JOURNAL OF
PETROLOGY
Journal of Petrology, 2016, Vol. 57, No. 5, 927–932
doi: 10.1093/petrology/egw027
Advance Access Publication Date: 6 June 2016
Reply
The Geochemical Complexity of Kimberlite
Rocks and their Olivine Populations: a Reply to
the Comment on Cordier et al. (2015) by
Andrea Giuliani & Stephen F. Foley
Carole Cordier1,2, Lucie Sauzeat1,2, Nicholas T. Arndt1,2*,
Anne-Marie Boullier1,2, Valentina Batanova1,2 and Fabrice Barou3
1
Université Grenoble Alpes, ISTerre, F-38041 Grenoble, France; 2CNRS, ISTerre, F-38041 Grenoble, France and
Université Montpellier 2, Géosciences Montpellier, Place Eugène Bataillon, 34095 Montpellier, Cedex 05, France
3
*Corresponding author. E-mail: [email protected]
Received February 2, 2016; Accepted April 25, 2016
We thank sincerely Giuliani & Foley for their discussion
of the textures and compositions of olivine in kimberlites. We do not agree with many of their interpretations
and therefore welcome the chance to respond, particularly because it provides an opportunity to explain once
again the characteristics of olivine in kimberlites that we
think are most important—features that have commonly
been misinterpreted or overlooked in many other
papers. In publications such as those by Kamenetsky
et al. (2008), Brett et al. (2009) or Pilbeam et al. (2013), the
emphasis has been on mineral compositions and internal zonation whereas in our paper (Cordier et al.,
2015) and in an earlier study (Arndt et al., 2010), we
emphasized the internal structures and fabrics of olivine
grains. In the earlier paper, we described structures produced by solid-state deformation and showed that these
could only have formed in the lithosphere. In our 2015
paper, we focused on grain boundary migration zones.
These features, which typically develop during fluidassisted recrystallization, may be unfamiliar to igneous
petrologists but are well known to metamorphic and
structural geologists. Like the internal deformation features, they form slowly or under high-stress conditions
and could only have been produced within the
lithosphere. We emphasize that these deformation or recrystallization features formed within rock of dunitic
composition: the orthopyroxene, clinopyroxene and
garnet–spinel that make up a large proportion of most
mantle xenoliths were absent when these textures originated. In our opinion, these observations indicate clearly
that orthopyroxene was eliminated before entrainment
of xenoliths into the ascending kimberlite magma. If this
interpretation is correct (and we have not heard or read
any arguments to negate it), it calls into question the
commonly accepted model that proposes loss of orthopyroxene within ascending kimberlite magma.
With this preamble we now discuss the comments of
Giuliani & Foley, which focus on mineral compositions,
and show that they are consistent with our model.
(1) Use of the term ‘nodule’. We are very aware that
this term is problematic and in retrospect we accept that another term—ovoid or ellipsoid?—might
have been better. But the terms in current usage—
macrocryst or xenocryst—allude to single grains
and they obscure the crucial fact that most of the
large olivine grains in kimberlites are not single
crystals but constituents of composite, multigranular, internally deformed xenoliths of dunite.
(2) Olivine proportions and compositions. Giuliani &
Foley submit that the mantle beneath western
Greenland contains a relatively high proportion of
dunite and propose that the olivine in kimberlites
may have come directly from unmodified lithospheric mantle. We reject this possibility with the
following arguments.
(a) Although olivine-rich lithologies are indeed common in suites of xenoliths in Greenland kimberlites, the lithologies of these rocks differ from those
of the source of olivine in the kimberlites. As documented by Arndt et al. (2010), polycrystalline olivine nodules (‘macrocrysts’) in kimberlites from
west Greenland contain >95% olivine, significantly
more than the 79–92% olivine measured in
Greenland xenoliths by Bizzarro & Stevenson
(2003). More tellingly, as also documented by
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Journal of Petrology, 2016, Vol. 57, No. 5
Fig. 1. Comparison of NiO and Fo contents of olivine in kimberlites and mantle xenoliths from two cratons. Data compiled in
GEOROC, mainly from Bizzarro & Stevenson (2003), Bernstein
et al. (2006), Arndt et al. (2010), Pilbeam et al. (2013) and
Cordier et al. (2015).
Arndt et al. (2010; see table 1 and fig. 2c–e of that
paper) and other researchers (e.g. Mitchell, 1986;
Brett et al., 2009; Moore, 2012; Bussweiller et al.,
2015), nodules in kimberlites from the Slave and
Kaapvaal cratons also are made up almost entirely
of olivine, in marked contrast to the more modest
olivine contents (50–70%) in harzburgite and lherzolite xenoliths from these cratons.
