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doi:10.2204/iodp.proc.309312.202.2009

Results

Results are presented in Tables T2 and T3.

There is no apparent difference in REE patterns and spider diagram patterns between analyzed shipboard AES samples and POOL samples (Figs. F1, F2), although the shipboard AES samples represent the freshest rocks in each igneous unit. This indicates that the original igneous property of typical immobile elements is well preserved. Positive Ta anomalies were observed only in shipboard AES samples and were most likely due to contamination from tungsten carbide during powdering. It has been shown that significant contaminants can be introduced from the grinding head during powdering (Webber et al., 2005). Tungsten carbide contains as much as 30 ppm Ta, which is 2 orders of magnitude higher than that in the analyzed samples. In contrast, Ta is not found in chrome steel, which explains the lack of Ta contamination in POOL samples. As no one sample was powdered by both methods, it is not possible to derive a direct evaluation of the contaminants from powdering at this stage. However, the observation on Ta does show the importance of contamination from the grinding head.

Compared with normal mid-ocean-ridge basalt (NMORB), the basaltic rocks from Hole 1256D have more-developed negative Sr anomalies. The positive Sr anomaly first occurred near the sheeted dike–gabbro transition and becomes dominant in the plutonic section (Fig. F3). Compared with the basaltic rocks in the upper crust section and NMORBs, gabbros show depleted patterns (Figs. F1, F2).

Samples 312-19A and 312-19B (Sample 312-1256D-214R-1, Piece 9, 26–35 cm) represent a gabbro (312-19B)/oxide diorite (312-19A) contact. As shown in Figure F4, the oxide diorite has much more enriched trace element contents along with a strong negative Sr anomaly compared with the gabbro with which it is in contact. This may suggest that the oxide diorite was formed from the upward intrusion of the inter-cumulous liquids during the cooling of crystal mush that solidified to gabbros. The intrusion and crystallization of late stage melts derived from the intercumulous liquids have also been documented at slow-spreading ridges, such as Ocean Drilling Program Leg 118 Hole 735B (Dick et al., 2000; Natland and Dick, 2001).

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