Chapter 6 Magmatic inclusions in minerals 147 Chapter 6 Magmatic inclusions in groundmass minerals of the Udachnaya-East kimberlite Research into identification of kimberlite liquids has been hampered by several factors. Most significant among these is the presence of variable amounts of mineral grains and mineral aggregates that are probably unrelated to the kimberlite crystallisation (xenocrysts and xenoliths), but are hard to separate from true phenocrysts and cognate inclusions especially, in altered rocks. A related problem is syn- crystallisation degassing of volatiles (CO 2 and H 2 O), which is assumed to be significant in kimberlitic magmas. It is now well accepted that inclusions of melt and fluid trapped in phenocrysts during crystallisation may represent true magmatic compositions. Magmatic (crystal, melt and fluid) inclusions in phenocrysts provide a snapshot of a particular evolutionary stage of the magma, as sampled at a particular time, place and physical and chemical conditions (e.g., reviews in Frezzotti, 2001; Kamenetsky et al., 2003; Lowenstern, 2003; Roedder, 1979; Roedder, 1984; Roedder, 1992; Sobolev, 1996). Magmatic inclusions within phenocrysts are present in almost all types of intrusive and volcanic rocks, and they have even been reported in mantle minerals. When crystals grow or recrystallise in magma or a fluid, they trap small batches of melt or fluid, or other crystals, preserving them as inclusions. The mechanisms and conditions of trapping can be very different, even for inclusions within a single crystal. One of the classification schemes describes the origin of inclusions and divides them into three different types. The most informative primary inclusions are trapped by a crystal from a medium from which it forms. This kind of inclusion provides information on the chemical and physical conditions at the moment of the crystal' s formation. To recognise them, one should look for individual inclusions, or for the clusters of two to several tens of randomly distributed inclusions, or for a group of inclusions which form a regular zoning pattern that follows relic crystal-melt, or crystal-fluid interfaces. Another type is secondary inclusions. Secondary inclusions are those that form by any process after crystallisation of the host mineral is essentially complete. If a
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Chapter 6 Magmatic inclusions in minerals
147
Chapter 6 Magmatic inclusions in groundmass
minerals of the Udachnaya-East kimberlite
Research into identification of kimberlite liquids has been hampered by several
factors. Most significant among these is the presence of variable amounts of mineral
grains and mineral aggregates that are probably unrelated to the kimberlite
crystallisation (xenocrysts and xenoliths), but are hard to separate from true phenocrysts
and cognate inclusions especially, in altered rocks. A related problem is syn-
crystallisation degassing of volatiles (CO2 and H2O), which is assumed to be significant
in kimberlitic magmas. It is now well accepted that inclusions of melt and fluid trapped
in phenocrysts during crystallisation may represent true magmatic compositions.
Magmatic (crystal, melt and fluid) inclusions in phenocrysts provide a snapshot of a
particular evolutionary stage of the magma, as sampled at a particular time, place and
physical and chemical conditions (e.g., reviews in Frezzotti, 2001; Kamenetsky et al.,
Magmatic inclusions within phenocrysts are present in almost all types of
intrusive and volcanic rocks, and they have even been reported in mantle minerals.
When crystals grow or recrystallise in magma or a fluid, they trap small batches of melt
or fluid, or other crystals, preserving them as inclusions. The mechanisms and
conditions of trapping can be very different, even for inclusions within a single crystal.
One of the classification schemes describes the origin of inclusions and divides them
into three different types.
The most informative primary inclusions are trapped by a crystal from a
medium from which it forms. This kind of inclusion provides information on the
chemical and physical conditions at the moment of the crystal's formation. To recognise
them, one should look for individual inclusions, or for the clusters of two to several tens
of randomly distributed inclusions, or for a group of inclusions which form a regular
zoning pattern that follows relic crystal-melt, or crystal-fluid interfaces.
Another type is secondary inclusions. Secondary inclusions are those that form
by any process after crystallisation of the host mineral is essentially complete. If a
Chapter 6 Magmatic inclusions in minerals
148
crystal is fractured in the presence of melt or fluid, then the fractures could be filled
with the material, which often has no genetic relation to the crystal. Secondary
inclusions are often aligned along fractures, and this helps to identify them.
Usually we think of primary inclusions as forming during the growth of the
crystal and of secondary inclusions as forming at some later time, from entirely different
magmas or fluids. Pseudosecondary inclusions are broadly intermediate between
primary and secondary. They look secondary, but are formed during the growth of the
host crystal, if the crystal fractures during formation.
Another inclusion classification scheme is based on phase composition and
appearance of inclusions. These subtypes are: crystal, melt, and fluid, but all three
phases may occur within one inclusion. In general, any given magmatic inclusion can
contain several different crystal phases, one or two different silicate glasses, immiscible
sulphide globules, aqueous saline liquid and one to several vapour bubbles.
From magmatic inclusion studies, combined with suitable theoretical,
experimental and numerical treatment, the composition, temperature and pressure of the
primary melt can be successfully reconstructed. Thus, the study of different types of
inclusions in the groundmass minerals, especially olivine, may overcome the problems
mentioned above and put constraints on the origin and evolution of kimberlitic magma.
Phenocryst-hosted melt and fluid inclusions in kimberlite offer a new window to
deciphering the geochemical budget of parental magmas, and therefore may refine our
understanding of the composition and evolution of this important magma type. The
great advantage of inclusion studies compared to an experimental and theoretical
approach is that they directly apply to natural melts and fluids, and the use of modern
microbeam analytical techniques, vital for success of this project, can put solid
constraints on element partitioning and the ultimate concentrations and fate of volatile
elements.
Only a few studies of melt and fluid inclusions in kimberlite groundmass olivine
have been published to date, and all of them concern the Udachnaya-East pipe. The first
study by Popivnyak and Laz'ko (1979) reported the discovery of extremely rare
primary, partly crystallized melt inclusions, fluid (“gas-glass” or “gas” inclusions) and
Chapter 6 Magmatic inclusions in minerals
149
sporadic pseudosecondary inclusions in the groundmass olivine of the Udachnaya pipe.
The authors stated that most studied inclusions were small (less than 1 µm) and
homogenized at temperatures 780-1100oC. The reliability of these results is
questionable, because it is impossible to observe any changes in such small inclusions
during experiment. No chemical compositions of melt and fluid inclusions were
reported in this study.
