Analyses of ^3He, a tracer of interplanetary dust particles (IDPs), in marine carbonates from the Italian Apennines exclude large, long-lived enhancement in the IDP flux between 75 Ma and 39 Ma, These data in combination with previous ^3He measurements preclude recurrent comet showers with periods less than 38 Myrs. The IDP accretion rate at the K-T boundary (65 Ma) is invariant and has, therefore, been used as a chronometer to estimate the duration of the K-T extinction event. Our calculation suggests that the duration of the boundary event is 10000 ± 2000 years, and the deposition time of the impact ejecta layer is less than 60 years. These results indicate that the mass-extinction at the K-T boundary was catastrophic, ruling out volcanism and sea level changes as important factors and requiring an extremely rapid faunal turnover rate.
Because extraterrestrial ^3He is preserved in the sedimentary record for at least 480 Ma, the ^3He carrier phase(s) must be chemically stable on the seafloor and resist diffusion over geologic time, Our chemical leaching and step-heating experiments indicate that the carrier phase(s) may be magnetite or more probably associated with magnetite, The association may be in the form of composite grains or nanometer thick rims.
We measured He, Sr, Nd, Pb, and Os isotope ratios and major and trace element concentrations in shield lavas from Kauai, Hawaii. The range of 3He/4He ratios (17-28 R_A) measured from the Kauai shield is similar to that reported from Loihi Seamount and is clearly distinct from other Hawaiian shield volcanoes. These results challenge the prevailing notion that high 3He/4He ratios are restricted to the pre-shield stage of Hawaiian volcanism. 3He/4He ratios in Kauai shield lavas vary erratically with stratigraphic position and on timescales of 100 years. These variations in 3He/4He ratios are correlated with variations in radiogenic isotope ratios, suggesting rapid changes in parental magma composition with time.
Our new geochemical data from Kauai shield lavas require the involvement of a depleted mantle component that is normally sampled only during the post-shield and post-erosional stages of Hawaiian volcanism. In addition, the Kauai data support the existence of a single high 3He4He reservoir in the Earth’s mantle and suggest that the proportion of the high 3He/4He component in the Hawaiian plume has varied significantly with time. The long-term evolution of the Hawaiian plume and the temporal variability recorded in Kauai lavas cannot be explained by a steady-state cylindrically zoned plume and require more complex time-varying heterogeneities.
PART I: Phase relationships have been determined for the dehydration-melting of a (powdered and solid) calcic, low-K, olivine tholeiitic amphibolite (hornblende 70%, plagioclase 30%), in runs at 10 kbar, 750 to 1000°C,fO2 ~= Ni-NiO, and for 1 to 21 days. Hornblende is involved in a sliding reaction: hornblende + anorthitic plagioclase -> clinopyroxene + liquid + aluminous hornblende + calcic hornblende + orthopyroxene + garnet. The liquid fraction varies from <1% at 750°C to ~47% at 1000°C, with the big increase occurring above 875°C. Liquids are tonalitic but have very high Al2O3 contents (18-21 wt.%). At high liquid fractions (~0.5), liquids are high-alumina basaltic. Liquids become more sodic with increasing temperature, but the compositional trends reverse direction, and liquids become more calcic above 975°C, where garnet is unstable. The water contents of liquids range from over 7 wt.% at low liquid fractions to 2 wt.% at high liquid fractions. In the solid amphibolite runs, liquid interconnectivity may be attained at 875°C with only 2 vol.% liquid and dihedral angles less than 60°. The removal of water-rich tonalitic liquids from a substantially melted amphibolitic source could help generate a relatively dry mafic granulite terrane, with densities up to 3.5 gm/cm3. Delamination of this dense lower crust is possible. PART II: A cogenetic and coeval tonalitic and mafic dike swarm has been identified within a southern fragment (the Owens Mountain area) of the western Foothills terrane (Sierra Nevada, California). The swarm was mylonitized and transposed during emplacement, from 155 to 148 m.y. (U-Pb zircon data), at an estimated depth of 10 km. Steeply SE-plunging fold axes and S-fold geometries indicate a left-lateral sense of shear. The Late Jurassic Nevadan orogeny is a manifestation of dramatic changes in magnitude and direction of North American motion. The Cordilleran dike swarms record a complex pattern of sinistral-sense transtension-transpression that developed during this period of change, at the J2 (~150 m.y.) apparent polar wander cusp, and during subsequent, rapid northwestward acceleration of North America. EXTENDED ABSTRACT (PART I): Phase relationships and morphologies and reaction kinetics have been determined for the dehydration-melting of a natural amphibolite (mode: hornblende 70%, plagioclase 30%) with no added water, at 10 kbar and 750 to 1000°C, and for durations of l to 21 days, using both finely-powdered and solid starting materials. The amphibolite composition is equivalent to a calcic, low-K, olivine tholeiite. Experimental conditions simulated the dehydration-melting of deep mafic continental crust and hot, subducted oceanic crust. Experiments were conducted in unbuffered Au capsules at oxygen fugacities probably just above the Ni-NiO buffer. Hornblende is involved in the following sliding reaction (the order of the product phases represents the order of appearance with increasing temperature): hornblende + anorthitic plagioclase -> clinopyroxene + liquid + aluminous hornblende + calcic hornblende + orthopyroxene + garnet. The liquid fraction ranges from <1% at 750°C to ~47% at 1000°C, with most of the increase occurring above 875°C. The liquids are generally tonalitic but have very high Al2O3 contents (18-21 wt.