). Tube vesicular pumice (50%-60% vesicles) and elongate bubble wall shards are the principal components of the ash, but rare blocky mafic clasts have <10% vesicles. Mean grain size for these blocky shards is fine sand (200 µm; 2.25 ).
The tephra layers recovered from Hole 1138A (Fig. F2) are 1-3 cm thick, massive, and vary in color from light to dark gray. All four tephra layers sampled for this study are glass rich (95%-100% glass shards), crystal and lithic fragment poor, fine grained (mean grain size ranges from very fine to medium sand), and well sorted. Major element analyses yielded totals of 95.0 ± 2.4 wt%, suggesting the presence of ~5 wt% H2O in the glass as a result of postemplacement low-temperature hydration. In grain mount, felsic shards are typically pale brown to translucent gray. Mafic shards tend to be blocky (vesicle poor) and yellowish brown in color. Small flakes of brown biotite are present in Samples 183-1138A-13R-1, 22-24 cm, and 19R-1, 110-112 cm, and most tephra layers contain minor plagioclase and/or sanidine.
The tephra from interval 183-1138A-12R-3, 5-7 cm (105.36 mbsf), (see Pl. P4, fig. 1) contains glass shards with a wide range of compositions, including basaltic trachyandesite, trachyandesite, trachyte, and rhyolite (Fig. F4). The mean grain size for the vitric concentrate is fine to medium sand (250 µm; 2.0 ). Tube vesicular pumice (50%-60% vesicles) and elongate bubble wall shards are the principal components of the ash, but rare blocky mafic clasts have <10% vesicles. Mean grain size for these blocky shards is fine sand (200 µm; 2.25 ).
The tephra from Section 183-1138A-13R-1, 22-24 cm (112.23 mbsf), contains glass shards that are dominantly trachyte in composition (Fig. F4). The mean grain size for the vitric concentrate is very fine sand (75 µm; 3.75 ). The sample is composed of tube vesicular pumice (50%-60% vesicles) with brittle fractures subperpendicular to tube orientation.
The tephra from Section 183-1138A-15R-1, 73-75 cm (131.94 mbsf) (see Pl. P4, fig. 2), contains glass shards that vary from trachyte to rhyolite (Fig. F4). The mean grain size for the vitric concentrate is fine to medium sand (250 µm; 2.0 ). The sample is composed of tube vesicular pumice (highly vesicular; 60%-70%) with abundant small tubes. Shard morphology is strongly vesicle controlled, with brittle fractures parallel and subperpendicular to tube orientation.
The tephra from interval 183-1138A-19R-1, 110-112 cm (170.81 mbsf), contains glass shards that are dominantly high-silica rhyolite in composition with less abundant trachyte shards (Fig. F4). The mean grain size for the vitric concentrate is medium sand (350 µm; 1.5 ). The sample is composed of elongate tube vesicular to platey clasts with a few large tube vesicles. Less abundant, equant tricuspate shards and shards with spherical vesicles are also preserved.
The fine-grained, well-sorted, and massive nature of the tephras from Hole 1138A is consistent with deposition from a distal ash cloud or fines elutriated from a subaqueous flow and dispersed in the water column. In order for glass shard-rich massive tephra horizons up to 3 cm thick to be preserved in marine sediments, a considerable influx of volcaniclastic material is required. The tephras have a dominantly felsic composition, and high vesicularities indicate a large proportion of exsolved volatiles in the erupting magma. Eruption of gas-charged viscous felsic magma is typically highly explosive, and a large volume of ejecta is commonly dispersed over a large area, particularly downwind from the eruption (Cas and Wright, 1988). An alternative mechanism for dispersion of fine-grained, well-sorted ash into the water column is as a byproduct of the entry of pyroclastic flows into the sea, or the subsidence of volcaniclastic deposits into the sea, initiating subaqueous gravity flows (ash turbidites). The associated clouds of elutriated fines could settle and form deposits that would appear similar to those formed from primary fall activity.
Tephras preserved in marine sediments older than those considered in this study contain a greater proportion of mafic and blocky-shaped glass shards (Coffin, Frey, Wallace, et al., 2000). This pattern of evolution from mafic to more felsic compositions, and from clast morphologies reflecting interaction with water to morphologies that are more typical of subaerial eruption, reflects the general evolution of magmatism on the Kerguelen Plateau. The tephras sampled for this study are dominantly trachytic to rhyolitic in composition and are generally alkalic. When compared with basement lavas of the Kerguelen Plateau and volcanic rocks from Heard Island and the Kerguelen archipelago, compositions appear broadly similar (Fig. F4) (Coffin, Frey, Wallace, et al., 2000). The likely source for the tephra deposits at Site 1138 is Heard Island, located 180 km to the northwest. Bathymetric surveys indicate that a submarine canyon runs south of Heard Island and passes near Site 1138. This observation suggests that some ash sampled for this study may have been contributed from fines associated with subaqueous flows originating at Heard Island (Coffin, Frey, Wallace, et al., 2000), as well as from primary ash fall.
