THE INSCRUTABLE EJECTA FROM OCEANIC IMPACTS

At 2.5 Ga, continental crust covered ~ 33% of the Earth, meaning that 2 out of 3 impacts hit oceanic crust. Unfortunately, most of these impacts did not leave shocked quartz. Oceanic crust is not quartz normative, and hydrothermal silica does not have the same crystal structure as quartz. Thus, we need other criteria to recognize ejecta from oceanic impacts. Because the largest impacts into the ocean basins excavate the mantle, the dominant mineral in impact spherules will be olivine. In the ejecta from a 150 km wide, late Miocene crater in the mid-Pacific (Ewing crater), the dominant spherical form is a serpentinized olivine spherule. This spherule type may be the most common form of ejecta from large oceanic impacts in the Precambrian. If so, these spherules will be difficult to recognize. Although the Ewing impact was relatively recent (7-11 Ma), these impact spherules are already very altered. Although the spherules appear glassy in visible light, the surfaces of the spherules appear quite rough in scanning electron photomicrographs. This is due to the positive volume change that accompanies serpentinization of olivine. As a result, the surface of the spherules expands and deforms. There are other minerals in the sub-oceanic mantle that will form impact spherules, plagioclase in the shallow mantle, and pyroxene throughout the mantle. These latter spherules are more recognizable and less altered, but they are very rare. We have found some plagioclase microkrystites in the Ewing layer, but they are much less common that the serpentine spherules. Furthermore, although they are more resistant than olivine, plagioclase and pyroxene both weather quite readily. What will remain after large amounts of alteration? The Ewing layer has shocked silicon carbide (1) and phlogopite mica (2), both resistant to alteration. Both of these phases are otherwise found only in meteorites (SiC) and the lamprophyre suite (phlogopite), respectively. These are the phases to look for in prospective Precambrian impact layers. (1) Leung et al., 2004, Shocked SiC from the Canyon Diablo Meteorite and the Ewing Impact Layer, EOS, Trans. AGU. (2) Leung et al., 2003. MARID Suite Minerals in Ejecta Layer from Ewing Crater (Core PLDS-111P) in the Central Equatorial Pacific, EOS, Trans. AGU.