The 3He/4He ratio in a clay from the K-T boundary,Hokkaido, Japan

We measured the He content and the ^3He/^4He ratio of a K-T boundary clay from Hokkaido. If some meteoritic debris which escaped total melting or evaporation at the K-T event remains in the clay, it may be possible to observe some ^3He in it. However, we could determine only the upper limits of the ^3He/^4He ratois ( for a bulk sample) because of the small amount of ^3He. Three alternatives are considered to explain the result

The role of compaction contrasts in sediments in décollement initiation in an accretionary prism

International audienceTo understand how décollements develop into the pristine sedimentary succession entering subduction zones, we have performed mechanical tests on samples from the sediment column entering the Nankai accretionary prism, Japan (ODP site 1173). Both poroelastic compliance and plastic shrinkage upon application of a large effective pressure sharply decrease with depth in a ~ 100 m-thick domain in the upper section of the Lower Shikoku Basin unit, i.e. in a domain stratigraphically close to the actual location of the décollement near the toe of the prism. These property contrasts provide a potential explanation for the outward migration of the décollement into the incoming sediments. When approaching the deformation front, a given material particle is affected by an increase in stress, which has a component of vertical loading due to the deposition of overburden trench sediment, and also a component of lateral compression transmitted from the accretionary wedge. Depending on its initial mechanical state, the amount of lateral shortening in the incoming Nankai sediment column varies with depth and causes horizontal velocity gradients that concentrate into the mechanical transition zone (upper section of the Lower Shikoku Basin at appx. 450-550 m depth) into which the décollement eventually propagates. Future work has to assess the role of this plastic deformation relative to other governing factors such as friction coefficient and excess pore pressure, both at Nankai and along other active margins

Drift assesment of pressure gauges for longterm subseafloor observation

Poster OS11A-1466 presented at 2011 Fall Meeting, AGU, San Francisco, Calif., 5-9 Dec. / Poster title: Depthmeter drift calibration for longterm obseervatio

TIARES Project -Tomographic investigation by seafloor array experiment for the Society hotspot

http://www.godac.jamstec.go.jp/darwin/cruise/mirai/mr08-06_leg1/

Acoustic and mechanical properties of Nankai accretionary prism core samples

International audienceWe studied undeformed sediment and accreted strata recently recovered by Ocean Drilling Program/Integrated Ocean Drilling Program (ODP/IODP) drilling in Nankai Trough convergent margin to unravel the changes in physical properties from initial deposition to incipient deformation. We have derived acoustic (Vp) and mechanical (uniaxial poroelastic compliance, compaction amplitude) properties of samples from various drill sites along the Muroto (ODP 1173) and Kii transects (IODP C0001, C0002, C0006, and C0007) from isotropic loading tests where confining and pore pressure were independently applied. We quantified the dependence of Vp on both effective (Peff) and confining (Pc) pressure, which can be used to correct atmospheric pressure measurements of Vp. Experimental Vp obtained on core samples extrapolated to in situ conditions are slightly higher than logging-derived velocities, which can be attributed either to velocity dispersion or to the effect of large-scale faults and weak zones on waves with longer wavelength. In the high-porosity (30%-60%) tested sediments, velocities are controlled at first order by porosity and not by lithology, which is in agreement with our static measurements of drained framework incompressibility, much smaller than fluid incompressibility. Rather than framework incompressibility, shear modulus is probably the second-order control on Vp, accounting for most of the difference between actual Vp and the prediction by Wood's (1941) suspension model. We also quantified the mechanical state of Nankai samples in terms of anisotropy, diagenesis, and consolidation. Both acoustic and mechanical parameters reveal similar values in vertical and horizontal directions, attesting to the very low anisotropy of the tested material. When considering the porous samples of the Upper Shikoku Basin sediments (Site 1173) as examples of diagenetically cemented material, several mechanical and acoustic attributes appeared as reliable experimental indicators of the presence of intergrain cementation. We also detected incipient cementation in samples from IODP Site C0001 (accretionary prism unit). In terms of consolidation, we distinguished two classes of material response (shallow, deformable samples and deep, hardly deformable ones) based on the amount of compaction upon application of a Peff large with respect to the inferred in situ value, with a transition that might be related to a critical porosity

Porosity and Compressional-Wave Velocity Measurement of Antarctic Meteorites

The intrinsic and bulk densities, porosities and compressional ultrasonic-wave velocities (V_p) in three mutually perpendicular directions have been measured in antarctic meteorites Allan Hills-769 and ALHA77231. V_p measurements were made at the room temperature and under one atmosphere pressure. The intrinsic and bulk densities and porosities are 2.89g/(cm)^3,3.59g/(cm)^3 and 19.4% for the Allan Hills-769 and 3.07g/(cm)^3,3.58g/(cm)^3 and 14.3% for the ALHA77231. The measured velocities were relatively low (the mean values of the three directions were 2.37km/s for the Allan Hills-769 and 3.52km/s for the ALHA77231) and showed up to 4% for the Allan Hills-769 and 10% for the ALHA77231 departures from the mean value of the three directions. The low values of velocity are attributed mainly to the high porosity of the sample. The velocity anisotropy might be due to a difference in crack shape between wavetransmitting directions. The porosity versus petrologic type diagram does not indicate a systematic variation of the porosity with the petrologic type

(Table 1) Anisotropy of magnetic susceptibility of basalts at DSDP Hole 89-462A

Measurements of the anisotropy of magnetic susceptibility (AMS) in basalts cored from 1070 to 1209 m sub-bottom depth in Hole 462A reveal that samples with a high ratio of maximum (K1) to minimum (K3) principal susceptibility show a prolate type of anisotropy in which K1 axes are nearly vertical. For other prolate types, K1 axes vary widely. Although the oblate type of AMS is less common among these basalts, K3 axes are nearly vertical in the upper units and horizontal in the lower units. The anisotropy of compressional wave velocities measured for the same specimen shows no correlation with the degree and type of AMS. This finding indicates that AMS could have been caused by variations in flows but not by compressive stress. Processes of emplacement of basaltic sills and flows must have been complex, because directions of principal axes of AMS vary from unit to unit

NRM intensity, MDF, stable inclination and susceptibility of cement samples of ODP Hole 109-648B (Table 1)

The remanence properties of 62 cement samples from Cores 7R, 11R, and 12R recovered from Hole 648B were examined. The remanent magnetization consists of two components, where the soft component is carried by the basalt fragments in the cement sample and the hard component is carried by fine magnetic grains originally contained in cement. The hard component was used to estimate the variation of the magnetic field inclination within the hole. The result indicates that some parts of the basalt section within the sub-bottom depth between 15 and 20 m are reversely magnetized, which can be attributed to the displacement of pillow basalt after its solidification