Later, Sobolev et al. (1989) performed a more detailed study of magmatic
inclusions in the groundmass olivine, but mainly focused on melt and fluid inclusions in
lamproitic olivine. Lesser details were provided from the study of kimberlite olivine-
hosted melt inclusions. They showed that inclusions in olivine-II were represented by
different crystal phases (chromite, low-Ca pyroxene, olivine, phlogopite, ilmenite and
rutile), nearly pure CO2 fluid with density as high as 0.75 g/cm-3 (no H2O was detected
in fluid inclusions), multiphase inclusions consisting “mainly of high-density fluid with
minor melt”, and secondary melt inclusions. Heating stage experiments with secondary
melt inclusions demonstrated low temperatures of homogenization (600-650 oC) and
authors envisaged a non-silicate nature (concentrated brines) for these inclusions based
on their low viscosity. “Normal” silicate melt inclusions were not been found in this
study. The temperature (1100 ± 30oC) of the olivine-II crystallization was also
estimated using the olivine-spinel geothermometer of Fabries (1979) and the graphical
version of the two-pyroxene thermometer of Lindsley (1983). The pressure of
crystallization (4-5 kbar, probably the lowest limit) was deduced from the density of
CO2-rich fluid inclusions.
The most recent melt inclusion study of the Udachnaya-East kimberlite (Golovin
et al., 2003) was performed at the same time as my research and on the same samples.
The publication by Golovin et al. (2003) postdates the presentation of my melt inclusion
studies in 2002 at the Goldschmidt Geochemical Conference in Davos, Switzerland and
at the Workshop-Short Course on volcanic systems “Melt inclusions: methods,
applications and problems” in Napoli, Italy (Kamenetsky et al., 2002a; Kamenetsky et
al., 2002b). Several coauthors of my publication (N.P. Pokhilenko and N.V. Sobolev)
also participated in the paper by Golovin et al. (2003), without my consent. Melt
inclusions in both generations of olivine described by Golovin et al. (2003) were
Chapter 6 Magmatic inclusions in minerals
150
interpreted to be secondary, and composed of finely crystallized aggregates of
carbonates (calcite, dolomite, siderite, and shortite-zemkorite), different sulphates, Na-
K chlorides, silicates (tetraferriphlogopite, phlogopite, olivine, humite-clinohumite,
diopside), oxides (magnetite series), and sulphides (Ni-pyrrhotite and djerfisherite). As
it shown below, many results of my study and those of Golovin et al. (2003) overlap,
but my study is considerably more detailed and supported by abundant high-quality
analytical data.
6.1 Inclusions in groundmass olivine
Magmatic inclusions in groundmass olivine are abundant in some grains, but
rare in others. Inclusion sizes are variable (<1 to ~300 µm) and the distribution of
inclusions within a single olivine crystal is very heterogeneous, with some parts totally
devoid of inclusions, and some parts so crowded with inclusions as to make olivine
almost opaque (Fig. 6.1A). The highest density of inclusions is observed along internal
fractures and growth planes (Fig. 6.1A, C). As a rule, the number of inclusions increases
from the cores to rims of olivine. Three main types of magmatic inclusions are
recognized in the studied samples: crystals, fluid and melt. Crystal inclusions are always
primary, whereas fluid and melt inclusions show features reminiscent of both primary
and secondary origin.
6.1.1 Crystal inclusions
The previous study Sobolev et al. (1989) has revealed a variety of crystal (low-
Ca pyroxene, phlogopite, ilmenite, olivine, Cr-spinel and rutile) and fluid (mainly CO2)
inclusions in groundmass olivine-II. Here I report discovery of previously unknown
inclusions of high-Ca pyroxene. Brief descriptions of other crystal inclusions – low-Ca
pyroxene, olivine, phlogopite, and two unidentified phases are also given.
Pyroxene inclusions are not common but are genetically important, and they
provide new information on the nature of the original kimberlitic melt. Pyroxene
inclusions are represented by both high-Ca and low-Ca varieties.
Chapter 6 Magmatic inclusions in minerals 151
Figure 6.1. Distribution of inclusions in groundmass olivine. A – inclusions spread throughout olivine-II grain; B – inclusion distribution in olivine-I; C – inclusions along healing fractures in olivine core; D – large multiphase inclusion in olivine core; E - large multiphase inclusion in olivine rims
D
EC
B
A
Chapter 6 Magmatic inclusions in minerals 152
Figure 6.2. High-Ca pyroxene (cpx) inclusions in groundmass olivine. A –distribution and zoning of cpx inclusions in the groundmass olivine; B – cluster of three inclusions, single subhedral and rounded cpx inclusions; C – Pyroxene inclusion in association with phlogopite inclusion in olivine core, insert – melt inclusions hosted by pyroxene inclusion:
20 μm 50 μm
B
50 μm
host ol
cpx
phl
C
Ca Na20 μm
A
Ca
Ca Cr Na20 μm
10 μm
Figure 6.3. High-Ca pyroxene inclusions in the core of groundmass olivine. Element distribution maps show the presence of alkali-Cl-C rich melt inclusions, partly exposed at surface by the electron beam, and the rim around pyroxene
Na K
CCl
Chapter 6 Magmatic inclusions in minerals 153
FeMg
Ca
50 μm Ca
min max
Chapter 6 Magmatic inclusions in minerals
154
High-Ca pyroxene inclusions occur as a single crystal or as clusters of several
crystals, and are restricted to the olivine cores (Fig. 6.2A). In the present set of data
olivine Fo86.3-87 and Fo91-93 are found only as hosts to high-Ca pyroxene. Pyroxene
inclusions are in Fe-Mg equilibrium with the host olivine, which can be seen in a
positive co-variation of pyroxene Mg# and host olivine Fo compositions (Fig. 6.4).
These inclusions are represented by crystals of different size (25-200 µm) and colour
(emerald-green to greyish-green). Based on shape, two types of high-Ca pyroxene
inclusion can be recognised: 1 - rounded crystals with smooth edges, and 2 - euhedral-
subhedral grains (Fig. 6.2B). The latter, in most cases, have slightly resorbed edges and
contain a large amount of fluid and melt inclusions (Fig. 6.2B, C). All high-Ca pyroxene
inclusions are intimately associated with a vapour-rich substance that forms a coating
around them. X-ray mapping of the element distribution shows high concentrations of
chlorine, carbon, potassium and sodium in these coats and in pyroxene-hosted
inclusions of melt/fluid (Fig. 6.3). CaO content in olivine sharply increases from ~0.02
wt% to 0.05-0.08 wt% towards high-Ca pyroxene inclusions (Figs. 6.2A and 6.3).
Compositionally, high-Ca pyroxene inclusions are similar to Cr-diopside, and
are characterised by low Al2O3 (0.6-2.9 wt%), and high CaO (19.5-23.8 wt%), Na2O
crystals show pronounced fine-scale compositional zoning, with a general pattern of
MgO and CaO increase, and Na2O, Cr2O3 and in some cases Al2O3, decrease towards
the rims (Figs. 6.3 and 6.5). Such zoning is atypical for pyroxenes from the Udachnaya-
pipe mantle and crustal xenoliths (Shimizu et al., 1997).
