A NEUTRON GONIOMETER STUDY OF THE PREFERRED ORIENTATION OF CALCITE INFINE-GRAINED DEEP-SEA CARBONATE

In weakly indurated, nannofossil-rich, deep-sea carbonates compression al wave velocity is up to twice as fast parallel to bedding than norma l to it. It has been suggested that this anisotropy is due to alignmen t of calcite c-axis perpendicular to the shields of coccoliths and shi eld deposition parallel to bedding. This hypothesis was tested by meas uring the preferred orientation (fabric) of calcite c-axis in acoustic anisotropic, calcareous DSDP sediment samples by X-ray goniometry, an d it was found that the maximum c-axis concentrations are by far too l ow to explain the anisotropies. The X-ray method is subject to a numbe r of uncertainties due to preparatory and technical shortcomings in we akly indurated rocks. The most serious weaknesses are: sample preparat ion, volume of measured sample (fraction of a mm3), beam defocusing an d background intensity corrections, combination of incomplete pole fig ures, and necessity of recalculation of the c-axis orientations from o ther crystallographic directions. Goniometry using thermal neutrons ov ercomes most of these difficulties, but it is time consuming. We test the interferences made about velocity anisotropy by X-ray studies abou t the concentration of c-axes in deep-sea carbonates by employing neut ron texture goniometry to eight DSDP samples comprising mostly nannofo ssil material. Fabric and sonic velocity were determined directly on t he core specimens, thus from the same rock volume and requiring no pre paration. The c-axis orientation is obtained directly from the [0006] calcite diffraction peak without corrections. The fabrics are clearly defined, but weak (1.1 to 1.86 times uniform) with the maximum about n ormal to bedding. They have crudely orthorhombic symmetry, but are not axisymmetric around the bedding normal. The observed c-axis intensiti es, although higher than determined by the X-ray method on other sampl es, are by far too low to explain the observed acoustic anisotropies.