Previous   |    Next

doi:10.2204/iodp.proc.330.104.2012

Geochemistry

Igneous rocks

Ten samples of igneous rocks from Hole U1373A were analyzed for concentrations of major elements and several trace elements (Table T8) 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). Two samples were from clasts of lava within the Unit I conglomerate. Eight were from lava flows of Units II, IV, VI, and VII. Of these samples, four were from the >22 m thick massive basalt flow of Unit VII.

As at Site U1372, total weight percentages for the major element oxides vary significantly, from 97.35 to 103.92 wt%. Possible reasons for this behavior are discussed in “Geochemistry” in the “Methods” chapter (Expedition 330 Scientists, 2012a). In order to better compare results with one another and with data from the literature, we normalized raw major element values to 100 wt% totals. The normalized values are presented below the raw data in Table T8 and are used in the figures and in the discussion below.

Weight loss on ignition (LOI), an indicator of the overall level of alteration, varies between 0.9 and 2.4 wt% for all but one sample. Values for unaltered basalt are usually <1 wt% (e.g., Rhodes, 1996). The LOI values thus indicate that these samples are only moderately altered as a group, consistent with their petrography (see “Alteration petrology” and “Igneous petrology and volcanology”). The exception is Sample 330-U1373A-2R-3 (Piece 4B, 135–137 cm) from Unit II, which has an LOI value of 7.2 wt%. As at Site U1372, the main effect of alteration on the measured group of elements appears to be on K₂O, but the effect appears to be very significant only for the high-LOI sample, which also has the highest K₂O content (2.67 wt%). The other samples have K₂O between 0.44 and 1.36 wt%, and for them no significant correlation is present between K₂O and LOI. Rather, K₂O in these samples correlates positively with the other minor elements Na₂O, TiO₂, and P₂O₅ (r² varies between 0.58 and 0.67).

In a total alkalis (Na₂O + K₂O) vs. SiO₂ diagram (Fig. F40), data for all but two Site U1373 samples fall in the field of basalt and overlap considerably with Site U1372. One Site U1373 data point lies (barely) within the field of basanite and tephrite. This data point is for the highly altered sample from Unit II whose K₂O content, in particular, appears to have been elevated significantly by alteration, as noted above. The other data point outside the basalt field is for Unit IV Sample 330-U1373A-7R-2 (Piece 7B, 125–127 cm) and lies within the field of picro-basalt. This sample is one of three Site U1373 basalts that have high MgO contents (10.50–13.46 wt% vs. 4.89–6.62 wt% for the other samples). All three of these basalts appear to contain excess olivine and augite (see below). The effect of olivine addition is to move data points in Figure F40 to lower SiO₂ and lower Na₂O + K₂O values (olivine accommodates virtually no Na₂O or K₂O in its crystal lattice, whereas the SiO₂ content of olivine varies with forsterite content but is typically relatively low at ~40 wt% in moderately evolved basalts). This olivine addition effect indeed is observed because points for all three high-MgO samples fall at the lower left corner of the Site U1373 data distribution in the figure. Values for one high-MgO sample (Unit I clast Sample 330-U1373A-1R-2 [Piece 2, 121–123 cm]) lie slightly below the line dividing alkalic and tholeiitic rocks of Hawaii. Like several samples from Site U1372, it appears to be a transitional basalt. Depending on the amount and composition of excess olivine in this sample, however, the liquid composition may have been somewhat more alkalic than suggested by the position of the bulk rock’s data point in the figure. The other Site U1373 rocks originated from alkalic basalt magmas. Depending on the amount and composition of excess olivine in the two high-MgO Unit IV samples, their liquids may have been more alkalic than indicated by their data points in the figure.