Trace element compositions of exposed high-Ca pyroxene inclusions in
groundmass olivine were determined by ion microprobe (SIMS) (Cameca ims-3f) at the
Max-Planck-Institut für Chemie, Mainz, Germany. Cores of the pyroxene inclusions are
highly enriched in most incompatible trace elements, but depleted in heavy rare-earth
elements. The general pattern of enriched trace element compositions of core regions
(Table 6.1 and Fig. 6.6A) is complicated by relative depletion of the most incompatible
elements (e.g., Ba), high-field strength elements (e.g., Nb, Zr, Ti) and moderately
compatible elements (e.g., Gd is strongly fractionated from Yb). Such trace element
compositions are different from those of pyroxenes in the Udachnaya mantle xenoliths
Chapter 6 Magmatic inclusions in minerals 155
Figure 6.4. Composition of high-Ca pyroxene inclusions in groundmass olivine, shaded field shows the compositions of host olivine rims, (abundances in wt%).
53.9
54.1
54.3
54.5
54.7
54.9
87 89 91 93 95
SiO2
0.05
0.10
0.15
0.20
0.25
87 89 91 93 95
TiO2
0.5
1.0
1.5
2.0
2.5
3.0
87 89 91 93 95
Al2O3
CaO
0.5
1.0
1.5
2.0
2.5
87 89 91 93 95
Na2O
0.8
1.2
1.6
2.0
2.4
2.8
87 89 91 93 95
Cr2O3
Fo, host olivine
Mg#
, cpx
incl
usi
on
88
90
92
94
86 88 90 92 94
Mg#
19
20
21
22
23
24
87 89 91 93 95
MgO
14.6
14.8
15
15.2
15.4
15.6
15.8
16
CaO
19.2
19.4
19.6
19.8
20
20.2
20.4
20.6
Na2O
1.5
1.7
1.9
2.1
2.3
2.5
Cr2O3
0
0.5
1
1.5
2
2.5
MgO
14.815
15.215.415.615.8
16
CaO
19.419.619.8
2020.220.420.6
Na2O
1.5
2
2.5
Cr2O3
0.5
1
1.5
2
2.5
Chapter 6 Magmatic inclusions in minerals 156
Figure 6.5. Profiles of element distributions within high-Ca pyroxene inclusions in groundmass olivine
BA
Chapter 6 Magmatic inclusions in minerals 157
MgO
15.6
15.8
16
16.2
TiO2
0.1
0.2
0.3
K2O
0.015
0.02
0.025
0.03
0.035
0.04
0.045
Cr2O3
2.3
2.4
2.5
2.6
2.7
2.8
Al2O3
0.4
0.6
0.8
1
1.2
1.4
1.6
1.8
Na2O
0.6
0.7
0.8
0.9
1
1.1
1.2
Cr2O3
0.8
1
1.2
1.4
Figure 6.5 (cont). Profiles of element distributions within high-Ca pyroxene inclusions in the core of groundmass olivine
C D
Figure 6.6. Primitive/mantle (Sun & McDonough, 1989) - normalized compositions of (A) high-Ca pyroxene inclusions in cores of Udachnaya groundmass olivine (red lines) and clinopyroxenes from diamondites (blue lines with diamonds, Dobosi & Kurat, 2002 ), a field (ligh blue) of compositions of high-Ca pyroxenes from garnet peridotite xenoliths in the Udachnaya kimberlites (Shimizu et al, 1997); B -hypothetical melts (blue lines) in equilibrium with high-Ca pyroxene inclusions in the olivine and composition of the groundmass (red lines with circles).
Chapter 6 Magmatic inclusions in minerals 158
B
sam
ple/
prim
itiv
e m
antl
e
A
sam
ple/
prim
itiv
e m
antl
e
0.01
0.10
1.00
10.00
100.00
Ba Nb La Ce Pr Sr Nd Sm Zr Eu Ti Gd Tb Dy Y Ho Er Yb
0.1
1.0
10.0
100.0
1000.0
Ba Nb La Ce Pr Sr Nd Sm Zr Eu Ti Gd Tb Dy Y Ho Er Yb
Chapter 6 Magmatic inclusions in minerals
159
and xenocrysts in having higher concentrations of moderately incompatible elements
(Zr, Eu, Sm) (Fig. 6.5A). On the other hand, a compositional similarity between olivine-
hosted high-Ca pyroxene inclusions and clinopyroxenes from diamond-bearing rocks -
diamondites (Dobosi and Kurat, 2002) should be noted.
The composition of hypothetical melts parental to these clinopyroxenes was
calculated using Kd (distribution coefficients) for low–Al clinopyroxene and melt
(Sobolev et al., 1996) for rare-earth elements and Y, and Kd = 0.008, 0.13, and 0.12 for
Nb, Sr, and Zr, respectively, and a comparison with the trace element composition of
the Udachnaya-East kimberlite groundmass is shown in Figure 6.6B.
Carbonate- and chloride-rich material decorating inclusions of high-Ca pyroxene
is difficult to confidently ascribe to a particular origin (i.e. primary vs secondary);
however inclusions of the same material inside pyroxene grains are likely to have been
coeval with pyroxene crystallisation. Another phase, co-trapped with high-Ca pyroxene
in olivine cores, is phlogopite. Only one occurrence of attached grains of high-Ca
pyroxene and phlogopite is recorded (Fig. 6.2C), and phlogopite was studied for its
water content by SIMS (Cameca ims-3f, Max-Planck-Institut für Chemie, Mainz), using
the method described in (Sobolev and Chaussidon, 1996). Unlike phlogopite inclusions
in the rims of olivine-II, this phlogopite in the olivine core is water-saturated (4.3 wt%
H2O) (Table 6.2).
The rims and marginal zones of olivine-II grains show abundant inclusions of
different minerals that are never present in the cores. Among them, magnetite,
phlogopite and rutile are relatively abundant, whereas perovskite and spinel are less
common. Inclusions of low-Ca pyroxene and olivine can occur in both cores and rims.
Phlogopite inclusions usually occur in the rim regions of olivine grains and are
either brownish euhedral grains 30-200 µm in size, or clusters of a few inclusions.
Phlogopite inclusions frequently coexist with low-Ca pyroxene (Fig. 6.7A). Phlogopite
grains are compositionally homogeneous (Table 6.2) and resemble groundmass
phlogopite, although they have less TiO2 (2.2-2.6 wt%), Al2O3 (13.1-13.4 wt%) and
FeO (4.4-4.6 wt%). A specific feature of phlogopite inclusions from olivine rims is a
Chapter 6 Magmatic inclusions in minerals
160
significant undersaturation in water (2.7 wt%) that distinguishes them from phlogopite
inclusions found in the core (Table 6.2).
Rutile occurs as euhedral needle-like crystals up to 100 µm long (Fig. 6.7C).
Commonly, one olivine grain can contain numerous randomly oriented rutile crystals,
often forming “L”-like intergrowths.
