Calcareous nannofossil biostratigraphic study of forearc basin sediments: Lower to Upper Cretaceous Budden Canyon Formation (Great Valley Group), northern California,USA

The results of a calcareous nannofossil biostratigraphic investigation of the North Fork Cottonwood Creek section of the Budden Canyon Formation (BCF; Hauterivian–Turonian) in northern California are summarized using the Boreal – cosmopolitan Boreal Nannofossil Biostratigraphy (BC) – Upper Cretaceous Nannofossil Biostratigraphy (UC) nannofossil zonal schemes of Bown et al. and Burnett et al. Sixteen intervals, ranging from the BC15 to UC8 zones, were established in the section. Combined biostratigraphic and magnetostratigraphic studies suggest a Hauterivian to mid‐Turonian age for the studied sequence. The Hauterivian–Barremian, Barremian–Aptian, Aptian–Albian, Albian–Cenomanian, and Cenomanian–Turonian stage boundaries were delineated near the top of the Ogo Member, below the Huling Sandstone Member, within the upper Chickabally Member, in the upper portion of the Bald Hills Member and within the Gas Point Member, respectively. Unconformities probably exist at the base of the Huling Sandstone Member and the upper part of the upper Chickabally Member. The nannofossil assemblage in the North Fork Cottonwood Creek suggests that the study area was under the influence of cold‐water conditions during the Barremian to Lower Aptian interval, shifting to tropical/warm‐water conditions during the Albian to Turonian interval as a result of the mid‐Cretaceous global warming. Although oceanic anoxic events have not yet been reported in the BCF, preliminary total organic carbon, along with nannofossil data, suggest the presence of the global Cenomanian–Turonian boundary oceanic anoxic event 2.     

Morphological analysis of olivine grains annealed in an iron‐nickel matrix: Experimental constraints on the origin of pallasites and on the thermal history of their parent bodies

Abstract— Two types of pallasites can be distinguished on the basis of the grain shape of olivine (rounded or angular). It has been suggested that these two types of textures resulted from different degrees of annealing at high temperature in the parent body. In order to characterize the kinetics of rounding of olivine grains in an Fe‐Ni matrix, we carried out a series of annealing experiments using a mixture of olivine and Fe‐Ni powder. We were able to reproduce, at a miniature scale, the range of textures in pallasites. The rate of rounding was rapid enough to be observed and measured at the scale of a few micrometers to 20 μm, even though the experiments were performed below the solidus of the Fe‐Ni metal. For instance, grains ?14 mm in diameter became nearly spherical within 7 days at 1400°C. For the morphological analysis of olivine grains, we used two independent techniques: the “critical diameter method” and the “Gaussian diffusion‐resample method,” a new technique specially developed for our study. Both techniques indicate that the rounding time scale is proportional to the cube of the grain size and that morphological adjustments in our experiments occurred by volume diffusion in the olivine lattice, not by surface diffusion along the olivine‐metal boundaries. We used our experimental data to estimate the time scales required for the development of olivine‐metal textures in natural pallasites. We determined that small scale rounding of olivine grains in a solid metal matrix can be produced within relatively short time intervals: ?100 years to produce rounded olivine grains 0.1 mm in radius at 1300–1400°C.     

Origin of polymict breccias on asteroids deduced from their pyroxene fragments

Abstract— Cumulate eucrite, noncumulate eucrite, and diogenite meteorites are considered to have come from the crust of one (or similar) parent asteroid. Howardites are regarded as regolith breccias of eucrites and diogenites, and polymict eucrites are regarded as polymict breccias of eucrites. These polymict breccias show many textural and chemical features. In order to gain a better understanding of the origin of polymict breccias and the origin of their components, we investigated four polymict breccias, Yamato (Y)-791439, Y-791192, Y-82009, and Y-82049 with a scanning electron microscope (SEM) equipped with a chemical mapping system, and by electron probe microanalysis (EPMA). We analyzed all pyroxene grains with chemical maps, classified them by chemical composition, and observed their chemistry and mineralogy in detail. The characteristics of pyroxenes suggest that the polymict breccias were generated by gathering locally ordinary eucrites and cumulate eucrites. The chemical-evolutionary features of the pyroxenes (such as homogenization, chemical zoning, and exsolution lamellae) suggest that there were at least two long annealing events and one short (or low-temperature) annealing event, separated by mixing events. Local heterogeneity on the asteroidal crust is also suggested.