%). At high liquid fractions (~0.5), the liquids have a composition of high-alumina basalt. Fractionation of the plagioclase would be necessary to reduce both the CaO and Al2O3 contents of the liquids to calc-alkaline compositions. The liquid compositions become more sodic with increasing temperature, but the compositional trends reverse direction, and the liquid compositions become more calcic above 975°C, as garnet disappears. The water contents of the liquids range from over 7 wt.% at low liquid fractions to 2 wt.% at high liquid fractions. The high-temperature mineral assemblage that coexists with the liquid is clinopyroxene, orthopyroxene and plagioclase ± garnet ± aluminous hornblende. Thus, dehydration-melting of the amphibolite can reproduce natural granulite and garnet pyroxenite mineral assemblages. The removal of the water-rich tonalitic liquids from a substantially melted amphibolitic source would help generate a relatively dry granulite terrane. The stability of garnet plays a major role in determining the REE composition of the liquids. Garnet modes from these runs are consistent with REE patterns of Archean tonalites. Delamination of the garnet clinopyroxenite restite is possible due to the very high densities (up to 3.5 gm/cm3) of these assemblages after liquid segregation. Garnet phenocrysts show syn-growth compositional zoning. The total alumina in hornblende geobarometer appears to work for this mafic mineral assemblage. The solid amphibolite runs indicate that anisotropic crystal structures and rock texture control liquid morphology and distribution during dehydration-melting. The shapes of most liquid pockets are crystallographically-controlled, with many corners having angles greater than 60°. Few crystal/liquid triple-junctions develop the interfacial energy-controlled dihedral angles ([theta]), which form in experiments using finely-ground powders of minerals with poor cleavage. Liquid interconnectivity probably is attained at 875°C with only 2 vol.% liquid, indicating that dihedral angles less than 60° may not be necessary to achieve interconnectivity in partially melted metamorphic rocks. The surfaces between elongated grains in lineated rocks can become pathways for the migration of liquid or the diffusion of components. EXTENDED ABSTRACT (PART II): The geology, petrology and geochronology (U-Pb zircon) of a southern fragment of the western Foothills terrane has been studied (the Owens Mountain area of the western Sierra Nevada foothills, northeast of Fresno, California). A previously unrecognized dike swarm/shear zone is identified within the steeply dipping, Callovian to Kimmeridgian metavolcanic and metasedimentary strata. The dike swarm consists predominantly of cogenetic tonalitic and mafic dikes and tonalitic tabular bodies. Mutually cross-cutting relationships indicate that the tonalitic and mafic dikes also were coeval. Some of the tonalitic dikes range up to ~100 m in thickness, and individual dikes of both tonalitic and basaltic composition can be followed for up to 3 km. The dike swarm is sheeted in places, comprising almost 100% of some outcrops. Textures and fabrics within the dike swarm range from partially recrystallized igneous to strongly deformed S and L metamorphic tectonites, implying that dike emplacement occurred during ductile deformation. Hot subsolidus mylonitization has transposed layering parallel to foliation and has greatly thinned many of the dikes to centimeter to meter thicknesses. Layering and parallel foliation dip subvertically and strike NNW-SSE. Post-tectonic annealing has destroyed most microscopic shear indicators, but macroscopic intrafolial folds are common and have steeply SE-plunging fold axes and S-fold geometries that indicate a left-lateral sense of shear. The geochronological data on the tonalite dikes reveal that emplacement and crystallization of the coeval tonalitic and mafic magmas at Owens Mountain occurred over an 8 m.y. period, from 155 to 148 Ma, at an estimated depth of 10 km. Thus the beginning of intrusion occurred within 5 m.y. of deposition of the metavolcanic and metasedimentary strata into which the dikes were emplaced. A correlation between age and degree of deformation and recrystallization of the tonalites implies syntectonic dike emplacement. Undeformed granitic dikes that cut the strata are younger than 124 Ma. The regional tectonics of the Owens Mountain and other Cordilleran dike swarms can be related in a broad dynamic sense to the absolute motion of North America by using the apparent polar wander (APW) analysis of May and Butler (1986). The Late Jurassic Nevadan orogeny is the manifestation of the drastic changes in magnitude and direction of North American motion (from ~45 km/m.y. to the NNE to ~200 km/m.y. to the NW; May and Butler, 1986). The Late Jurassic dike swarms record a complex pattern of sinistral-sense transtension-transpression that developed at the J2 (~150 Ma) APW cusp and during subsequent, rapid northwestward acceleration of North America.
}, address = {1200 East California Boulevard, Pasadena, California 91125}, advisor = {Wyllie, Peter J. and Saleeby, Jason B.}, }