We encountered basement at 42.7 mbsf in Hole 1205A and penetrated 283.2 m into a sequence of subaerially erupted lava flows and interbedded sediment and soil horizons, of which 160.0 m was recovered (recovery = 56.5%). The minimum age of the basement in Hole 1205A is constrained to late Paleocene-early Eocene (nannofossil Zone NP10 [~53.6-55.0 Ma]) on the basis of three nannofossil assemblages recovered in Core 197-1205A-5R immediately above basement (see "Biostratigraphy").
Basement in Hole 1205A was divided into 30 lithologic units, of which 25 are lava flows and 5 are composed of soil or sandstone (Table T4; Fig. F7). The lava flow units include a'a and pahoehoe, as well as transitional flow types. They range from aphyric to highly plagioclase- and olivine-phyric basalt and are variably affected by posteruption alteration. Shipboard geochemical analysis showed that two of the lava flow units recovered (Subunits 18b and 19b [230-250 mbsf]) consist of tholeiitic basalt, whereas the other units are all composed of alkalic basalt. Clasts of hawaiite were recovered from a conglomerate immediately overlying basement in Section 197-1205A-5R-2; this rock type was not found deeper in the basement section. The lithology and major element composition of the lavas from Nintoku Seamount are similar to those of lavas erupted during the postshield stage of young Hawaiian volcanoes, such as Mauna Kea Volcano. In detail, there are differences in the trace element compositions of lavas from Nintoku Seamount and volcanoes from the Hawaiian Islands, which may result either from differences in source composition or from variations in the degree of mantle melting.
Hole 1205A is located ~100 m southwest of Hole 432A (Fig. F1), which was drilled 31.9 m into basement during DSDP Leg 55 (Jackson, Koizumi, et al., 1980). Parts of three lava flow units were identified in the basement section in Hole 432A. The two upper flows are composed of feldspar-phyric alkali basalt with a combined thickness of 2.6 m, separated from underlying sparsely olivine-plagioclase-phyric alkali basalt by a 10-cm-thick layer of red clay soil. Dalrymple et al. (1980b) obtained an 40Ar-39Ar age of 56.2 ± 0.6 Ma for one sample of alkali basalt from the lowermost flow unit. The basement units from Hole 432A are very similar in lithology and chemistry to the youngest basement rocks recovered from Hole 1205A (see "Geochemistry").
In this section, we describe the different lithologies of the basement sequence in Hole 1205A. Detailed descriptions of each core section can be found in the "Site 1205 Core Descriptions".
The Hole 1205A basement was divided into 30 lithologic units (Table T4; Fig. F7), composed of lava flows capped by red soil or deeply weathered flow tops, and one sandstone unit (Unit 4). We used several criteria for determining flow unit boundaries: major changes in lithology; the occurrence of weathered flow tops, soil horizons, or sedimentary interbeds; and the presence of basal flow breccia, glass-rich zones, or abrupt changes in vesicularity. We considered soil horizons (11 were recovered) to be subunits of the underlying flow, except in the few examples where the soil overlay relatively unaltered lava and no fragments of the underlying flow could be found in the soil (Fig. F8). In these examples, the soil was given a separate unit number (Units 2, 7, 23, and 25). Of the volcanic units, Units 9 and 10 and Subunits 12b, 15b, and 18b were composed of two or more lobes. These were identified by the presence of vesicular, glassy lobe boundaries (see "Physical Volcanology"); however, because the mineralogy and lithology of the different lobes were in all cases very similar, they were not divided into separate subunits.
The major lithologic features of each of the volcanic units in Hole 1205A are summarized in Figure F7. The thickness and vesicularity of the flows and the presence of oxidized flow tops and soil horizons indicate that the flows were erupted subaerially (see also Fig. F56 in the "Leg 197 Summary" chapter).
