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Rates of clay formation in three watersheds located at the Coweeta Hydrologic Laboratory, western North Carolina, have been determined from solute flux-based mass balance methods. A system of mass balance equations with enough equations and unknowns to allow calculation of secondary-mineral formation rates as well as the more commonly determined primary-mineral dissolution rates was achieved by including rare earth elements (REE) in the mass balance. Rates of clay-mineral formation determined by mass balance methods have been used to calculate the time needed for a 5% (50 g [kg.sup.-1]) change in relative clay abundance in the saprolite at Coweeta; this corresponds to the 'response time' of the clay mineral to, for example, a change in climate. The 5% change in relative clay abundance is the smallest change that can generally be detected using X-ray diffraction (XRD). Response times range from tens of thousands to hundreds of thousands of years. Extrapolating the Coweeta clay formation rates to other southern Appalachian regoliths, the time required to form measured clay abundances ('production times') in eastern Blue Ridge and Inner Piedmont regolith have been calculated. The production times of clay-mineral assemblages range from 2 k.y. to 2 m.y., with mean values ranging from 50 k.y. to 1 m.y. The results of this study are consistent with the arguments of Thiry (2000) that the best resolution of the paleoclimatic record in marine clay-rich sediments and mudrocks is ~1 or 2 m.y. Keywords: clays, rates, mass balance, Appalachians, regolith.

Allanite and epidote occur in the parent rocks of weathered regolith at the Coweeta Hydrologic Laboratory in North Carolina and exhibit different responses to weathering. Petrographically, epidote and allanite are identical at Coweeta, and only with additional analytical techniques (e.g., EDS or LAICP-MS) can the two be distinguished. Allanite is more abundant in unweathered bedrock but weathers readily below the weathering front. In contrast, the much less abundant epidote persists into the solum with only minimal evidence of weathering. The rapid dissolution of allanite is likely influenced by its metamict nature. Both epidote and allanite at Coweeta dissolve by interface-controlled dissolution kinetics, evidenced by etch pits on grain surfaces. Etch pits appear to be either large 'negative crystals,' or small, shallow, and elongated. The incipient stage of allanite weathering is characterized by AI mobility and Fe precipitation as goethite. During the initial stage of allanite weathering, carbonate precipitates, but with progressive weathering the carbonate is dissolved. Based on electron microprobe analyses and point-count data of the Ca-bearing phases in the Coweeta bedrock, accessory (

Calcic cores in plagioclase of Karymsky andesite of the 1996-2000 eruptive cycle texturally and compositionally (both trace and major elements) mimic the plagioclase phenocrysts of basalt erupted 6 km away at the onset of the cycle. These observations support the view that simultaneous eruption of andesite and basalt at Karymsky in the beginning of the cycle represents an example of replenishment and eruption triggering of an andesitic reservoir. Homogeneity of andesitic output occurred within two months. This suggests to us that blending of injected basalt into reservoir magma was thorough and rapid. Keywords: plagioclase, andesite, mixing, Kamchatka Peninsula.

Over the last forty years, research has revealed the importance of magma mixing as a trigger for volcanic eruptions, as well as its role in creating the diversity of magma compositions in arcs. Sensitive isotopic and microchemical techniques can reveal subtle evidence of magma mixing in igneous rocks, but more robust statistical techniques for bulk chemical data can help evaluate complex mixing relationships. Polytopic vector analysis (PVA) is a multivariate technique that can be used to evaluate suites of samples that are produced by mixing of two or more magma batches. The Papagayo Tuff of the Miocene-Pleistocene Bagaces Formation in northern Costa Rica is associated with a segment of the Central American Volcanic Arc. While this segment of the arc is located on oceanic plateau, recent (<8 Ma) ignimbrites bear the chemical signatures of upper continental crust, marking the transition from oceanic to continental crust. The Papagayo Tuff contains banded pumice fragments consistent with one or more episodes of mixing/mingling to produce a single volcanic deposit. The PVA solution for the sample set is consistent with observations from bulk chemistry, microchemistry and petrographic data from the rocks. However, without PVA, the unequivocal identification of the three end-member solution would not have been possible. [ABSTRACT FROM AUTHOR]

