ALBERT

Description: The northwest trending walls of the Pito Deep Rift (PDR), a tectonic window in the southeast Pacific, expose in situ oceanic crust generated ?3 Ma at the superfast spreading southern East Pacific Rise (SEPR). Whole rock analyses were performed on over 200 samples of dikes and lavas recovered from two ~8 km**2 study areas. Most of the PDR samples are incompatible-element-depleted normal mid-ocean ridge basalts (NMORB; (La/Sm)N 〈 1.0) that show typical tholeiitic fractionation trends. Correlated variations in Pb isotope ratios, rare earth element patterns, and ratios of incompatible elements (e.g., (Ce/Yb)N) are best explained by mixing curves between two enriched and one depleted mantle sources. Pb isotope compositions of most PDR NMORB are offset from SEPR data toward higher values of 207Pb/204Pb, suggesting that an enriched component of the mantle was present in this region in the past ?3 Ma but is not evident today. Overall, the PDR crust is highly variable in composition over long and short spatial scales, demonstrating that chemically distinct lavas and dikes can be emplaced within the same segment over short timescales. However, the limited spatial distribution of high 206Pb/204Pb samples and the occurrence of relatively homogeneous MgO compositions (ranging 〈2.5 wt %) within a few of the individual dive transects (over distances of ~1 km) suggests that the mantle source composition evolved and magmatic temperatures persisted over timescales of tens of thousands of years. The high degree of chemical variability between pairs of adjacent dikes is interpreted as evidence for along-axis transport of magma from chemically distinct portions of the melt lens. Our findings suggest that lateral dike propagation occurs to a significant degree at superfast spreading centers.

Description: Records of long-term sediment deposition in the Caribbean Sea were recovered during Ocean Drilling Program Leg 165. Samples from the Cayman Rise (Site 998), the Colombian Basin (Site 999), and the Hess Escarpment (Site 1001) were analyzed for calcium carbonate (CaCO3) by coulometry and for selected major and trace elements by X-ray fluorescence and inductively coupled plasma-emission spectrometry. These data were used to quantify in the bulk sediment the absolute concentrations of CaCO3, terrigenous matter, and dispersed ash (as opposed to discrete ash layers). The weight percent of terrigenous matter was computed using a Cr-based normative calculation, and dispersed ash content was calculated by difference; the assumption of a three component system (CaCO3, ash, and terrigenous matter) is justified by and consistent with petrographic analysis. Sites 998 and 999 broadly exhibit the same pattern of terrigenous accumulation. Both show a general decrease in terrigenous accumulation rates during the Oligocene and early Miocene, except for a sharp increase at Site 998 during the early Oligocene (30-40 Ma) and significant increases in the late Miocene and late Pliocene/early Pleistocene. The same pattern in terrigenous accumulation is recorded at Sites 925 and 929 in the Ceara Rise (Atlantic Ocean), which receives input from an Amazon River source, demonstrating that Sites 925, 929, 998, and 999 collectively provide a circum-Andean record of tectonic uplift, with the Leg 165 sites responding to inputs from the Magdelena River system. Both Sites 998 and 999 appear to be responding to South and Central American inputs, particularly after the middle Miocene; however, the variation in the terrigenous, carbonate, and dispersed ash at Site 998 point to an erosional event during the Oligocene that is apparently unique to this site's location. Dispersed ash commonly accounts for 15-20 wt% of the bulk sediment, and in some cases up to 45 wt%. The accumulation of dispersed ash typically leads the accumulation of discrete layers by 2-10 m.y. These changes in sediment composition could signify (1) the distance from the source of volcanism, (2) periods of small volume volcanic activity preceding the large eruptions, or (3) the transportation to the deep sea of terrestrially deposited ash preceding the large eruptions.