Lava flow units above 230 mbsf are all composed of alkali basalt and are aphyric (Unit 9 and Subunits 11b, 12b, and 13b), plagioclase-phyric (Unit 10), or olivine- and plagioclase-phyric (Units 6, 16, and 17 and Subunits 3b, 5b, 8a, 14b, and 15b). Phenocrysts generally make up <7 modal% of the rock, although plagioclase phenocrysts comprise up to 15 modal% of the rock in Unit 1 (Fig. F9). In several Units (e.g., Units 9 and 27), plagioclase and olivine crystals up to 1 cm in diameter are present as xenocrysts and are intergrown in a cumulate texture. These crystals often have rounded and embayed outlines. Olivine phenocrysts are partially to completely altered to Fe oxyhydroxide, carbonate, and clay, and plagioclase is partially altered to sericite along cracks in the crystal structure (see "Alteration and Weathering"). The groundmass of the alkali basalt lava is composed of plagioclase, olivine, clinopyroxene, titanomagnetite, and glass. The glass is generally completely replaced by secondary minerals. Many of the flows are highly vesicular (up to 50%), with 1- to 10-mm-diameter vesicles, especially close to unit boundaries. Pipe vesicles are present close to flow bases. Vesicles are generally filled with secondary minerals, including carbonate, clay, and zeolite (see "Alteration and Weathering").
Subunits 18b and 19b are composed of sparsely to highly olivine-phyric tholeiitic basalt. Olivine is present both as microphenocrysts and as larger phenocrysts up to 4 mm in diameter, which are partially to completely replaced by secondary minerals. The interior portion of Subunit 19b contains an olivine-enriched layer, consisting of up to 20 modal% altered olivine crystals. The groundmass of the tholeiitic units is composed of plagioclase, clinopyroxene, titanomagnetite, and minor glass; olivine is absent as a groundmass phase. Subunit 19b is underlain by a relatively thick breccia and soil horizon (Subunit 19c; 74 cm recovered).
Several units contain auto-breccia horizons, which were formed during emplacement of the flow and which may occur either at the base of the unit (e.g., Units 8, 19, 24, 26, and 28), at the top of the unit (Units 3 and 30), or both (Unit 29). Breccias are generally poorly sorted, clast supported, and composed of angular clasts of the associated lava unit cemented by carbonate, clay, zeolite, and Fe oxyhydroxide (Fig. F10).
The conglomerate immediately overlying basement in Section 197-1205A-5R-2 contains rounded clasts, up to 8 cm in diameter, of fine-grained aphyric lava with trachytic texture (Fig. F11). Shipboard inductively coupled plasma-atomic emmission spectroscopy [ICP-AES] analysis of two of these clasts showed them to be composed of hawaiite, a relatively evolved rock type that was not found deeper in the basement section.
The uppermost parts of lava flows range from slightly weathered (slight discoloration of the flow top) to deeply weathered (highly altered upper part of the flow), with an overlying red soil (Fig. F12). Soils are up to 30 cm thick (although recovery of these poorly consolidated deposits is likely to be much lower than for the volcanic rocks), and they may represent substantial time intervals. Soils are present throughout the basement sequence; however, they tend to be more common in the upper 250 m of the basement (Fig. F7). The soils are red brown, generally structureless or poorly bedded, and usually contain round, highly altered clastlike domains composed of the underlying lava unit in a clay- to silt-sized matrix. Microscopic examination reveals that the matrix is composed of opaque minerals, rare unidentified organic material, amorphous iron oxide/oxyhydroxide, feldspar, and palagonite and smectite after volcanic glass, together with basaltic lava fragments in various stages of alteration. Most soil samples listed on Table T5 have total organic carbon contents close to the analytical detection limits (0.01 wt%) for shore-based analyses. The highest values were found in Unit 10 (0.12 wt%), Unit 20 (0.08-0.10 wt%), and Unit 23 (0.08-0.11 wt%). Organic carbon reported on Table T5 was determined by the difference between the total carbon and carbonate carbon. The analytical procedures involved are described in Shipboard Scientific Party (1998). The basal breccia of Subunit 19c also contains a significant amount of organic carbon (up to 0.10 wt%).
Unit 4 is a 12-cm-thick laminated vitric sandstone composed of fragments of vesicular lava and tephra fragments (the latter almost completely replaced by smectite and palagonite), plagioclase, iron oxide, and opaque minerals.
In this section we describe the internal architecture and key flow structures of the Site 1205 lavas and briefly discuss their volcanological implications. The terminology used in this section to describe the lavas is outlined in "Physical Volcanology and Igneous Petrology" in the "Explanatory Notes" chapter.
