Samples analyzed from Legs 170 and 205 are characterized by small variations in major element oxide composition typical for ocean floor igneous rocks (Tables T2). These samples are low- to medium-K (0.07–0.53 wt%) subalkalic rocks of basaltic composition (Fig. F5) and show a Fe-enrichment trend characteristic of mid-ocean-ridge tholeiites (Fig. F6). All samples contain between 46 and 50 wt% SiO2 with Mg# ranging from 0.44 to 0.63 and have therefore experienced a significant degree of fractionation. Samples from the Leg 205 Hole 1253A interval ~460–509 mbsf (Subunits U4B-1–U4B-4) generally have larger proportions of pyroxene to plagioclase phenocrysts (Morris, Villinger, Klaus, et al., 2003) and higher whole-rock Mg# (0.58–0.63) than Subunit 4A and the lower part of Subunit 4B (0.48–0.57).
Overall LOI is low (–0.12 to 1.46 wt%), consistent with the low degree of alteration of these samples. The abundances of fluid-mobile trace elements (Cs, Sb, Pb, Li, Rb, Ba, and Sr) vary downhole, especially near subunit boundaries. The samples with the highest abundance of these elements generally have an elevated 87Sr/86Sr ratio, which also correlates with ratios of fluid mobile/immobile trace elements and generally occur near subunit boundaries (Chavagnac et al., in prep). This suggests that geochemical modification may have accompanied localized seawater interaction. Trends in major and compatible trace element abundances are consistent with phase equilibria control by olivine, plagioclase, clinopyroxene, and Fe-Ti oxides, all of which are phenocrysts, although olivine is rare (Morris, Villinger, Klaus et al., 2003). A combination of fractionation and alteration processes cannot encapsulate all variations in major and compatible trace elements of Subunits 4A and 4B. For example, Figure F7 shows the covariation of V and Ti within Subunits 4A and 4B. The positive linear variation reflects enrichment in the magma during fractional crystallization, but the offset trends (Subunit 4A shifted to higher TiO2) suggest that the units are not derived from a common melting event.
The two stratigraphic Subunits 4A and 4B form distinct geochemical groups that can be distinguished by abundances and ratios of fluid-immobile incompatible trace elements that are minimally affected by secondary alteration. Despite similar ranges in SiO2 and Mg# for both units, the abundances of fluid-immobile incompatible trace elements (La, Zr, Nb, Hf, and Th) are generally higher in the upper igneous Subunit 4A. This bimodal distribution is more clearly developed in ratios of immobile, incompatible trace elements that are not controlled by degree of fractionation as gaged by Mg# (Fig. F8).
The distinct groups are also apparent in the REE abundances of the Leg 170 and 205 igneous rocks (Fig. F9). The upper Subunit 4A is characterized by a tight range in abundances, with light rare earth elements (LREEs) ~50–60x chondrites and heavy rare earth elements (HREEs) ~15x chondrites. Subunit 4B has a range of LREE (15–30x) and HREE (7–15x) concentrations. Patterns for Subnit 4B are nearly parallel, and lower REE abundances correlate generally, though not systematically, with higher Mg#. Samples from Subunit 4A are characterized by higher LREEs than Subunit 4B, with partly overlapping HREE abundances. Normalized REE abundances of the Leg 170 and 205 igneous rocks lie almost entirely within the combined fields of regional spreading center (EPR and CNS) and Galápagos Islands basalts. REE patterns of the samples are distinct from spreading center basalts, having lower HREE and higher LREE abundance patterns, and are more similar to moderately enriched basalts of the Galápagos Islands.