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Interpretation and conclusions

The Shipboard Scientific Party (2002) of Leg 198 reported that the igneous rocks recovered from Hole 1213B are intrusive sills, using three arguments based on previous ODP cruise results:

  1. Extrusive lava flows are usually accompanied by highly fractured and brecciated flow tops, yet these phenomena are rare or absent in the basement rocks of Hole 1213B;

  2. Extrusive lava flows generally have thin (or no) chilled margins, yet chill zones exist and are relatively thick (as thick as ~20 cm) in Hole 1213B; and

  3. Gradational changes in crystallinity toward coarsely crystalline interiors are typical of mafic sill intrusions.

However, from our careful descriptions we conclude that the Hole 1213B igneous rocks are massive submarine lava flows based on the following observations:

  1. Although highly fractured and brecciated rock is not common in the igneous rocks of Hole 1213B, part of the lower chill zone in stratigraphic Subunit IVb is weakly brecciated in interval 324(198)-1213B-32R-4, 62–65 cm, indicating that this zone is presumably the base of a submarine lava flow. Several fragments in the lower chill zone of Subunit IVb and upper chill zone of Subunit IVc are likely autobrecciated fragments typically found associated with the eruption as lava flows.

  2. The relatively thick chill zones in Hole 1213B are also seen in similar massive flows cored at ODP Leg 192 Sites 1185 and 1186 on Ontong Java Plateau (Mahoney, Fitton, Wallace, et al., 2001). For example, the thickest massive lava flow of Site 1186 has a ~20 cm thick cryptocrystalline chill zone at the top of lithologic Unit 4.

  3. Thin section descriptions of the chill zones and interiors of all three cooling units suggest that the cooling units have a submarine origin, as is confirmed in one instance by the occurrence of coalesced and bow-tie spherulites. Systematic differences in grain size, crystal morphologies and crystallinity are apparent in all three cooling units from the chill zones to the more crystalline and coarse-grained interiors and point into the direction of massive extrusive basalt flows.

In this report, we thus specifically note that the three cooling units are more likely to represent "massive lava flows" instead of a series of "sills" as described in the Site 1213 chapter in Leg 198 Scientific Results (Shipboard Scientific Party, 2002). Seismic profiles from site survey Cruise TN037 (Klaus and Sager, 2002; Sager et al., 2009) over the flank of Tamu Massif also show that massive flows are present all the way down the southwest flank and beyond Site 1213. Therefore, the massive submarine flows (as thick as 15 m) were probably emplaced as inflated compound sheet flows during eruptions similar to those in large oceanic plateaus (Greene et al., 2010) and continental flood basalts (e.g., Self et al., 1997) worldwide. The flows likely erupted from a limited number of fissures along the flank of Shatsky Rise and may have traveled a long distance. Massive flows in oceanic plateaus are generally <15 m thick (Nixon et al., 2008), similar to the thicknesses of the three massive flow units at Site 1213. From comparative land studies in submarine flood basalt provinces, it is evident that massive submarine flows are rarely marked by amygdaloidal horizons or brecciated flow tops and that flow contacts are not easily discernible where interflow sediments are absent (Greene et al., 2009).