The volcanic basement at Site 1205 consists of at least 25 eruptive units, which range in thickness from 0.6 to ~31.5 m. The division into eruptive units is based principally on the presence of soils and deeply weathered lava tops, indicating that there was a significant time break between the emplacement of each subaerial lava flow unit (Table T4; Fig. F7). The greater abundance and thickness of the soils toward the top of the succession may indicate a longer time between emplacement of successive lava flows in that part of the section.
Because of good recovery of the lava units (average = ~70%; range = 33%-100%), we obtained sufficient information about the internal architecture of individual lava units to classify them according to flow type. Using the type of flow or lobe contacts, along with the arrangement of internal flow structures, we identified four flow types in the Site 1205 succession (see also "Physical Volcanology and Igneous Petrology" in the "Explanatory Notes" chapter). The Site 1205 lavas are categorized as compound pahoehoe (five units), simple pahoehoe (five units), or a'a (five units). A further 10 units were classified as transitional because the flows exhibit characteristics of both pahoehoe and a'a flows. The key features of the lava flow units are listed in Table T6, and the main characteristics of each lava type are summarized below.
At Site 1205, the compound pahoehoe lava flow units range in thickness from 4 to 25 m (average = 13 m) (see Table T6) and account for 24.5% of the total cored thickness of flows in the succession. Each unit consists of 2 to >10 vertically stacked lobes, which range from 0.4 to 7 m in thickness.
The pahoehoe lobes generally have smooth glassy lobe margins grading sharply to aphanitic (hypohyaline to hypocystalline) groundmass with intersertal texture, whereas the lobe interiors are typically fine grained and are characterized by an intergranular texture. Most commonly, the lobes have highly vesicular upper and lower crusts and sparsely vesicular or nonvesicular lobe interiors. Some of the smaller lobes are vesicular throughout. Viscous flow fabric is not present in these lavas, and the vesicle fabric is isotropic, which is consistent with laminar flow of low viscosity lava beneath a stationary insulating crust. These lobes also commonly feature segregation structures, such as small pipe vesicles, vesicle cylinders, and megavesicles, which are indicative of lava flow inflation during emplacement (Fig. F13A).
Five of the lava flow units at Site 1205 are categorized as "simple" pahoehoe because in addition to their characteristic pahoehoe morphology they consist of only one lobe. Simple pahoehoe accounts for ~18% of the total thickness of flows in the succession, and lobe thicknesses range from ~4 to 14 m (average = ~9.5 m). The morphology and internal structures of these flows are identical to those of the lobes in the compound pahoehoe flows (Fig. F13), the most significant differences being greater lobe thickness and the presence of a single lobe in the sampled section.
Five lava flow units in the Site 1205 basement succession were tentatively identified as a'a flows, and these account for ~17% of the total flow thickness (Table T6). The a'a lava flow units range in thickness from 0.5 to 17 m (average = 8.4 m) and are most common in the lower part of the succession (below 275 mbsf). At Site 1205 these units consist of a single lobe and have massive interiors that rest on and are capped by a thin breccia (5 to 95 cm recovered thickness) consisting of spinose and clinkerlike lapilli-sized basalt clasts (Fig. F14). The flow-top breccia of Subunit 14b and Unit 24 was not recovered (Table T6). Internally, these lavas are characterized by a central region with isotropic flow fabric that is flanked by zones showing distinct viscous flow fabric (Fig. F14). This fabric is expressed by subparallel arrangement of flow bands (present as 1- to 2-mm-thick dark gray and subhorizontal wispy bands), by alignment of tabular plagioclase phenocrysts, and by elongate vesicles drawn out in response to viscous flow at high strain rates. The observed flow fabric is best explained by sheared laminar flow around a flow center that was transported within a viscous flow as a plug.
Ten lava flow units are classified as transitional flow types. Consequently, they are the most common flow type at Site 1205, comprising ~40% of the total lava flow thickness in the succession (Table T6). Cored thicknesses of the transitional flows range from ~3 to 32 m (average = 11 m), and all units consist of a single lobe in the cored section.
The morphology and structure of these lava units are highly variable, but all exhibit characteristics that are transitional between pahoehoe and a'a flows (Fig. F15). For example, Unit 3 has a pahoehoe flow base but is capped by breccia. Subunit 5b is capped by a smooth pahoehoe surface, but the internal flow fabric is similar to that of a'a flows. Subunit 11b is capped and floored by flow breccia and thus exhibits a surface morphology typical of an a'a flow; however, its internal flow architecture is more like that of a simple pahoehoe flow (Fig. F15).
