Samples selected for ICP-AES analysis from the cores of Hole 1223A included four from the crystal vitric tuff of Unit 5, five from the palagonitized crystal vitric tuff of Unit 11, and seven siltstones and claystones from Units 8, 10, 13, and 14. All major oxides and the trace elements Ba, Sr, Y, Zr, Ni, V, Cr, and Sc were measured. Iron is reported as Fe2O3 (Table T7). Loss on ignition (LOI) is mainly a measure of volatile (H2O + CO2) loss on heating of the sample to 1000°C, but it also includes some addition of oxygen resulting from oxidation of iron. The analyses shed light on the diagenesis and the alteration of the sedimentary rocks and crystal vitric tuffs, the possible contribution of detrital clays to the sedimentary rock bulk compositions, and the provenance of the sedimentary rocks and the crystal vitric tuffs. These topics are discussed below.
Observations on cores, smear slides, and thin sections indicate a basaltic provenance for most of the rocks and sediments recovered from Hole 1223A. Basaltic glass, both fresh and altered, is ubiquitous, and it is accompanied by varying proportions of olivine, pyroxenes, and plagioclase as separate grains or fragments. Many olivine crystals include tiny Cr spinel. The abundance of olivine and Cr spinel in the bulk rock analyzed is the cause of the high MgO, Ni, and Cr contents (Table T7). There are also lithic fragments that individually combine these several minerals together with titanomagnetite. Much of the matrix of tuffs, siltstones, and claystones is very fine clay minerals. Whether clay minerals are detrital, authigenic, or both is important to establish. The tuff matrix has a great deal of authigenic clay and several zeolites. These are the principal cementing minerals of the upper tuff of Unit 5, in which most glass is still fresh. Clays and zeolites also cement the dominantly palagonitized glass in the lower tuff of Unit 11. Much of the clay in the siltstones and claystones appears to be detrital, as discussed below.
Both subaerial tropical weathering and submarine authigenesis produce the high K2O clay minerals smectite, illite, or mixed-layer smectite-illite. Because clay minerals are hydrous, high LOI in our samples is an indication of clay content. The correlation between K2O and LOI contents among our samples (Fig. F52) indicates a high proportion of potassic clay minerals. Therefore, K2O should not be used as a geochemical discriminant to establish provenance.
The bulk compositions of samples from Hole 1223A are proportionately aluminous compared with other basaltic protoliths. A ternary diagram of CaO-Al2O3-K2O (Fig. F53A) shows that the sample compositions are shifted toward the compositions of aluminous subaerial clay minerals such as illite, montmorillonite, and kaolinite. This shift toward the clay field is a consequence of lower CaO rather than higher Al2O3 (Table T7). The effect of the K2O increase during alteration, even though substantial (Fig. F52), is proportionately small compared with the variation in Al2O3, and thus only produces a minor shift toward the K2O axis in this ternary plot.
Analyses of glass-palagonite pairs from DSDP Hole 504B by Noack et al. (1983) suggest that palagonitization produces a similar relative effect to that of addition of detrital clays (Fig. F53A). However, when the same comparisons are made using total iron as Fe2O3T, Al2O3, and MgO on a ternary MAF diagram (Fig. F53B), the addition of clays and formation of palagonite have opposing results. Samples of Hole 1223A, circled as Group 2 in Figure F53, likely contain a significantly higher proportion of detrital clay than those circled as Group 1. Both groups are displaced away from MORB glasses and tholeiitic basalt glasses from Kilauea Volcano, Hawaii, toward Al2O3 in Figure F53A. None of the samples are displaced away from Hawaiian glasses toward marine secondary clays (Fig. F53B). Thus, the claystones, siltstones, and the vitric tuffs have at least some detrital, but not marine, secondary clay. If palagonitization in the lower tuff caused the glass to become enriched in Fe2O3 and MgO with respect to Al2O3, then the effect was more than compensated by the presence of detrital clays in the samples.
Therefore, to a first order, the analyzed samples from Hole 1223A are composed of components of basaltic rock diluted to varying extents by aluminous detrital clays. Such clays were produced by subaerial tropical weathering and carried by streams to offshore regions. Emergent Hawaiian volcanoes are the only potential sources for such material in the vicinity.
