In this section we describe the primary texture, mineralogy, and geochemistry of the three basaltic units at Site 1136 (Fig. F12). Volcanic structures are described in "Physical Volcanology" and secondary mineralogy in "Alteration and Weathering".
Unit 1 extends from 128.1 to 140.6 mbsf and comprises the lower part of a massive flow. The base of this unit is oxidized and finer grained than the interior and contains ~10% clay-filled vesicles. The uppermost and lowermost recovered parts of Unit 2, which continues to 159.3 mbsf, are fine grained with relatively high abundances of vesicles. This unit comprises the massive interior and most of the base of a ~18-m-thick flow. From Unit 3, the lowermost unit, only 53 cm of vesicular breccia was recovered. This unit extends to the base of the hole at 161.4 mbsf.
All lavas are sparsely to moderately phyric basalts containing phenocrysts of plagioclase (An65-80) with lesser amounts of clinopyroxene and olivine, in an intersertal to granular or intergranular groundmass consisting of plagioclase (An55-65), clinopyroxene, titanomagnetite, and variable proportions of altered glass (Fig. F13). The flow interiors are largely crystalline, whereas samples from marginal zones or from vesicle-rich segregations contain a higher proportion of altered glass. The extent of alteration varies from complete in the breccia of Unit 3 to slight in the massive lava in the lower part of Unit 2. In this region, plagioclase and clinopyroxene are well preserved, and secondary minerals are found only in altered glass and within vesicles and fractures.
The igneous mineralogy of the three lava flows is similar. The only significant differences are variations in the proportion of the phenocryst phases. Olivine is a persistent phenocryst mineral throughout Unit 1 but is absent from the interior of Unit 2, which contains a higher proportion of phenocryst clinopyroxene (Fig. F12). In Unit 3 the mineralogy is difficult to establish because of the severe alteration and very fine grain size of the brecciated flow top, but one fragment consists of glomerophyric basalt similar to that in Units 1 and 2.
Phenocrysts are found in three forms: as isolated grains, as loose glomeroporphyritic clusters made up of euhedral grains, and as compact, dense intergrowths of grains with diverse habits. Plagioclase is the dominant phenocryst mineral, accompanied in some cases by clinopyroxene or olivine. The loose clusters enclose abundant groundmass material between the aggregated phenocrysts and resemble typical glomerocrysts in basaltic lavas, but the morphology of the dense intergrowths and their constituent crystals is unusual (Figs. F14, F15, F16, F17). The intergrowths are up to 12 mm across and have angular to subrounded shapes (Fig. F14). Larger plagioclase grains commonly display fine oscillatory zoning and corroded cores and contain abundant, small (10-50 µm) glass inclusions (Figs. F14, F15). Also included within large plagioclase grains are small (0.5-1 mm) rounded grains of untwinned plagioclase (Fig. F16). Some intergrowths are surrounded by a thin (100-200 µm) discontinuous plagioclase rim (Fig. F17). The shape of the larger grains and the form of the oscillatory zoning (Fig. F14) indicate that these grains grew radially outward from the center of the cluster. This feature, in particular, suggests that the intergrowths are glomeroporphyritic clusters that formed during the evolution of the magma. We do not know why the grains in these clusters grew together to form a compact mass while those of the other type of glomeroporphyritic cluster remained loosely packed.
Small (~1 cm) subangular microgabbro xenoliths are present in the lower part of Unit 2 (Fig. F18). They mainly consist of medium-grained plagioclase and clinopyroxene with minor olivine. Altered glass forms films between mineral grains and has accumulated at interstices between plagioclase and clinopyroxene grains, which also contain abundant small glass inclusions. These textures may have resulted from remelting of xenoliths after incorporation in the basalt. The corroded plagioclase grains in intergrowths resemble those in these xenoliths and could have come from the same source.
Vesicle-rich segregations form 1- to 2-cm-wide bands or patches in Units 1 and 2. They are commonly horizontal (rarely subvertical) and contain 10%-30% vesicles in a fine- to medium-grained, glass-rich matrix (Figs. F9, F19). Neither large phenocrysts nor glomerocrysts are present in the matrix of these bands, which is coarser grained and richer in titanomagnetite and glass than the surrounding rock. Some grains of plagioclase and many grains of titanomagnetite have skeletal habits indicative of rapid crystallization (Fig. F20). The contacts of the vesicle-rich segregations have several notable features. They are sharp but lack textures such as broken crystals or chilling of one unit against the other that indicate intrusion of magma into solidified rock, and plagioclase crystals have nucleated at the contact and grown into the segregation (Fig. F21). These observations suggest that the segregations formed when late-stage, volatile-rich interstitial liquids migrated and accumulated within the cooling flow.
