Sulfur isotope rations in oceanic basement samples

Low temperature alteration of oceanic basement rocks is characterized by net gain of sulfur, which commonly yields low d34S values, suggesting involvement of microbial sulfate reduction. In order to test whether secondary sulfide minerals are consistent with a biogenic source, we apply high precision multiple sulfur isotope analysis to bulk rock sulfide and pyrite isolates from two contrasting types of altered oceanic basement rocks, namely serpentinized peridotites and altered basalts. Samples from two peridotite sites (Iberian Margin and Hess Deep) and from a basalt site on the eastern flank of the Juan de Fuca Ridge yield overlapping d34S values ranging from 0 per mil to -44 per mil. In contrast, sulfides in the basalt site are characterized by relatively low D33S values ranging from -0.06 per mil to 0.04 per mil, compared to those from peridotite sites (0.00 per mil to 0.16 per mil). The observed D33S signal is significant considering the analytical precision of 0.014 per mil (2 sigma). We present a batch reaction model that uses observed d34S and D33S relationships to quantify the effect of closed system processes and constrain the isotope enrichment factor intrinsic to sulfate reduction. The estimated enrichment factors as large as 61 per mil and 53 per mil, for peridotite and basalt sites respectively, suggest the involvement of microbial sulfate reduction. The relatively high D33S values in the peridotite sites are due to sulfate reduction in a closed system environment, whereas negative D33S values in the basalt site reflect open system sulfate reduction. A larger extent of sulfate reduction during alteration of peridotite to serpentinite is consistent with its higher H2 production capacity compared to basalt alteration, and further supports in-situ microbial sulfate reduction coupled with H2 production during serpentinization reactions.

(Table 2) Estimated basement temperatures at DSDP Hole 69-504B

The achievement of deep penetration (562 m) of seafloor basalts at Hole 504B, near the Costa Rica Rift (1°13.63'N, 83°43.81'W), on DSDP Legs 69 and 70 presented a rare opportunity to examine the structure of young (6 m.y.) oceanic crust. In addition to the recovery of samples for laboratory studies, an extensive suite of downhole logs and experiments was carried out at this site, for two main purposes: (1) to allow reliable deductions about the nature of the entire section of penetrated crust, because recovery of samples was far from complete (-25%); (2) to probe the physical state of rock around the drilled hole on a scale of tens of meters to kilometers. Information on the latter large-scale phenomena at Hole 504B were provided mainly by the oblique seismic experiment, utilizing a bore-hole seismometer (Stephen 1983), and by the large-scale-electrical- resistivity experiment described below.

Physical models of dilatant normal faulting in jointed cohesive rocks

Dilatant faults often form in rocks containing pre-existing joints, but the effects of joints on fault segment linkage and fracture connectivity is not well understood. We present an analogue modeling study using cohesive powder with pre-formed joint sets in the upper layer, varying the angle between joints and a rigid basement fault. We analyze interpreted map-view photographs at maximum displacement for damage zone width, number of connected joints, number of secondary fractures, degree of segmentation and area fraction of massively dilatant fractures. Particle imaging velocimetry helps provide insights on deformation history of the experiments and illustrate the localization pattern of fault segments. Results show that with increasing angle between joint-set and basement-fault strike the number of secondary fractures and the number of connected joints increases, while the area fraction of massively dilatant fractures shows only a minor increase. Models without pre-existing joints show far lower area fractions of massively dilatant fractures while forming distinctly more secondary fractures.

Petrographical classification and mineral assemblage of basement clasts from the Quaternary and Miocene sections of sediment core CRP-1 (Table 1)

Petrographical and mineral chemistry data are described for the mist representative basement lithologies occurring as clasts (pebble grain-size class) from the CRP-1 drillhole. Most pebbles consits of either undeformed or foliated biotite with or without hornblende monzogranites. Other rock types include biotite with or without garnet syenogranitr, biotite-hornblende granodiorite, tonalite, monzogranitic porphyries, haplogranite, quartz-monzonite (restricted to the Quaternary section), Ca-silicate rocks and biotite amphibolite (restricted to the Miocene strata). The common and ubiquitous occurence of biotite with or without hornblende monzogranite pebbles, in both the Quaternary and Miocene sections, apparently mirrors the dominance of these rock types in the granitoid assemblages which are presently exposed in the upper Precambrian-lower Paleozoic basement of the south Victoria Land. The other CRP-1 pebble lithologies show petrographical features which consitently support a dominant supply from areas of the Transantarctic Mountains located to the west and south-west of the CRP-1 site, and they thus furthercorroborate a model of local provenance for the supply of basement clasts to the CRP-1 sedimentary strata.