The pahoehoe flows make up ~40% of the lava flow succession at Site 1205 and represent the low-viscosity end-member of the lava flow types. All of the pahoehoe flows exhibit an internal flow architecture identical to that found in modern pahoehoe flow fields in Hawaii and elsewhere (Hon et al., 1994; Self et al., 1998; Thordarson and Self, 1998). Consequently, an endogenous mode of emplacement (transport of lava beneath an insulating crust and growth by lava flow inflation) was an important mechanism for dispersing lava from source vents during the postshield stage at Nintoku Seamount. This mode of emplacement would have facilitated transport of lava in preferred pathways (e.g., lava tubes) over long distances without much cooling, which may account for the large thicknesses of the compound pahoehoe flows at Site 1205.
The a'a lavas represent flows that had the highest bulk viscosity at the time of emplacement. However, this should not be taken as an indication of higher initial melt viscosities, especially since the composition of the a'a lavas is identical to that of the pahoehoe lavas (see "Geochemistry"). The transition from pahoehoe to a'a is a common occurrence in modern basaltic lavas and, in principle, is controlled by two factors: viscosity and strain rate. These in turn are controlled by a range of parameters such as eruption or flow rates, slope, cooling rates, dissolved gas, and bubble content (e.g., Peterson and Tilling, 1980). Active pahoehoe flows have a plastic fluid surface that is stretched like rubber as the lobe advances, ensuring transport beneath a stationary and insulating crust. However, such ductile behavior breaks down if, for example, the lava is subjected to sufficiently high strain rates (e.g., by flowing over steeper terrain) that cause the hot plastic lava to be ripped apart, exposing the incandescent flow interiors. This process forms the spinose clinkerlike chunks that accumulate to form a flow-top breccia. This breccia rides on top of the flow and is dumped at the flow front, where it is later buried beneath the advancing lava.
Given the hybrid nature of the transitional lavas at Site 1205, it is possible that they are flows that solidified during different stages of the pahoehoe-to-a'a transition. However, some (e.g., Unit 3) exhibit flow structures (pahoehoe base and breccia top) similar to those found in rubbly pahoehoe flows. This implies a fundamentally different mode of emplacement, where the coherent crust on an inflating pahoehoe flow is disrupted by surges of lava through the molten flow interior and consequently causes large changes in the lava heat budget and rheology (Keszthelyi et al., 2000; Keszthelyi and Thordarson, 2000).
All the lavas from Site 1205 are aphyric to plagioclase- and/or olivine-phyric basalt. Plagioclase is typically the most abundant phenocryst phase (up to 15 modal% of the rock) and occurs as phenocrysts up to several centimeters in size (Fig. F16A). Olivine phenocrysts form up to 20 modal% of the rock and are generally smaller (up to 8 mm in size). In most cases, phenocrysts show evidence of being out of equilibrium with the groundmass. Plagioclase phenocrysts frequently display rounded edges (Fig. F16B), optical zonation (Fig. F16C), straining (Fig. F16D), fractures (Fig. F16E), resorption rims, and embayments (Fig. F16F). Olivine crystals occur in glomerocrysts with plagioclase (Fig. F16G), and olivine phenocrysts are also often embayed (Fig. F16H), rounded, or have hollow centers. Olivine phenocrysts in Subunits 18b and 19b contain inclusions of Cr spinel. Subunit 19b and Unit 20 appear to contain two generations of olivine phenocrysts: larger phenocrysts that are typically 1 mm in diameter and microphenocrysts that are of a similar size to the groundmass. Consequently, it is difficult to distinguish phenocrystic olivine from groundmass olivine in these rocks. In general, olivine microphenocrysts are recognized by their euhedral shape.
Olivine phenocrysts are partly to completely altered (see also "Alteration and Weathering"). In some cases, an unaltered relict remains, but fractures and crystal margins are altered to iddingsite, Fe oxyhydroxide, or green-brown clay (Fig. F16I). Completely altered crystals are replaced by Fe oxyhydroxide, iddingsite, or, rarely, calcite. In Unit 27, olivine phenocrysts are partially deuterically altered to biotite or phlogopite along crystal margins and fractures (Fig. F16J).