Olivine inclusions in olivine-II are widespread in this kimberlite. They usually
occur as single euhedral grains of variable size (30-200 µm). A coating of fluid-rich
material helps to optically distinguish olivine inclusions from the olivine-host (Fig.
6.7B). Compositions of olivine inclusions (Fo85-91) are almost identical to those of their
host olivine-II (Table 6.3; Fig. 6.7B).
Low-Ca pyroxene is typically present in clusters of several (10-30) euhedral
grains in olivine-II cores and rims (Fo86-91). A common association of low-Ca pyroxene
includes numerous melt and fluid inclusions, and a volatile-rich substance coating
surfaces of pyroxene crystals (Fig. 6.8). A positive correlation between Mg# of
pyroxene inclusions and host olivine Fo is indicative of their local equilibrium in terms
of Mg-Fe. The compositions of low-Ca pyroxene (Fig. 6.9 and Table 6.4) are distinctly
different from mantle orthopyroxene in having high SiO2 (53.3-58 wt%), Na2O (0.1-0.9
wt%), elevated TiO2 (0-0.5 wt%), and low Al2O3 (0.7-1.4 wt%), CaO (0.7-1.7 wt%) and
Cr2O3 (0.1-0.6 wt%), and thus resemble primitive orthopyroxene in Si-oversaturated
volcanic rocks (e.g., boninites). There is a tendency for Al2O3, CaO and Na2O to
increase and Cr2O3 decrease in the grains with Mg# <89 mol%.
84
86
88
90
92
84 86 88 90 92 94
C
Figure 6.7. Crystal inclusions in groundmass olivine. A – phlogopite inclusion, note the association of phlogopite inclusion with low-Ca pyroxene (opx) inclusion; B –olivine inclusions and Fo in host olivine vs Fo of olivine inclusions, (abundances in wt%), shaded field shows the compositions of host olivine rims; C – putile; D –perovskite.
Chapter 6 Magmatic inclusions in minerals 161
phl
phlA
opx
phl
B
D
20 μm
Fo, host ol
Fo, ol inclusions
ol
ol
Figure 6.8. Low-Ca-pyroxene (opx) inclusions in the groundmass olivine. A –distributions of opx inclusions; B – unheated single opx inclusions; C– heated opxinclusions with large CO2 rich bubble and silicate glass
Chapter 6 Magmatic inclusions in minerals 162
B
10 μm
C
10 μm
20 μm
A
opx
87
88
89
90
91
92
86 87 88 89 90 91
Al2O3
TiO2
CaO
Na2O
Cr2O3
SiO2
Chapter 6 Magmatic inclusions in minerals 163
Fo, host olivine
Mg#
, opx
incl
usi
ons
Mg#
Figure 6.9. Composition of low-Ca pyroxene inclusions in groundmass olivine, shaded field shows compositions of host olivine rims, (abundances in wt%).
53
55
57
59
87 88 89 90 91 92
0.6
0.9
1.2
1.5
87 88 89 90 91 92
0.0
0.2
0.4
0.6
87 88 89 90 91 92
0.6
1.0
1.4
1.8
87 88 89 90 91 92
0.1
1.0
87 88 89 90 91 92
0.0
0.2
0.4
0.6
87 88 89 90 91 92
Ca Mg
KNa
opxcpx
glass
opx opx
Figure 6.10. Heated opx inclusions with large bubble and silicate glass, and associated cpx inclusions hosted by groundmass olivine.
Chapter 6 Magmatic inclusions in minerals 164
cpx
cpx
Chapter 6 Magmatic inclusions in minerals
165
Table 6.1 Compositions of high-Ca pyroxene inclusions and their host olivine
Heating of grains containing low-Ca pyroxene inclusions to 1250oC reveals
some changes occurring in the material around inclusions. After quenching, a film of
clear glass, containing CO2-rich bubbles (some with visible liquid-vapor boundary) is
always present around low-Ca pyroxene crystals (Figs. 6.8A, C and 6.10). The glass is
characterised by high SiO2 (60-70 wt%), moderately enriched alkalis (K2O+Na2O = 3-6
wt%), low CaO (3.5-6 wt%), and exceptionally low Al2O3 (2-4 wt%) (Table 6.4). In one
instance several grains of high-Ca pyroxene are present in association with a group of
low-Ca pyroxene crystals and the silicate glass (Fig. 6.10). This high-Ca pyroxene (Mg#
Chapter 6 Magmatic inclusions in minerals
168
87.3-88 mol %) has higher TiO2 (0.92 wt%) and lower CaO (16.22 wt%) abundances in
comparison with individual Cr-diopside inclusions in the olivine cores.
Perovskite and Cr-spinel, together with magnetite (individual grains and rims
around Cr-spinel), occur at the very edge of olivine crystals and form encrustations
around the olivine grains. Perovskite often occurs as aggregates of euhedral zoned
crystals (Fig. 6.7D). Cr-spinel forms euhedral, often zoned crystals (Fig. 6.11B),
mantled by Ti-magnetite. Rarely, spinel grains contain melt inclusions (Fig. 6.11C). The
cores of Cr-spinel inclusions have compositions typical of kimberlitic spinel (Table 6.
5). These are most depleted in Al2O3 (9.4-12.3 wt%), and at the same time most
enriched in TiO2 (4.4-5 wt%) among magmatic Cr-spinels in the terrestrial rocks
(Kamenetsky et al., 2001a). They are more enriched in TiO2 (at similar Al2O3) than
spinel in subduction-related volcanics, and more depleted in Al2O3 (at similar TiO2)
than spinel from oceanic island basalts (Fig. 6.11A). Their overall resemblance to Cr-
spinel from primitive flood basalts and meimechites (highly magnesian enriched picrites
from Meimecha-Kotui province, northern Siberia) should be noted.
Figure 6.11. Spinel inclusions in groundmass olivine. A – composition of spinelinclusions compared to spinel from LIP, OIB, ARC and MORB (Kamenetsky et al., 2001), (abundances in wt%); B – zoned spinel inclusion; C – spinel inclusion, containing melt inclusion, .
Chapter 6 Magmatic inclusions in minerals 169
0.01
0.10
1.00
10.00
0 10 20 30 40 50
LIP
MORB
OIB
ARC
Al2O3, spinel
TiO
2, s
pin
elA
CB
Chapter 6 Magmatic inclusions in minerals
170
Table 6.3 Representative analyses of olivine inclusions and their host olivine
Table 6.4 Representative analyses of low-Ca pyroxene (opx) inclusions, their host olivine, and associated high-Ca pyroxene (cpx*) and melt compositions
Daughter olivine (Fo88.6-81.5) is characterised by higher CaO (up to 0.18 wt%) and lower
Figure 6.15. Fluid inclusions in groundmass olivine. A – distribution of the inclusions; B – partly decrepitated low-density CO2 fluid inclusions, rarely with boundary between liquid and gaseous CO2 (Th ~14oC).