The overall effects of this dilution on bulk compositions are as follows. A higher fraction of detrital clays in the samples would produce lower CaO, TiO2, P2O5, MnO, and Sr in the analyses. Typical illite, montmorillonite, and kaolinite in tropical soils have very little or none of these components. On an anhydrous basis, addition of such clays to a volcaniclastic component would slightly increase SiO2 and MgO and slightly lower V, Sc, and Fe2O3. It would cause higher K2O and Ba, which tend to be leached from basalts by subaerial tropical weathering and thus are carried away in solution and in clays. All of these effects are apparent in correlative trends of these constituents to one another in our data. The only anomalous oxide is Na2O, which is present only in minor amounts in tropical detrital clays. It would be relatively diluted in samples with a high proportion of such clays. Where the CaO content is low, for example, the Na2O contents would be low as well and have a positive correlation. However, all samples analyzed from Hole 1223A have high Na2O contents that are anticorrelated with CaO. Analyzed samples that have a large detrital clay component have more Na2O than samples with a small clay component. In some samples, especially the tuffs, this may be related to the abundance of sodic zeolites (e.g., analcime) in vesicles, cavities, and other void spaces. High Na2O would therefore be an authigenic effect. Another possibility is that sea salt, especially in the porous claystones and siltstones, caused the elevated Na2O concentrations. After several days of desiccation in the core laboratory, salt crystals formed on the sawed surfaces of some of these rocks.
Two very closely spaced samples (200-1223A-4X-2, 53-56 and 56-58 cm) have slightly higher SiO2 and P2O5 than any of the others. This may be the result of a higher proportion of radiolarian-bearing pelagic clay in these samples. The radiolarians would cause the SiO2 enrichment, whereas a high proportion of icthyoliths (fish teeth) in the pelagic clay would provide the elevated P2O5 concentrations. The lower SiO2 and P2O5 contents in the other tuffs, siltstones, and claystones indicate that pelagic clay is a smaller component and that it is more likely that their clay component is detrital.
The effects of palagonitization on the bulk composition of the lower tuff seem to have been fairly small, whereas in some subaerial settings, it causes wholesale reconstitution of the composition of glassy tuffs (e.g., Hay and Iijima, 1968). The chemistry of the lower tuff is similar to that of the upper tuff, the siltstones, and the claystones. The main effect of the palagonitization is that the lower tuffs have higher LOI and are reddish brown rather than greenish gray. This conclusion may need revision, or at least it will become better understood when actual compositions of glass pairs and palagonite can be determined on the samples and additional literature sources are investigated. However, for now, we can say that several samples of both tuffs and the claystones and siltstones circled as Group 1 in Figure F53 have comparatively low proportions of detrital clay and fairly minimal effects of alteration. Thus, these samples are the most useful for comparison with potential igneous sources in order to determine the provenance of their volcaniclastic components.
Provenance is an important issue because of the possibility that the volcaniclastic rocks at Site 1223 are related to the Nuuanu Landslide, which originated by the collapse of a major portion of the island of Oahu. An island provenance would be a prerequisite if this was indeed the case. However, the problem with an island provenance is that heat seemed to be required to explain the induration of the two tuffs so near the seafloor. A local provenance for the tuffs, perhaps a nearby seamount, would be consistent with this hypothesis. Accordingly, we compared bulk compositions of samples from Hole 1223A with Hawaiian basalts, with lavas of the North Arch volcanic field some 300 km to the northwest, and with both normal and enriched mid-ocean-ridge basalts (N-MORB and E-MORB). Our initial petrographic interpretation was that the olivine-bearing glass shards in the tuffs resembled types of Hawaiian tholeiite, but the issue still remained whether tholeiites might have erupted recently along the Hawaiian Arch and thus provided a local source for the tuffs essentially identical to the islands.
The presence of a significant component of aluminous detrital clay, produced by subaerial erosion and confirmed by chemical analyses, clarifies these questions. The source of the aluminous component in all of the sediments and tuffs was clearly the islands, making it unlikely that a separate local source provided the volcanic glass shards and associated minerals and lithic fragments. But can the chemical analyses tell us which type of Hawaiian basalt contributed to the sedimentary materials? Tholeiitic basalt is the most voluminous type of Hawaiian lava (e.g., Macdonald and Katsura, 1964; Clague and Dalrymple, 1987). However, alkalic olivine basalts, basanites, and olivine nephelinites erupt during both the earliest and latest stages of Hawaiian volcanism, and it is possible that these contributed volcaniclastic materials to the sedimentary succession at Site 1223.
