Leung, I.S. and Abbott, D.H., 2003, December. MARID suite minerals in ejecta layer from Ewing Crater (Core PLDS-111P) in the central equatorial Pacific. In AGU Fall Meeting Abstracts (Vol. 1, p. 0471).
The Ewing crater is a 150 km diameter impact crater located between the Clarion and Clipperton fracture zones in the central equatorial Pacific about 10 degrees east of the longitude of Hawaii. The crater is early part of the late Miocene in age (7-11 Ma). The typical excavation depth of impact craters is about 1/10th of their diameter. Thus, the Ewing crater should have excavated down to 15 km, well below the oceanic Moho at 6 to 7 km depth. We test the impact origin of the Ewing crater by looking at the composition, morphology, and X-ray diffraction characteristics of minerals in the top part of core PLDS-111P. These minerals appear with abundant impact spherules and microtektites, at least 40 total in a 6-gram sample. We found high Ti amphibole with etching on the ends of the crystals. Etching of crystal terminations is characteristic of placer deposits and impact deposits. High Ti amphibole is characteristic of the MARID suite (Mica (Phlogopite)-Amphibole-Rutile -Ilmenite-Diopside). We have also found phologopitic mica, ilmenite and diopside in our sample. We did not find rutile, but rutile can be absent in the MARID suite. We have also found zircon and apatite, both of which are accessory minerals of the MARID suite. The MARID suite is confined to areas where the mantle has been heavily metasomatized. Previously documented occurrences of the MARID suite are from kimberlites and veins in metasomatized periodotites. Because the oceanic crust beneath the Ewing crater is Eocene (34-55 Ma) in age, kimberlites are unlikely to form. The most likely alternative is that the MARID suite minerals are from mantle that was metasomatized by normal processes of hydrothermal alteration within the oceanic plate. This metasomatized material was brought to the surface by excavation of the suboceanic mantle during the impact of the bolide that formed the Ewing crater. The impact hypothesis is supported by microcraters (10 to15 microns in diameter) found in phlgopite and ilmenite, similar to ones on surfaces of associated microtektites.