Abstract: Silicic pyroclastic flows and related deposits are abundant along the Central American volcanic front. These silicic magmas erupted through both the non-continental Chorotega block to the southeast and the Paleozoic continental Chortis block to the northwest. The along-arc variations of the silicic deposits with respect to diagnostic trace element ratios (Ba/La, U/Th, Ce/Pb), oxygen isotopes, Nd and Sr isotope ratios mimic the along-arc variation in the basaltic and andesitic lavas. This variation in the lavas has been interpreted to indicate relative contributions from the slab and asthenosphere to the basaltic magmas [Carr, M.J., Feigenson, M.D., Bennett, E.A., 1990. Incompatible element and isotopic evidence for tectonic control of source mixing and melt extraction along the Central American arc. Contributions to Mineralogy and Petrology, 105, 369–380.; Patino, L.C., Carr, M.J. and Feigenson, M.D., 2000. Local and regional variations in Central American arc lavas controlled by variations in subducted sediment input. Contributions to Mineralogy and Petrology, 138 (3), 265–283.]. With respect to along-arc trends in basaltic lavas the largest contribution of slab fluids is in Nicaragua and the smallest input from the slab is in central Costa Rica — similar trends are observed in the silicic pyroclastic deposits. Data from melting experiments of primitive basalts and basaltic andesites demonstrate that it is difficult to produce high K2O/Na2O silicic magmas by fractional crystallization or partial melting of low-K2O/Na2O sources. However fractional crystallization or partial melting of medium- to high-K basalts can produce these silicic magmas. We interpret that the high-silica magmas associated Central America volcanic front are partial melts of penecontemporaneous, mantle-derived, evolved magmas that have ponded and crystallized in the mid-crust — or are melts extracted from these nearly completely crystallized magmas. [Copyright &y& Elsevier]

Abstract: Textural and geochemical characteristics of plagioclase phenocrysts from the eruptive products of Mount Unzen (SW Japan) record repeated intrusions of basaltic magma into a dacitic host magma chamber over the lifetime of the volcano. An important aspect of this mixing style is the exchange of phenocrysts between the intruding basalt magma and host dacite magma, and the effect that this wide-reaching mixing style has on the mineralogical diversity of the erupted products. Plagioclase phenocrysts that originally crystallized from the host dacite magma are identified by oscillatory zoning patterns, low An content cores (An45 to An60), and low Sr/Ba ratios. Host-derived plagioclase phenocrysts are engulfed during intrusion of basaltic magma, evidenced by their presence in basaltic to andesitic enclaves. In response to changes in temperature and composition of the surrounding melt, the engulfed plagioclases develop resorption zones, which are composed of a densely packed network of micron-sized glass inclusions and high An content plagioclase (An72–An92) with high Sr/Ba ratios that match those of plagioclase microphenocrysts inherent to the enclave-forming magma. Over time, host-derived plagioclase phenocrysts that were once engulfed during replenishment events are recycled back to the host as enclaves disaggregate (e.g.[ Clynne, M.A., 1989. The disaggregation of quenched magmatic inclusions contributes to chemical diversity in silicic lavas of Lassen Peak, California. Bull New Mexico Bureau of Mines and Mineral Resources, 131: 54]). An eruption of andesite lava with no enclaves, something particularly unique for Unzen, occurred in 1663. Similar to enclaves, all plagioclase phenocrysts in this lava flow are surrounded by resorption zones suggesting that the 1663 lava may represent a magma that was erupted after thoroughly mixing with the intruding basaltic. Using experimentally calibrated crystallization rates, we estimate that phenocrysts exist in the Unzen chamber a minimum of 0.5–3 months between the time of their encounter with a basaltic intrusion and eruption. [Copyright &y& Elsevier]