(Table 1) Sr-isotope values of representative sediment samples from ODP sites at the Costa Rica margin

Two sealed borehole hydrologic observatories (CORKs) were installed in two active hydrogeochemical systems at the Costa Rica subduction zone to investigate the relationship between tectonics, fluid flow, and fluid composition. The observatories were deployed during Ocean Drilling Program (ODP) Leg 205 at Site 1253, ~ 0.2 km seaward of the trench, in the upper igneous basement, and at Site 1255, ~ 0.5 km landward of the trench, in the décollement. Downhole instrumentation was designed to monitor formation fluid flow rates, composition, pressure, and temperature. The two-year records collected by this interdisciplinary effort constitute the first co-registered hydrological, chemical, and physical dataset from a subduction zone, providing critical information on the average and transient state of the subduction thrust and upper igneous basement. The continuous records at ODP Site 1253 show that the uppermost igneous basement is highly permeable hosting an average fluid flow rate of 0.3 m/yr, and indicate that the fluid sampled in the basement is a mixture between seawater (~ 50%) and a subduction zone fluid originating within the forearc (~ 50%). These results suggest that the uppermost basement serves as an efficient pathway for fluid expelled from the forearc that should be considered in models of subduction zone hydrogeology and deformation. Three transients in fluid flow rates were observed along the décollement at ODP Site 1255, two of which coincided with stepwise increases in formation pressure. These two transients are the result of aseismic slip dislocations that propagated up-dip from the seismogenic zone over the course of ~ 2 weeks terminating before reaching ODP Site 1255 and the trench. The nature and temporal behavior of strain and the associated hydrological response during these slow slip events may be an analog for the response of the seaward part of the subduction prism during or soon after large subduction zone earthquakes.

Geochemistry of basement basaltic rocks of ODP Hole 131-808C

Major- and trace-element analyses, mineral chemistry, and Sr-Nd isotopic determinations were obtained on representative igneous rocks drilled from the Nankai accretionary complex (Site 808) during Ocean Drilling Program Leg 131. For the first time, the oceanic basement of the subducting plate below an accretionary prism has been reached. The Nankai Trough basement was encountered at a depth of 1289.9 mbsf and a total of 37.1 m of igneous rocks, middle Miocene (15.6 Ma) in age, was penetrated. Two main lithological units have been distinguished from the top downward; sill-like rocks (Unit I: Cores 105, 106, 107) and pillow lavas (Unit II: Core 108). Basalts are predominantly nonvesicular, hypocrystalline, aphyric to slightly phyric with intersertal to intergranular textures. Alteration is generally slight to moderate. All the basaltic rocks are cut by ramifying veins of varying widths. Secondary mineral assemblages (including vein fillings) are typical of submarine alteration and zeolite to low greenschist facies metamorphism. The order of crystallization of primary minerals is: olivine, plagioclase, clinopyroxene. This, together with mineral chemistry, characterized by forsteritic olivine (Fo 84-85), highly anorthitic Plagioclase (up to An 90), and in particular the composition of clinopyroxene, are typical of normal mid-ocean ridge basalts (MORB). In terms of Zr/Y (2.9-3.8) and Zr/Nb (21-58), all the analyzed samples plot in the normal MORB field. The chondrite-normalized REE patterns confirm the close affinity with normal MORB type (LaN/SmN: 0.6-0.8). Note that such magmatism does not reveal any evidence of subduction-related geochemical components. The 87Sr/86Sr isotopic ratios range from 0.70339 in pillow lavas to 0.70317 in the least-altered basalts of sill units (ratios reduced to 0.70265-0.70271 by HC1 2.5 N hot leaching), whereas 143Nd/144Nd ratios are 0.51314-0.51326. These values conform with those of normal MORB. Stratigraphy, petrography, and geochemistry of the basaltic rocks recovered at Site 808 appear very similar to those from the Shikoku Basin basement (particularly Sites 442 and 443, DSDP Leg 58), analogously identified as normal MORB.

