Koga Kenneth T.

No Thumbnail Available
Last Name
Koga
First Name
Kenneth T.
ORCID

Filters

1999 - 1999 1 2010 - 2014 2
2 1
3
Reset filters

Settings

Search Results

Now showing 1 - 3 of 3
  • Article
    Volatile (F and Cl) concentrations in Iwate olivine-hosted melt inclusions indicating low-temperature subduction
    (Springer, 2014-08-01) Rose-Koga, Estelle F. ; Koga, Kenneth T. ; Hamada, Morihisa ; Helouis, Thomas ; Whitehouse, Martin J. ; Shimizu, Nobumichi
    Investigation of olivine-hosted melt inclusions provides information about the abundance of volatile elements that are often lost during subaerial eruptions of lavas. We have measured the abundances of H2O, CO2, F, Cl, and S as well as Pb isotopes in 29 melt inclusions in the scoria of the 1686 eruption of the Iwate volcano, a frontal-arc volcano in the northeast Japan arc. Pb Isotope compositions identify that Iwate magma is derived from a mixture of depleted mantle, subducted basalt, and sediment. Systematics of F in comparison to MORB and other arc magma indicates that (1) the slab surface temperature must be among the lowest on Earth and (2) hydrous minerals, such as amphibole, humites, and/or mica, must be present as residual phases during the dehydration of the slab.
  • Article
    Volatile cycling of H2O, CO2, F, and Cl in the HIMU mantle : a new window provided by melt inclusions from oceanic hot spot lavas at Mangaia, Cook Islands
    (John Wiley & Sons, 2014-11-28) Cabral, Rita A. ; Jackson, Matthew G. ; Koga, Kenneth T. ; Rose-Koga, Estelle F. ; Hauri, Erik H. ; Whitehouse, Martin J. ; Price, Allison A. ; Day, James M. D. ; Shimizu, Nobumichi ; Kelley, Katherine A.
    Mangaia hosts the most radiogenic Pb-isotopic compositions observed in ocean island basalts and represents the HIMU (high µ = 238U/204Pb) mantle end-member, thought to result from recycled oceanic crust. Complete geochemical characterization of the HIMU mantle end-member has been inhibited due to a lack of deep submarine glass samples from HIMU localities. We homogenized olivine-hosted melt inclusions separated from Mangaia lavas and the resulting glassy inclusions made possible the first volatile abundances to be obtained from the HIMU mantle end-member. We also report major and trace element abundances and Pb-isotopic ratios on the inclusions, which have HIMU isotopic fingerprints. We evaluate the samples for processes that could modify the volatile and trace element abundances postmantle melting, including diffusive Fe and H2O loss, degassing, and assimilation. H2O/Ce ratios vary from 119 to 245 in the most pristine Mangaia inclusions; excluding an inclusion that shows evidence for assimilation, the primary magmatic H2O/Ce ratios vary up to ∼200, and are consistent with significant dehydration of oceanic crust during subduction and long-term storage in the mantle. CO2 concentrations range up to 2346 ppm CO2 in the inclusions. Relatively high CO2 in the inclusions, combined with previous observations of carbonate blebs in other Mangaia melt inclusions, highlight the importance of CO2 for the generation of the HIMU mantle. F/Nd ratios in the inclusions (30 ± 9; 2σ standard deviation) are higher than the canonical ratio observed in oceanic lavas, and Cl/K ratios (0.079 ± 0.028) fall in the range of pristine mantle (0.02–0.08).
  • Thesis
    Kinetic processes of mantle minerals
    (Massachusetts Institute of Technology and Woods Hole Oceanographic Institution, 1999-09) Koga, Kenneth T.
    This dissertation discusses the experimental results designed to constrain the processes of MORB generation. The main focus of this study is to investigate the location and the related processes of the transformation boundary from spinel to garnet peridotite facies at subsolidus conditions, because the presence of garnet in melting residues has significant influence to the conclusion drawn from geochemical/geophysical observations. Using an approach that monitors the rate of reaction progresses, the experimental results confirmed the presence of a region that garnet and spinel coexist in peridotite compositions. The trace element distribution among the product phases (opx and cpx) subsequent to the garnet breakdown reaction is in disequilibrium, due to the differences of diffusivity between major and trace elements. The presence of disequilibrium distribution in nature may be used to infer time scales of geodynamic processes. Diffusion coefficients of A1 in diopside are experimentally determined, and used for modeling the equilibration of major elements in pyroxene during MORB genesis. In summary, this dissertation contributes two major inferences: the location of the transformation boundaries of the gamet-spinel peridotite; the presence of disequilibrium trace elements distribution with equilibrium major elements distribution in mantle pyroxenes.