Gabbroic xenoliths are present in Subunit 5b and consist of 2- to 15-mm-sized glomerocrysts of plagioclase, olivine, and interstitial ophitic clinopyroxene (Fig. F16K). The similarity in mineralogy of these xenoliths to the host rock suggests that they may be cognate in origin. Plagioclase crystals in these xenoliths are rounded and show compositional zoning and are similar in appearance to those found as phenocrysts in the basalt, possibly indicating a common origin for both.
The groundmass typically contains plagioclase, clinopyroxene, devitrified glass, titanomagnetite, and olivine. Most rocks display a subophitic texture, with large (up to 4 mm) elongate clinopyroxene crystals enclosing smaller plagioclase laths and, more rarely, olivine (Fig. F16L). Clinopyroxene is typically pink in plane-polarized light and displays moderate pleochroism, reflecting a relatively high titanium content. Olivine and clay (replacing glass) tend to occur in patches around the ophitic clinopyroxene. Groundmass olivine was not always identified in thin section. In some units, olivine is probably not a groundmass phase (e.g., Subunits 18b and 19b), and in others, the degree of alteration renders recognition difficult. In addition, microphenocrystic olivine may be difficult to distinguish from groundmass olivine. The absence of groundmass olivine in Subunits 18b and 19b indicates that petrographically these units are composed of tholeiitic basalt, in contrast to the alkalic basalt that makes up the rest of the sequence. Groundmass olivine is occasionally unaltered but is typically replaced by Fe oxyhydroxide, iddingsite (e.g., Fig. F16M), or brown clay. In Unit 27 and Subunit 30b, groundmass olivine has been altered to strongly pleochroic biotite or phlogopite, although the Cr spinel inclusions remain.
Glass is usually completely altered to green and brown clay, Fe oxyhydroxide, zeolite, or chlorite. In Sample 197-1205A-35R-4, 77-79 cm (Subunit 19b), interstitial glass is replaced by a bright green and completely isotropic mineral, which may be palagonite.
Many basalts display a poorly to well-developed trachytic texture in which plagioclase crystals are aligned parallel to one another and frequently wrap around phenocrysts, indicating the flow direction. In Sample 197-1205A-10R-2, 73-75 cm (Fig. F16N), the foliation undulates on a millimeter scale. In the macroscopically flow-banded units (Units 28 and 29), this trachytic texture is particularly well developed and even the phenocrysts are aligned with their long axes parallel to the local flow direction. Some olivine phenocrysts (altered to clay) appear to have been sheared and deformed by the process (Fig. F16O).
Basalt from Site 1205 differs from that from Site 1203 (Detroit Seamount) in that phenocrysts are not generally glomerocrystic and also that they tend to be out of equilibrium with the surrounding groundmass. These disequilibrium features (resorbed margins, zoning, etc.) also characterize many of the phenocrysts in basalt from Holes 1204A and 1204B.
The opaque mineralogy of the Site 1205 basement lavas is summarized in Table T7. The sequence is dominated by titanomagnetite. Exceptions are extensive development of maghemite in oxidized flow tops (e.g., Sample 197-1205A-24R-2 [Piece 9, 124-130 cm]) (Fig. F17A), basal flow breccias, and in the tholeiitic basalt that comprises Subunit 19b (Fig. F17B, F17C). Note that here, as in other Leg 197 site reports, we do not distinguish between maghemite and titanomaghemite (or Ti-bearing maghemite), as they are indistinguishable using reflected-light microscopy. We use the term Fe oxyhydroxide to include a number of Fe-rich secondary minerals that have poor crystal form, including amorphous goethite (-FeOOH), akaganeite (ß-FeOOH), lepidocrocite (-FeOOH), ferroxyhyte (-FeOOH), and ferrihydrite (5Fe2O3·9H2O) (see Waychunas, 1991).
