143
S.Afr.1 .GeoI., 1993,96(3),143-148
Interaction of carbonate-phosphate melts with mantle peridotites at 20-35 kbar
I.D. Ryabchikov
Institute for Geology of Ore Deposits, Petrography, Mineralogy and Geochemistry (lGEM), 35 Staromonetny Pereulok,
Moscow 109017, Russia
D.L. Hamilton
Department of Geology, University of Manchester, Manchester M13 9PL, United Kingdom
Accepted 19 February 1993
Partial melting of carbonated phosphate-bearing peridotites was studied experimentally between 20 and 35 kbars.
Liquids equilibrated at about HOO°C are characterized by the predominance of a dolomitic component with substantial
concentrations of phosphate (about 20 wt % P2 0 S) and sodium carbonate. With decreasing temperature and increasing
pressure the Ca/(Mg+Fe) ratios of these melts become lower. Uraninite .was added to some of the experimental
charges. Apatite-liquid partition coefficients of uranium were found to be in the range 0.15 - 0.3, while for
clinopyroxene, orthopyroxene, and olivine they are below c. 0.01. Thus, carbonate-phosphate melts would extract
uranium intensely from the rock-forming minerals of mantle peridotites. The crystallization of these melts would result
in the fixation of uranium in apatite. This is consistent with apatite being the main concentrator of uranium in some of
the metasomatized mantle rocks.
Die parsiele smelting van gekarbonatiseerde fosfaathoudende peridotiete is tussen 20 en 35 kbar eksperimenteel ondersoek. Vloeistowwe wat by omtrent 1100°C geekwilibreer is, word gekenmerk deur 'n oorwig van die dolomitiese
komponent met substansiele konsentrasies van fosfaat (ongeveer 20 gewigs % P2 0 S) en natriumkarbonaat. Met
afnemende temperatuur en toenemende druk word die Ca/(Mg+Fe) verhoudings van hierdie smelte laer. Uraniniet is
by sommige eksperimentele mengsels gevoeg. Apatiet-vloeistof partisiekoeffisiente van uraan is tussen 0.15 en 0.3
gevind terwyl dit vir klinopirokseen, ortopirokseen, en olivien laer as omtrent 0.01 is. Gevolglik sal karbonaatfosfaatsmelte uraan intensief uit die rotsvormende minerale van mantel-peridotiete onttrek. Die kristallisasie van
hierdie smelte sal daartoe lei dat uraan in apatiet gefikseer word. Dit is in ooreenstemming met die feit dat apatiet die
hoofkonsentreerder van uraan in sommige metasomaties-veranderde mantelgesteentes is.
Introduction
Experimental and isotope geochemistry studies reveal a very
strong differentiation of incompatible elements within the
Table 1 Composition of materials used for starting
mixtures
2
3
4
5
6
7
Si02
Ti02
43.30
52.30
55.64
40.43
n.d.
n.d.
n.d.
0.09
0.65
0.13
n.d.
n.d.
n.d.
n.d.
A1 20 3
FeO*
3.25
7.09
4.65
n.d.
0.02
n.d.
n.d.
8.85
2.90
6.45
10.24
0.03
3.3
0.11
MgO
41.78
14.94
32.98
48.79
n.d.
17.5
46.63
CaO
1.50
19.40
0.70
0.06
54.66
29.0
1.54
Na20
0.16
2.09
n.d.
n.d.
0.15
n.d.
Cr203
NiO
0.12
0.67
0.29
n.d.
n.d.
n.d.
n.d.
0.27
n.d.
0.13
0.39
n.d.
n.d.
n.d.
P20 S
CO2
n.d.
n.d.
n.d.
n.d.
39.18
0.10
n.d.
n.d.
n.d.
n.d.
n.d.
49.8
n.d.
n.d.
52.1
Note:
nd = not detected
FeO* = total iron recalculated as FeO
1 = lherzolite MNR-1 (xenolith from Shavaryn-Tsaram volcano,
Mongolia)
2 = clinopyroxene from (1)
3 = orthopyroxene from (1)
4 = olivine from (1)
5 = apatite from the Khibina intrusion (Kola Peninsula, Russia, also
includes 0.88% Ce203 and 0.13% La203
6 = dolomite, Djeruy deposit, Kirghizia
7 = magnesite, Larga deposit, Transbaykilia
mantle. The extreme melt/crystal partItIon coefficients of
these incompatible components, coupled with mass-balance
calculations, require that their extraction and transport must
be accomplished by very small melt fractions. Such melts
would strongly concentrate volatile components, and their
compositions must be drastically different from the more
familiar magmas formed by large-scale melting of mantle
material. In fact, it has been shown that in the system
peridotite-CQ near-solidus melts are represented by
carbonate-rich liquids (Wyllie & Huang, 1975; Eggler,
1978; Wyllie et al., 1990).
