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doi:10.2204/iodp.proc.309312.204.2009

Results

Comparison of analyses by different dissolution procedures

Most of the elements analyzed by both dissolution methods (acid digestion and alkali fusion for ICP-MS analyses) show similar values within analytical error ranges. However, Zr, Hf, Th, and U concentrations in samples recovered during Expedition 312 show systematic differences between the two dissolution methods. Figure F3 shows a comparison of analyses of the same sample (312-1256D-184R-1, 0–8 cm) dissolved by acid digestion and following alkali fusion. It is apparent that Zr, Hf, Th, and U concentrations are higher in the sample split prepared by alkali fusion compared to that treated by acid digestion. This relationship is true for all samples from Expedition 312 treated by both methods. The discrepancy between the two dissolution methods is interpreted by the presence of acid-resistant minerals containing Zr, Hf, Th, and U, and is discussed below. On the other hand, Zn, Cs, and Pb show slightly different values between two dissolution methods. As mentioned above, procedural blanks for Pb are high even for the acid digestion method. The Pb blanks are further increased when samples are prepared by alkali fusion. Pb abundances in Hole 1256D samples are significantly affected by high blank levels, but as the Pb data from acid digestion appear to be more accurate with lower blank levels, these values are used for all samples in this study.

The most remarkable differences in Zr concentrations yielded by the two dissolution methods is encountered in samples from deeper than 1300 mbsf. The Zr values by acid digestion are much lower than the Zr values by alkali fusion for the samples from a deeper level. It is also apparent that the Zr abundances by acid digestion are much lower than those returned during shipboard analyses of samples from deeper levels than 1300 mbsf (Fig. F4). The Zr abundances by alkali fusion yield concentrations consistent with the shipboard analyses. Furthermore, they show good correlation with XRF data (R² = 0.9802) (Fig. F5). This indicates that the Zr abundances by acid digestion are incorrect because of the incomplete dissolution of acid-resistant minerals. Consequently, the Zr values by alkali fusion are regarded as correct values.

Because acid-resistant minerals such as zircon and titanite are not completely dissolved by acid digestion, the elements contained within these minerals will not be correctly analyzed in samples containing these minerals. For example, zircon contains high concentrations of Zr and Hf, so abundances of Zr and Hf are poorly analyzed by acid digestion for the samples containing zircon. Because Th and U are also highly compatible with zircon (Mahood and Hildreth, 1983), these concentrations may also be lower than the true values for zircon-bearing samples. Analyses of samples from deeper than 1300 mbsf, prepared by acid digestion only, show discernibly lower Th and U concentrations than the analyses by alkali fusion (Fig. F4). We conclude that the presence of zircon in samples deeper than 1300 mbsf has affected our ICP analyses of splits prepared by acid digestion.

The presence of titanite, another common acid-resistant mineral, may result in low abundances of heavy rare earth element (HREE) concentrations in samples prepared by acid digestion due to very high compatibility of HREE into titanite. However, rare earth element (REE) concentrations by the two dissolution methods are in good agreement with each other, suggesting that the presence of titanites has not affected the REE abundances in this study.

In conclusion, we found that three samples (309-1256D-146R-1, 30–54 cm; 146R-2, 80–88 cm; and 155R-2, 60–80 cm) at above 1300 mbsf contain acid-resistant minerals because of much lower values for splits prepared by acid digestion than the splits prepared by alkali fusion. Therefore, the solutions dissolved by alkali fusion were used for these three samples to determine Zr, Hf, Th, and U concentrations.

Therefore, we use the Zr, Hf, Th, and U analyses which were dissolved by alkali fusion for the samples containing acid-resistant minerals. However, because the accuracy of analyses using acid digestion is generally higher than those of alkali fusion, for most trace elements we use data yielded by the standard acid digestion approach. Our preferred trace element analyses are shown in Table T4 and variations of trace element abundances are shown in Figure F7.

Zircons are usually contained in felsic rocks as accessory minerals. The analyses described above suggest that zircons may be contained in the dike rocks deeper than 1300 mbsf, though zircons were not reported on board (see the “Expedition 309/312 summary” chapter). This may be explained by the different degrees of differentiation in the dike complex, but a systematic difference in Mg-number, where Mg# = 100 × Mg/(Mg + Fe), is not recognized in samples above 1300 mbsf (Fig. F6). Higher degrees and different styles of hydrothermal alteration may be responsible for the appearance of zircon at the deeper level.

Comparison of data from this study and shipboard data

During Expedition 309/312, 10 major elements and 11 trace elements (Co, Zn, Sc, Cr, V, Cu, Zr, Y, Sr, Ba, and Ni) were analyzed by ICP-AES. Figures F6 and F7 compare the variations in major element and trace element abundances with depth using both the shipboard data and this study. For the major elements, downhole variations are generally consistent between the shipboard data and this study.

For the trace element analyses, comparison with the shipboard data is possible for the 10 trace elements analyzed on board; however, Cu, V, Cr, Ni, Zn, Y, and Zr values from this study are generally similar to the shipboard data. Sc values are significantly different between shipboard data and this study with Sc concentrations determined during Expedition 309 being much lower than our data. Co abundances determined aboard ship for samples in the vicinity of 1200 mbsf are much higher than our results. Sr abundances from Expedition 312 are in good agreement with our results, but Expedition 309 shipboard data tend to have higher Sr concentrations than those of this study. Shipboard Ba abundances from 800 to 900 mbsf tend to be higher, and those from 1100 to 1200 mbsf are lower than our results. Some of these discrepancies may be due to the different samples analyzed on board and in this study, but a significant component must be due to differences in analytical methods and analytical equipment and the difficulties of shipboard analysis. Table T5 shows results of BAS-206 and BAS-312 by shipboard and this study for major and trace elements. Major elements by shipboard and this study show generally similar values. However, our data tend to have higher values for some trace elements.

Brief summary of bulk rock chemistries of the basaltic rocks of Hole 1256D

Almost all basaltic lavas from Expedition 309/312 show normal mid-ocean-ridge basalt (N-MORB) signatures with flat, light rare earth element (LREE)-depleted REE patterns (Fig. F8). Only one analysis (Sample 309-1256D-80R-2, 92–102 cm; 781.47 mbsf) from the lava succession of Hole 1256D exhibits an LREE-enriched pattern similar to an enriched MORB pattern. Most analyses of the sheeted dike complex including granoblastic dikes also yield N-MORB-like REE patterns. However, two analyses of samples from the same core (Samples 312-1256D-189R-1, 0–13 cm; and 189R-1, 71–89 cm) return LREE-enriched patterns (Fig. F8). The late-stage dike (Sample 312-1256D-234R-1, 26–29 cm) recovered in the bottom-most core from Hole 1256D shows N-MORB patterns but is characterized by highly evolved features (e.g., Mg# = 39; TiO₂ = 2.2 wt%). This analysis is one of the most evolved analyses throughout Hole 1256D. It is noticed that the uppermost lava pond, suspected to have solidified some 5–10 km off-axis, also shows relatively evolved chemistry (see the “Expedition 309/312 summary” chapter). This may imply that late-stage magmatism at Site 1256, as displayed by the late dike of the bottom of hole and lava pond of the top of lava sequence, is characterized by evolved magmas.

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