Figure F54 shows the similarity of analyzed samples from Hole 1223A to Hawaiian tholeiite represented by basalt glasses from Kilauea Volcano and its undersea extension, Puna Ridge (Clague et al., 1995). The diagrams also show strong differences between our samples (as well as Kilauea tholeiites) and Hawaiian alkalic olivine basalts, basanites, and olivine nephelinites from three localities—the North Arch volcanic field (Dixon et al., 1997), the Honolulu Volcanic Series of Oahu (Jackson and Wright, 1970; Clague and Frey, 1982), and the Hana Volcanic Series of Haleakala Volcano, Maui (Chen et al., 1991). Data are also plotted for a representative suite of abyssal tholeiites from the East Pacific Rise (Fig. F54C, F54D) (J. Natland, Y. Niu, and P. Castillo, unpubl. data). This suite includes a wide range of primitive and differentiated abyssal tholeiites (N-MORB).
Because we compare bulk sediment compositions to compositions of glasses from Kilauea and the East Pacific Rise, the effects of abundant olivine in Site 1223 samples need to be taken into account. Olivine forms 9-13 wt% of the mode of the vitric tuffs. If its composition is about Fo85, the effect of subtracting 13 wt% of olivine from the bulk composition of a tuff with 10.5 wt% Al2O3, 47.5 wt% SiO2, and 11 wt% iron as Fe2O3 is shown by the arrows in Figure F54A and F54B. This is approximately the composition of Group 1 tuff in Figure F53 with the lower amount of detrital clay component. The tips of the arrows in Figure F54A and F54B are approximately that of an aphyric basalt or basalt glass still very closely resembling the composition of Kilauea tholeiite. It differs greatly from the compositions of Hawaiian alkalic basaltic lavas and MORB.
In Figure F54C and F54D, compositions of samples from Hole 1223A are additionally compared with E-MORB glasses, using a compilation drawn from the literature, and with eight analyses of magnesian tholeiites and tholeiitic picrites from Koolau Volcano (Frey et al., 1994). E-MORBs resemble N-MORBs, except they are slightly more aluminous. Most of the Koolau lavas can be distinguished from Kilauea lavas by their slightly higher SiO2 and lower iron (as Fe2O3) contents. On these diagrams, the effects of addition of detrital clays and authigenesis make it difficult to ascertain which Hawaiian volcano is the more likely source for the samples from Hole 1223A.
Figure F55 and Table T7 provide additional information about the provenance of samples from Hole 1223A. Data from N-MORB, E-MORB, Kilauea-Puna Ridge, and Koolau Volcano are plotted for comparison. Again, samples from Hole 1223A resemble Hawaiian tholeiites rather than N-MORB or E-MORB. Several of the tuffs from Hole 1223A, including those falling in Group 1 in Figure F53, have higher SiO2, lower Ba, and lower Zr than Kilauea glasses at given MgO content (Fig. F55A, F55B, F55C). For these elements, they more closely resemble Koolau Volcano. SiO2 and Ba concentrations reflect the presence of some detrital clay, but in two samples Ba clearly is too high. However, this comparison holds for those samples with the least amount of clay and highest CaO contents. In addition, Zr is an element that is usually unaffected by alteration. It is also usually precisely and consistently measured from one laboratory to the next. The measurements should give a relatively accurate estimation of its original concentration in volcanic glass and lithic fragments in the sediments and tuffs, diluted by up to 13 wt% with olivine and only small amounts of clay in several of the tuffs. The effect of subtraction of olivine (with ~45 wt% MgO and no Zr) is shown by the arrow in Figure F55C. Addition or subtraction of olivine cannot direct residual liquid compositions from Koolau into the field of Kilauea tholeiites. Dilution by clays in samples of Hole 1223A will draw compositions nearly toward the origin (no Zr; <1 wt% MgO), but in several samples this effect is minimal. The diagram thus suggests a Koolau provenance for the vitric tuffs. The higher SiO2 and lower Ba and Fe2O3T (Fig. F54C) of the same samples support this contention.
These data are not definitive. Preliminary electron microprobe data of glasses from the upper vitric tuff show chemistry similar to Koolau Volcano (S. Sherman, unpubl. data). The high MgO content in the bulk analyses is due to the accumulation of olivine and is not due to high MgO in the glass (preliminary microprobe analyses have MgO contents between 6 and 8 wt%). The evaluation here, however, suggests that certain trace elements, including Ba and Zr, should also be measured on glasses in the final provenance evaluation.