(Table T1) Trace-element abundances for igneous basement at ODP Site 206-1256

In this report, I present trace element data for basement samples at Ocean Drilling Program (ODP) Site 1256. The samples analyzed represent a subset of the group ("pool") samples from ODP Leg 206, and these trace element data are part of a more comprehensive data suite for the same samples, with analyses of stable and radiogenic isotopes (e.g., Sr, Li, and O) in progress or recently completed that will be presented elsewhere. The trace element analyses were performed in the GeoAnalytical Lab at Washington State University. The following elements were analyzed: La, Ce, Pr, Nd, Sm, Eu, Gd, Tb, Dy, Ho, Er, Tm, Yb, Lu, Ba, Th, Nb, Y, Hf, Ta, U, Pb, Rb, Cs, Sr, Sc, and Zr. Trace element data indicate that the igneous basement at Site 1256 is geochemically normal mid-ocean-ridge basalt. A massive ponded flow sampled in both Holes 1256C and 1256D is distinguished by higher abundances of rare earth elements (REE) and most of the other trace elements analyzed. One interval of highly altered basalt has significantly higher concentrations of Cs, Rb, and Ba and lower concentrations of Sr, Pb, Zr, Hf, Sc, and most REE than the samples of background alteration or halos. No correlation is obvious between trace element abundance and macroscopic type of alteration within the background alteration or halos.

Geochemical composition of minerals obtained from ODP Sites 180-1109, 180-1117 and 180-1118

During the last 8 m.y. the Papuan Peninsula region of Papua New Guinea has been affected by extension which opened the Woodlark Basin. The present-day spreading tip is located at the foot of the Moresby Seamount, a crustal block whose northern flank is an active low-angle normal fault related to this extension. During Ocean Drilling Program Leg 180 (7 June-11 August 1998), 11 sites (1108-1118) were drilled along a north-south-trending transect across the Woodlark Basin just ahead of the spreading tip. Four of these sites (1118, 1109, 1114, and 1117) reached the crystalline basement, which is composed of diabase and gabbro. Sites 1118 and 1109, located on the Woodlark Rise, belong to the hanging wall block, and Sites 1114 and 1117, located on the crest of the Moresby Seamount, belong to the footwall block and the fault zone itself. Most of the basalt, diabase, and gabbro that were recovered show a well-preserved magmatic texture. The diabase, which is the most abundant rock type, has a coarse-grained ophitic texture composed of poikilitic clinopyroxene including radiating, locally skeletal plagioclase laths with interstitial iron oxide grains. Secondary mineralogy consists of chlorite, zeolite, calcite, albite, and quartz. The gabbro shows a medium-grained granular texture. The magmatic mineralogy consists of euhedral laths of plagioclase and anhedral interstitial clinopyroxene. Secondary mineralogy consists of a magnesio to actinolitic hornblende, chlorite, clinozoisite, zeolite, quartz, and calcite. The retrograde metamorphic evolution of both gabbro and diabase occurred under low amphibolite to subgreenschist facies conditions associated mainly with brittle deformation and the development of a local low-temperature shear zone. This shows no evidence for high thermal gradient in the crust during the continental rifting.

Table 1: U-Pb analyses of zircons and monazites of paragneisses which belong to the high grade metamorphic basement of Wilson Terrane

Samples of high grade metamorphic basement rocks of Wilson Terrane cropping out in the Deep Freeze Range and on Kay Island were collected during GANOVEX VI to study their isotopic evolution. The age and origin of granulite facies gneisses and of their migmatite host rocks are especially of interest for the interpretation of the geological and tectonic development of North Victoria Land. Another important research aspect is the influence of the polyphase metamorphic evolution on the isotopic systems of whole rocks and minerals like zircon, garnet, orthopyroxene, amphibole and feldspar.