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doi:10.2204/iodp.proc.324.103.2010

Geochemistry

Major and trace element analysis

We analyzed 22 samples of igneous rocks from stratigraphic Unit V of Hole U1346A for concentrations of major element oxides and several trace elements (Table T7) by inductively coupled plasma–atomic emission spectroscopy (ICP-AES) (see "Geochemistry" in the "Methods" chapter for information on analytical procedures, instrumentation, and data quality). Stratigraphic units above Unit V are not igneous and were not sampled for ICP-AES analysis. Most of the samples were highly altered, although a range of alteration from moderate to nearly total was represented (see "Alterations and metamorphic petrology"), in part to investigate the chemical effects of alteration. Two samples of 1–2 cm thick carbonate-rich, light green veins (Samples 324-U1346A-7R-1 [Piece 3, 28–30 cm] and 7R-2 [Piece 7, 104–107 cm]) were also analyzed (Table T7).

Total weight percentages for the major element oxides in the igneous samples vary rather widely, from 93.50 to 101.65 wt% (and for one carbonate-rich vein, the total is only 85.09 wt%). This variation may partly be a result of the inability of the muffle furnace to attain a temperature above 1000°C during the ignition step of sample preparation (see "Geochemistry" in the "Methods" chapter). In order to better compare our results with one another and with data from the literature, we normalized the raw major element values to 100 wt% totals; the normalized values are presented below the raw data in Table T7 and are used in the figures and in the discussion below.

Weight loss on ignition (LOI) serves as a good general indicator of overall alteration level in these rocks. Relative to LOI values for unaltered tholeiitic basalt (typically <1 wt%), values for the Site U1346 igneous samples are all very high, ranging from 3.12 to 13.85 wt% (Fig. F44A). LOI values for the two carbonate-rich veins are 30.97 and 34.34 wt%. In a total alkalis versus SiO₂ diagram (Fig. F45), data for the igneous samples fall in the fields of both tholeiitic basalt and more alkalic compositions. However, the samples with the lowest total alkali values are also those with the lowest LOI values (<5.1 wt%), and data for these samples all lie in the field of tholeiitic basalt. The principal effect of alteration on the alkali elements appears to have been on K₂O, as Na₂O shows little covariation with LOI and a limited range of variation (2.28 to 3.14 wt%, except for one sample with 1.48 wt%), whereas K₂O exhibits a rough positive correlation with LOI and a very large overall range (0.10 to 4.77 wt%). Likewise, a general decrease of SiO₂ with increasing alteration is recorded in a negative correlation of SiO₂ with LOI. Other elements that were strongly affected by alteration include Sr, Ni, and Co, all showing variable but often large increases in concentration, and CaO, for which both increases and decreases are evident relative to the least-altered samples (most of the variation in CaO/Al₂O₃ in the Site U1346 data in Figure F44B reflects variation in CaO content). Lesser, but still significant, effects are evident in variable increases in Ba and P₂O₅ concentrations, moderate decreases in Fe₂O₃^T (the superscript indicates total iron as Fe₂O₃), and both moderate increases and decreases in MgO. Despite the visually impressive differences between the brown and gray alteration observed at Site U1346, no systematic chemical differences between the two types are evident; we infer that the brown alteration, which came after the gray alteration (see "Alteration and metamorphic petrology"), had relatively little effect on element concentrations at the bulk rock scale.

Elements that appear to have been affected relatively little by alteration include TiO₂, Zr, Y, Sc, V, and Cr. These elements display only small ranges, particularly TiO₂ and Zr, for which concentrations vary from 1.48 to 1.79 wt% and 73 to 87 ppm, respectively. For this group of elements, the concentrations and interelement ratios of the Site U1346 lavas are similar to those of basalts recovered from the Tamu Massif of Shatsky Rise at Site 1213 (e.g., Fig. F44C, F44D, F44E) and the Early Cretaceous (paleomagnetic Chron M8, ~132.5 Ma; Ogg et al., 2008) ocean-ridge basalts recovered at ODP Leg 191 Site 1179 to the north of Shatsky Rise. (Site 1179 serves as a basement "reference site" for nonplateau oceanic crust formed in the same region and during the same period of time as Shatsky Rise.) More generally, data for these elements for the Site U1346 lavas cluster in or very near the field for ocean-ridge basalts, such as those of the present-day East Pacific Rise (Figs. F44, F45). Like the Site 1213 lavas, the Site U1346 basalts consistently look more like ocean-ridge basalts than they do those of other Pacific plateaus, although some compositional overlap is present (e.g., see the field for OJP basalts in the figures). In short, when erupted, the Site U1346 lavas appear to have been compositionally much like those at Site 1213, ~870 km away and probably ~5 m.y. older.

Total carbon and carbonate carbon

Six samples of sedimentary material from Cores 324-U1346A-4R and 5R were analyzed in replicate for carbonate content (percent carbonate). The carbonate values are baseline-corrected for CO₂ in equilibrium with acid and scaled to a 100% CaCO₃ standard (see "Geochemistry" in the "Methods" chapter for an explanation of analytical procedures and instrumentation used for carbonate, total carbon, and organic carbon measurement). The samples were also analyzed for total carbon, although not in replicate. The content of organic carbon was estimated by subtracting the percentage of measured carbonate from that of total carbon. Results are presented and discussed in "Sedimentology."

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