I. Extraterrestrial ³He in the Sedimentary Record. II. Geochemistry of Shield Stage Lavas from Kauai, Hawaii

Citation

Mukhopadhyay, Sujoy (2002) I. Extraterrestrial ³He in the Sedimentary Record. II. Geochemistry of Shield Stage Lavas from Kauai, Hawaii. Dissertation (Ph.D.), California Institute of Technology. doi:10.7907/XPM2-1Q79. https://resolver.caltech.edu/CaltechTHESIS:02232012-115811346

Abstract

(I) Extraterrestrial ^3He in the sedimentary record:

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.

(II) Geochemistry of shield stage lavas from Kauai, Hawaii:

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 ^3He^4He 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.

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