Petrology and geochemistry of peridotites from IODP Site 304-U1309

Peridotite samples recovered from IODP Site U1309 at the Atlantis Massif in the Mid-Atlantic Ridge were examined to understand magmatic processes for the oceanic core complex formation. Original peridotite was fragmented, and the limited short peridotite intervals are now surrounded by a huge gabbro body probably formed by late-stage melt injections. Each peridotite interval has various petrographical and geochemical features. A spinel harzburgite in contact with gabbro shows evidence of limited melt penetrations causing gradual compositional change, in terms of trace-element compositions of pyroxenes, as well as modal change near the boundary. Geochemistry of clinopyroxenes with least melt effects indicates that the harzburgite is originally mantle residue formed by partial melting under polybaric conditions, and that such a depleted peridotite is one of the components of the oceanic core complex. Some of plagioclase-bearing peridotites, on the other hand, have more complicated origin. Although their original features were partly overprinted by the injected melt, the original peridotites, both residual and non-residual materials, were possibly derived from the upper mantle. This suggests that the melt injected around an upper mantle region or into mantle material fragments. The injected melt was possibly generated at the ridge-segment center and, then, moved and evolved toward the segment end beneath the oceanic core complex.

Average dip directions from intervals of dipmeter log in drill hole CRP-2A

Bedding dips in the CRP-2A drillhole were determined in two ways (1) analysis of a dipmeter log, and (2) identification of bed boundaries on digital images of the outer core surface. The two methods document the downhole increase in structural dip, to a maximum of 15° in the lowest 150 m of the hole. Dipmeter data, which are azimuthally oriented, indicate a 75° azimuth for structural tilting, in agreement with seismic reflection profiles. Core and log dips indicate that structural dip increases by 5-7° between 325 and 480 mbsf. Both, however, also exhibit high dip inhomogeneity because of depositional (e.g., cross bedding) and post-depositional (e.g., softsediment deformation) processes. This variability adds ambiguity to the search for angular unconformities within the CRP-2A drillhole. Dip directions of different lithologies are generally similar, as are dip directions for the four kinds of systems tracts. Downdip azimuths of sands and muds are slightly different from those of diamicts, possibly reflecting the divergence between ENE offshore dip and ESE glacial advance.

Cretaceous-Tertiary ichthyoliths of brown clay intervals

The pattern of ichthyolith distribution established in sequences with stratigraphies based on calcareous or siliceous microfossils is used to provide age correlations for three deep-sea pelagic clay intervals that lack the better known microfossils. At Site 637, approximately 25 m of brown clay in Cores 103-637A-21R through 103-637A-23R underlies upper Miocene sediments and is of Paleocene to early Eocene age. At Site 639, 1.7 m of brown clay in Core 103-639C-2R is Eocene to Oligocene. At Site 640, 3.5 m of clay in Cores 103-640A-1R and 103-640A-2R contains a Cretaceous to Paleocene sequence, with the Cretaceous/Tertiary boundary between 84 and 103 cm in Section 103-640A-2R-1.

Geochemical and mineralogical analyses of clay-rich intervals in ODP Sites 166-1006 and 166-1007

The western flank of the Great Bahama Bank, drilled during ODP Leg 166 at seven sites, represents a prograding carbonate sequence from late Oligocene to Holocene [Eberli et al., Proc. ODP Init. Reports 166 (1997)]. The signatures of the detrital input and of diagenetic alteration are evident in clay enriched intervals from the most distal Sites 1006 and 1007 in the Straits of Florida. Mineralogical and chemical investigations (XRD, TEM, SEM, ICP-MS) run on bulk rocks and on the clay fractions enable the origin and evolution of silicate parageneses to be characterized. Plio-Pleistocene silt and clay interbeds contain detrital clay assemblages comprising chlorite, illite, interstratified illite smectite, smectite, kaolinite and palygorskite. The greater smectite input within late Pliocene units than in Pleistocene oozes may relate either varying source areas or change in paleoclimatic conditions and weathering intensity. The clay intervals from Miocene-upper Oligocene wackestone sections are fairly different, with prevalent smectite in the fine fraction, whose high crystallinity and Mg contents that point towards an authigenic origin. The lower Miocene section, below 1104 mbsf, at depths where compaction features are well developed, is particularly characterized by abundant authigenic Na-K-clinoptilolite filling foraminifer tests. The authigenic smectite and clinoptilolite paragenesis is recorded by the chemical trends, both of the sediment and the interstitial fluid. This diagenetic evolution implies Si- and Mg rich fluids circulating in deeper and older sequences. For lack of any local volcaniclastic input, the genesis of zeolite and the terms of water rock interaction are discussed. The location of the diagenetic front correlates with that of the seismic sequence boundary P2 dated as 23.2 Ma. This correspondence may allow the chronostratigraphic significance of some specific seismic reflections to be reassessed.