Chapter 6 Magmatic inclusions in minerals 179
5μm 10μm
1250 1300 1350 1400 1450 Wavenumber (cm-1)
CO2
CO2
10μm
10μm
B
A
50μm
50μm
50μm
20μm
50μm
Figure 6.16. Large multiphase melt inclusions in groundmass olivine. These inclusions usually have irregular shapes.
Chapter 6 Magmatic inclusions in minerals 180
20μm20μm
20μm
20μm50μm
20μm
Figure 6.17. Relatively small, well-formed (some euhedral) multiphase melt inclusions in groundmass olivine. Dominant phases: low-density CO2 bubbles, Na-K-Ca carbonates and Na-K-chlorides
Chapter 6 Magmatic inclusions in minerals 181
Figure 6.18. A-C - elemental maps of the exposed large multiphase melt inclusions in groundmass olivine., alk-carb – alkali-rich carbonate, phl – phlogopite; D –compositional variations of the alkali-rich carbonates
Chapter 6 Magmatic inclusions in minerals 182
68
1012141618
6 8 10 12 14
K2O
Na2O
35
40
45
50
55
60
10 15 20 25 30
K2O+Na2O
CaO
2468
101214
60 65 70 75 80 85 90Total oxides
SO3
D
phl
alk-
carb
C
Al CaK
Ca
K Na
clc
alk-carb A
Ca
ClNa
alk-carb
NaCl
B
Chapter 6 Magmatic inclusions in minerals
183
NiO (0.11 wt%) contents compared to host olivine. A number of other different
silicate phases (SiO2 25-38 wt%) are recorded by electron microprobe analisis (Table
6.8a #4-8), but cannot be confidentially identified. All but one are aluminosilicates. The
Al-free phase has a total ~ 100 wt%, and is characterised by the lowest SiO2, but highest
MgO (28 wt%), K2O (16.5 wt%), and SO3 (9 wt%) and very high Cl (6.8 wt%). Other
silicates have broadly similar abundances of Na2O (3-4.5 wt%), but widely variable
concentrations in Al2O3, CaO, MgO, K2O and volatile elements, and most likely belong
to the sodalite and cancrinite groups of minerals.
Carbonate daughter minerals are dominant and represented by calcite and alkali-
Ca-S rich species (Fig. 6.18 A-C and Table 6.8b). Alkali elements in these carbonates
are correlated with each other, and the total of alkalies shows antithetic correlation with
CaO (R= -0.89; Fig. 6.18D). Sulphur is very variable (3-12 wt% SO3), and its positive
correlation with the microprobe totals (R = 0.8; Fig. 6.18D) suggests that sulphur
substitutes for CO3, probably as a sulphate. Sulphur is very high in one analysis (27
wt% SO3, Table 6.8b, #30), which is also remarkable in having very high soda (32
wt%). A number of analyses of carbonate- or /sulphate-like phases show the presence of
SiO2 and MgO (up to 18 and 22 wt%, respectively, Table 6.8b, #32-38), and although
the incorporation of olivine in the analyses cannot be excluded, the Si/Mg ratios are
unlike those in olivine.
Chlorides are found as halite (NaCl) and less abundant sylvite (KCl). Because of
their high solubility in water, chlorides usually dissolve when melt inclusions are
exposed to the surface, even though precautions were taken. If the inclusions were
analysed immediately after exposure, then chlorides were noticed in melt inclusions as
fine-grained aggregates (Figs. 6.18B and 6.26). After several heating experiments,
chlorides become more evident in the inclusions, forming fields with orthogonal face
boundaries (Fig. 6.21D).
As kimberlite is a silicate-rich, although Si-undersaturated rock, a special effort
was made to locate silicate glasses among melt inclusions. However, despite a search in
several thousand grains, only one glass-like inclusion was found (Fig. 6.19, Table 6.9).
The abundances of most elements in this phase are basalt-like apart from negligible
Al2O3 (1.2 wt%) and exceptionally high Na2O (13 wt%) concentrations. In close
Chapter 6 Magmatic inclusions in minerals
184
proximity to this inclusion, a film (or thin embayment in olivine), of what looks like a
silicate glass, was noted and analysed (Fig. 6.19, Table 6.9). Although very different in
many details to the inclusion, this film is indeed a silicate phase (35-37 wt% SiO2), but
unusual for a silicate glass in having low Al2O3 (3-4 wt%), and very high TiO2 (7-10
wt%) and Na2O (8-15 wt%). A few chemical components of this phase correlate
positively or negatively with SiO2 and each other (Fig. 6.19), but the nature of these
variations and the identity of this phase(s) remain unclear.
Inclusions of sulphide melt are very rare in the groundmass olivine (only three
such occurrences are recorded), but they are present in clusters of several tens (Fig.
6.20A). They are either spherical or cylindrical, confirming their origin as immiscible
liquids. The composition of sulphides is mainly pyrrhotite, which is rimmed by
djerfisherite where a film or a pool of chloride-rich material is present around sulphide
(Fig. 6.20B).
Figure 6.19 Glassy silicate melt inclusion and associated embayment of silicate melt. The diagrams show compositional variations of this embayment, abundances in wt%
Chapter 6 Magmatic inclusions in minerals 185
SiO2
8
10
12
14
16
34 35 36 37 38
Na2O
6
7
8
9
10
11
34 35 36 37 38
TiO2
1.5
2.0
2.5
3.0
3.5
34 35 36 37 38
K2O
9
10
11
12
13
14
34 35 36 37 38
FeOt
9
10
11
12
13
14
34 35 36 37 38
CaO
10 μm
Figure 6.20. Sulphide inclusions in groundmass olivine.
Chapter 6 Magmatic inclusions in minerals 186
Cl
Ni
KNa
S
NaCl
djer
10 μm
40 μm
B
A
Chapter 6 Magmatic inclusions in minerals
187
Table 6.8 a Representative analyses of different silicate and phosphate minerals in
F 1.00 0.93 0.08 0.00 0.04 1.92 2.22 Sr 0.06 0.00 0.14
BaO 0.00 0.65 0.66 Total 94.83 93.77 94.79 98.58 88.55 85.68 99.18 88.80 91.60 98.59
Chapter 6 Magmatic inclusions in minerals
188
Table 6.8 b Representative analyses of calcite (1-2), alkali-Ca-S-bearing carbonate (3-29) and unidentified minerals (30-36) in olivine-hosted multiphase melt inclusions
heterogenisation etc) and the temperature of the above mentioned events.
