TEM investigation of shock-induced polymorphic transformation of olivine

Introduction: The occurrence of ringwoodite as highpressure
polymorph of shocked olivine within shock veins in meteorites
is relatively common, e.g., [1]. In some cases, shocked
olivine displays a complex structure, with high-Fe ringwoodite
rimming low-Fe olivine and fine-grained lamellae of undefined
phases occurring in the olivine core [2-4]. Similar features were
observed in the sample A09584 and were further investigated by
scanning electron microscopy, electron microprobe, Raman spectroscopy,
and transmission electron microscopy.

Shock veins and olivine clasts: In the investigated sample,
classified as L6 [5], shock veins are 1-2 mm wide blackish portions
under transmitted light, generally localized along grain
boundaries. The shock veins consist of clasts, mostly of olivine
and pyroxene, suspended in a glassy matrix, partially crystallized
in 10 µm microlites with olivine composition. Olivine clasts are
rimmed by a 50 µm thick layer of ringwoodite, which has a higher
Fe/Mg ratio than the unshocked olivine (UO). The core of these
clasts contains a dense network of dark (in BSE-SEM images)
lamellae and whitish domains, whose nature could not be determined
other than with TEM.

TEM results: The ringwoodite rim consists of an aggregate
of hypidiomorphic grains, which have an average size of 500 nm
and exhibit internal features that resemble stacking faults. The
clast core contains: (a) domains with nanocrystals of either olivine
or wadsleyite, with strong shape preferred orientation and
lower Fe/Mg ratio than the UO, and (b) veinlets of maximum 500
nm in thickness, composed of equigranular nanocrystals of olivine,
with higher Fe/Mg ratio than the ringwoodite and the UO
and with random orientation. No amorphous material has been
detected.

Discussion: Our observations are in agreement with the most
accepted hypothesis for the formation of the ringwoodite rim,
which is solid state transformation due to diffusion controlled
growth under high temperature conditions [2-4, 6]. An alternative
explanation is fractional crystallization from olivine melt under
shock pressure conditions [7], but an intermediate layer of wadsleyite
should have formed. However, a pressure-composition
phase diagram calculated for an ambient temperature of 1600°C
[8], might explain also the different Fe/Mg ratios in the coexisting
olivine, wadsleyite and ringwoodite. The peak pressure in the
compression stage corresponds to a "triple point", where ringwoodite
and wadsleyite, with respectively high and low Fe/Mg
compositions, formed from olivine. The following release wave
triggered melting of the remaining olivine along veinlets. The
melt, enriched in Fe, lately crystallized as olivine