PRIMARY PETROGRAPHIC AND MINERALOGIC FEATURES OF PILLOW BASALTS

In this section we describe and summarize the primary mineralogic and petrographic features of the Leg 168 pillow basalts. For a more detailed description see the Leg 168 Initial Reports (Davis, Fisher, Firth, et al., 1997).

Pillow basalts were recovered at the basement-sediment interface at nearly all sites (Sites 1023-1029 and 1031-1032; Table 1). Shorebased whole-rock major element analyses, together with shipboard analyses aboard the JOIDES Resolution (Davis, Fisher, Firth, et al., 1997) and shorebased analyses of other components of the Leg 168 Scientific Party (M. Constantin, pers. comm., 1998), indicate that most of the igneous rocks are low-K tholeiites. Four distinct geochemical groups have been distinguished on the basis of Mg#:

  1. primitive pillow basalts from Site 1023 and from the lower subunits at Sites 1027 and 1032 (Mg# = 66-63);
  2. slightly more fractionated pillow basalts from the upper subunits of the pillow lava sequence at Sites 1027 and 1032 and the pillow basalts from Sites 1024, 1026, 1028, and 1031 (Mg# = 63-56);
  3. fractionated ferrobasalts from Site 1029 (Mg# = 53-40); and
  4. highly fractionated ferrobasalts from the upper pillow unit overlying the massive lava sequence at Site 1025 (Mg# = 50-45).

The pillow basalts consist of aphyric (<1% phenocrysts) to moderately phyric (2%-10% phenocrysts) plagioclase ± olivine ± pyroxene or olivine ± plagioclase basalts and show a low degree of vesicularity (<2%), with vesicles having diameters generally <1 mm.

Pillows commonly exhibit a distinct concentric structure as a consequence of the fast cooling conditions, which progressively decreases toward the pillow interior. From margin to core, five main zones (Fig. 2) have been distinguished, in good agreement with the previous observations of other authors (e.g., Baragar et al., 1977; Scott and Hajash, 1976; Juteau et al., 1980; Natland, 1980, Davis, Mottl, Fisher, et al., 1992): (1) glassy zone, (2) variolitic zone, (3) subvariolitic zone, (4) spherulitic zone, and (5) holocrystalline pillow core. Very small concentric cracks commonly mark the transitions from Zones 1 to 4, whereas the transition from Zone 4 to 5 is always graded without any sharp boundary.

  1. The glassy zone consists of pale yellow to very light brown isotropic glass with scattered olivine microlites, plagioclase microlaths, and rare olivine ± plagioclase phenocrysts (0.1-0.4 mm). The observed thickness in the recovered samples varies from <1 to 4 mm, but most of the glassy zones could have been broken up during the drilling operation. Nevertheless, only one section from Hole 1026B (Section 168-1026B-3R-1) consists of a strongly altered basalt-hyaloclastite breccia, containing angular clasts of aphyric basalt (with a variolitic to subvariolitic texture) and glassy shards, set within a consolidated clays ± zeolites ± carbonates matrix. The texture throughout the breccia varies from matrix supported to clast supported, and the dimension of the clasts generally is 6 mm.
  2. The variolitic zone, 1-3 mm thick, consists of scattered ovoid brown varioles (0.03-0.08 mm in the longest dimension) isolated in the fresh isotropic glass. Varioles consist of a thin rim of fibrous anisotropic minerals (SEM analyses indicate that they probably correspond to intimate intergrowths of plagioclase and pyroxene fibers) centered on a plagioclase microlites.
  3. The subvariolitic zone, 1-2 mm thick, is characterized by the progressive increase in the variole number and size from the outer margin towards the pillow interior. As the variole number increases, they progressively coalesce and the glass remains only in interstitial scattered areas.
  4. The spherulitic zone, 3-8 mm, is marked by the complete coalescence of the varioles that commonly form a dense polygonal net (honeycomb textures). The transition to the holocrystalline pillow interior (Zone 5) is marked by the progressive disappearance of varioles and by the development of different and various types of quench textures, such as plumose, branching, or sheaf spherulitic, indicating different nucleation and growth mechanisms mainly as a consequence of progressively slower cooling rates (McKenzie et al., 1982).
  5. In the pillow interior, the groundmass textures change from glomeroporphyritic, to intersertal, and finally to intergranular as the distance from the spherulitic zone increases. Simultaneously, the abundance of undifferentiated mesostasis areas decreases, and, when present, they occur in scattered, variably shaped, and irregularly distributed areas.

Euhedral plagioclase crystals (up to 1.5-1.7 mm in size) are the most abundant phenocryst species, often forming glomeroporphyritic aggregates; conversely, submillimetric euhedral to subhedral clinopyroxene and olivine phenocrysts (the latter completely replaced by secondary phases in almost all sites) commonly occur as isolated crystals or in bimineralic clots associated with plagioclase.

The groundmass mainly consists of plagioclase laths and clinopyroxene micrograins with minor olivine, variable amounts of opaque oxides, and rare primary sulfides (pyrite, pyrrhotite, and chalcopyrite). Apatite whiskers commonly occur within or around plagioclase phenocrysts and laths.

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