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6.5 The Ascent Rate of Diamond and Phlogopite-Bearing
Olivine Lamproite or a Kimberlitic Magma
Barua, Gupta, Mandal and Singh (2013) estimated the ascent rate of a phlogopite
and dolomite-bearing ultramaic magma (the assemblage contained olivine, enstetite
and diopside in appropriate proportion mimicking a model kimberlitic magma)
containing diamond. Their experiments were conducted using a Walker-type multi-
anvil apparatus. The experiments were made with a piece of diamond, placed within
such a synthetic assemblage in each set of an experiment, keeping an initial pressure
and temperature of 6 GPa and 1,350
C. The assemblage was then cooled at
different rates (along 1st, 2nd and 3rd curve, see Fig.
6.14
). The topology of the
solidus of peridotite is after Wyllie (1979). It was observed that the volume fraction
of diamond to graphite conversion strongly depended on the ascent rates. Using
electrical resistivity and X-ray diffraction studies, Barua et al. measured the degree
of graphitization as a function of the ascent rate (u). For u < 3 m/s, diamond
underwent almost complete graphitization (conversion > 90 %), whereas it
remained nearly intact (conversion of 10 %) when u is >10 m/s. Their theoretical
calculations of the settling velocity of mantle xenoliths again con
°
rm that diamond
cannot exist when u is as small as 3 m/s. They also performed numerical experi-
ments with
finite element (visco-elastic) models to analyse the dynamics of tensile
failure at the tip of magma pools, leading to dilatational vertical fractures for
magma transport. Considering the tensile strength of mantle in the order of 0.5 kb,
their models show this failure process as a function of the critical shape (A
r
ratio of
vertical and horizontal dimensions) and density contrast (
Δρ
) of magma pools. The
was estimated to be nearly 200 kg/m
3
when A
r
is considered to be very
large (>4) (Fig.
6.14
).
critical
Δρ
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