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[1] We collected 56 marine gravity cores from the Pacific seafloor offshore Central America which contain a total of 213 volcanic ash beds. Ash-layer correlations between cores and with their parental tephras on land use stratigraphic, lithologic, and compositional criteria. In particular, we make use of our newly built database of bulk-rock, mineral, and glass major and trace element compositions of plinian and similarly widespread tephras erupted since the Pleistocene along the Central American Volcanic Arc. We thus identify the distal ashes of 11 Nicaraguan, 8 El Salvadorian, 6 Guatemalan, and 1 Costa Rican eruptions. Relatively uniform pelagic sedimentation rates allow us to determine ages of 10 previously undated tephras by their relative position between ash layers of known age. Linking the marine and terrestrial records yields a tephrostratigraphic framework for the Central American volcanic arc from Costa Rica to Guatemala. This is a useful tool and prerequisite to understand the evolution of volcanism at a whole-arc scale.

A Pliocene submarine series of alkali basaltic pillow lavas, hyaloclastites, and breccias (A), a sheeted dike swarm (B), and a basal suite of gabbro and ultramafic rocks (C) from La Palma (Canary Islands) is interpreted as a cross section through an uplifted seamount. This series has been tilted to its present orientation of 50ø/230 ø (plunge and azimuth), probably by upwarping due to intrusions in the central portion of the island. The basal plutonic complex (C) also includes intrusives coeval with up to 2000 m of younger subaerial alkali basaltic lavas unconformably overlying the submarine series. The plutonic suite (C) is overlain abruptly by more than 1800 m of sills (B), 0.4-1 m thick on average, with minor screens of lavas and breccias. Extrusives (A) form a 1750 m thick sequence of pillow lavas, breccias, and hyaloclastites. The clastic rocks increase in abundance upward and are of four main types: (1) breccias, consisting of partly broken pillows, formed nearly in situ, (2) heterolithologic pillow fragment breccias, (3) hyaloclastites composed dominantly of highly vesicular lapJill and ash sized shards, the latter thought •to have formed by near surface explosive eruptions and been subsequently transported downslope by mass flows, (2) and (3) being interpreted to have been resedimented, and (4) pillow scoria breccias from the upper 700 m of the extrusive section consisting of amoeboidal, highly vesicular "pillows" and lava stringers and local bombs, probably formed by cracking and "bleeding" of gas-rich expanding pillow lava and some shallow submarine/subaerial lava fountaining. The extrusive series is chemica!ly and mi•eralogically crudely zoned, with the most differentiated rocks (metatrachytes and mugearires) at the base and most picritic: lavas occurring near the top of the series. Subsequent to emplacement, the entire extrusive and intrusive series has been hydrothermally altered, the lower part to greenschist and the upper part to smectite--zeolite facies mineral assemblages. The La Palma succession, combined with evidence from surface studie s of seamounts, suggests that seamounts are formed by intrusive and extrusive processes in approximately equal portions. The nature of eruptive clasti c and depositional mechanisms changes drastically during growth of a seamount if the critical depth for major magmatic degassing is surpassed and especially if magmatic explosive processes can occur at very shallow water depth, the critical depth dependi'ng on magma and thus volatile composition. •Changes in slopes of a seamount influence depositional processes. Based on these factors, at least three major depositional sites develop as a seamount grows' summir, flank, and apron facies. Nonexplosive, extrusive processes prevail in the Deep Water Stage, dominant•!y producing pillow lavas (75%). These consist of individual pillow volcanoes'Up to 200 m high, with large pillows near the base and decreasing pillow size toward the top Of a volcano. Pillow breccias•, and pillow fragment breccias comprise approximately 20...

Palagonite is the first stable product of volcanic glass alteration. It is a heterogeneous material, usually with highly variable optical and structural properties, ranging from a clear, transparent, isotropic, smooth and commonly concentrically banded material, commonly called "gel-palagonite", to a translucent, anisotropic, slightly to strongly birefringent material of fibrous, lathlike or granular structure, commonly called "fibropalagonite". The color of palagonite ranges from shades of yellow to shades of brown. Palagonite forms rinds of variable thickness on every mafic glass surface exposed for some time to aquatic fluids. It is formed by either incongruent dissolution or by congruent dissolution of glass with contemporaneous precipitation of insoluble material at the glass-fluid interface. The process of palagonitization is accompanied by extensive mobilization of all elements involved in the alteration process, resulting in the depletion or enrichment of certain elements. The extent and direction of element mobility and the palagonitization process itself (including the rate of palagonitization) depend on a number of different, complex interacting properties: e.g. (1) temperature, (2) the structure of the primary material, (3) the reactive surface area of the primary material, (4) the structure of the precipitating secondary phases, (5) the growth rates of the secondary phases, (6) time, and (7) fluid properties such as fluid flow rates, pH, Eh, ionic strength, and oxygen fugacity. The fluid properties themselves are affected by different hydrogeological properties such as porosity, permeability, and pressure gradients.

