Jianzhong Zhang1,2 Liping Wang1, and Yusheng Zhao2
1 SUNY at Stony Brook;
2Los Alamos Neutron Science Center, Los Alamos National Laboratory)
jzhang@lanl.gov
NSLS-X17B (MAP) and APS-GSECARS
One of the most important goals in studying the olivine (a)-wadsleyite
(b) transformation is to understand the now well-accepted
seismic discontinuity near the depth of 410 km in the Earth's mantle. Although
one school of thought attributes such a discontinuity to radical chemical changes
from lherzolite to picritic eclogite (Anderson and Bass, 1986), it has widely
been viewed that this discontinuity is caused by the a-b
transformation in an isochemical peridotitic mantle (Ringwood, 1975 and 1979).
If this interpretation is valid, the composition and temperature of the mantle
can be inferred at this depth, providing useful information for understanding
the present state of the Earth's transition zone. Although there have been numerous
experimental investigations of this transformation, most studies were conducted
either using quench method or in the simple Mg2SiO4-Fe2SiO4
system. No efforts have been directed to study the kinetic barrier of the olivine-wadsleyite
transformation under normal mantle conditions. In addition, recent studies have
demonstrated increasing needs for the study of this transformation in complex
system relevant to the Earth's mantle.
A two-stage multi-anvil press (T-cup) was utilized on the superconducting wiggler
beamline X-17B of National Synchrotron Light Source and on the bending magnet
at beamline 13-BM-D of the Advanced Photon Source. The KLB-1 spinel lherzolite,
a xenolith from Kilborne Hole crater in New Mexico, is chosen as starting material
because it represents one of the most undepleted mantle compositions and thus
a suitable rock specimen for simulating the Earth?s mantle. The powder KLB-1
sample was pre-annealed in the temperature range of 1200-1600°C at the pressures
in close proximity to the a to a
+ b and a + b to b
phase boundaries but outside the a + b loop.
The present experimental results have provided a lower bound for the stability
field of b phase (b-out)
at 1215 °C and an upper bound for the stability field of a phase (a
out) at 1505°C. In addition, the nucleation barriers determined from reversal
experiments at 1300°C and higher temperatures seem to be about 0.2-0.3 GPa
for both a ® a + b and a
+ b ® b phase boundaries, and the width of the two-phase loop appears
to be no more than 0.4 GPa. Based on the petrologic barometry and thermometry,
the mantle temperatures near the 410-km depth were estimated to be in the range
of 1300-1400°C (e.g., Mercier and Carter, 1991; Nisbet et al.,
1993). The middle point of these temperatures intersects the two-phase loop
of Fig. 7 at pressures of 13.5-13.9 GPa, which is comparable to the pressure
of 13.7 GPa at the discontinuity. This match provides additional confidence
in the experimental results that have been achieved. The findings obtained from
these pilot experiments demonstrate an experimental feasibility to resolve a
pressure difference of less than 0.4 GPa for the two-phase loop, even when the
effect of nucleation barrier is taken into account.
This work is jointly supported by the NSF-funded Consortium for Materials Properties
Research in Earth Sciences and by the Department of Energy.