Melting in the inclusions is inferred to begin in the temperature interval of 160-
450oC, as indicated by jolting movements of either solid phases or vapour bubbles
inside inclusions. In fact, initial melting may have occurred at even lower temperature,
but the accurate determination of this exact point was impossible because of very small
“amplitude” of visible changes. Further heating enhances phase boundaries through the
increasing amount of interstitial liquid, which also causes bubbles to coalesce at 420-
580oC. At 540-600oC daughter phases experience some changes in their relative
position, shape and colour that are interpreted to be related to recrystallisation and
melting. At temperature above 600oC two distinct phases, apart from vapour bubbles
and opaque minerals, are clearly visible. One is pale-blue and constitutes the “matrix”
for a large number of pinkish globules that move freely and change shape continuously.
The shape of a single globule is always very smooth: it can instantaneously change from
perfectly spherical to cylindrical, embayed or lopsided, similar to an amoeba. With
further heating, the number and size of floating and shape-changing pinkish globules, as
well as the number and size of vapour bubbles, gradually decrease. Homogenisation of
the inclusions (except some opaque crystals) occurs when the pinkish globules and
vapour bubbles disappear almost simultaneously (within 20-30oC) at temperatures of
660-760oC (Figs. 6.21A, B).
These temperatures were achieved in eight experiments, and only in one run did
homogenisation take place at a much higher temperature (955oC). A number of
inclusions that did not homogenise below 760oC were further heated. Most decrepitated
(expansion of vapour phase filling the whole volume of inclusions) at some instant at
higher temperatures (770-1000oC), whereas in other inclusions decrepitation did not
show up as a dramatic event, and the bubble was still present even at 1250oC.
Figure 6.21 A. Heating stage experiments with multiphase melt inclusions in olivine-II. Run 14 – complete melting and bubble dissolution (homogenisation) occur almost simultaneously at temperature as low as 680oC. Carbonate-chloride immiscibility occurs at 580oC
Chapter 6 Magmatic inclusions in minerals 191
629oC20oC
680oC463oC
618oC564oC
580oC614oC
1
2
3
4
5
6
7
8
Figure 6.21 B. Heating stage experiments with multiphase melt inclusions in olivine-II. Run 27B – homogenization occurs at 720oC, immiscibility at ~600oC
Chapter 6 Magmatic inclusions in minerals 192
25oC
568oC
622oC
654oC
710oC
720oC
646oC
610oC
1
2
3
4
5
6
7
8
Figure 6.21. Heating stage experiments with multiphase melt inclusions in olivine-II. C – orientation of immiscible globules; D - chlorides and carbonates are more evident in the inclusions after several heating experiments
Chapter 6 Magmatic inclusions in minerals 193
D
C
before experiment after experiment
after experiment
Chapter 6 Magmatic inclusions in minerals
194
During slow cooling (5-20oC/min) cooling, a vapour bubble(s) nucleates at 690-
650oC and then progressively increases in size (Fig. 6.21A, B). Almost at the same time
(within 30-50oC), formation of a colourless (?), needle-like crystal(s) was noticed in
several runs. Nucleated on the inclusion walls, the growing crystal rapidly propagates
into the melt, and after reaching the opposite wall continues growing in width. Cooling
to 610-580°C results in a spontaneous process when for a split second inclusions
acquire a ‘foggy” appearance. This process can be best described as the formation of
emulsion, i.e, microglobules of liquid in another liquid (melt immiscibility).
Microglobules coalesce immediately into larger, elongate, sausage-like pinkish
globules. The neighbouring globules (“boudins”) are subparallel, and are grouped into
regularly aligned formations with a common angle of ~75-80o (Fig. 6.21C). A
resemblance to the skeletal or spinifex texture is evident for several seconds, after which
the original “pinch-and-swell structure” pulls apart giving rise to individual blebs of
melt. The latter coalesce and become more spherical with time or further cooling. They
continue floating that slows down with decreasing temperature and further coalescence
until two distinctive phases are forced (by slow cooling 3oC/min) to aggregate in
different parts of inclusions. The exact moment of crystallisation or complete
solidification is not recorded, but finally a fine-grained phase with high birefringence
and an isotropic phase with orthogonal grain boundaries form (Fig. 6.21D). These
phases can be recognized as the carbonate and chloride, respectively, on the basis that
these minerals are dominantly present in the unheated melt inclusions.
In every experiment, all inclusions present in a field of view show similar phase
transformations at nearly the same temperature. All successful experiments
demonstrated very similar pattern in inclusion behaviour. Importantly, the results can be
reproduced for a single inclusion many times, and a change in the heating/cooling rate
has no significant influence on the results.
Heating stage experiments with inclusions hosted by porphyroclastic olivine-I
show similar melting behaviour, similar temperatures of phase transformations and
homogenisation, as well as analogous carbonate-chloride melt unmixing during cooling
(Fig.6.22).
Figure 6.22. Heating stage experiments with multiphase melt inclusions in olivine-I. Run 2. Homogenization occurs at 660oC, recrystallisation at 560oC. Note similar behavior and temperatures of homogenization and heterogenizationfor inclusions in olivine of both generations.
Chapter 6 Magmatic inclusions in minerals 195
475oC25oC 510oC
590oC 640oC 655oC
660oC 560oC
1 2 3
4 5 6
7 8
Chapter 6 Magmatic inclusions in minerals
196
Overheating (to 1250oC) and fast quenching (>100oC/sec) of the inclusions
results in the formation of homogeneous “frosted glass” (Fig. 6.23), providing those
inclusions survived decrepitation (a large volume vapour bubble in these inclusions
inclusions that appear as “frosted glass” at room temperature are ideally suited for
analysis of bulk melt composition. However, inclusions of homogeneous “frosted glass”
decompose and recrystallise immediately after being exposed at the surface, even if
precautions are taken during lapidary work. This suggests that the “frosted glass” melt
inclusions are a highly metastable melt when fast quenched and then exposed at room
conditions.
The immiscible liquids formed in a fast quenching experiment were identified as
predominantly Ca-carbonate and Na-K chloride (Fig. 6.25) using PIXE imaging
(CSIRO-GEMOC Nuclear Microprobe, North Ryde, analysed by Dr Chris Ryan) and
laser Raman spectroscopy (Geoscience Australia, Canberra, analysed by Dr Terry
Mernagh). Laser Raman spectroscopy (GFZ Potsdam, analysed by Dr Rainer Thomas)
of a “frosted glass” inclusion at room temperature (Fig. 6.24) shows bands of hydrogen-
bearing carbonate and common carbonate mixtures. The band at 1046 cm-1 corresponds
to nahcolite (NaHCO3) and the broad band, centred at about 1077 cm-1, results from
different carbonate species. Quantitative deconvolution of the overlapping bands gives
three components centred at 1068, 1077, and 1087 cm-1, respectively. The first two
bands are typically for Na2CO3 with a weak band at 1067.5 and a strong band at 1077.6
cm-1. The weak band at 1087 cm-1 is probably from Mg-bearing calcite.
