In Hole 1256C, where an almost complete succession of the lava pond was recovered, the basalt is a macroscopically homogeneous, massive microcrystalline to fine-grained lava with a coarse variolitic to intergranular texture, apart from the uppermost 0.7 m and the basal 1.7 m. Neither the top nor the base of the lava pond were recovered in Hole 1256D. Although Unit 1256D-1 is more than twice as thick as Unit 1256C-18, the lava is similar to Unit 1256C-18 both in hand specimen and under the microscope, consisting of microcrystalline to fine-grained basalt with similar groundmass textures. The basalt lava has 0.5–11 vol% olivine with only traces of plagioclase and clinopyroxene as phenocrysts. The groundmass consists of plagioclase and augite with a subordinate amount of magnetite and a small amount of pigeonite and with or without glass. Besides the folded surface crust of the lava pond in the uppermost 0.7 m and the recrystallized base of Unit 1256C-18, the massive lava contains interstitial mesostasis of quartz-albite granophyric to vermicular intergrowths, quartz, clinopyroxene, apatite, and dendritic to skeletal magnetite with ilmenite lamellae. Granophyric veins and pods may also appear as late-stage magmatic products.
A common groundmass texture prevails through almost all basalt lava samples from Site 1256, which consists mainly of clinopyroxene and plagioclase ± disseminated magnetite. Both clinopyroxene and plagioclase are radially arranged to form spheroidal or fan-shaped crystal aggregates, finer-grained varieties of which have been traditionally called varioles (Fig. F1). MacKenzie et al. (1982) used the term "varioles" for fanlike varieties and "spherulites" for spheroidal crystal aggregates irrespective of the constituent mineral species and the host rock compositions. However, spherulites are traditionally used for those consisting essentially of feldspars in felsic rocks, and similar textures that occur in mafic rocks are termed varioles. I prefer the conventional usage of varioles but broaden its definition to include fan-shaped varieties of prismatic to platy crystal aggregates of clinopyroxene and plagioclase similar to spheroidal varioles.
Devitrification of a quenched flow surface begins within a few millimeters of the surface as patches of fine cervicon-plumose crystallites, predominantly clinopyroxene, form. Both the number density of the crystallite patches and the crystal density in individual patches rapidly increase toward the flow interior and occupy the entire groundmass with dark brownish varioles that are barely translucent under the microscope. Many varioles near the front of the dense variole zone have tiny microlites of plagioclase or olivine in their cores and are elongated to the orientation of the microlites, showing that they nucleated on the preexisting microlites. Typical varioles occur within a zone 1–2 cm beneath the surface and are composed of very fine grained fibrous aggregates of clinopyroxene with minor amounts of thin plagioclase laths. Tiny magnetite grains may be disseminated in and between the pyroxene-plagioclase aggregates. Samples taken further into the interior of lava flows show preferential growth of clinopyroxene over plagioclase where clinopyroxene forms larger curved sheaves of acicular crystals with the intergrown plagioclase laths much less abundant (fine varioles) (Fig. F1A, F1B). As the groundmass crystals coarsen, both plagioclase and clinopyroxene thicken to form more stubby and prismatic crystals. Eventually the growth rate of plagioclase and clinopyroxene is reversed, and further into the interiors of thick massive flows plagioclase becomes skeletal to platy with bowtie-like crystals larger than clinopyroxene forming subhedral to euhedral prisms broadening toward the exterior of varioles (medium varioles). There is also a transitional type that has both plagioclase and clinopyroxene crystals similarly developed in the cores of thick sheet flows and massive lava.
Further development of variolitic texture is only observed in the thick massive lavas from Units 1256C-18 and 1256D-1, which are interpreted as a lava pond by the Leg 206 Shipboard Scientific Party (Wilson, Teagle, Acton, et al., 2003). Stubby clinopyroxene coagulates near the center of a larger plagioclase crystal to form a hub from which a few crystals of platy plagioclase radiate (coarse varioles) (Fig. F1C, F1D). The variole hub is usually composed of several clinopyroxene crystals, but in some cases only one or two clinopyroxene crystals showing a patchy extinction pattern are intergrown with radiating plagioclase crystals. This suggests that gradual coalescence and reorientation of several clinopyroxene crystals into a single continuous crystal took place in the slowly cooling lava. An extremely developed variety can be seen in the coarsest part of the lava pond, where large plagioclase crystals poikilitically enclose clinopyroxene crystals with different crystallographic orientations (Fig. F1E, F1F).