Seafloor alteration of the basaltic upper oceanic crust provides one of the major geochemical pathways between the mantle, the ocean/atmosphere and subduction zone regimes. Yet, no reliable mass balances are available, in large part because of the extremely heterogeneous distribution of altered materials in the oceanic crust but also because of the limited availability of high recovery drill cores. In this paper, we document the feasibility of determining the bulk altered and fresh composition of the oceanic crust on a !0-500 m length scale, from a region in the western Atlantic Ocean (DSDP/ODP Sites 417-418). Unaltered compositions were obtained from glass and phenocryst data and altered compositions were determined through analysis of composite samples. Most of the alteration-related chemical inventory resides preferentially in the upper oceanic crust and in highly permeable volcaniclastics. Most major elements (Si, A1, Mg, Ca, and Na) and many trace elements (Sr, Ba, LREE's) experience substantial large scale redistribution, but fluxes are relatively low. Overall, 12 wt % are added to the crust, mostly H20, CO 2, and K, but the distribution varies widely. High field strength elements, Th, Ti and Fe remain essentially immobile during low temperature alteration, while most other elements are affected to some degree. While the total fluxes are relatively small, the re-distribution of alteration -sensitive elements in the ocean crust is much larger, even on length scales exceeding 100m. The bulk composition of the upper 500m at Sites 417/418 can be used to constrain the impact of ocean crust subduction on element recycling to volcanic arcs. Flux balances indicate that the altered domains within the upper basaltic crust may contribute a very large proportion of some element fluxes recycled to the arc (H20, CO 2, K, Rb, U), while other element fluxes require additional contributions from sediments and deeper oceanic crust. whereby approximately 5 x 1() •6 grmns of basalt m'e generated and recycled per yem'. This cycle provides a pathway for •nanfle comlx)nents into the hydrosphere, and lbr sea water derived elements into subduction zones ,and the mm•tle. Chemical fluxes in fl•ese pafl•ways ,'u'e extremely poorly constrained, including the extent of high te•nperature altera-' tion at ridges, as well as off-axis low temperature chemic,'d exchange. This lack of &xta provides a major stumbling block in our understranding of earth chemic,-d dynamics. The alteration or' the ocemfic crust on the se•dloor, ,-red its subsequent met,'unorphism m•d chemical losses during subduction have a major impact on the loci ,'red composition of arc magmatism. Sever,'d recent studies have pointed to the altered b•salfic crust specificsally a,s a major source of elements recycled to volcanic arcs during subduc-fion (e.g. H20 [Peacock, 1990; Ph-mk, 1994]; B [Ishikawa m•d Nakamura, 1993]; Pb [Miller et ,'d, 1992; Peucker-Ehrenbrinck et aL, 1995]). Despite the recognition of the ,altered oceanic 20 GEOCHEMICAL FLUXES DURING SEAFLOOR ALTERATIO...

[1] Abstract: The structural and chemical evolution of palagonite was studied as a function of glass composition, alteration environment, and time by applying a range of analytical methods (electron microprobe, infrared photometry, atomic force microscopy, X-ray fluorescence, and X-ray diffraction). Palagonitization of volcanic glass is a continuous process of glass dissolution, palagonite formation, and palagonite evolution, which can be subdivided into two different reaction stages with changing element mobilities. The first stage is characterized by congruent dissolution of glass and contemporaneous precipitation of``fresh,'' gel-like, amorphous, optically isotropic, mainly yellowish palagonite. This stage is accompanied by loss of Si, Al, Mg, Ca, Na, and K, active enrichment of H 2 O, and the passive enrichment of Ti and Fe. The second stage is an aging process during which the thermodynamically unstable palagonite reacts with the surrounding fluid and crystallizes to smectite. This stage is accompanied by uptake of Si, Al, Mg, and K from solution and the loss of Ti and H 2 O. Ca and Na are still showing losses, whereas Fe reacts less consistently, remaining either unchanged or showing losses. The degree and direction of element mobility during palagonitization was found to vary mainly with palagonite aging, as soon as the first precipitation of palagonite occurs. This is indicated by the contrasting major element signatures of palagonites of different aging steps, by the changes in the direction of element mobility with palagonite aging, and by the general decrease of element loss with increasing formation of crystalline substances in the palagonite. Considering the overall element budget of a water-rock system, the conversion of glass to palagonite is accompanied by much larger element losses than the overall alteration process, which includes the formation of secondary phases and palagonite aging. The least evolved palagonitized mafic glass studied has undergone as much as 65 wt% loss of elements during palagonite formation, compared to $28 wt% element loss during bulk alteration. About 33 wt% element loss was calculated for one of the more evolved, in terms of the aging degree, rocks studied, compared to almost no loss for bulk alteration.

The lithostratigraphic framework of Lake Van, eastern Turkey, has been systematically analysed to document the sedimentary evolution and the environmental history of the lake during the past ca 600 000 years. The lithostratigraphy and chemostratigraphy of a 219 m long drill core from Lake Van serve to separate global climate oscillations from local factors caused by tectonic and volcanic activity. An age model was established based on the climatostratigraphic alignment of chemical and lithological signatures, validated by 40 Ar/ 39 Ar ages. The drilled sequence consists of ca 76% lacustrine carbonaceous clayey silt, ca 2% fluvial deposits, ca 17% volcaniclastic deposits and 5% gaps. Six lacustrine lithotypes were separated from the fluvial and event deposits, such as volcaniclastics (ca 300 layers) and graded beds (ca 375 layers), and their depositional environments are documented. These lithotypes are: (i) graded beds frequently intercalated with varved clayey silts reflecting rising lake levels during the terminations; (ii) varved clayey silts reflecting strong seasonality and an intralake oxic-anoxic boundary, for example, lake-level highstands during interglacials/interstadials; (iii) CaCO 3 -rich banded sediments which are representative of a lowering of the oxic-anoxic boundary, for example, lake level decreases during glacial inceptions; (iv) CaCO 3 -poor banded and mottled clayey silts reflecting an oxic-anoxic boundary close to the sediment-water interface, for example, lake-level lowstands during glacials/stadials; (v) diatomaceous muds were deposited during the early beginning of the lake as a fresh water system; and (vi) fluvial sands and gravels indicating the initial flooding of the lake basin. The recurrence of lithologies (i) to (iv) follows the past five glacial/interglacial cycles. A 20 m thick disturbed unit reflects an interval of major tectonic activity in Lake Van at ca 414 ka BP. Although local environmental processes 1830