Chemical composition and isotopic ratios of basic lavas from Iceland and the surrounding ocean floor

Major and trace dement data are used to establish the nature and extent of spatial and temporal chemical variations in basalts erupted in the Iceland region of the North Atlantic Ocean. The ocean floor samples are those recovered by legs 38 and 49 of the Deep Sea Drilling Project. Within each of the active zones on Iceland there are small scale variations in the light rare earth elements and ratios such as K/Y: several central complexes and their associated fissure swarms erupt basalts with values of K/Y distinct from those erupted at adjacent centres; also basalts showing a wide range of immobile trace element ratios occur together within single vertical sections and ocean floor drill holes. Although such variations can be explained in terms of the magmatic processes operating on Iceland they make extrapolations from single basalt samples to mantle sources underlying the outcrop of the sample highly tenuous. 87Sr/86Sr ratios measured for 25 of the samples indicate a total range from 0.7028 in a tholeiite from the Reykjanes Ridge to 0.7034 in an alkali basalt from Iceland and are consistent with other published ratios from the region. A positive correlation between 87Sr/86Sr and Ce/Yb ratios indicates the existence of systematic isotopic and elemental variations in the mantle source region. An approximately fivefold variation in Ce/Yb ratio observed in basalts with the same 87Sr/86Sr ratio implies that different degrees and types of partial melting have been involved in magma genesis from a single mantle composition. 87Sr/86Sr ratios above 0.7028, Th/U ratios close to 4 and La/Ta ratios close to 10 distinguish most basalts erupted in this part of the North Atlantic Ocean from normal mid-ocean ridge basalt (N-type MORB) - although N-type MORB has been erupted at extinct spreading axes just to the north and northeast of Iceland as well as the presently active Iceland-Jan Mayen Ridge. Comparisons with the hygromagmatophile element and radiogenic isotope ratios of MORB and the estimated primordial mantle indicate that the mantle sources producing Iceland basalts have undergone previous depletion followed by more recent enrichment events. A veined mantle source region is proposed in preference to the mantle plume model to explain the chemical variations.

Mineralogy and geochemistry of Miocene tuff from ODP Hole 129-802A

Drilling at Ocean Drilling Program Site 802 in the central Mariana Basin, northwest Pacific Ocean, revealed an unexpected 222-m-thick sequence of well-cemented tuff of Miocene age. The deposits are unusual in that their source is presumably an unmapped seamount and they exhibit several peculiar petrological and mineralogical features. The well-developed secondary mineral sequence which includes analcime is rare in such relatively young, unburied deposits, in an area where there is little other evidence of hydrothermal activity. The massive tuff section also contains abundant fissure veins made of a rare silicate carbonate sulfate hydroxide hydrate of calcium, called thaumasite, which has not before been described in deep submarine deposits. The smectite-zeolite-thaumasite paragenesis coincides with the presence of chloride and calcium-enriched interstitial waters. The diagenetic evolution of the deposit appears to have been largely controlled by the depositional mode. The discharges of disaggregated and rejuvenated volcaniclasts seem to have been abrupt and repeated. The Miocene tuff at Site 802 thus provides new insights on the interactions between basaltic glass, biogenic phases, and seawater, in a specific deep-sea environment.

Geochemistry, isotopes, trace elements and fluid inclusion geothermometry at DSDP Hole 83-504B

Stockwork-like metal sulfide mineralizations were found at 910-928 m below seafloor (BSF) in the pillow/dike transition zone of Hole 504B. This is the same interval where most physical properties of the 5.9-m.y.-old crust of the Costa Rica Rift change from those characteristic of Layer 2B to those of Layer 2C. The pillow lavas, breccias, and veins of the stockwork-like zone were studied by transmitted and reflected light microscopy, X-ray diffraction, and electron microprobe analysis. Bulk rock oxygen isotopic analyses as well as isolated mineral oxygen and sulfur isotopic analyses and fluid inclusion measurements were carried out. A complex alteration history was reconstructed that includes three generations of fissures, each followed by precipitation of characteristic hydrothermal mineral parageneses: (1) Minor and local deposition of quartz occurred on fissure walls; adjacent wall rocks were silicified, followed by formation of chlorite and minor pyrite I in the veins, whereas albite, sphene, chlorite and chlorite-expandable clay mixtures, actinolite, and pyrite replaced igneous phases in the host rocks. The hydrothermal fluids responsible for this first stage were probably partially reacted seawater, and their temperatures were at least 200-250° C. (2) Fissures filled during the first stage were reopened and new cracks formed. They were filled with quartz, minor chlorite and chlorite-expandable clay mixtures, traces of epidote, common pyrite, sphalerite, chalcopyrite, and minor galena. During the second stage, hydrothermal fluids were relatively evolved metal- and Si-rich solutions whose temperatures ranged from 230 to 340° C. The fluctuating chemical composition and temperature of the solutions produced a complex depositional sequence of sulfides in the veins: chalcopyrite I, ± Fe-rich sphalerite, chalcopyrite II ("disease"), Fe-poor sphalerite, chalcopyrite III, galena, and pyrite II. (3) During the last stage, zeolites and Mg-poor calcite filled up the remaining spaces and newly formed cracks and replaced the host rock plagioclase. Analcite and stilbite were first to form in veins, possibly at temperatures below 200°C; analcite and earlier quartz were replaced by laumontite at 250°C, whereas calcite formation temperature ranged from 135 to 220°C. The last stage hydrothermal fluids were depleted in Mg and enriched in Ca and 18O compared to seawater and contained a mantle carbon component. This complex alteration history paralleling a complex mineral paragenesis can be interpreted as the result of a relatively long-term evolution of a hydrothermal system with superimposed shorter term fluctuations in solution temperature and composition. Hydrothermal activity probably began close to the axis of the Costa Rica Rift with the overall cooling of the system and multiple fracturing stages due to movement of the crust away from the axis and/or cooling of a magmatic heat source.

Composition of basalts from the north part of the Bougault anomaly, Mid-Atlantic Ridge 15°N

A quantitative model of development of magmatic and ore-magmatic systems under crests of mid-ocean ridges is constructed. Correct physical models of melting zone formation in approximation to active spreading, non-stationary dynamics of magma intrusion from a center of generation, filling of magma chambers of various shapes, feeding of fissure-type volcanoes, and retrograde boiling of melts during solidification of intrusive bodies beneath axial zones of spreading in crests of ridges are proposed. Physicochemical and mathematical theories of disintegration of multi-component solutions, growth of liquational drops of ore melts, and sublimation of components from magmatic gases are elaborated. Methods for constructing physically correct models of heat and mass transfer in heterophase media are devised. Modeling of development of magmatic and ore-magmatic systems on the basis of the Usov-Kuznetsov facies method and the Pospelov system approach are advanced. For quantitative models numerical circuits are developed and numerical experiments are carried out.