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doi:10.2204/iodp.proc.330.103.2012

Geochemistry

Igneous rocks

Twenty-two samples of igneous rocks from stratigraphic Units II–XVII of Hole U1372A were analyzed for concentrations of major elements and several trace elements (Table T9) by inductively coupled plasma–atomic emission spectroscopy (ICP-AES; see “Geochemistry” in the “Methods” chapter [Expedition 330 Scientists, 2012] for information on analytical procedures, instrumentation, and data quality). With one exception, the samples were of basaltic lava, including samples of flows, clasts in hyaloclastite-rich volcaniclastic rocks, and clasts in the Unit II basalt conglomerate and breccia. A single sample of bulk hyaloclastite from Unit XIII was also analyzed (Sample 330-U1372A-19R-1 [Piece 5A, 37.5–40.5 cm]).

Total weight percentages for the major element oxides vary rather widely (95.97–101.57 wt%). Possible reasons for this behavior are discussed in “Geochemistry” in the “Methods” chapter (Expedition 330 Scientists, 2012). In order to better compare our 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 T9 and are used in figures and in the discussion below.

Weight loss on ignition (LOI) serves as a rough indicator of the overall level of alteration in these rocks. Relative to LOI values for unaltered basalt (typically <1 wt%; e.g., Rhodes, 1996), values for Site U1372 samples are high as a whole, ranging from 0.3 to 6.7 wt%. However, all but two samples have LOI ≤ 3.1 wt%; the other two samples have LOI > 6 wt% (the highest value is for the sample of Unit XIII bulk hyaloclastite). The principal effects of alteration on the suite of elements measured appear to be on K₂O concentrations. Potassium shows no significant correlation with LOI overall, but samples from the portion of the igneous section inferred to have been erupted below sea level (i.e., below ~92 mbsf; see “Alteration petrology” and “Igneous petrology and volcanology”) define a rough positive correlation of K₂O with LOI (r² = 0.59, excluding the sample of bulk hyaloclastite). Moreover, K₂O values of samples from the shallow-marine and subaerially erupted portion of the section exhibit a significantly greater total range (0.53–1.94 wt% vs. 0.46–1.02 wt% for the submarine portion) and tend to be higher overall (Fig. F50A). Because such a difference is not seen in alteration-resistant elements or their interelement ratios (e.g., Ti and Ti/Zr; Fig. F50B, F50C), we infer that subaerial or shallow-marine alteration tended to have a greater effect on K₂O contents than did submarine alteration. Samples from the shallow-marine and subaerially erupted part of the section also have slightly lower SiO₂ contents than those from the submarine portion (44.82–47.41 wt% vs. 47.38–51.73 wt%); however, it is unclear whether this is an alteration effect or not. Other chemical effects of alteration generally appear to be rather minor, but alteration appears to have affected CaO levels significantly in the two samples with the highest LOI (>6 wt%). Unit VI Sample 330-U1372A-12R-2 (Piece 1, 10–12 cm), from the lower part of the subaerial portion of the section, has 15.56 wt% CaO, probably reflecting a small amount of secondary CaCO₃ in the sample. In contrast, the Unit XIII bulk-hyaloclastite Sample 330-U1372A-19R-1 (Piece 5A, 37.5–40.5 cm) has only 5.38 wt% CaO. For the other samples, CaO is between 8.47 and 13.18 wt%.

In a total alkalis (Na₂O + K₂O) vs. SiO₂ diagram (Fig. F51), the data overlap with, but define a much smaller range than, samples from dredge hauls along the Louisville Seamount Trail. Despite the effects of alteration in some of the samples, most of the Site U1372 data fall squarely in the field of alkalic basalt, consistent with the petrography described in “Igneous petrology and volcanology.” The data point for Sample 330-U1372A-9R-6 (Piece 2, 39–41 cm), which appears to contain excess olivine phenocrysts (see below), lies barely outside this field and just within the field of alkalic picro-basalt. Interestingly, values for four samples lie slightly below the line dividing the alkalic and tholeiitic series of Hawaii. One of these samples is the bulk hyaloclastite specimen. Because this sample has the highest LOI value measured (6.75 wt%), it is possible that its position in the diagram is at least partly a result of alteration (this sample also has the highest SiO₂ content). The other three samples, however, are much less altered (LOI = 0.3–1.9 wt%). These three samples (Unit XI Sample 330-U1372A-17R-1 [Piece 6, 76–78 cm], Unit XVI Sample 31R-1 [Piece 15, 105–107 cm], and Unit XVII Sample 38R-3 [Piece 2D, 80–82 cm]) contain titanaugite and thus are unlike true tholeiites (see “Igneous petrology and volcanology”). These three samples are best classified as transitional basalts. In summary, the Site U1372 igneous rocks range from transitional to alkalic basalt.