The distribution, morphology, and size of varioles appear to change in response to the distance from the chilled margin, the main factor that determines the degree of undercooling. In the core of the lava pond, fine and medium varioles tend to form as randomly spaced isotropic spheroids, consistent with crystallization under static conditions. However, varioles beneath the deforming surface crust and recrystallized base of the lava pond are flattened and oriented subparallel to the shear plane or disintegrated into bands of granular pyroxene and platy plagioclase intercalated with flattened variole-rich bands.
In Unit 1256C-18, the average maximum plagioclase grain size rapidly increases with depth in the upper 2 m from the surface of the lava pond (Fig. F2A) and then shows stepwise increases up to 2.7 and 0.53 mm in length and width, respectively, at 289 meters below seafloor (mbsf). From this depth to 293 mbsf, the plagioclase grain size widely fluctuates from 1.4 to 2.6 mm in length (0.7–1.8 mm in diameter of a circle with an equivalent area to the crystal [EQD]). Below this depth range, the plagioclase grain size remains almost constant (1.56 ± 0.21 mm in length and 0.83 ± 0.16 mm in EQD) with little variation. The width/length ratio of plagioclase varies in accordance with the size variations. The ratio remains low (0.11–0.12) in the uppermost 2 m of the unit but increases quickly to 0.35, from which it varies between 0.13 and 0.41 to 293 mbsf. Below this depth, the width/length ratio remains almost constant at ~0.23 but shows an overall slight decrease downhole. As the width/length ratio indicates, the zone of coarse-grained plagioclase at 289–293 mbsf has larger, more equant plagioclase than elsewhere in Unit 1256C-18.
The average maximum augite grain size shows a similar pattern to plagioclase, and the peak in grain size also occurs at 289 mbsf (Fig. F2B). The width/length ratio of augite shows different variations from those of plagioclase, especially in the lower half of the unit. Below the zone of coarse-grained augite, the width/length ratio further increases to 0.64 at 301 mbsf and then remains constant at 0.56 ± 0.05 to 310 mbsf. The coarse augite zone is due to the abundant elongate prismatic augite crystals with pigeonite core that have low width/length ratios. The lower half of Unit 1256C-18 has some elongate augite prisms; however, they are much less abundant than the stubby to equant augite crystals.
Plagioclase and augite from Unit 1256D-1 generally show similar variations in maximum grain size as Unit 1256C-18 (Fig. F2C, F2D). Unlike Hole 1256C, the largest grain size occurs near the top of the unit (282 mbsf), which may be ascribed to the unrecovered uppermost part of the lava pond in Hole 1256D. However, the width/length ratio of plagioclase has the highest value of 0.36 below the depth of the peak maximum grain size and then rapidly decreases to 0.18, as in Hole 1256C. The largest plagioclases at 282 mbsf in Hole 1256D form long but thin platy crystals, suggestive of rapid growth under a large degree of undercooling.
The coarse-grained plagioclase and augite zones in Holes 1256C and 1256D are enriched in incompatible elements and coincide with the high magnetic susceptibilities (Wilson, Teagle, Acton, et al., 2003), which are explained by the concentration of differentiated melt in the upper part of the massive lava body. This indicates that the lava body solidified mainly from the bottom and the residual melt became more enriched in incompatible elements and volatiles as it accumulated into the upper part of the body, promoting the growth of large grains of plagioclase and augite. Such a bottom-up solidification of a lava body differs from that of subaerial inflating sheet flows (Kauahikaua et al., 1998), but is well known from observations based on direct drilling into solidifying lava lakes in Hawaii (Helz et al., 1989).
