Except for Sections 209-1269B-1R-1 (Piece 1) and 209-1269C-1R-1 (Pieces 3–5), all pieces from Holes 1269A, 1269B, and 1269C are small, nonoriented, and composed of rather fresh porphyritic to aphyric basalt. Phenocrysts in basalt include olivine and plagioclase with minor amounts of clinopyroxene and spinel. The texture is glomerocrystic in some basalts from Holes 1269A and 1269B. A sector of pillow basalt with variolitic structure toward the rim that includes a few chips of fresh glass is found in Section 209-1269C-1R-1 (Piece 1). Vesicles make up >15% of the volume in some basalt from Holes 1269B and 1269C.
The black vesicular basalts drilled in Holes 1269A, 1269B, and 1269C are essentially unaltered. Locally, olivine phenocrysts are slightly oxidized and one clay + iron oxyhydroxide veinlet with an irregular greenish gray halo was observed in Hole 1269C at 0.12 mbsf. The basalts are not deformed and display no structural features.
None of the core sections display high natural gamma radiation or magnetic susceptibility. The mean magnetic susceptibilities of cores from Holes 1269A and 1269B are 222 ± 85 x 10–5 and 177 ± 92 x 10–5 SI, respectively.
Microcrystalline intersertal to intergranular textures (Fig. F3) are present in thin sections of two basalts (Samples 209-1269B-1R-1, 2–5 cm, and 1R-1, 44–46 cm). Acicular plagioclase laths and quench clinopyroxene with minor amounts of fresh brown glass and skeletal opaque minerals make up the groundmass. The fine-grained, branching clinopyroxene crystals are indicative of relatively rapid quenching (Fig. F4). Some large spinel phenocrysts (<0.8 mm) have melt inclusions and/or inclusions of other minerals. Small euhedral to subhedral spinel grains are also included within olivine and plagioclase phenocrysts.
Mid-ocean-ridge basalt (MORB) erupted below 2500 m water depth typically has vesicularities (volume fraction of vesicles) of 1%–2% due to the large hydrostatic pressure (Moore et al., 1977). The 14°–15°N section of the Mid-Atlantic Ridge is an area where relatively high volatile contents in basalts have been observed (e.g., Staudacher et al., 1989; Javoy and Pineau, 1991). Rare examples of "popping rocks" with very high carbon dioxide contents and unique noble gas characteristics were recovered from this region (Staudacher et al., 1989; Sarda and Graham, 1990; Javoy and Pineau, 1991; Burnard et al., 1997; Moreira et al., 1998). Popping rocks are peculiar samples notable for their active popping on the ship's deck soon after recovery from the seafloor as their trapped gases are rapidly released (Hekinian et al., 1973; Pineau et al., 1976). We studied the vesicle size distribution of two basalts from Hole 1269B (intervals 209-1269B-1R-1 [Piece 1, 0–8 cm] and 1R-1 [Piece 7, 39–47 cm]) using digital image analysis. The vesicularity for these two basalts is very similar to that for the 14°N popping rocks (~16%) (Sarda and Graham, 1990).
The theory and application of crystal size distribution is now an active area of study in igneous petrology (e.g., Marsh, 1988, 1998; Cashman, 1993). Although vesicle size distribution (VSD) studies of basalts reveal important clues about magma dynamics and the style of degassing (e.g., Cashman et al., 1994; Mangan and Cashman, 1996), there have been only a few such studies and only one on MORB (Sarda and Graham, 1990). For two Hole 1269B basalts, digital photo images from the split core face were imported into an image analysis program (NIH Image, version 1.63) and the sizes of all vesicles were determined. These images were edited to separate "touching particles" that could clearly be discerned as individual bubbles and to remove "digital noise." Image resolution was 100 pixels/mm2. We used a minimum threshold of 3 pixels for the area of particles. From the particle areas we calculated individual bubble diameters assuming spherical geometry. A comparison of digital images of the two core samples with this idealized model derived from the computer image processing is shown in Figure F5. The vesicle size results were then binned logarithmically according to their diameter, beginning at the largest size of 2.23 mm observed in Piece 7 and 2.17 mm in Piece 1. The total number of objects having diameters between 2.2 and 0.2 mm counted by this procedure were 729 and 843 for Pieces 1 and 7, respectively. The number of vesicles in each size class was converted to number per unit volume (Nv), following the Saltykov method for stereological analysis as outlined in Sarda and Graham (1990). The population density (n) is Nv divided by the bin width and has the units of cm–4. The results are displayed on a diagram of the logarithm of population density vs. vesicle diameter in Figure F6, where they are also compared to the earlier popping rock results.
The modal percent vesicles for these two Hole 1269B basalts determined from the image analysis are very similar (Piece 1 = 15.2% and Piece 7 = 16.2%). Both also display curved VSDs. This VSD shape has several possible origins, related to variations in nucleation and growth rates of bubbles (Marsh, 1998). Piece 7 could also be described as a "kinked" VSD, having two quasi-linear intervals (0.8–1.5 and 0.3–0.7 mm). Such kinked patterns are found in crystals from historical eruptions at Mt. Etna and were suggested to reflect changes in nucleation and growth during the transition from magma storage to ascent (Armienti et al., 1994). By analogy to the Etna study, the larger vesicles (~0.8–1.5 mm) in Piece 7 may reflect bubble nucleation and growth deeper in the magma plumbing system. In comparison, the smaller (0.3–0.7 mm) vesicles may have formed during later stages of magma ascent and eruption. In contrast, Piece 1, which also shows a curved VSD, has a notable deficiency of vesicles in the size class centered around 1.23 mm. This pattern may be produced by coalescence of smaller bubbles (Marsh, 1998). At small bubble sizes (0.3–0.7 mm) this second sample has a slope similar to that for Piece 7. Intriguingly, the slope at small bubble sizes for both samples is similar to that for the 14°N popping rock (approximately –5.5). This popping rock has been interpreted, on the basis of its VSD, relative abundance of rare gas, and high CO2 content, to represent a rare case of undegassed MORB (Sarda and Graham, 1990). The relative positions on the VSD diagram of the two basalts and the popping rock may represent a temporal evolutionary pattern similar to what has been described for crystals in volcanic rocks. Marsh (1998) demonstrated that the negative log-linear slope of natural crystal size distributions is consistent with nucleation and growth in a system in which nucleation rate increases with time. This produces a VSD that migrates systematically up the ln(n) axis and across the size (D) axis at a rate that is determined by the bubble growth rate. Following this reasoning, the three cases shown in Figure F6 might be viewed as snapshots of maturing bubble systems in MORB with the popping rock representing the most primitive system and Sections 209-1269B-1R-1 (Pieces 1 and 7) representing more evolved systems.