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doi:10.2204/iodp.proc.330.108.2012

Geochemistry

Igneous rocks

Concentrations of major elements and several trace elements (Table T7) were measured for one Site U1377 sample 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). This sample (330-U1377A-3R-2 [Piece 1, 2–4 cm]) is from a moderately olivine-phyric lava flow of stratigraphic Unit III.

The total for the major element oxides is 98.68 wt%. As with other Expedition 330 analyses, we normalized the raw major element values to a 100 wt% total. The normalized values are presented below the raw data in Table T7 and are used in figures in the “Site U1376” chapter (Expedition 330 Scientists, 2012c) and in the discussion below.

Weight loss on ignition (LOI) is 3.0 wt%, indicating a relatively high level of alteration, consistent with petrographic observations (see “Alteration petrology” and “Igneous petrology and volcanology”). The LOI measurement was made after the chemical analysis, contrary to the procedure explained in “Geochemistry” in the “Methods” chapter (Expedition 330 Scientists, 2012a). This was a time-saving measure that allowed us to include the sample in the final batch of powders analyzed during the expedition.

Silica concentration is 55.00 wt%, the highest encountered at any of the Expedition 330 sites. In a total alkalis (Na₂O + K₂O) vs. SiO₂ diagram, data for the sample fall in the field of basaltic trachyandesite, very close to the line dividing alkalic and tholeiitic compositions (see Fig. F38 in the “Site U1376” chapter [Expedition 330 Scientists, 2012c]). However, neither the alteration-resistant elements nor the mineralogy of the rock (see “Igneous petrology and volcanology”) provides any evidence of tholeiitic affinities, and it is likely that alteration has modified the sample’s K₂O and possibly Na₂O concentration. Concentrations of incompatible elements much less susceptible (Zr and TiO₂) or generally less susceptible (Sr, P₂O₅, and Ba) to alteration than K₂O are near the high end of values measured for other Expedition 330 samples (e.g., Fig. F40A–F40C in the “Site U1376” chapter [Expedition 330 Scientists, 2012c]). This characteristic suggests that the rock represents highly differentiated alkalic to transitional magma that evolved from a more mafic composition rather like those samples cored at other Expedition 330 sites. Yttrium concentration is relatively low (see Fig. F40D in the “Site U1376” chapter [Expedition 330 Scientists, 2012c]).

Major element characteristics are also consistent with highly differentiated magma. In addition to high SiO₂, the sample has the highest Al₂O₃ (18.65 wt%), the lowest Fe₂O₃^T (7.01 wt%; the superscript indicates total iron as Fe₂O₃), and the second-lowest MgO (3.00 wt%), CaO (6.18 wt%), and CaO/Al₂O₃ (0.33) values of the Expedition 330 samples (see Fig. F39 in the “Site U1376” chapter [Expedition 330 Scientists, 2012c]). However, several other features are inconsistent with an origin by simple differentiation of a basaltic parent magma. In particular, concentrations of the compatible trace elements Ni (421 ppm), Cr (686 ppm), and Co (122 ppm) are exceptionally high. Such high values are unusual even in basalts, and during Expedition 330 broadly similar concentrations were found only in the highly olivine-phyric high-MgO basalts of Site U1376 (see Table T9 in the “Site U1376” chapter [Expedition 330 Scientists, 2012c]). Mg number (Mg# = 100 × Mg²⁺/[Mg²⁺ + Fe²⁺], assuming Fe₂O₃/FeO = 0.15) also is comparatively high (49.9) for a rock with 55 wt% SiO₂, as are V (281 ppm) and Sc (29 ppm) concentrations.

The combined results suggest a relatively complicated petrogenesis, perhaps involving contamination of ascending evolved magma with a small amount of olivine-rich material from ultramafic wall rock or a crystal mush zone. If so, the uncontaminated magma would have been even more siliceous than the resulting contaminated mixture. Detailed studies on shore will be required for a fuller understanding of this unusual rock.

Carbon, organic carbon, nitrogen, and carbonate

No samples from Site U1377 were analyzed for carbonate, total carbon, total organic carbon, or total nitrogen content.

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