Description: This study investigates the internal anatomy and petrogenesis of the Tuolumne Intrusive Suite (TIS), which comprises metaluminous, high-potassium, calc-alkaline granitoids typical of the Sierra Nevada batholith. Although the TIS has often been cited as an example of a large magma chamber that cooled and fractionated from the margins inward, its geochemistry is inconsistent with closed-system fractionation. Most major elements are highly correlated with SiO2, but the scattered nature of trace elements and variations of initial Sr and Nd isotopic ratios indicate that fractional crystallization is not the predominant process responsible for its chemical evolution. Isotopic data suggest mixing between melts of mantle-like rocks and a granitic melt similar in composition to the highest-silica TIS unit. Monte Carlo models of magma mixing confirm that such processes can reproduce the observed variations in major elements, trace elements and isotopic ratios. Thermobarometry suggests emplacement at depths near 6 km and crystallization temperatures ranging from 660 to 750 {degrees}C. Feldspars, hornblende, biotite and magnetite exhibit evidence of extensive low-temperature subsolidus exsolution. The TIS as a whole trends toward more evolved isotopic compositions and younger U-Pb zircon ages passing inward. This pattern indicates a general increase in the proportion of felsic, crustally derived melt in the mixing process, which may have resulted from net accumulation of heat added to the lower crust by intrusion of mantle-derived mafic magma. However, the bulk geochemical and isotopic compositions of the equigranular Half Dome Granodiorite, the porphyritic Half Dome Granodiorite and the Cathedral Peak Granodiorite overlap one another and the contacts between them are commonly gradational. We interpret these map units to represent a single petrological continuum rather than distinct intrusive phases. The textural differences that define the units probably reflect thermal evolution of the system rather than distinct intrusive events.

Description: Zircons in transport in the modern Amazon River range from coarse silt to medium sand. Older grains are smaller on average: Mesozoic and Cenozoic grains have average equivalent spherical diameter (ESD) 122 {+/-} 42 {micro}m (lower fine sand), whereas grains 〉2000 Ma have average ESD 67 {+/-} 14 {micro}m (upper coarse silt). As a full Wentworth size class separates the two values, zircons in these age populations are hydraulically distinct. Host sand size is correlated with average size of co-transported zircons, implying hydrodynamic fractionation. Zircon size is positively correlated with percent medium sand, and inversely correlated with percent very fine sand (p 50% medium sand, average zircon size is 100 {micro}m, compared with 80 {micro}m in samples with 〉50% very fine sand. We infer from these data that zircon deposition is not size-blind, and that zircons track with hydraulically comparable sand grains. As different aged grains tend to have different characteristic sizes, this indicates the possibility of hydrodynamic fractionation of age populations. Five samples representing different hydrodynamic microenvironments of a single dune present significantly different detrital zircon age spectra, apparently the result of hydraulic processes. Peak mismatch (age peaks failing to overlap at 2{sigma} level) is the most common disparity; but age populations present in some samples are missing from other samples. The lack of correspondence among the samples appears to exceed that attributable to random sampling. We conclude that hydrodynamic fractionation of zircons and zircon-age populations does occur. Zircon size should therefore be taken into consideration in detrital zircon provenance analysis.

Description: Ian Carmichael wrote of an “andesite aqueduct” that conveys vast amounts of water from the magma source region of a subduction zone to the Earth’s surface. Diverse observations indicate that subduction zone magmas contain 5 wt % or more H2O. Most of the water is released from crystallizing intrusions to play a central role in contact metamorphism and the genesis of ore deposits, but it also has important effects on the plutonic rocks themselves. Many plutons were constructed incrementally from the top down over million-year time scales. Early-formed increments are wall rocks to later increments; heat and water released as each increment crystallizes pass through older increments before exiting the pluton. The water ascends via multiple pathways. Hydrothermal veins record ascent via fracture conduits. Pipe-like conduits in Yosemite National Park, California, are located in or near aplite–pegmatite dikes, which themselves are products of hydrous late-stage magmatic liquids. Pervasive grain-boundary infiltration is recorded by fluid-mediated subsolidus modification of mineral compositions and textures. The flood of magmatic water carries a large fraction of the total thermal energy of the magma and transmits that energy much more rapidly than conduction, thus enhancing the fluctuating postemplacement thermal histories that result from incremental pluton growth. The effects of water released by subduction zone magmas are central not only to metamorphism and mineralization of surrounding rocks, but also to the petrology and the thermal history of the plutons themselves.