The lowermost 1.6-m-thick core of Unit 1256C-18 is an aphyric cryptocrystalline basalt with an unusual groundmass texture of equigranular clinopyroxene and magnetite with sparse plagioclase laths (Fig. F3, F4). The sample from the deepest portion of Unit 1256C-18 (Sample 206-1256C-11R-7, 130–133 cm) has a groundmass with a variolitic texture, where varioles 0.1–0.3 mm in diameter are composed of slightly elongate, subround clinopyroxene crystals 10–15 µm in length aligned in curved lines radiating from the center of varioles and uncommon plagioclase laths 10–40 µm in length with disseminated granular magnetite (Fig. F4S–F4X). Unlike cryptocrystalline basalt with similar fine variolitic textures and grain sizes, skeletal magnetite is less common than equant crystals and clinopyroxene crystals are granular instead of fibrous-dendritic forms. In contrast, plagioclase tends to retain elongate skeletal forms. Plagioclase phenocrysts and larger laths have an embayed subhedral outline where original plagioclase is disintegrated into tiny (<10 µm) granular crystals of plagioclase. Recrystallization is more completely advanced upward, and at 1 m above the base of Unit 1256C-18 (Sample 206-1256C-11R-7, 33–36 cm), clinopyroxene is completely recrystallized and magnetite barely preserves elongate dendritic forms (Fig. F4G–F4L). At 1.2 m above the base (Sample 206-1256C-11R-7, 9–12 cm), clinopyroxene in the original groundmass is completely recrystallized into equant equigranular neoblasts and magnetite scarcely shows the remnant of skeletal crystal forms. Nevertheless, the igneous variolitic texture is still identifiable from the alignment of granular clinopyroxene and elongate blebs of plagioclase. This is especially true for larger varioles, suggesting that recrystallization is strongly dependent on the original grain size and mineral species (Figs. F5, F6, F7). Figure F8 shows the groundmass grain size variations through the recrystallized base of Unit 1256C-18. Both clinopyroxene and magnetite show steady increases in grain size toward the main body of the lava pond. This can be explained if the driving force of recrystallization was heat supplied from the thick ponded lava above. In contrast, plagioclase does not show any trend of grain size variation with depth, which is consistent with the view that plagioclase retains primary igneous textures and is most resistive to recrystallization.
Coarser-grained late magmatic veins are seen in interval 206-1256C-11R-7, 20–110 cm (see Figs. F4, F5, F6). These veins are composed of plagioclase, quartz, magnetite, brownish clinopyroxene with pale to dark green rims, and granophyric to vermicular intergrowths of sodic plagioclase and quartz. This is an identical mineral assemblage to the mesostasis in the fine-grained basalt of the massive lava pond. Samples 206-1256C-11R-7, 33–36 cm, and 11R-7, 47–50 cm, show progressive recrystallization of more intensely deformed vein minerals than later, less-deformed veins with chilled margins against the host basalt (Fig. F4G–F4L). Earlier veins are more progressively recrystallized into equigranular neoblasts and show evidence of subsolidus intracrystalline deformation such as undulose extinction and kink bands. This, together with the undulating margins of the veins, suggests that either the deformation took place under hypersolidus conditions or the rate of replacement of deformed crystals with neoblasts always exceeded the rate of intracrystalline deformation. Examination of core sections cut subparallel to and normal to the core show that the deformed veins have sheath foldlike structures.
Plagioclase and clinopyroxene in three stratigraphically different samples from the recrystallized base of Unit 1256C-18 were analyzed by EPMA and are shown in Figure F9 and Table T3. All samples have minimal differences in ranges of An mol% of plagioclase and Mg# of clinopyroxene and show the same overall differentiation trend. Plagioclase shows a slight enrichment in FeO with decreasing An from 80 to 60 and a wide scatter in FeO from 0.7 to 2.1 wt% at lower An contents. Most plagioclase phenocryst cores are An80–68, whereas rims have a similar range in composition to the groundmass laths and are as low as An43. Plagioclase in veins mostly plots in the same range as the phenocryst rims and groundmass laths, although some grains have significantly lower An contents (An43–6) (Table T3). Recrystallized plagioclase neoblasts plot in the lower An range (An50.6–52.3) of the groundmass plagioclase.
Clinopyroxene phenocrysts are exclusively augite, whereas crystals in the groundmass and veins plot in both augite and pigeonite fields (Fig. F9B). Some have Wo content as low as hypersthene; however, no orthopyroxene was identified under the microscope. Although the number of analyses of phenocrysts is few, they show three distinct clusters (Fig. F9C, F9D): two cores with high Mg# (~82), two cores and three rims with low Mg# (58–66), TiO2 (0.5–0.8 wt%), and Al2O3 (1.2–1.5 wt%), and one core with low Mg# (59) and high TiO2 (>1.3 wt%) and Al2O3 (2.4 wt%). The groundmass augite overlaps the latter two and extends to lower Mg#s. Augite in veins also plots in the same range as the low-Mg# phenocrysts and the groundmass crystals. Pigeonite in the groundmass and veins shows lower Mg#, TiO2, and Al2O3 contents than the majority of augite, and it appears to plot on the same trend together with those augite. Recrystallized augite neoblasts have Mg# 59.3–61.6, TiO2 0.61–0.84 wt%, and Al2O3 1.26–1.61 wt%, which are within the groundmass compositions.