All samples analyzed were selected to exclude obvious signs of alteration (i.e., veined material); however, some alteration characteristics, such as brown color, were observed in the rocks, especially those from Units 2 and 3.
The effect of alteration on the bulk rock chemistry of pillow basalts have been studied extensively in Hole 504B (e.g., Kempton et al., 1985; Sparks, 1995). They concluded that K and Sr are mobile in the upper pillow section where seawater interaction is greatest. Emmermann (1985) divided alteration in pillow lavas into upper oxidative and lower nonoxidative zones, with K, S, and iron oxidation ratios being the most effective discrimination measures between these zones.
Fig. F3A shows LOI vs. H2O(–) for Site 1224 rocks. Many samples, especially those from Unit 1, show negative LOI results. This characteristic indicates that the weight increase arising from oxidation of Fe2+ to Fe3+ (FeO to Fe2O3) is higher than the weight loss caused by removing volatiles from the mineral structures. All samples have low H2O(–) (<1.4 wt%) and LOI (<0.8 wt%). Fig. F3B is K2O vs. H2O(–). Some samples with high water content show high K2O. However, K2O content does not increase with increasing water content (Fig. F3B). We therefore considered the Site 1224 rocks to show a low degree of alteration. Nevertheless, we recognize that K and Rb contents were affected by alteration processes.
In Site 1224 basalt, SiO2 ranges from 48 to 53 wt% as MgO varies over narrow range of 5 to 7 wt% (Fig. F4A). FeO/MgO ratios of rocks from all three units are higher (1.3–3.0) than typical mid-ocean-ridge basalt (MORB) ratios, which are normally 1–1.5. Unit 2 has the lowest (1.2–1.8), Unit 3 has the highest (2.3–3.0), and Unit 1 has intermediate (1.6–2.6) (Fig. F4A).
Site 1224 rocks show interesting trace element characteristics (Fig. F4B). Most compatible and incompatible trace elements of the three units overlap, and they do not show clear crystal fractionation trends. However, the HFSE (e.g., Y, Zr, and Nb) compositions of the three units are remarkably different: Unit 2 has the lowest concentration, Unit 3 has the highest, and two different concentrations are observed in Unit 1.
The most important trace element characteristic is that Site 1224 rocks have higher HFSE content than typical normal (N-) and enriched (E-) MORB, as compiled by Sun and McDonough (1989). Y and Zr concentrations are two to three times higher than those of N- and E-MORB, and Y is nearly twice that of oceanic island basalt (OIB) (Fig. F5A). The average Y/Zr ratios in these rocks are similar to that in E-MORB. In detail, Unit 2 has a slightly higher ratio than the other two units. This higher Y/Zr ratio for Unit 2 at similar Y and Zr content reflects a different petrogenesis for this unit. It is likely that the high trace element contents of these rocks are a result of extensive crystal fractionation.
Significant chemical stratigraphic (chemostratigraphic) differences among the three units at Site 1224 are clearly shown in element concentration vs. depth diagrams (Fig. F6). The highest FeO/MgO ratios and abundances of TiO2, Y, Zr, and Nb are in Unit 3; the lowest are in Unit 2, which also has the highest Y/Zr. Also, the two flows in Unit 1 are indicated by abrupt downhole decreases in abundance of TiO2, Y, Zr, and Nb.
It is important to note that each lithologic unit has different P-wave velocity, bulk density, and other physical properties (Shipboard Scientific Party, 2003a). It is thought that physical properties are associated with lithologic features (massive flows and breccia, degree of alteration, etc.).
At Site 1224, plagioclase and clinopyroxene are the major mineral phases, and they are potential fractionating phases. Clinopyroxene groundmass composition is consistent with bulk composition; that is, groundmass clinopyroxene in Unit 2 has higher Mg# than in Unit 3 (Fig. F7). In contrast, these units have no systematic differences in their plagioclase groundmass compositions (anorthite content= 40–70; mean = ~60).
As a group, Site 1224 basalts display positive correlation between abundances of the incompatible trace elements Y, Zr, and Nb and the FeO/MgO ratio and negative correlations between FeO/MgO and Sr, Ni, and Cr (Fig. F8), which are compatible in plagioclase, olivine, and clinopyroxene, respectively.
Sun and McDonough (1989) compiled chemical data for N-MORB, E-MORB, and OIB and published typical compositions of these basalts. Studies of basalt from deep basements on the Costa Rica Rift (Hole 504B and Leg 148) showed that the compositional variation of these basalts dominantly reflect crystal fractionation (e.g., Brewer et al., 1996). In comparison, Site 1224 basalt is much more evolved (FeO/MgO of 1.3–3.0), and abundances of Zr and Y are higher by a factor of two. Clearly, Site 1224 lavas experienced unusually large amounts of crystal fractionation within the crust.