Step-heating degassing data for argon-39 from volcanic rocks drilled in the Shikoku Basin and Mariana trench, DSDP Legs 58 and 60

From the experimental data on stepwise thermal release of neutron induced 39Ar (39K (n, p) 39Ar) from rocks and minerals, Arrhenius plots were constructed, which gave activation energies for the thermal release process. The activation energies for DSDP Leg 58 and Leg 60 submarine volcanic rocks range from 12 to 20 kcal/mol, whereas those for granodiorites and the K-feldspar separates have activation energies ranging from 37 to 48 kcal/mol. The smaller activation energies for the submarine volcanic rocks reflect the grain boundary diffusion process, while the thermal diffusion of 39Ar from granodiorites and K-feldspar is essentially controlled by a volume diffusion. The grain boundary diffusion for the submarine volcanic rocks suggests that K resides essentially in the grain boundaries.

Composition of sediment pore water and isotopes in carbonate and benthic foraminifera at DSDP Holes 68-501, 69-505 and Site 69-504

Two sites on the southern flank of the Costa Rica Rift were drilled on DSDP Legs 68 and 69, one on crust 3.9 m.y. old and the other on crust 5.9 m.y. old. The basement of the younger site is effectively cooled by the circulation of seawater. The basement of the older site has been sealed by sediment, and an interval in the uppermost 560 meters of basement recently reheated to temperatures of 60 to 120°C. Although the thickness of the sediments at the two sites is similar (150-240 m versus 270 m), the much rougher basement topography at the younger Site 505 produces occasional basement outcrops, through which 80 to 90% of the total heat loss apparently occurs by advection of warm seawater. This seawater has been heated only slightly, however; the temperature at the base of the sediments is only 9°C. Changes in its composition due to reaction with the basement basalts are negligible, as indicated by profiles of sediment pore water chemistry. Bacterial sulfate reduction in the sediments produces a decrease in SO4 (and Ca) and an increase in alkalinity (and Sr and NH3) as depth increases to an intermediate level, but at deeper levels these trends reverse, and all of these species plus Mg, K, Na, and chlorinity approach seawater values near basement. Si, however, is higher, and Li may be lower. At the older site, Site 501/504, where heat loss is entirely by conduction, the temperature at the sediment/basement contact is 59°C. Sediment pore water chemistry is heavily affected by reaction with the basaltic basement, as indicated by large decreases in d18O, Mg, alkalinity, Na, and K and an increase in Ca with increasing depth. The size of the changes in d18O, Mg, alkalinity, Ca, Sr, and SO4 varies laterally over 500 meters, indicating lateral gradients in pore water chemistry that are nearly as large as the vertical gradients. The lateral gradients are believed to result from similar lateral gradients in the composition of the basement formation water, which propagate upward through the sediments by diffusion. A model of the d18O profile suggests that the basement at Site 501/504 was sealed off from advection about 1 m.y. ago, so that reaction rates began to dominate the basement pore water chemistry. A limestone-chert diagenetic front began to move upward through the lower sediments less than 200,000 yr. ago.

Geochemistry of DSDP Holes 60-453 and 60-454A

Pliocene and Pleistocene volcanic glass fragments from Mariana Trough sediments at Sites 453 (16 samples) and 454 (4 samples), located near the western edge of the trough and just west of the spreading axis, respectively, have been analyzed for major elements with an electron microprobe. They derive from volcanic activity on the present Mariana active arc. The glasses from Site 453 are all tholeiitic with a wide range of SiO2 contents. Those less than 2 m.y. old have slightly lower TiO2 and higher K2O contents than the older ones. The glasses from Site 454 are all Pleistocene and resemble the younger glasses at Site 453. Major element compositions of the older basaltic glasses at Site 453 are similar to those of the Mariana Trough basalts drilled on Leg 60. Both older and younger suites of glasses differ from the composition of rocks exposed on the active arc, which are assumed to be younger than any of the samples studied (i.e., about 200,000 y.). A third suite is represented by the arc rocks exposed on the volcanic islands. These have a smaller range of SiO2 contents and contain more A12O3 but less K2O, TiO2, and FeO1 (total Fe as FeO) than the sediment glasses studied. Further, a plot of FeO1 against MgO for the arc rocks does not follow the island arc tholeiite trend of the trough sediment glasses. Using the major element compositions of the arc rocks and sediment glasses, we can recognize three phases of volcanic activity, as indicated. The first evidence of the oldest phase of activity occurs 5 Ma, about 4.5 m.y. after the trough started to form. The second commenced about 2 Ma, and the last, including present-day activity, began within the last 200,000 y. Initially the rocks had major element affinities with the tholeiitic Mariana Trough seafloor, but this influence declined as the trough widened.

Geochemistry at DSDP Site 60-456

Basalts in two holes spaced 200 meters apart at DSDP Site 456 in the Mariana Trough both show a downward sequence of nonoxidative and oxidative zones of alteration, each 10 to 15 meters thick, overlying fresh basalts. Basalts in the nonoxidative zone have been extensively chloritized and have vein and vesicle fillings of quartz, opal, chlorite, calcite, and pyrite. Minor sulfides are chalcopyrite and digenite. Basalts in the oxidative zone have abundant smectites and iron hydroxides and are variably enriched in K, Rb, and Ba, unlike the nonoxidative basalts above them. We propose that the oxidative zone was a zone of mixing between high-temperature, reduced hydrothermal fluids moving horizontally beneath impermeable sediments at the top of the pillowed basement lavas and cold, oxygenated seawater in interpillow voids deeper in the basement. Recrystallized vitric tuffs immediately above the basalts containing authigenic quartz and wairakite, as well as occurrence of chlorite, epidote, and chalcopyrite in the basalts, suggest temperatures of alteration in excess of 200°C.

A high-resolution record of the atmospheric CO2 concentration from 60-20 kyr BP from the Taylor Dome ice core, Antarctica

A high-resolution record of the atmospheric CO2 concentration from 60 to 20 thousand years before present (kyr BP) based on measurements on the ice core of Taylor Dome, Antarctica is presented. This record shows four distinct peaks of 20 parts per million by volume (ppmv) on a millennial time scale. Good correlation of the CO2 record with temperature reconstructions based on stable isotope measurements on the Vostok ice core (Antarctica) is found.