6.4 Compositions of melt inclusion in groundmass olivine
The bulk compositions of melt inclusions were recalculated using analyses of
individual daughter crystals and mineral aggregates within thoroughly exposed
inclusions and their estimated volume and weight ratios (Fig. 6.26 and Table 6.10). It
should be noted that there is a large uncertainty in the estimate of the olivine component
present, because olivine, crystallised on the inclusion walls (daughter olivine) is
optically and compositionally indistinguishable from the host olivine.
Figure 6.23. Dominantly carbonate-chloride melt inclusions in olivine-II from the Udachnaya kimberlite, quenched into “frosted glass” after heating to 1150oC. Scale bars are 20 μm.
Figure 6.24. Raman spectrum of a “frosted glass” inclusion at room temperature, showing bands of nahcolite (NaHCO3) and carbonate mixtures.
1020 1040 1060 1080 11000
20
40
60
80
100
120
Chapter 6 Magmatic inclusions in minerals 197
1087
1077
1068
1046
Inte
nsity
(a.u
.)
Wavenumber (cm-1)
nahcolite
Na2CO3
Mg-bearing carbonate
K
Br
S
Ca
Sr
Rb
Cl
20μm
Figure 6.25. Optical images and PIXE (proton-induced X-ray emission) element maps of the “frosted glass” inclusion.
Chapter 6 Magmatic inclusions in minerals 198
min max
Chapter 6 Magmatic inclusions in minerals
199
Table 6.10 Representative compositions of daughter minerals in melt inclusions in
Note to Table: Daughter minerals in the olivine-hosted melt inclusion: (1) - Na-K chloride; (2) – olivine; (3) – phlogopite; (4) - Na-K-Ca carbonate. * - elemental wt%; **- calculated on the basis of stoichiometry (OH+F+Cl=2.0 apfu); n.d. – not determined.
Although the analysed sections of individual inclusions are different in terms of
phase assemblage and mineral proportions, the calculated bulk melt compositions are
distinct from the kimberlite groundmass in having very high Na and K and Cl contents
(Table 6.11 #5). This is a consequence of the contribution from Na-K chlorides and
sulphur-bearing alkali carbonates present in melt inclusions. Similarities between melt
inclusions and the groundmass include relatively low Si and Al, enrichment in volatile
components and presence of water-soluble alkali chlorides, carbonates and sulphates.
The variability in the melt components is likely due to post-entrapment evolution of
melt inclusions (e.g., necking down). Table 6.11 shows that the estimates of the
kimberlitic melt are markedly different from the composition of the groundmass of
studied kimberlites and the typical composition of the type-I kimberlites in having a
significant amount of Na-K chloride component (>20 wt%). This previously unknown
component carries considerable amounts of Cl, Na, and K in the original kimberlite
magma.
Figure 6.26. Multiphase melt inclusion in transmitted (A) and reflected light (B), respectively. Principal daughter phases are indicated - alk-carb - Na-K-Ca carbonate; chl - K-Na chloride; phl – phlogopite; ol – olivine, and their average compositions are given in the text, Table 6.10.
Chapter 6 Magmatic inclusions in minerals 200
chlol
50μmalk-carb
phl chlchl
olol
ol
phl
B
A
Ca Cl
Na+K 1
2
5
3
4
Figure 6.27. Compositions of olivine-hosted melt inclusions and estimated parental kimberlitic melts (1) - melt inclusions hosted in olivine-II, analysed by LA-ICPMS; (2) - calculated bulk composition of the melt inclusion (Fig. 6.19; Table 6.x); (3) - bulk composition of the melt inclusion calculated on the basis of estimated mass proportions of chloride and carbonate phases (Fig. 6.14); (4) -Udachnaya pipe kimberlite groundmass; (5) - diamond-hosted brine inclusions (Izraeli et al., 2001).
Figure 6.28. Primitive mantle normalised (Hofmann, 1988) compositions of olivine-hosted melt inclusions, analysed by LA-ICPMS and normalised to La in the Udachnaya kimberlite groundmass YBK-0 in comparison with average groundmass
1
10
100
1000
Nb La Ce Pr Sr Nd Zr Sm Eu Gd Tb Dy Y
groundmass
individual melt inclusionaverage melt inclusion
Chapter 6 Magmatic inclusions in minerals 201
Chapter 6 Magmatic inclusions in minerals
202
Table 6.11 Compositions of group-I kimberlites and calculated parental melt of the Udachnaya pipe kimberlite
Note to Table: (1)- average Siberian kimberlite (Mitchell, 1989); (2)-average South African on-craton kimberlite (Price et al., 2000); (3)-Jericho pipe kimberlite JD82, Canada (Price et al., 2000); (4)-groundmass of studied Udachnaya-East pipe kimberlite YBK-0; (5)- bulk composition of the melt inclusion (Fig 6.26) calculated using relative abundances and average compositions of daughter phases. FeOt – total Fe; n.d. – not determined.
Unexposed melt inclusions were analysed by laser-ablation ICP-MS at CODES
ARC Special Research Centre, University of Tasmania. The setup includes a UP213 UV
laser with a custom-built small-volume (~4 cm3) ablation cell, coupled to a HP4500
ICP-MS. Samples were ablated using 30-60 µm spots, a repetition rate of 5 Hz and laser
energy of 14 J/cm2. The measurements were calibrated using the NIST612 standard.
Because the signal from the ablated melt inclusions included unknown contributions
from host olivine, major constituents of olivine (Si, Mg, and Fe) could not be estimated
in the melt inclusions. In the absence of an internal standard (independently determined
concentration of an element), the compositions of melt inclusions were recalculated
based on two assumptions. First, the major (Cl, Na, K, Ca, Al, and Ti) and lithophile
trace (e.g., REE) element contribution from the host olivine was taken to be negligible.
Second, the total of analysed non-olivine elements and CO2, related to Na-K-Ca
carbonate, should be 100 wt% (Table 6.12).
In an attempt to calculate a bulk kimberlite melt from the melt inclusions,
relative proportions of the observed daughter phases and immiscible liquids were
recombined, taking into account data from heating experiments and the element ratios
derived from the LA-ICPMS analyses of unexposed inclusions. When recast in terms of
Ca, Cl and (Na+K), the composition of the melt inclusions is enriched in Cl and alkalies
compared to the kimberlite groundmass (Fig. 6.27). As mentioned above, the observed
variability in the major melt components is likely due to post-entrapment evolution of
Chapter 6 Magmatic inclusions in minerals
203
melt inclusions (e.g., necking down), but nevertheless the melt inclusions have the
enriched trace element signature similar to that in the kimberlite groundmass (Fig.
6.28).
6.5 Inclusions in other groundmass minerals and carbonate-
chloride nodules
6.5.1 Inclusions in groundmass calcite, anhydrite and chlorides
Individual groundmass grains of calcite, as well as mineral segregations of
calcite contain crystal and fluid inclusions. Crystal inclusions are mainly represented by
perovskite and fine-grained magnetite aggregates (Fig. 6.29A #1,2). A number of
perovskite inclusions occur in association with sulphide blebs and a vapour-rich phase.