(b) The compositions of olivine in kimberlites differ
from those of olivines in mantle xenoliths. As
shown in Fig. 1, in kimberlites, the Fo content of
olivine cores varies continuously from Fo93 to 88,
with less frequent grains extending the range to
Fo82. Most of the low-Fo grains have low Ni contents, unlike those in most xenoliths from the cratonic mantle (Fig. 1). The mismatch between
kimberlite and xenolith olivine is most obvious in
samples from Greenland; in these, the cores of
olivines in kimberlites define a sloping trend from
low Fo and Ni to moderate Fo and high Ni, which
overlaps only partially with the flatter trend of
xenolith olivines. Figure 2 shows that the peak in
Fo contents of kimberlite olivines lies at 905, significantly lower than that of the more magnesian
olivines in xenoliths from cratonic lithospheric
mantle. As Giuliani & Foley have observed, the
low-Fo low-Ni characteristics of olivine at the end
of the kimberlite trend are similar to those of megacrysts from Monastery and Jagersfontein in South
Africa; this similarity reinforces the idea that these
compositions are peculiar to kimberlites. Small
populations of olivines with low Fo but high Ni are
found both in xenoliths and kimberlites, and in the
latter at least some may have formed during metasomatic reaction with silicate melt within the
lithosphere.
(3) Grain size. We based our conclusion that the grain
size of olivine in kimberlite nodules (before recrystallization into tablets) is greater than the grain size
Fig. 2. Comparison of the Fo contents of olivine from kimberlites and mantle xenoliths from the Greenland, Slave,
Kaapvaal and Siberian cratons. Data compiled in GEOROC,
mainly from Moore & Lock (2001), Bizzarro & Stevenson
(2003), Simon et al. (2003) Aulbach et al. (2004), Eccles et al.
(2004), Menzies et al. (2004), Gibson et al. (2008), Brett et al.
(2009), Patterson et al. (2009), Arndt et al. (2010), Bernstein
et al. (2006), Pilbeam et al. (2013), Giuliani et al. (2014) and
Cordier et al. (2015).
in mantle xenoliths on statements in standard texts
such as the book by Mitchell (1986). Some xenoliths no doubt contain larger grains, but Nixon
(1987) and Tabor et al. (2010) quoted mean sizes
less than 05 mm, which is less than the sizes
observed in kimberlites. To provide a firmer basis
would require detailed analysis. Suffice to say that
the common terms for olivine nodules, including
those that consist of a single olivine grain, are
‘macrocryst’ or ‘megacryst’, implying a larger than
normal grain size.
(4) Transition zones. We are not sure how to respond
to the remarks about the transition zones. Giuliani &
Foley have selected and plotted two profiles that are
not representative of our data; most of our measured profiles conform to the pattern in fig. 1d of
Cordier et al. (2015), which shows a rapid change
in Fo but near-constant Ni in the transition zone
(Fig. 3), in contrast to near-constant Fo but rapidly
decreasing Ni in the marginal zone. We note as well
that the zone boundaries shown in their figure do
not correspond to the divisions we proposed in our
Journal of Petrology, 2016, Vol. 57, No. 5
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Fig. 3. Chemical profile 22 across marginal zones in olivine.
Analyses are given in the Supplementary Data of Cordier et al.
(2015).
Supplementary Data. Giuliani & Foley state:
‘Neither the olivine cores nor the transition zones
were produced by kimberlite metasomatism at
mantle depths. Even if this were the case, it would
be difficult to reconcile how the same process generated olivine (i.e. cores and transition zones) with
such distinct features at the same time and in the
same mantle domain.’ We do not say that the olivine cores and the transition zones formed synchronously. Our observations are that (a) olivine cores,
grain boundary zones and transition zones show
similar chemical gradients, (b) grain boundary
zones must have formed by fluid–crystal interaction
in a deforming mantle, and (c) dunites with contrasting olivine composition are typical of reaction zones
in peridotite massifs (e.g. Tursack & Liang, 2012).
We therefore maintain that the transition zones result from a process that occurred in the lithosphere,
prior to or shortly after entrainment of the dunite
fragments by the kimberlite.
(5) Metasomatism. We do not deny that metasomatism has changed the compositions of large portions of the lithosphere, as is well described in the
examples selected by Giuliani & Foley. We thank
them for pointing out that silicate melts at depths
and pressures corresponding to those of the lower
part of the lithosphere would have ultramafic and
not basaltic compositions. However, even highly
magnesian magma containing around 20% MgO
(that of a low-degree mantle melt at 6 GPa; Walter,
1998) would crystallize pyroxene in addition to olivine, which is what is required to explain the elevated Ni contents of the high-Ni low-Fo olivines. In
Fig. 4, we show that the trend to high-Ni low-Fo
olivine can equally well be explained by crystallization of a high-Mg liquid.
(6) Estimation of liquid composition. Giuliani & Foley
have three criticisms of the manner in which we
estimated the composition of the parental kimberlite liquid.
Fig. 4. Ni vs Fo diagram showing margin crystallization models
from Cordier et al. (2015) together with the crystallization of
ultramafic melt with MgO ¼ 20 wt %, FeOTOT ¼ 135 wt % and
Ni ¼ 750 ppm. We assumed FeO/Fe2O3 ¼ 09 and the ratio of
crystallizing clinopyroxene to olivine as 15. The amount of
crystallization required to reproduce the trend observed in the
M1 zone is 48 wt %.
