Previous   |    Next

doi:10.2204/iodp.proc.330.106.2012

Geochemistry

Igneous rocks

One sample of the microgabbro from Hole U1375B (Unit I) was analyzed for major elements and several trace elements (Table T6) by inductively coupled plasma–atomic emission spectroscopy (see “Geochemistry” in the “Methods” chapter [Expedition 330 Scientists, 2012a] for information on analytical procedures, instrumentation, and data quality).

The major element oxides total only 95.68 wt%. Possible reasons for totals significantly different from 100 wt% are discussed in “Geochemistry” in the “Methods” chapter [Expedition 330 Scientists, 2012a]. As with other Expedition 330 analyses, we normalized the raw major element values to 100 wt% totals. The normalized values are presented below the raw data in Table T6 and are used in geochemistry figures in the “Site U1376” chapter (Expedition 330 Scientists, 2012c) and in the discussion below.

Despite abundant veining in the microgabbro (see “Alteration petrology”), the sample chosen for analysis is only moderately altered, as indicated by a weight loss on ignition value of 1.4 wt%. Data for the sample fall in the field of basanite and tephrite on a total alkalis (Na₂O + K₂O) vs. SiO₂ diagram (see Fig. F38 in the “Site U1376” chapter [Expedition 330 Scientists, 2012c]). With a total alkali content of 5.34 wt% and a SiO₂ concentration of 44.33 wt%, this sample is one of the most alkalic rocks recovered during Expedition 330. It is also one of the more evolved, with an Mg number (Mg# = 100 × Mg²⁺/[Mg²⁺ + Fe²⁺], assuming Fe₂O₃/FeO = 0.15) of 38.2 and MgO, Ni, and Cr contents of 3.84 wt%, 60 ppm, and 39 ppm, respectively. In an Al₂O₃ vs. MgO diagram, the data point for this sample lies near the high-Al₂O₃, low-MgO end of the array defined by the other Expedition 330 rocks (see Fig. F39A in the “Site U1376” chapter [Expedition 330 Scientists, 2012c]). This array is consistent with olivine being the major control on magmatic differentiation (see “Geochemistry” in the “Site U1372” chapter [Expedition 330 Scientists, 2012b]). The sample also has a rather low Sc concentration (17 ppm) and relatively low CaO/Al₂O₃ ratio (0.64; see Fig. F39B in the “Site U1376” chapter [Expedition 330 Scientists, 2012c]), suggesting that fractionation of augite was also important. The relative unimportance of plagioclase during differentiation is indicated by the sample’s high Al₂O₃ and Sr content (see Fig. F40B in the “Site U1376” chapter [Expedition 330 Scientists, 2012c]).

The microgabbro has the highest TiO₂ concentration measured during Expedition 330 (4.46 wt%). On diagrams of TiO₂ vs. different incompatible elements, data for the sample generally lie at the high-TiO₂ extension of the arrays defined by other Expedition 330 rocks (see Fig. F40A–F40C in the “Site U1376” chapter [Expedition 330 Scientists, 2012c]). However, Y and Zr concentrations are slightly low for the sample’s TiO₂ content (e.g., Fig. F41D in the “Site U1376” chapter [Expedition 330 Scientists, 2012c]). With 259 ppm Zr, the rock is probably unsaturated in zircon and is unlikely to have lost zircon, or a more exotic Zr-Y-rich phase, by crystal fractionation. Instead, the Hole U1375B microgabbro may represent a different magma type than those seen at Sites U1372–U1374 or U1376.

Carbon, organic carbon, nitrogen, and carbonate

No samples from Site U1375 were analyzed for carbonate, total carbon, total organic carbon, or total nitrogen content.

Top of page   |    Previous   |    Next