19. Pedogenic Alteration of Basalts Recovered During Leg 144

Basalts erupted to form the atolls and guyots of the Western Pacific have been altered in various ways, ranging from hydrothermal alteration to subaerial weathering by meteoric waters in a tropical environment. Subaerial weathering has been moderate to extreme. Moderate subaerial weathering is expressed by dissolution or replacement of primary minerals (olivine, pyroxenes, plagioclase feldspar) and alteration of glassy or aphanitic matrix to clay minerals, goethite, and hematite. The clay minerals are kaolinite or a brown smectite. Kaolinite concentrations decrease downhole and smectite concentrations increase. Although primary minerals are generally not preserved, primary structures, such as vesicles (generally filled by secondary or tertiary minerals), flow structure, or relict breccia structure, remain evident. Extreme subaerial weathering is expressed by the loss of all primary minerals as well as all traces of primary structures, presumably by plant root and soil infaunal bioturbation, resulting in recognizable soil. Kaolinite, goethite, and hematite have replaced all primary components. Hydrothermal alteration is expressed by the presence of analcime and delicate zonation in vesicles and veins of secondary minerals. The hydrothermal clay in this case is a green smectite; analcime and other zeolites are present. Soil development has occurred at Sites 871 (28.6 m thick), 873 (22.5 m), 874 (15 m), 877 (5 m), and possibly 878 (9.6 m) and 879 (40.2 m). Sites 871 and 873 exhibit the most extreme weathering and pedogenesis, expressed by soil horizonation, loss of parent relict structure, and distinct vertical zonations of mineral and chemical compositions, from kaolinitic at the surface to smectitic at the base. B horizons were identified from these paleosols, and a possible reworked A horizon occurs at Site 871. The relict parent structure is still very evident in the clays at Sites 874, 877, 878, and 879 as well as in the deeper parts of Sites 871 and 873. These are interpreted as C horizons. The parent materials—lava flows and epiclastic volcanic deposits—are completely altered to clay: to kaolinite at Site 874 and to kaolinite and smectite at the others. The clays at Site 877 are vertically zoned; they are not vertically zoned at Sites 878 or 879. The clays at Sites 878 and 879 may have been altered by groundwater, are very deep parts of soil profiles, or else are the products of multiple deposition and pedogenic events. The depth of soil development and the mineral composition (absence of gibbsite) suggest that at Site 871 on Limalok Guyot, soils developed in upland areas were subjected to annual rainfall of less than 1 m. Relief was at least tens of meters and possibly more. The presence of a drab mottled horizon at Site 873 indicates it formed in a poorly drained, possibly low-lying area on Wodejebato Island. Rainfall here was also insufficient to generate gibbsite. Similar depths of weathering mantles and the degree of soil development that occurs on older land surfaces of the older Hawaiian Islands suggest a duration for weathering and pedogenesis at Site 871 of 1 to perhaps 3 m.y.