Tab. 1+2: Representative electron-microprobe analyses of minerals from Du Toit Nunataks

An example of cordierite-bearing gneiss that is part of a high-grade gneiss-migmatite sequence is described from the Hatch Plain in the Read Mountains of the Shackleton Range, Antarctica, for the first time. The cordierite-bearing rocks constitute the more melanosomic portions of the metatectic and migmatitic rocks that are associated with relict granulite facies rocks such as enderbitic granulite and enderbitic garnet granulite. The predominant mineral assemblage in the cordierite-bearing rocks is chemically homogeneous cordierite (XMg 0.61) and biotite (XMg 0.47), strongly zoned garnet (XMg 0.18-0.11), sillimanite, K-feldspar (Or81-94Ab5-18An0.6), plagioclase (An28), and quartz. Inclusions of sillimanite and biotite relics in both garnet and cordierite indicate that garnet and cordierite were produced by the coupled, discontinuous reaction biotite + sillimanite + quartz = cordierite + garnet + K-feldspar + H2O. Various garnet-biotite and garnet-cordierite geothermometers and sillimanite-quartz-plagioclase-garnet-cordierite geobarometers yield a continuous clockwise path in the P-T diagram. The P-T conditions for equilibrium between garnet core and cordierite and between garnet core and biotite during peak metamorphism and migmatization were estimated to be 690 °C at 5-6 kb. This was followed by cooling and unloading with continuously changing conditions down to 515 °C at 2-3 kb. This low-pressure re-equilibration correlates with the pressure conditions evaluated by SCHULZE (1989) for the widespread granitic gneisses of the Read Group in the Shackleton Range. The associated relict enderbitic granulites representing low-pressure type granulite (8 kb; 790 °C) are comparable to similar low-pressure granulites from the East Antarctic craton. They were either formed by under-accretion processes after collision (WELLS 1979, p. 217) or they are a product of remetamorphism at P-T conditions intermediate between granulite and amphibolite facies. A model of a multiple imbrication zone with crustal thickening (CUTHBERT et al. 1983) is discussed for the formation of the relict granulites of the central and eastern Read Mountains which show higher pressure conditions (8-12 kb, SCHULZE & OLESCH 1990), indicating a Proterozoic crustal thickness of at least 40 km.

Granulometry, mineralogy and geochemistry of sediments from ODP Hole 169-1037B and Site 169-1038

Central Hill is in the northern part of the Escanaba Trough, which is a sediment-filled rift of southern Gorda Ridge. Central Hill is oriented north-south and is associated with extensive sulfide deposits. Hydrothermal alteration of sediment from Site 1038 was studied through analyses of mineralogy and the chemistry and oxygen isotopic compositions of one nearly pure clay sample. In addition, Site 1037 was drilled to establish the character of the unaltered sedimentary sequence away from the hydrothermal centers of the Northern Escanaba Trough Study Area (NESCA). Mineralogy of the clay-size fraction of turbiditic and hemipelagic sediments of Hole 1037B are predominantly quartz, feldspar, pyroxene, illite, chlorite, and smectite, representing continental-derived material. Cores from Hole 1038I, located within the area of Central Hill but away from known active vent areas, recovered minor amounts of chlorite/smectite mixed-layer clay in the fine fraction, indicating a low-temperature hydrothermal alteration. The 137.4-m-thick sediment section of Hole 1038G is located in an area of low-temperature venting. The uppermost sample is classified as chlorite/smectite mixed layer, which is underlain by chlorite as the dominant mineral. The lowermost deposits of Hole 1038G are also characterized by chlorite/smectite mixed-layer clay. In comparison to Hole 1038I, the mineralogic sequence of Hole 1038G reflects increased chloritization. Intensely altered sediment is almost completely replaced by hydrothermal chlorite in subsurface sediments of Hole 1038H. Alteration to chlorite is characterized by depletion in Na, K, Ti, Ca, Sr, Cs, and Tl and enrichment in Ba. Further, Eu depletion reflects a high-temperature plagioclase alteration. A chlorite 18O value of 2.6 indicates formation at a temperature of ~190°C. It is concluded that the authigenic chlorite in Hole 1038H formed by an active high-temperature fluid flow in the shallow subsurface.

Major element oxides of minerals from a lava pond at ODP Site 206-1256

At Ocean Drilling Program Site 1256 (6°44.2'N, 91°56.1'W), during Leg 206, a thick massive unit was cored in two neighboring penetrations of the uppermost basement, Holes 1256C and 1256D. This thick massive lava flow, commonly referred to as the "Lava Pond," is identified as Unit 18 (>30 m thick) in Hole 1256C and Unit 1 (>74.2 m thick) in Hole 1256D (Wilson et al., 2003, doi:10.2973/odp.proc.ir.206.2003). In the coarse-grained basalt that comprises this lithological unit, low-temperature "background" alteration events are present. This report provides microprobe analyses of both primary and secondary minerals present in this massive lava pond. The analyses of typically magmatic minerals (titanomagnetite, plagioclase, and clinopyroxene) are given for comparison with secondary minerals.

Geochemistry of minerals from lavas of ODP Leg 135 holes

We use Nomarski differential interference contrast imaging to reveal the wealth of complex detail in plagioclase zoning for selected samples from Sites 834, 839, and 841. All sites contain some plagioclase with the very complex internal core zoning, convolute zoning, or very fine-scale euhedral oscillatory zoning of the sort generally considered typical of island-arc volcanic rocks. Plagioclase with contrasted zoning styles may coexist within a single lithologic unit or even within a single thin section. Especially notable is the presence of scattered plagioclase phenocrysts with complex zoning throughout Unit 7 in Hole 834B, which in other respects is relatively uniform in composition and appears to have had little or no differential sorting of crystals and liquid. Although our study is by no means comprehensive, it is sufficient to indicate that magmatic conditions have been variable during crystallization of these rocks, and mixing or at least minor contamination may be required to explain some of the relations observed. By analogy with experimental studies, it is possible that variations in water content, either over time or within different parts of a chamber or conduit system, have contributed to the observed contrasts in zoning.