Oxygen isotopic and geochemical evidence for a short-lived, high-temperature hydrothermal event in the Chegem caldera, Caucasus Mountains, Russia

18O/16O studies of fossil fissure fumaroles from the Valley of Ten Thousand Smokes, Alaska

“…This preservation of primary magmatic 8180 values in the VTTS feldspar phenocrysts is clearly a consequence of the extremely short time span (i.e., 1912 to = 1923) of vigorous, high-temperature, fumarolic activity in the 1912 ash-flow sheet. These 180/160 systematics are strikingly similar to those discovered in the 2.8-Ma intracaldera Chegem Tuff (Gazis et al 1996) and in the fossil fumaroles in the outflow sheet of the 0.76 Ma Bishop Tuff (Holt and Taylor 1998), thus confirming that a similar type of fumarolic meteoric-hydrothermal activity occurred above the zone of intense welding in all three of these ash-flow tufts. This is particularly important, because it provides a direct linkage between the older tufts and the actual observations at the VTTS of steam chemistry, water/rock interaction, circulation geometry, flow velocities, and fumarolic temperaEditorial responsibility: T.H.…”

“…Hildreth (1987) for the rhyolite (solid gray; n=6) and the dacite/andesite (gray stipple; n=5) parts of the ashflow sheet and the overlying ash-fall deposit. Together with the rhyolite t80#60 data of Hildreth (1987), samples MLA-1, MLA-2, MLA-3, RL-2, MLB-3, and KC-5 largely define the same kind of steep trajectory that is characteristic of the Bishop Tuff (Holt and Taylor 1998) and the Chegem Tuff (Gazis et al 1996). However, the feldspars in samples KC-2 and KC-I3 have undergone an anomalous shift to higher 8180 values (as a result of low-tempera-18 ture alteration 9 see text).…”

“…Here, steam temperatures as high as 645~ (Allen and Zies 1923) were measured in fumaroles emanating from the surface of the 20-km-long, 4-to 10-km-wide, 200-m-thick ash-flow sheet. Following up on this work by Gazis et al (1996), Holt and Taylor (1998) reported that 180/160 patterns analogous to those discovered in the Chegem Tuff could also be found in the outflow facies of the Bishop Tuff in California. However, it remained to be determined if the 1912 VTTS ashflow sheet, the only known historical example of this particular kind of fumarolic activity, also shares this characteristic 180/160 signature.…”

“…This volcanic glass contains up to 4.5 wt.% H20 and has a very low (i.e., meteoric) 8D value, clearly demonstrating that this zone in the upper part of the tuff interacted in some way with large amounts of low-t80, low-D meteoric waters at high temperatures (>300~ In virtually all previous studies of volcanic rocks from either active or fossil geothermal areas (e.g., see Criss and Taylor 1986;Larson and Taylor 1986), both the feldspar phenocrysts and the coexisting groundmass are: (a) at least partially hydrothermally altered and replaced by OH-bearing minerals such as, for example, sericite, saussurite, chlorite, and kaolinite; and/or (b) significantly depleted in tsO relative to their coexisting quartz phenocrysts (quartz is one of the more resistant minerals to hydrothermal t80 exchange, and unlike feldspar, it usually closely retains its primary magmatic 8~80 value in such situations). Gazis et al (1996) postulated that this unique combination of 180 signatures in feldspar and glass at Chegem was the result of an intense, short-lived (=25-year duration), very high-temperature (500-650~ meteoric-hydrothermal event, the surface expression of which might have been analogous to the vigorous fumarolic activity observed during [1916][1917][1918][1919] in the Valley of Ten Thousand Smokes (VTTS), Alaska (Fig. 1).…”

“…Although 180/160 and D/H studies have been carried out on more than a hundred active and fossil meteorichydrothermal systems during the past few decades (see review by Criss and Taylor 1986), it was only recently that a new and conceptually different kind of meteoric-hydrothermal oxygen-isotope signature was discovered by Gazis et al (1996) in their study of the 2.8 Ma Chegem caldera (llx15 km) in the Caucasus Mountains. Those authors describe a 750-m-thick stratigraphic zone in the Hildreth and Fierstein 1987), and the three sites that were sampled in this study: River Lethe (RL): Knife Creek (KC); and Mageik Lakes (ML).…”