The morphology of the titanomagnetite crystals differs between the alkalic and tholeiitic basalt units recovered at Site 1205. The alkalic basalt contains almost exclusively equant, subhedral, skeletal octahedra (Fig. F17D, F17E), whereas the tholeiitic basalt contains dendritic morphologies, suggestive of high cooling rates, in addition to the equant octahedra (Fig. F17F, F17G). The hawaiite clasts from the conglomerate immediately overlying volcanic basement (Unit V) (see "Lithostratigraphy") contain goethite and Fe oxyhydroxide and unaltered titanomagnetite (Fig. F17H). A dendritic form of titanomagnetite is also present and occurs in discrete layers that may be parallel or transverse to the plagioclase laths that define the trachytic texture. These dendritic forms distinguish the opaque mineralogy of these hawaiite cobbles from the basalt flows below (Fig. F17I). Titanomagnetite in other units and away from oxidized flow tops and basal breccias is generally unaltered, although ilmenite oxidation features (Buddington and Lindsley, 1964; Haggerty, 1991) are more extensive toward the bottom of the basement section. For example, in Subunit 3b ilmenite oxidation lamellae are present (Fig. F17J, F17K). In Unit 27 the titanomagnetite appears "twinned" because of ilmenite oxidation (Fig. F17L), but this feature disappears when the microscope stage is rotated (Fig. F17M), consistent with the bireflectant nature of ilmenite. Rarely, maghemite is developed along cleavage planes and around crystal rims in those crystals of titanomagnetite that have been partially altered to ilmenite (Fig. F17N, F17O, F17P, F17Q).
Olivine is a ubiquitous silicate phase throughout the Site 1205 basement lava units and often contains inclusions of opaque minerals. The compositions of these inclusions are different in tholeiitic and alkalic basalt lava types. In alkali basalt, opaque mineral inclusions in the groundmass and phenocryst olivine are of titanomagnetite (Fig. F17R, F17S). However, in the tholeiitic Subunit 18b, such inclusions are of Cr spinel and are unaltered, although the host olivine crystals have been entirely replaced by clay and, occasionally, Fe oxyhydroxide (Fig. F17T). In Sample 197-1205A-35R-1 (Piece 1A, 32-34 cm), inclusions of Cr spinel have reacted with the melt to form rims of titanomagnetite (Fig. F17U, F17V).
Primary sulfide is rare in the basement lavas (Table T7). It occurs as <0.01-mm inclusions in silicate and oxide minerals in Subunits 3b (Sample 197-1205A-8R-1, 59-61 cm), 5b (Sample 197-1205A-15R-3, 37-39 cm), 8a (Sample 197-1205A-20R-5, 67-68 cm), and 13b (Sample 197-1205A-28R-3, 4-6 cm). These bleblike inclusions are probably composed of pentlandite. The presence of pentlandite indicates that subsolidus reequilibration has occurred (Augustithis, 1979).
Major and trace element abundances were determined by ICP-AES (see "Physical Volcanology and Igneous Petrology" in the "Explanatory Notes" chapter) for 27 samples of lava flows from the basement at Site 1205 and 2 lava clasts from the conglomerate overlying basement in Section 197-1205A-5R-2 (Table T8).
In the total alkali vs. silica diagram (Fig. F18), only two of the lava flows classify as tholeiitic basalt (Subunits 18b and 19b). The other flow units sampled are all composed of alkalic basalt. Compared to the other samples, the two lava clasts from the conglomerate have the highest total alkali contents (Fig. F18), the lowest MgO contents (1.6 and 3.2 wt%), relatively high Al2O3 and low CaO (Fig. F19), and are composed of hawaiite (West et al., 1988). The sequence of intercalated alkalic and tholeiitic basalt overlain by evolved alkalic lavas, such as hawaiites, found at Site 1205 is typical of the postshield stage of Hawaiian volcanism. For example, a similar sequence of subaerially erupted lava flows is exposed on the northeast flank of Mauna Kea Volcano (West et al., 1988; Frey et al., 1990, 1991).
The 32 m of alkalic basalt recovered from Nintoku Seamount at Site 432 during DSDP Leg 55 is very similar in composition to the volcanic rocks in the upper 184 m of the igneous basement at Site 1205 (Figs. F18, F19). Conglomerate containing clasts of hawaiite is present immediately above the volcanic basement at both Sites 1205 and 432. However, tholeiitic basalt was not recovered at Site 432.