The Site U1373 basalts are also similar to those of Site U1372 in other major element characteristics and in trace element characteristics. For example, the Site U1373 data largely overlap the Site U1372 array in diagrams of MgO vs. Al₂O₃ (Fig. F41A), Na₂O (Fig. F41B), and Fe₂O₃^T (total iron as Fe₂O₃), when excluding the highly altered Unit II sample on the basis of its high 7.2 wt% K₂O abundance. The incompatible elements TiO₂, P₂O₅, Y, and Zr also define closely similar trends to those of the Site U1372 basalts (e.g., Fig. F42A, F42B). Likewise, the Zr/Ti ratio (Fig. F43D) is within the same restricted range as that found for Site U1372 (average = 0.012 ± 0.001 in each case). The same is true of Zr/Y (average = 7.8 ± 0.9 at Site U1373 vs. 8.0 ± 0.9 at Site U1372). As at Site U1372, the MgO vs. Al₂O₃ array indicates the importance of olivine control on magmatic compositions at Site U1373 and that plagioclase played a rather small role, consistent with the relative scarcity of plagioclase phenocrysts in the rocks (see “Igneous petrology and volcanology”). The three high-MgO Site U1373 samples are all highly olivine-augite-phyric, and their high MgO and Ni (277–381 ppm; Fig. F43B) contents appear to be a result of the presence of excess olivine phenocrysts, as is also the case for their high-MgO counterparts at Site U1372. Indeed, clear petrographic evidence of olivine addition is seen in the strain-banded olivine observed in the high-MgO Unit I clast from which Sample 330-U1373A-1R-2 (Piece 2, 121–123 cm) was taken (Fig. F13).

Despite their very considerable similarities, however, the Site U1373 basalts do differ somewhat from those of Site U1372. For example, Mg number (Mg# = 100 × Mg²⁺/[Mg²⁺ + Fe²⁺], assuming Fe₂O₃/FeO = 0.15) varies from 45.0 to 72.7 among the Site U1373 basalts (Fig. F43A). This range is substantial but smaller than that found at Site U1372 (34.9–73.5; see “Geochemistry” in the “Site U1372” chapter [Expedition 330 Scientists, 2012b]). More significantly, the three high-MgO Site U1373 basalts have somewhat higher CaO and CaO/Al₂O₃ (Fig. F41C) for a given MgO value than do the high-MgO basalts of Site U1372. This characteristic suggests that in addition to excess olivine, excess augite phenocrysts are also present in these basalts, consistent with their highly olivine-augite-phyric nature (see “Igneous petrology and volcanology”). Scandium provides further support for excess augite in the high-MgO samples in that Sc contents do not decrease with increasing MgO at Site U1373 (note that Sc data were not obtained for Site U1372). Addition and/or removal of olivine alone produces a negative correlation between Sc and MgO because Sc is incompatible in olivine. In contrast, Sc is compatible in clinopyroxene (e.g., Bacon and Druitt, 1988), explaining the elevated (but constant) Sc in all the Site U1373 basalts.

Another difference between the basalts at Sites U1372 and U1373 is illustrated in Figure F42C and F42D. Whereas most of the Site U1373 samples are very similar to those of Site U1372 in Ba and Sr, just as they are in P₂O₅, Zr, TiO₂, and Y, the two high-MgO basalts from Unit IV (Samples 330-U1373A-7R-2 [Piece 7B, 125–127 cm] and 7R-3 [Piece 6B, 106–108 cm]) have distinctly high Sr and Ba concentrations relative to their TiO₂ contents. The introduction of calcium carbonate to a rock during alteration can raise Sr and Ba levels substantially, and carbonate veins were observed in Unit IV (see “Alteration petrology”). However, the samples taken for chemical analysis were from locations well removed from any veins. Furthermore, CaCO₃ contains ~44 wt% CO₂, so basalts containing even small amounts of carbonate are characterized by elevated LOI values. Yet, the LOI values of the two Unit IV samples (1.9 and 2.3 wt%) are not particularly high. We infer that these two samples may represent a slightly different magma type than that represented by Site U1372 and other Site U1373 basalts.

The four samples analyzed from the thick lava flow of Unit VII are relatively homogeneous in most of their elemental characteristics (e.g., Fig. F43A, F43B, F43D). However, their Fe₂O₃^T content varies significantly, from 11.44 to 13.69 wt% (Fig. F43C). The highest concentration is in Sample 330-U1373A-9R-2 (Piece 5B, 115–118 cm) (44.46 mbsf, top depth), which is from a core interval characterized by a peak in magnetic susceptibility (see “Physical properties”). The lowest concentration is in Sample 9R-3 (Piece 1B, 41–43 cm) (45.17 mbsf, top depth) from a nearby interval of unusually low magnetic susceptibility. The differences in Fe₂O₃^T content are consistent with differences in the proportion of opaque minerals observed in these two intervals (see thin section descriptions for Samples 330-U1373A-9R-2, 113–115 cm [Thin Section 101], and 9R-3, 39–41 cm [Thin Section 102]).

Carbon, organic carbon, nitrogen, and carbonate

No samples from Site U1373 were analyzed for carbonate, total carbon, total organic carbon, or total nitrogen content.

Top of page   |    Previous   |    Next