This study uses in situ characterization techniques to analyze five solid inclusions found in diamonds from Siberia. The inclusions are identified as upper mantle minerals including olivine, enstatite orthopyroxene, omphacite clinopyroxene, and two grains of majoritic garnet. Synchrotron X-ray microfluorescence, diffraction, and Raman spectroscopy indicate the inclusions have a homogeneous eclogitic composition and formed under high temperatures and reducing conditions in the upper mantle. X-ray absorption near edge spectroscopy shows the iron in the inclusions is predominantly in the ferrous state, suggesting low oxygen fugacity during formation.

1. In situ crystal chemical study of solid diamond inclusions
from Quaternary alluvial deposit in the Siberian craton
P. Dera1*
, M.H. Manghnani1
, A. Hushur1
, N.V. Sobolev2
, A.M. Logvinova2
, M. Newville3
and A. Lanzirotti3
1
Hawaii Institute of Geophysics and Planetology, University of Hawaii at Manoa, Honolulu, HI, USA
2
Institute of Geology and Mineralogy, Russian Academy of Sciences, Novosibirsk, Russia
3
GSECARS, University of Chicago, Argonne, Illinois, USA
*E-mail: pdera@hawaii.edu
Introduction
Kimberlites belong to rare rock type available only within the
Earth’s cratonic areas and have been a subject of detailed
studies because of the great depth of their origin in the mantle.
Kimberlitic diamonds often contain pristine inclusions derived
from significant depths with different histories of their origins.
Many of kimberlitic diamonds were formed in ultramafic
(peridotitic) and mafic (eclogitic) environments of the upper
mantle. Thus far only a handful of comprehensive in situ studies
including single-crystal X-ray diffraction characterization of
pristine diamond solid inclusions have been reported (e.g. Kunz
et al. 2001, Nestola et al. 2011, Joswig 2011). In situ synchrotron X-ray
microfluorescence
Synchrotron X-ray microfluorescence (SµXRF) is
a novel approach which allows to probe the
chemical composition of mineral inclusions in situ,
with 1 micrometer spatial resolution, without the
necessity to expose the inclusion surface (e.g.
Sitepu et al. 2005). Quantitative analysis of the
data and exact determination of the stoichiometry
are not trivial, because of the X-ray absorption
and extinction phenomena caused by the
presence of host diamond. For all of the
inclusions investigated in this study SµXRF maps
suggest high compositional homogeneity, lack of
zonation, and presence of fairly significant
amounts of Ca2+
, Ni2+
and Mn2+
.
Synchrotron X-ray diffraction and
Raman spectroscopy
The mineral nature and crystal structure of the inclusions were
investigated by Raman spectroscopy and single-crystal synchrotron X-ray
diffraction. For the diffraction experiments, the methodology developed for
diamond anvil cell experiments was implemented (Dera et al. 2013). All of
the investigated inclusions demonstrate very high diffraction quality and
close to hydrostatic environment. High accuracy of the unit cell
parameters and Raman mode frequencies can be used to constrain the
residual stress and estimate the pressure and temperature conditions of
the diamond crystallization, once quantitative chemical composition is
established. Crystal chemical evidence for oEn (distribution on iron
between M1 and M2 sites, Stimpfl et al. 1999) and cOm (C2/c space
group, Fleet et al. 1978) indicate temperatures of formation in excess of
1000K.
Petrology and geologic setting
In this study five single-crystal solid inclusions from diamonds
found in the Quaternary alluvial deposit in NW of the Siberian
craton have been investigated using a combination of in situ
single-crystal X-ray diffraction, Raman spectroscopy,
synchrotron X-ray microfluorescence and X-ray Absorption Near
Edge Spectroscopy (XANES). We focused on in situ
characterization approaches which preserve the inclusions in
their pristine environment. The grains (found in separate
diamonds from the same locality) were identified to be a suite of
major upper mantle minerals including olivine, enstatite
orthopyroxene (oEn), C2/c omphacite clinopyroxene (cOm) and
majoritic garnet (two grains Ga1 and Ga2), indicating eclogitic
origin.
XANES and oxidation state of iron
Eclogitic minerals, particularly pyroxenes, take part in
redox reactions with mantle fluids, and play significant
role in controlling the oxidation sate of carbon in the
mantle (Luth 1993). XANES spectroscopy is a very
sensitive probe of the oxidation state of elements, and
can be used as a proxy of oxygen fugacity in deep
Earth environments (Berry et al. 2010, 2013). The
inclusions of oEn and Ga examined in this study
exhibit significantly reduced conditions of formation
with almost all of the iron present in the ferrous form.
References
Berry, A.J. (2010) et al. Chemical Geology, 278(1–2): p. 31-37.
Dera, P. et al. (2013) High Press. Res., 33: p. 466-484.
Fleet, M.E. et al. (1978) American Mineralogist, 63: p. 1100-1106.
Joswig, W. Z. (2011) Kristallogr., 226: p. 226-228.
Kunz, M. (2002) et al. Earth and Planet. Sci. Lett., 198: p. 485-493.
Luth, R.W. (1993) Science, 261: p. 66-68.
Nestola, F. et al. (2011) Earth and Planet. Sci. Lett., 305: p. 249-255.
Sitepu, H. et al. (2005) Amer. Mineral., 90: p. 1740-1747.
Stimpfl, M. et al. (2005) Contrib. to Mineralogy an
Fig. 1. Photomicrographs of the diamond inclusions investigated in this study
(a,b) oEn, (c,d) cOm, (e,f) Ga1, (g,h) Ga2
Fig. 2. Single crystal synchrotron X-ray diffraction data collected on the oEn inclusion demonstrate very high quality of the
grain and close to hydrostatic environment. Unit cell parameters of oEn indicate composition slightly departing from the
typical upper mantle opx, enriched in Fe and Ca (the unit cell volume is 0.7% larger than for oEn studied by Joswig(2011)).
Fig. 5. Raman spectroscopy provides fast and reliable tool for identification of the mineral nature of solid inclusions in diamonds.
The spectral feature ~900 cm-1
(green arrow) in Ga was interpreted by Kunz et al. (2002) as indicative of majoritic nature of
garnet crystals.
Fig. 3. Fe XANES spectra of garnet, oEn and cOm indicate very
reduced state or iron, with Fe3+
/ΣFe close to 0.
Fig. 4. SµXRF maps of all grains suggest high compositional
homogeneity, lack of zonation, and presence of fairly
significant amounts of Ca2+
, Ni2+
and Mn2+
.
Conclusions
• All of the investigated inclusions share similar
compositional and oxidation state
characteristics, consistent with common
eclogitic origin.
• XANES indicates reducing conditions, and low
oxygen fugacity during formation of the
inclusions.
• Oxygen geothermometers suggest
temperatures in excess of 1000K during the
inclusion formation.
a) b)
c) d)
e) f)
g) h)