(a) They question whether all Fe was present as Fe2þ,
noting that some spinels in kimberlites contain
some Fe3þ. In our paper we considered all iron to
be FeO because of the reduced state of kimberlite
magmas (DNNO –3 to –2, where NNO is nickel–
nickel oxide; Fedortchouk & Canil, 2004). The presence of a small amount of Fe3þ makes little difference to the estimated composition.
(b) Giuliani & Foley suggest that our calculations may
have been compromised by crustal assimilation or
alteration. As emphasized by Arndt et al. (2010)
and documented in figs 1 and 2 of that paper, the
Kangamiut kimberlites are remarkably well preserved. Neither the olivine nor the groundmass is
altered. The samples we analysed contain rare
megacrysts of garnet and orthopyroxene but no
crustal xenoliths were observed.
(c) Giuliani & Foley object that volatile components
were ignored. This is true—we calculated the
volatile-free composition because it is impossible
to quantify the loss of H2O and CO2 that accompanied emplacement. If the magma contained 10%
volatiles, this would reduce the calculated MgO
content from 19 to 17%, which is within the error of
the calculation. Giuliani & Foley state that ‘the volatile components . . . must be a major factor in identifying the various types of melt at source’. This too
is correct—but not entirely relevant. The composition we calculated represents only that of the melt
930
that transferred its load of olivine fragments from
the site of interaction in the lower part of the lithosphere to the surface, not that of the magma at its
source.
(7) Modelling of compositional variations in the margins
of olivine grains. Giuliani & Foley state: ‘The main
outcome of their modelling is that orthopyroxene dissolution in ascending kimberlite magmas does not
influence the composition of olivine margins, which
contrasts with recent findings (Pilbeam et al., 2013;
Bussweiler et al., 2015). The corollary is that orthopyroxene in mantle wall-rocks was largely dissolved during metasomatism by proto-kimberlite melts or fluids
shortly before kimberlite eruption.’ We have the impression that Giuliani & Foley did not read sufficiently carefully the arguments on page 1789 of
Cordier et al. (2015). We explain there that, if a high
Ni partition coefficient is adopted, the peculiar trend
of rapidly decreasing Ni at near-constant Fo can be
explained by crystallization of either olivine alone or
olivine plus orthopyroxene. The value of the Ni partition coefficient we used is appropriate for silicocarbonatite melts and similar to that used by Pilbeam
et al. (2013). To discriminate between the two options
(ol alone or ol þ opx) we used our estimates of the
proportion of cognate olivine (i.e. olivine that crystallized from the kimberlite and is now preserved as
overgrowths and matrix grains). If the trends were
produced by olivine crystallization and orthopyroxene dissolution, 37% olivine must crystallize and 22%
orthopyroxene must dissolve. These values are far
greater than the estimated proportion of cognate olivine (6 wt % in the Greenland samples and only 4 wt
% in other kimberlite samples; e.g. Brett et al., 2009).
We cannot totally rule out orthopyroxene dissolution
at the time of crystallization of the overgrowths, but
believe that it is not necessary because the trends are
satisfactorily explained by the crystallization of olivine without other silicate minerals.
(8) Olivine was the only mineral on the liquidus. G&S
submit that our modelling was based on the assumption that olivine was the only mineral on the
liquidus and speculate that the small grains in an
olivine margin could be spinel. This is incorrect—
we subsequently analysed the grains in question
and found that they are mainly ilmenite with some
titaniferous magnetite. In fig. 8e of Cordier et al.
(2015) we showed that Ti increases outwards in an
olivine margin, indicating that the Ti content of the
liquid increased during olivine crystallization. This
provides evidence that ilmenite did not crystallize
together with olivine. We note that only rarely do
overgrowths contain ilmenite grains and that ilmenite is known to form by metasomatic processes (e.g. Robles-Cruz et al., 2009). Most of the
ilmenite grains in the olivine margins probably are
xenocrysts, perhaps from the same source as the
ilmenite megacrysts that are common in
Kangamiut kimberlites.
Journal of Petrology, 2016, Vol. 57, No. 5
The proportion of spinel and other oxides is very
small (<1%) and their presence would not materially affect the calculation of the parental melt composition, nor
the modelling of the olivine overgrowths. We therefore
are puzzled by Giuliani & Foley’s conclusion at the end of
this section: ‘We contend that the proposal of Cordier
et al. (2015) that (partial) dissolution of orthopyroxene in
mantle wall-rocks precedes xenolith incorporation and
ascent of kimberlite magmas is not substantiated by the
observations presented and their models.’
To conclude, we submit that none of Giuliani &
Foley’s observations and arguments negates our model
and we maintain our position that metasomatic processes that acted within the lithosphere eliminated
orthopyroxene and other minerals from mantle peridotite. Olivine nodules in kimberlites, not only from west
Greenland but also from many other localities, represent recrystallized and partially disaggregated fragments of dunite that was produced by metasomatism
prior to entrainment by ascending kimberlite.
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