In MgO variation plots (Fig. F19), lavas from Nintoku Seamount define trends that overlap with those defined by basalt from Site 433 Suiko Seamount (Kirkpatrick et al., 1980; M. Regelous et al., unpubl. data); such trends are typical of Hawaiian lava flows. The lowest abundances of Al2O3, Na2O, K2O, TiO2, and Zr are in the two samples of tholeiitic basalt (Subunits 18b and 19b). Abundances of these oxides and Zr are inversely correlated with MgO and, except for TiO2, they reach a maximum in lavas with the lowest MgO content (the hawaiite that occurs as clasts in the conglomerate overlying the basement lava flows). In contrast, the hawaiite lava has relatively low CaO content (Fig. F19). A notable contrast to data from Detroit Seamount (see "Physical Volcanology and Igneous Petrology" in the "Site 1203" chapter) is that the variation in K2O content reflects magmatic rather than postmagmatic processes (see "Alteration and Weathering").
In the uppermost 184 m of the basaltic sequence (Units 1 through 17), MgO abundances vary only from 5.03 to 6.98 wt% (Table T8). These alkali basalt units, together with the underlying tholeiitic basalt of Subunits 18b and 19b, show systematic compositional variations with depth. For example, with increasing depth, the abundance of Zr, Ba, and alkalinity (a measure of the deviation of the sample from the tholeiitic-alkalic boundary line in Fig. F18) systematically decrease (Fig. F19). The systematic geochemical variations with depth do not extend below Subunit 19b; Units 20-30 are composed of alkali basalt with relatively high Zr and Ba contents (Fig. F20).
With increasing depth from Unit 1 to Subunit 19b, the Ti/Zr ratio systematically increases from ~80 to 130 but then decreases to ~85-106 in Units 20-30 (Fig. F20). Samples with the lowest Ti/Zr (from Unit 1 and Subunit 3b) also have relatively low Sc content (Fig. F21). The two hawaiite clasts also have a low Ti/Zr (~60) and low Sc, coupled with relatively low MgO and CaO content (Figs. F19, F21; Table T8). These geochemical characteristics are similar to those of postshield hawaiites from Mauna Kea Volcano (Frey et al., 1990) and indicate that clinopyroxene fractionation was an important process as the eruption rate decreased at Nintoku Seamount.
Although the geochemical characteristics of lavas from Nintoku and Suiko Seamounts and their variation with eruption age are broadly similar to Hawaiian lavas, there are important geochemical differences between lavas from these volcanoes. Such differences are not unexpected because even active Hawaiian volcanoes that are located close together, such as Kilauea and Mauna Loa Volcanoes, erupt geochemically distinct lavas (e.g., Frey and Rhodes, 1993). Lavas from Nintoku and Suiko Seamounts and Mauna Kea Volcano have significantly different Zr/Ba, Zr/Sr, Ti/Ba, and Ti/Sr ratios; in each case these ratios increase in the order Nintoku Seamount < Mauna Kea Volcano < Suiko Seamount (Fig. F22). These differences may be the result of systematic differences in the extent of mantle melting or differences in source composition. Resolving the effects of these processes will be a goal of shore-based research.
A plot of Y vs. Zr abundances clearly illustrates an important geochemical difference between East Pacific Rise mid-ocean-ridge basalt (MORB) and Hawaiian basalt; at a given Zr content the MORB has a higher Y content than Hawaiian lavas (Fig. F23). Since Y is a compatible element in garnet, the abundance of Y in a magma is controlled by the proportion of residual garnet present at the time of melt segregation, assuming similar source compositions. Therefore, garnet is a more important residual phase for Hawaiian basalt than for MORB. There are, however, differences between Hawaiian volcanoes. At a given Zr content, Y abundances increase in the order Mauna Kea Volcano = Koolau Volcano < Mauna Loa Volcano < Suiko Seamount. The trends for these Hawaiian volcanoes and for MORB define a fan-shaped array of lines. Surprisingly, the Nintoku Seamount trend clearly crosscuts the Suiko trend. Thus, the Zr/Y ratio is more variable (factor of 2.5) in lavas from Nintoku Seamount than in lavas from the Mauna Kea shield and from the East Pacific Rise (Fig. F23). The larger range for Nintoku Seamount lavas may result in part from variable extents of melting and proportions of residual garnet during the postshield stage of Nintoku Seamount and in part from the clinopyroxene-dominated fractionation that was important in creating the most evolved lavas with >200 ppm Zr (the hawaiite clasts and alkali basalt from Unit 1 and Subunit 3b).