (Fig. F8). This arises from the distribution of submicroscopic inclusions, probably of rutile, and from slight grain size differences in the clay-chlorite. The matrix is also replaced by abundant anhydrite, ranging from large round grains to dusty particles, and by scarcer pyrite as both larger subhedra and tiny grains (Fig. F8).

A millimeter-wide rim of paler clay-chlorite with finely disseminated anhydrite at the contact between the altered xenolith and its host (Fig. F9) retains a microfabric highly reminiscent of the epitaxial plagioclase outgrowths seen at this position in the fresh sample. A few more cloudy, needlelike pseudomorphs within this suggest some pyroxene accompanied the plagioclase.

Compositions of the spinifex-textured xenoliths and their hosts are provided in Table T1. The fresh xenolith (142421B) is basaltic with a high MgO content (13.4 wt%), not sufficient to apply the term komatiite (Arndt and Nisbet, 1982) despite the similar quench fabric. In major and trace element compositions, it conforms to but extends the closely constrained fractionation trend displayed by Pual Ridge lavas ranging from basaltic andesite to rhyodacite in composition (Binns et al., 2002), and its glassy dacite host (142421A) also conforms to that trend.

The altered spinifex xenolith (142652D) is significantly depleted in MgO but retains elevated V, Cr, and Ni relative to the fresh rock. High Ca and S reflect the abundance of anhydrite. The two subsamples of brecciated host rock, hand picked free of obvious veins from the white kernel (142652C) and greenish gray rims (142652E), are virtually identical in composition. Their Zr/Ti ratios indicate a dacite parent comparable with the host glass of the fresh sample.

Figure F10 plots gains and losses of particular elements during alteration relative to the appropriate portion of the fresh rock. This ignores the effects of volume changes and hydration but clearly shows extreme enrichment in S and Te during alteration of all three altered samples and substantial depletions in alkali elements, Mn, and Mg in most.

Figure F11 displays the differing behavior of the rare earth elements (REE). The flat chondrite-normalized REE pattern of the fresh spinifex xenolith compares closely with other basaltic rocks from the eastern Manus Basin, while the dacite glass shows the higher abundances and the light REE enrichment characteristic of the more fractionated lavas. Relative to its fresh equivalent, the altered spinifex-textured xenolith has similar light and mid-REE abundances but mildly depleted heavy REE. The two host rock samples have very different REE characteristics. The white kernel (142652C) shows pronounced overall reduction in abundance, with La and Ce being slightly more depleted relative to the mid-REE and a slightly greater progressive depletion in the heavier REE. The green rim material (142652E), despite its otherwise geochemical similarity with the white kernel, shows a greater level of overall depletion in REE and pronounced relative loss of the light REE from La to Nd.