He isotopes in Late Cretaceous sediments

Late Cretaceous (100-73 Ma) pelagic limestones were measured for helium concentration and isotopic composition to characterize the interplanetary dust flux using 3He as a tracer. In the Bottaccione section near Gubbio, Italy, three intervals of elevated 3He concentration were detected: K1 in the Campanian stage at ~79 Ma, K2 in the Santonian stage at ~ 85 Ma, and K3 in the Turonian stage at ~91 Ma. All three of these episodes are associated with high 3He/4He and 3He/non-carbonate ratios, consistent with their derivation from an enhanced extraterrestrial 3He flux rather than decreased carbonate sedimentation or dissolution. While K2 is modest in magnitude and duration and thus is of limited significance, K1 and K3 are each identified by a few myr interval with an ~4-fold enhancement in mean 3He flux compared with pre-event levels. Samples from ODP Hole 762C in the Indian Ocean spanning both K2 and K3 (93-83 Ma) confirm the presence of a peak in the Turonian stage, suggesting that K3 is a global event. The K1 and K3 3He events are similar in most respects to the two peaks previously detected in the Cenozoic, suggesting a similar origin. These have been attributed to a major asteroid collision in the Late Miocene and to a shower of either comets or asteroids in the Late Eocene. Based on the age and temporal evolution of K1, we suggest that it most likely records the collision which produced the Baptistina asteroid family independently dated at ~80 Ma. The K3 event is less easily explained. It is characterized by an unusually spiky and erratic temporal progression, suggesting an unusual abundance of very 3He rich particles not previously seen in the sedimentary 3He record. We suggest this episode arises either from a comet shower or from an asteroid shower possibly associated with dust-producing lunar impacts.

Magnetic properties of ODP Hole 109-670A

Magnetic fabrics of serpentinized peridotites are related to anisomorphic magnetite formed during serpentinization. In the less serpentinized facies they are, however, mainly mimetic of the high temperature deformation prior to serpentinization. In more serpentinized peridotites, the magnetic fabrics, related to magnetite veins which are more developed in this case, are superimposed on mimetic fabrics. Remanent properties, hysteresis loop parameters, and Curie temperatures were measured. Natural remanent magnetizations (NRM) have crystallization remanent magnetic (CRM) origin. Measured magnetic parameters suggest that pseudo-single domain (PSD) grains of magnetite are present in samples with low degree of serpentinization. The samples with high degree of serpentinization contain mainly multi-domain (MD) magnetite grains.

Magnetic measurements on ODP holes 178-1095A, 178-1095B and 178-1101A

We have determined the azimuth of bottom-current flow in drift deposit sediments recovered at ODP Sites 1095 and 1101, Antarctic Peninsula, using paleomagnetic reorientation of anisotropy of magnetic susceptibility (AMS) ellipsoids. A total of 38 cores from the two ODP sites have been measured, providing spatial and directional information on the physical record of the ACC (Antarctic Circumpolar Current) in the Plio-Pleistocene. Declination and inclination of the paleomagnetic vector of each core segment were used to reorient the AMS principal axes to the geographic coordinates. The cores were reoriented using the measured direction of the characteristic remanent magnetization (ChRM) with respect to a common reference line for the core, from which we are able to determine the orientation of the paleocurrent flow for Sites 1095 (Drift 7) and 1101 (Drift 4) relative to the geographic coordinates. Both sites have paleocurrent directions trending ~NW-SE, which in the former locality are parallel to a sediment wave field. Our study shows that a combination of magnetic fabric analysis and paleomagnetism allows deep-sea sedimentary fabric to be used as a long-term proxy of bottom-current flow history.

Physical properties and AMS characteristics of ODP Site 185-1149 sediments

During Ocean Drilling Program Leg 185, we studied progressive changes of microfabrics of unconsolidated pelagic and hemipelagic sediments in Holes 1149A and 1149B in the northwest Pacific at 5818 m water depth. We paid particular attention to the early consolidation and diagenetic processes without tectonic deformation before the Pacific plate subduction at the Izu-Bonin Trench. Shape, size, and arrangement of pores were analyzed by scanning electron microscope (SEM) and were compared to anisotropy of magnetic susceptibility (AMS) data. The microfabric in Unit I is nondirectional fabric and is characterized by large peds of ~10-100 µm diameter, which are made up of clay platelets (mainly illite) and siliceous biogenic fragments. They are ovoid in shape and are mechanically packed by benthic animals. Porosity decreases from 0 to 60 meters below seafloor (mbsf) in Unit I (from 60% to 50%) in association with macropore size decreases. The microfabric of coarser grain particles other than clay in Unit II is characterized by horizontal preferred orientation because of depositional processes in Subunit IIA and burial compaction in Subunit IIB. On the other hand, small peds, which are probably made of fragments of fecal pellets and are composed of smectite and illite (3-30 µm diameter), are characterized by random orientation of clay platelets. The clay platelets in the small peds in Subunit IIA are in low-angle edge-to-face (EF) or face-to-face (FF) contact. These peds are electrostatically connected by long-chained clay platelets, which are interconnected by high-angle EF contact. Breaking of these long chains by overburden pressure diminishes the macropores, and the clay platelets in the peds become FF in contact, resulting in decreases in the volume of the micropores between clay platelets. Thus, porosity in Subunits IIA and IIB decreases remarkably downward. The AMS indicates random fabric and horizontal preferred orientation fabric in Units I and II, respectively. This result corresponds to that of SEM microfabric observations.In Subunit IIB, pressure solutions around radiolarian tests and clinoptilolite veins with normal displacement sense are seen distinctively below ~170 mbsf, probably in correspondence to the transition zone from opal-A to opal-CT.

AMS and EBSD maps of rock salt - lamprophyre dyke contact zone, Loule diapir, Algarve basin, Portugal

A rock salt-lamprophyre dyke contact zone (sub-vertical, NE-SW strike) was investigated for its petrographic, mechanic and physical properties by means of anisotropy of magnetic susceptibility (AMS) and rock magnetic properties, coupled with quantitative microstructural analysis and thermal mathematical modelling. The quantitative microstructural analysis of halite texture and solid inclusions revealed good spatial correlation with AMS and halite fabrics. The fabrics of both lamprophyre and rock salt record the magmatic intrusion, "plastic" flow and regional deformation (characterized by a NW-SE trending steep foliation). AMS and microstructural analysis revealed two deformation fabrics in the rock salt: (1) the deformation fabrics in rock salt on the NW side of the dyke are associated with high temperature and high fluid activity attributed to the dyke emplacement; (2) On the opposite side of the dyke, the emplacement-related fabric is reworked by localized tectonic deformation. The paleomagnetic results suggest significant rotation of the whole dyke, probably during the diapir ascent and/or the regional Tertiary to Quaternary deformation.

(Table T1) Magnetic anisotropy of samples from ODP Hole 180-1117A

At Site 1117, drilled during Leg 180 of the Ocean Drilling Program in the Woodlark Basin, we cored a fault zone and recovered fault gouge, mylonitized and brecciated gabbros, and undeformed gabbro. We measured the anisotropy of magnetic susceptibility for the rock samples. The susceptibilities of the fault gouge samples were lower than those of the undeformed gabbro, and those of deformed gabbros were lowest. The anisotropy degrees of the fault gouge samples were higher than those of the deformed and undeformed gabbros. Oblate magnetic fabrics were dominant in the samples from the fault zone.