HR: 0800h
AN: V41A-1353 [Abstracts]
TI: In Situ Density Measurement of Basaltic Melts at High Pressure by X-ray Absorption Method
AU: * Ando, R
EM: andou_rt@ganko.tohoku.ac.jp
AF: Institute of Mineralogy Petrology and Economic Geology, Tohoku University, Aoba-ku, Sendai, 980-8578
Japan
AU: Ohtani, E
EM: ohtani@mail.tains.tohoku.ac.jp
AF: Institute of Mineralogy Petrology and Economic Geology, Tohoku University, Aoba-ku, Sendai, 980-8578
Japan
AU: Suzuki, A
EM: a-suzuki@mail.tains.tohoku.ac.jp
AF: Institute of Mineralogy Petrology and Economic Geology, Tohoku University, Aoba-ku, Sendai, 980-8578
Japan
AU: Urakawa, S
EM: urakawa@cc.okayama-u.ac.jp
AF: Deptment of Earth Science, Okayama University, 3-1-1 Tsushima-Naka, Okayama, 700-8530
Japan
AU: Katayama, Y
EM: katayama@spring8.or.jp
AF: Synchrotron Radiation Research Center, Japan Atomic Energy Research Institute, 1-1-1 Kouto, Mikaduki,
679-5148
Japan
AB:
Density of silicate melt at high pressure is one of the most important properties to understand magma migration in the
planetary interior. However, because of experimental difficulties, the density of magma at high pressure is poorly known.
Katayama et al. (1996) recently developed a new in situ density measurement method for metallic melts, based on the density
dependency of X-ray absorption in the sample. In this study, we tried to measure the density of basaltic melt by this
absorption method.
When X-ray is transmitted to the sample, the intensity of the transmitted X-ray beam (I) is expressed as follows;
I=I$_{0}$exp(-$\mu$$\rho$t), where I$_{0}$ is the intensity of incident X-ray beam, $\mu$ is the mass absorption coefficient,
$\rho$ is the density of the sample, and t is the thickness of the sample. If t and $\mu$ are known, we can determine the
density of the sample by measuring I and I$_{0}$. This is the principle of the absorption method for density measurement.
In this study, in order to determine t, we used a single crystalline diamond cylinder as a sample capsule, diamond is less
compressive and less deformable so that even at high pressure t (thickness of the sample at the point x) is expressed as
follows; t = 2*(R$_{0}$$^{2}$-x$^{2}$)$^{1/2}$, R$_{0}$ is the inner radius of cylinder at the ambient condition, and x is
distance from a center of the capsule. And diamond also shows less absorption so that this make it possible to measure the
density of silicate melt with smaller absorption coefficient than metallic melts.
In order to know the $\mu$ of the sample, we measured both densities ($\rho$) and absorptions (I/I$_{0}$) for some glasses
and crystals with same composition of the sample at the ambient condition, and calculated as fallows;
$\mu$=ln(I/I$_{0}$)/$\rho$.
Experiments were made at the beamline (BL22XU) of SPring-8. For generation of high pressure and high temperature, we used
DIA-type cubic anvil apparatus (SMAP180) there. We used tungsten carbide anvils with the edge-length of 6 mm. The energy of
monochromatic X-ray beam was 25 keV and the beam size was reduced to 0.1*0.1 mm$^{2}$ by two slits. Intensities of X-ray
beam were measured by ion chambers. The starting material was a glass with the MORB composition
(SiO$_{2}$-Al$_{2}$O$_{3}$-FeO-MgO-CaO-Na$_{2}$O).
Experiments were made from 1 atm to 5GPa, from 300 to 1873 K. We measured the density of basaltic glass, crystals (eclogite)
and melt. A density error of this method is less than 2 %. We calculated the bulk modulus of the glass at 773K, crystals at
1273K and melt at 1873 K, and obtained K$_{glass}$(773K)=46(4) GPa, K$_{crystals}$(1273K)=100(7) GPa,
K$_{melt}$(1873K)=16.5(1.5) GPa assuming K$\prime$=4. This K$_{melt}$(1873K) value is consistent with the previous study by
the sink-float method (Ohtani and Maeda (2001); K=18.7(2.1) GPa). We can conclude this method is applicable for silicate
melts.
DE: 3924 High-pressure behavior
DE: 0350 Pressure, density, and temperature
SC: Volcanology, Geochemistry, Petrology [V]
MN: 2004 AGU Fall Meeting