Fluid inclusions are relatively small (20-30 µm) with almost perfect negative crystal
shapes. Most are saline aqueous fluid with either a single chloride crystal or a mosaic-
textured aggregate of chloride grains and a vapour bubble (Fig. 6.A #4-6).
Abundant fluid inclusions were also found in groundmass anhydride and
chlorides. Numerous small (<10 µm) rounded fluid inclusions are trapped along the
growth planes and usually consist of single phase aqueous fluid. However, multiphase
saline fluid inclusions are also present. These inclusions have good hexagonal shapes
and consist of a perfectly shaped cubic chloride crystal, aqueous liquid and a vapour
bubble (Fig. 6.29B, C).
6.5.2 Inclusions in carbonate from carbonate-chloride nodules
The inclusion study reveals the presence of different mineral and multiphase
melt inclusions, trapped in Na-Ca carbonate. As mentioned in Chapter 3, mineral
inclusions are represented by abundant euhedral crystals of apatite, zoned crystals of
phlogopite, chlorides, Na-K-bearing sulphates and different carbonates (northupite, Ca-
Ba-Na carbonate and S-bearing alkali-Ca carbonate). Multiphase melt inclusions are
shown on Fig. 6.30-32. These inclusions occur in clusters along healed cracks and
growth planes, and thus are likely to be pseudo-secondary. Multiphase melt inclusions
have rounded elongated shapes, are 30-60 µm across, and consist of crystals, aqueous
Chapter 6 Magmatic inclusions in minerals 204
Figure 6.29. Different types of inclusions in groundmass calcite and carbonate aggregates (A); in groundmass anhydrite (B) and chloride (C)
chloride
perovskiteperovskite
sulphide
15μm
15μm
15μm
C
B
Amagnetite
1 2 3
4 5 6
Figure 6.30. Multiphase (crystals, aqueous fluid, vapor bubble) inclusions in Na-Ca-carbonate from chloride-carbonate nodules
Chapter 6 Magmatic inclusions in minerals 205
1
2
3
12 3
Figure 6.31. Exposed multiphase inclusions in Na-Ca-carbonate. Empty cavities were probably filled with fluid. ap – apatite, sy-sylvite, ha-halite, phl-phlogopite, phl*-tetraferriphlogopite (Fe-rich and Al-poor phlogopite), slf-sphalerite
fluid and a vapour bubble (Fig. 6.30). Daughter phases are represented mainly by
apatite, chlorides, phlogopite and tetraferriphlogopite (Fe-rich and Al-poor phlogopite).
Sulphides (Fig. 6.31) and sulphates (Fig. 6.32) are less common.
6.5.3 Heating experiment with melt inclusions in carbonate from
carbonate-chloride nodules
At room temperature the studied inclusion has an elongated euhedral
(hexagonal) shape and consists of several (3-4?) crystalline phases, interstitial liquid
(aqueous?) and a vapour bubble. The daughter crystals are euhedral and based on
optical and crystallographic features they are likely to be different phases. During the
heating stage experiment, upon heating, visible changes of crystal shape happen around
170oC. At this temperature, the crystals start melting slowly or possibly recrystallise.
Further heating from 240 to 340oC results in intensive melting and the disappearance of
some crystals at 348oC. At approximately 390oC the inclusion starts increasing in size,
and the remaining mineral phases melt. At ~ 400oC, it appears that at least two
immiscible substances have separated from each other. Based on the optical properties
one phase (A) occupies “left and right corners” of the inclusion, and another phase (B)
is in between (Fig. 6.33). At this particular moment the bubble was stretched between
these two phases (bottom right), and gradually poured to the right into phase A. The size
of the inclusion continues to grow significantly. At 415oC the bubble suddenly
disappears, and this coincides with drastic changes in the inclusion appearance. Melt or
fluid contained in the inclusion reacts intensively with the host mineral, causing a
further increase in the inclusion’s size (by the factor of ~4-5 compared to original
volume). The host mineral becomes unstable at >400oC (this temperature is consistent
with the upper termal stability of shortite and zemkorite ~427oC (Parthasarathy et al.,
2002), and the experiment was stopped. Cooling to room temperature causes inclusion
size to decrease and inclusion outlines to become jagged, possibly because of
crystallisation on the walls.
Figure 6.33. Heating stage experiment with inclusion in the alkali-bearing carbonate from chloride-carbonate nodules. Note, inclusion increases in size. Red outline shows the original size of inclusion before experiment
Chapter 6 Magmatic inclusions in minerals 209
415oC
125oC 170oC 240oC
295oC 335oC 340oC
390oC 400oC
415oC
22oC – after experiment
Chapter 6 Magmatic inclusions in minerals
210
6.6 Summary
The study of magmatic inclusions in kimberlite groundmass minerals provided
invaluable information about the crystallising mineral assemblage at different stages of
magma evolution, and the composition and temperature of the magma. The most
important observations and inferences are summarised below:
1. A variety of primary and pseudosecondary magmatic inclusions in
groundmass olivine is a clear indication of that olivine-II, at least their rims, crystallised
from the melt, and were not accidentally entrained during magma ascent and
emplacement;
2. The evolution of the magmatic system during ascent and cooling is recorded
in significant variability in the compositions and proportions of its constituents, such as
melt, crystals and fluid;
3. The early evolution of the kimberlite magma (protokimberlite) occurred at
deep mantle conditions (>150 km), possibly in the melting source regions, and resulted
in crystallisation of olivine cores and Na- and Cr-enriched high-Ca pyroxene;
4. The intermediate evolution of the kimberlite magma occurred during
emplacement in the crust, and was characterised by significant degassing of C-H-O
fluids and crystallisation of olivine-II rims, phlogopite, low-Ca pyroxene, perovskite,
Cr-spinel, rutile and magnetite. The absence of silicate melt inclusions in the studied
olivine indicates that in this case the crystallisation of Fe-Mg silicate minerals occurred
from an atypical melt. The evidence from the melt inclusions strongly implies that such
melt was particularly enriched in chloride and carbonate components, whereas the
amount of the aluminosilicate components was significantly reduced relatively to those
in common mafic magmas. This melt still crystallised olivine at temperatures as low as
~ 700oC;
5. A strong chloride-carbonate “flavour” of the kimberlite melt at the latest
stages of evolution is most pronounced in the compositions of melt inclusions in the
rims of olivine-II, segregations in the groundmass and nodule-like mineral aggregates.
The mafic silicate component is nearly exhausted from this melt following preceding
Chapter 6 Magmatic inclusions in minerals
211
crystallisation of abundant olivine, and the melt undergoes chloride-carbonate
immiscibility and crystallisation of chloride and carbonate minerals at <600oC. The
composition of fluids at this stage is likely to be brine.