We list XRF analyses of major and trace elements in six samples in Table T7. Four samples from the least-altered (loss on ignition = 0.9%-2.1%) and massive portions of Units 1 and 2 have very similar compositions. Their SiO2 contents vary from 50 to 51 wt%, MgO from 6.4 to 6.7 wt%, and TiO2 from 1.60 to 1.76 wt% (Table T7; Figs. F22, F23). The flows are quartz normative tholeiitic basalts with relatively low MgO and Ni contents and low Mg#, similar to basement rocks from other parts of the Kerguelen Plateau (Fig. F23). Mantle-normalized trace element patterns of the least-altered rocks are almost flat, with only slight enrichment of the more incompatible immobile trace elements (Nb and Ce) relative to the more compatible elements (Ti and Y) (Fig. F24).
Trace element abundances in the two analyzed flows differ slightly; the less altered sample from Unit 1 has marginally lower Ti, Nb, Zr, Y, and Ce, distinctly lower V, and higher Cr than the massive samples from Unit 2 (Fig. F25; Table T7).
Two samples are mineralogically and compositionally distinct. In Sample 183-1136A-15R-2 (Piece 2, 69-71 cm) in Unit 1, most of the primary phases have been replaced by secondary minerals (Fig. F12). This sample was analyzed to assess the chemical effects of alteration. As shown in Figures F23 and F25 and Table T7, the contents of MgO and CaO are slightly lower, and Fe2O3* (total iron as Fe2O3) is slightly higher, than in other samples. However, concentrations of the mobile elements K and Rb are 3.5 and 5.5 times greater than in the less-altered sample from Unit 1, and the Ce content is ~50% lower (Table T7; Fig. F25).
Sample 183-1136A-18R-4 (Piece 2, 89-92 cm) from a vesicle-rich segregation in Unit 2, has slightly lower SiO2, Al2O3, MgO, CaO, Ce, and V contents, distinctly lower Ni and Cr, and markedly higher Fe2O3*, TiO2, P2O5, Nb, Zr, and Y contents than other Unit 2 samples (Table T7; Figs. F23, F25). In thin section, the segregations are seen to be relatively rich in titanomagnetite and goethite, which, together with clay minerals, replace domains of altered glass. The low contents of Ni, Cr, and particularly V, elements that are compatible with Fe-Ti oxides, indicate that accumulation of titanomagnetite could not have been responsible for the high Fe2O3* and TiO2 contents. Instead, we propose that the vesicle-rich segregations formed from highly evolved interstitial liquids whose high Fe2O3*, TiO2, P2O5, Nb, Zr, and Y contents resulted from fractionation of olivine, clinopyroxene, and plagioclase. A problem with this explanation is the relatively low abundance of Ce, an element incompatible in the crystallizing assemblage that should have been relatively enriched in the segregated liquids. Resolution of this paradox must await shore-based analysis of the rare-earth elements.
We compare the compositions of basalts from Site 1136 with those of basalts drilled or dredged in other parts of the Kerguelen Plateau in Figures F23, F26, and F27. The least-altered samples from Site 1136 have compositions within the range of previously recovered basement basalts and distinct from those of the younger, more alkaline rocks on Heard Island and the Kerguelen Archipelago (see "Previous Sampling of Igneous Basement: Ages and Geochemical Characteristics" in "Study Area" and Fig. F8, both in the "Leg 183 Summary" chapter). Abundances of incompatible elements in Site 1136 basalts are intermediate between those basalts from Site 750 and the more enriched samples from Sites 747 and 738 (Fig. F26). They most closely resemble the low-Al2O3 basalts from Site 749 (Figs. F23, F26, F27).
Two elements analyzed with shipboard XRF are particularly useful for identifying a continental crust component in plateau basalts. The concentrations of Nb, and to a lesser extent Ti, are relatively low in continental crust, compared to concentrations of other elements with similar geochemical behavior. A continental component is therefore reflected in low ratios of Nb/Ce, Nb/Zr, and high Zr/Ti, or as relative depletions (troughs) in mantle-normalized trace element patterns. Such features have been used, in conjunction with isotope data, to identify a continental component in the Bunbury and Rajmahal continental flood basalts and in basalts from Site 738, which forms part of the SKP (see "Previous Sampling of Igneous Basement: Ages and Geochemical Characteristics" in "Study Area" in the "Leg 183 Summary" chapter). Figure F27 shows the three ratios, plotted against Zr/Y and Zr content. Basalts from Site 738, whose isotope compositions indicate a continental crust component, have low values of (Nb/Ce)N and (Nb/Zr)N and high values of (Zr/Ti)N. Basalts from Site 1136 show no evidence of such a component. Specifically, they are not relatively depleted in Nb or Ti (Fig. F24), and they do not overlap with crust-contaminated basalts from Site 738 in Figure F27. On the basis of shipboard data, the source of Site 1136 basalts does not contain a continental component, and their magmas did not interact significantly with continental lithosphere during their emplacement.