Hydrothermal Origin of the Clays from the Upper Slopes of Mauna Kea, Hawaii

-The soils of the summit region of Mauna Kea are similar to the soils of the high mountain deserts and to the soils of cold deserts. Dramatic differences, however, exist between the soils of the summit and other neighboring cones and the soils of the glaciated terrain. The soils of some of the cones of the summit area are clay rich and contain phyllosilicate minerals; the soils of the glaciated terrain are sandy and contain X-ray amorphous clay. Montmorillonite and a Mg-rich trioctahedral mineral identified as saponite are the clay minerals of the summit. Because the summit area of Mauna Kea supported an ice cap at the time of the formation of the cones, the origin of the smectite minerals could have resulted from the alteration of the tephra by steam and water released in the melting of the ice. Hypogene fluids are, however, more likely to be responsible for the genesis of the phyllosilicate minerals. I N T R O D U C T I O N The soils of the summit region of Mauna Kea (Fig, 1) are analogous to the soils of the high mountain deserts of the world and similar to the soils of the cold deserts of the polar regions. The climate of the high elevations ofMauna Kea, a dormant volcano rising some 4200 m above sea level, is cool to cold and extremely dry, with nocturnal freezing temperatures throughout the year (Raine, 1934; Woodcock et aL, 1966; Tagliaferro, 1971). Bioclimatically, the mountain top has been defined as a semi-arid, barren 'alpine desert tundra (MuellerDombois and Krajina, 1968) characterized by a very low density of vascular plants. Pleistocene ice caps have repeatedly covered the summit area and recent work of Porter (1970) indicates that three and possibly four episodes of glaciation can be recognized. The soils above 3500 m and formed on glacial deposits are poorly developed: they lack an organic surface layer, have a desert pavement, show coarse texture and display below the desert pavement a brown yellowishred horizon. Similarly, in general terms, their chemistry and mineralogy indicate incipient weathering as manifested by small clay content and the dominance of X-ray-amorphous colloids over crystalline material. These characteristics are expected considering the severe bioclimatic environment, the age, and the volcanic nature of the terrain. Notably, exceptions are found with respect to texture and mineralogy. The exceptions occur on the summit of Mauna Kea, Douglas Cone, Puu Waiau, and other sites on the north side of the summit (Fig. 1). In these areas the soils show dramatically different texture and mineralogy, indicating a more advanced stage of weathering CCM: Vol. 22, No. 2 D Fig, I. Map of the south slopes of Mauna Kea showing the pyroclastic cones on the summit plateau (shaded), and the limits of the Makanaka and Waihu ice caps. Contours are in meters (modified after Porter).