ALBERT

Description: Sixty-three samples representing 379 m of sheeted dikes from Deep Sea Drilling Project/Ocean Drilling Program Site 504B have been analyzed for major and selected trace elements by X-ray fluorescence. The samples range from microcrystalline aphyric basalts to moderately phyric (2%-10% phenocrysts) diabase that are typically multiply saturated with plagioclase, olivine, and clinopyroxene, in order of relative abundance. All analyzed samples are classified as Group D compositions with moderate to slightly elevated compatible elements (MgÆ-value = 0.65% ± 0.03%; Al2O3 = 15.5% ± 0.8%; CaO = 13.0% ± 0.3%; Ni = 114 ± 29 ppm), and unusually depleted levels of moderate to highly incompatible elements (Nb 〈 1 ppm; Zr = 44 ± 7 ppm; Rb 〈 0.5 ppm; Ba ~ 1 ppm; P2O5 = 0.07% ± 0.02%). These compositions are consistent with a multistage melting of a normal ocean ridge basaltic mantle source followed by extensive fractionation of olivine, plagioclase, and clinopyroxene. Leg 140 aphyric to sparsely phyric (0%-2% phenocrysts) basalts and diabases are compositionally indistinguishable from similarly phyric samples at higher levels in the hole. An examination of the entire crustal section, from the overlying volcanics through the sheeted dikes observed in Leg 140, reveals no significant trends indicating the enrichment or depletion of Costa Rica Rift Zone source magmas over time. Similarly, significant trends toward increased or decreased differentiation cannot be identified, although compositional patterns reflecting variable amounts of phenocryst addition are apparent at various depths. Below ? 1700 mbsf to the bottom of the Leg 140 section, there is a broadly systematic pattern of Zn depletion with depth, the result of high-temperature hydrothermal leaching. This zone of depletion is thought to be a significant source of Zn for the hydrothermal fluids depositing metal sulfides at ridge-crest hydrothermal vents and the sulfide-mineralization zone, located in the transition between pillow lavas and sheeted dikes. Localized zones of intense alteration (60%-95% recrystallization) are present on a centimeter to meter scale in many lithologic units. Within these zones, normally immobile elements Ti, Zr, Y, and rare-earth elements are strongly depleted compared with "fresher" samples centimeters away. The extent of compositional variability of these elements tends to obscure primary igneous trends if the highly altered samples are not identified or removed. At levels up to 40% (or possibly 60%) recrystallization, Ti, Zr, and Y retain their primary signatures. Although the mechanisms are unclear, it is possible that these intense alteration zones are a source of Y and rare-earth elements for the typically rare-earth-element-enriched hydrothermal vent fluids of mid-ocean ridges.

Description: Fifty samples of basalt recovered during ODP Leg 111 from the dikes (Layer 2C) of Hole 504B (1350.0-1562.3 m below seafloor) were analyzed by X-ray-fluorescence techniques. All of the samples are highly depleted in magmaphile elements relative to other mid-ocean ridge basalts, with TiO2 = 0.75-1.24 wt%, Na2O = 1.59-2.22 wt%, Zr = 38-64 ppm, Nb = 0.3-1.5 ppm, and Y = 20-30 ppm (for samples containing 0%-2% phenocrysts), but have ratios of highly incompatible elements similar to normal Type I mid-ocean ridge basalts (e.g., Zr/Nb 〉 30). Abundances of compatible elements are similar to those of typical mid-ocean ridge basalts, with MgO = 7.2-9.2 wt%, Fe2O3* = 9.3-12.5 wt%, Ni = 55-164 ppm, and Cr = 26-388 ppm. Approximately 2% of the samples recovered from the top part of Hole 504B are similar to normal Type I or Type II ocean floor basalts. However, all of the analyzed Leg 111 samples from Hole 504B are depleted basalts. Aphyric dike rocks from Leg 111 are virtually identical to the depleted aphyric samples recovered from the pillow lavas and dikes in the upper 1075 m of Hole 504B during DSDP Legs 69, 70, and 83, with the exception of elements readily altered by seawater (Sr, Rb, and K). These elements reach a maximum in both abundance and variability in the pillow lavas of the upper 571.5 m of Hole 504B and decline to more constant values in the dike system sampled on Legs 83 and 111, apparently as a result of a decrease in porosity and increase in alteration temperatures relative to the pillow lavas. Based on compositional similarities to the vast majority of the pillows and flows, the dikes sampled on Leg 111 appear to be the feeder system for the pillow lavas in the upper part of Hole 504B. The incompatible-element-depleted compositions of the Costa Rica Rift Zone basalts are consistent with multistage melting of a normal mid-ocean ridge source.

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: To test whether a catastrophic earthquake could affect an active magma system, mean abundances (adjusted for 'olivine control') of titanium, potassium, phosphorus, strontium, zirconium, and niobium of historic lavas erupted from Mauna Loa Volcano, Hawaii, after 1868 were analyzed and were found to decrease sharply relative to lavas erupted before 1868. This abrupt change in lava chemistry, accompanied by a halved lava-production rate for Mauna Loa after 1877, is interpreted to reflect the disruptive effects of a magnitude 7.5 earthquake in 1868. This interpretation represents a documentable case of changes in magmatic chemical variations initiated or accelerated by a major tectonic event.