Concentrations of the other elements generally also lie within the range of values measured for the dredged Louisville samples but show less overall variation (Figs. F52, F53). For example, Mg number (Mg# = 100 × Mg²⁺/[Mg²⁺ + Fe²⁺], assuming Fe₂O₃/FeO = 0.15) varies from 34.9 to 73.5 among the Site U1372 basalts. Although this is a significant range, values for the dredge samples vary between 20.5 and 78.6 (Hawkins et al., 1987; Vanderkluysen et al., 2007; Beier et al., 2011). The same sort of relationship is seen for SiO₂, Fe₂O₃^T (total iron expressed as Fe₂O₃), Al₂O₃, TiO₂, CaO (excluding the two aforementioned samples with LOI > 6 wt%), Na₂O, K₂O, P₂O₅, Ba, Zr, Y, V, Cr, Co, and Ni. This result is not surprising considering that the dredge samples represent sites from some 30 seamounts along most of the 4500 km length of the Louisville Seamount Trail (note that the dredge data shown in the figures are only for those samples with LOI < 6 wt%).

In addition to their general similarity to other samples from the Louisville Seamount Trail, the Site U1372 basalts are also similar to basalts of other intraplate oceanic islands. For example, the data overlap with one or more fields of recent shield and postshield lavas of the island of Hawaii in Figure F53. However, as a group the Site U1372 samples are relatively evolved, with an average Mg# of 48.3, and they are lower in MgO and higher in Al₂O₃ (Fig. F52A) than the Hawaiian shield basalts (which incidentally are tholeiites) and some Hawaiian postshield lavas (which are alkalic). Do the Site U1372 basalts represent a shield or postshield stage of Louisville volcanism? On the basis of the chemical data, they could be either, given what little is presently known about Louisville volcanic stages.

Evidence for significant amounts of plagioclase removal during magmatic evolution at Site U1372 is lacking. In particular, Al₂O₃ increases overall as MgO decreases (Fig. F52A), consistent with olivine (± clinopyroxene) control. Likewise, Sr concentrations generally increase as the concentrations of TiO₂ (Fig. F53A) and other incompatible elements increase (Sr is a compatible element in plagioclase but incompatible in clinopyroxene and olivine; e.g., Bindeman and Davis, 2000). These results are consistent with the relative scarcity of plagioclase phenocrysts at Site U1372 (see “Igneous petrology and volcanology”). However, a modest role for plagioclase during differentiation is suggested by the Sr/Ti ratio, which is higher in the lowermost two units of the hole (0.033 and 0.038, respectively) than in the overlying units (where values vary between 0.022 and 0.031). Clinopyroxene also appears to have played a role in the evolution of the Site U1372 magmas, as indicated by a decrease in CaO/Al₂O₃ with decreasing MgO below ~5 wt% (Fig. F52B), excluding the highly altered Unit VI sample discussed above, which has anomalously high CaO/Al₂O₃. Iron as Fe₂O₃^T varies from 10.26 to 14.27 wt%. This element shows no simple trend with MgO, but the greatest variation in Fe₂O₃^T is seen at low values of MgO (Fig. F52C), consistent with variation in the amounts of plagioclase and clinopyroxene in the fractionating mineral assemblage. Two samples appear to contain excess (accumulated) olivine phenocrysts: Sample 330-U1372A-9R-6 (Piece 2, 39–41 cm), from olivine-rich Unit IV, and Sample 7R-4 (Piece 4, 137–140 cm), an olivine-phyric clast (Type 2; see “Igneous petrology and volcanology”) from Unit II. These rocks have the highest values of MgO (15.75 and 13.07 wt%, respectively; Fig. F52), Mg number (73.5 and 70.0, respectively), and the compatible trace elements Cr and Ni. The least differentiated of the Site U1372 basalts appear to be the two samples taken in the lowermost part of the hole (Unit XVI Sample 330-U1372A-31R-1 [Piece 15, 105–107 cm] and Unit XVII Sample 38R-3 [Piece 2D, 80–82 cm]). These basalts are characterized by relatively high Mg number (65.8 and 58.5, respectively), moderate MgO values (8.89 and 6.72 wt%), and rather high Ni (168 and 189 ppm) and Cr (424 and 490 ppm) contents.

Downhole variations in element concentration and their interelement ratios do not lend themselves to any single simple generalization. The clasts of stratigraphic Unit II display the greatest intra-unit variation, but with the exception of K₂O, the total range of values in this unit is within the range defined by the basement units (e.g., Fig. F50). The Zr/Ti ratio shows a lack of significant downhole variation (Fig. F50B). Unlike K, both Ti and Zr are resistant to most types of alteration (e.g., Humphris and Thompson, 1978). Both elements are incompatible in olivine, clinopyroxene, and plagioclase (e.g., Salters and Longhi, 1999; Bindeman and Davis, 2000). Therefore, the Zr/Ti ratio is not affected greatly by differentiation in basaltic magmas (prior to removal of magnetite, which is not indicated by the variation of TiO₂ vs. Mg number in the Site U1372 basalts). However, the Zr/Ti ratio can be changed by variable amounts of partial melting of the mantle source and by a varying source composition. As such, the small observed range of Zr/Ti (average of 0.012 with a standard deviation of 0.001) suggests that the Site U1372 basalts were produced from a rather uniform source (at least with respect to Zr and Ti) and that the amount of partial melting varied within a relatively small range. The Zr/Y ratio is geochemically similar to Zr/Ti in basaltic systems, except that Y can be affected significantly by alteration and is not measured as well as Ti and Zr by the shipboard ICP-AES instrument. Among samples from Units II–XIV, Zr/Y varies over a small range between 6.9 and 8.7 (excluding values for the two highly altered samples with LOI > 6 wt%). However, the samples from Units XVI and XVII have slightly lower Zr/Y values (6.2 and 6.7), which might suggest that these two transitional basalts were formed by slightly larger mean amounts of partial melting than the overlying basalts.

In any case, magmatic differentiation is known to change concentrations of Ti, Zr, Y, and other incompatible elements, and much of the variation in the concentrations of these elements downhole (e.g., Fig. F50C) is probably a result of different amounts of differentiation. The same is true for compatible trace elements such as Ni (Fig. F50D; note also that the effect of olivine accumulation is illustrated nicely in this diagram in the anomalously high Ni concentrations of Unit IV Sample 330-U1372A-9R-6 [Piece 2, 39–41 cm] and Unit II clast Sample 7R-4 [Piece 4, 137–140 cm]). Unit XIV is distinctive, with high TiO₂ (Fig. F50C), Ba, Zr, Sr, P, Y, and Na₂O contents and low Mg numbers (38.9–44.1). This unit appears to be among the most evolved of the basement section. Finally, we note that if differentiation of a single parental magma type is in fact responsible for the bulk of the variation in TiO₂ and other incompatible element concentrations, a rather large total amount of crystal fractionation is implied (on the order of 40%–50%). More detailed data acquired in planned shore-based studies should permit a fuller evaluation of the amounts and effects of differentiation and partial melting, as well as an evaluation of mantle source composition.

Carbon, organic carbon, nitrogen, and carbonate

Eight samples of foraminiferal ooze from stratigraphic Units I and II (Cores 330-U1372A-2R and 3R) were analyzed for carbonate (percent carbonate as CaCO₃), total carbon, total organic carbon, and total nitrogen content (see “Geochemistry” in the “Methods” chapter [Expedition 330 Scientists, 2012] for an explanation of analytical procedures and instrumentation used for these measurements). The content of organic carbon was estimated by subtracting the percentage of measured carbonate from that of total carbon. The results can be found in the Laboratory Information Management System (LIMS) database (iodp.tamu.edu/tasapps/).

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