Because phosphorus is present in the mantle in amounts
comparable with those of carbon (Wanke, 1981; Sun, 1982;
Ryabchikov, 1987), a first melt in the carbonated mantle
may be represented by carbonate-phosphate liquids (Ryabchikov et al., 1991). Early studies in such systems permitted
a determination of the compositions of carbonate-phosphate
melts equilibrated with wehrli tic mineral assemblages
(Ryabchikov et al., 1991a; 1991b). The purpose of the
present work is to assess similar equilibria for melts
interacting with lhcrzolites and harzburgites by varying the
proportions of dolomite and magnesite in the starting
mixtures. Some of our experiments were done with compositions containing substantial amounts of V02 in order to
study the inter-phase distribution of uranium in volatilebearing peridotites.
Starting materials
Spinel lherzolite from a nodule found in Shavaryn-Tsaram
volcano, Mongolia (Press et aI., 1986), natural apatite
(Khibina alkaline massif, Kola peninsula), dolomite and
magnesite (compositions of these materials are given in
S.-Afr.Tydskr.Geol.,1993,96(3)
144
Table 1), as well as reagent Na2C03 were used for the
preparation of the starting materials. Lherzolite constituted
roughly 40% and apatite 25% of the starting mixture, while
carbonates were added in varying proportions in order to
find the conditions of carbonate-phosphate coexistence with
a Iherzolitic mineral assemblage. Two runs were done without the addition of apatite (runs 15 and 20), while in runs 2
and 8 reagent V02 was added to the starting mixture.
Starting materials were prepared by grinding the appropriate
ingredients under acetone. .
Experimental method
Approximately 20 mg of starting material was placed into
Ag40 Pd60 capsules (3 mm external diameter and length not
more than 5 mm). The capsules were sealed by welding in a
DC arc. Experiments were conducted in the piston-cylinder
high-pressure apparatus by the conventional quenching
method; NaCI cells were used. Oxygen fugacities were not
controlled. Temperatures were measured using Pt-Pt87Rh13
thermocouples, and no pressure corrections were introduced.
Capsules recovered after experiments were polished,
carbon-coated, and their contents were investigated by a
JEOL JSM 6400 scanning electron microscope with the
attached LINK energy dispersive system which allowed
quantitative chemical analyses to be done. Run products
containing uraninite were also analyzed by a CAMEBAX
electron microprobe. Operating conditions on both instruments included a 15 keY accelerating voltage and 15 nA
beam current.
Experimental results
Run conditions and inferred equilibrium phase assemblages
are given in Table 2, and analyses of solid phases and
quenched liquids are given in Table 3. Liquid phases from
the runs with apatite quenched to carbonate-phosphate
glasses which usually contained less than 3 wt % Si02. In
the absence of apatite, carbonate-rich liquids quenched to an
aggregate of crystalline phases with relatively large skeletal
dolomites and fine-grained sodium-rich phases (nyerereitelike and others) between them. Quenched liquids were
analysed by a scanning electron beam through relatively
large areas (usually 25x25 J,1m) to avoid loss of sodium
due to volatilization.
No attempts were made to reach equilibrium from various
directions and, therefore, the results of the reported experiments are considered to be preliminary. Baker & Wyllie
(1992) considered that equilibrium in a system of similar
composition is attained during c. 8 hours at llOO°C, and all
our experiments were carried out with longer run durations.
In many cases relics of spinel lherzolite minerals from the
starting mixtures were present in the run products. However,
in runs with a significant proportion of quenched liquid,
silicate grains are usually euhedral and obviously represent
newly-formed phases. Typical residual phases form cores of
grains surrounded by rims of newly formed minerals. Neoformations of orthopyroxene are easily distinguished from
the relict compositions at pressures above 20 kbar by their
significantly lower contents of Ah03. Garnets were absent
from starting mixtures, and they are represented in the run
products exclusively by newly formed grains. Table 3 gives
compositions of newly formed crystalline phases.
Silica-free glasses similar to those observed in the present
work and our previous publication (Ryabchikov et ai.,
1991b) were described by Jones & Wyllie (1983) for the
system CaCOrCa(OH)2-CaF2-BaS04-LA(OH)3.
The CO2 contents in quenched liquids given in Table 3
are calculated assuming that Si~ forms metasilicate components, P 20 S is bound with divalent cations by phosphates
with the general formula M3(P04)2 and all the other divalent
cations and sodium form carbonates. These calculated
values are consistent with the shortfalls of the microprobe
analyses of glasses within 5 wt %. In the case of quench
crystalline aggregates discrepancies are larger. Dolomite and
magnesite analyses were calculated in a similar manner, the
apatite analyses given in Table 2 are normalized to 98 wt
Table 2
Run conditions and equilibrium phase assemblages
Run
Duration
Pressure
numbers
hours
kbar
°C
Lhz
Ap
Na2C03
Dol
Mst
U02
13
21
24.5
23.5
20
25
30
1050
1100
1100
37
36
40
24
22
22
8
4
4
22
26
24
9
11
10
0
0
0
18
19
14
15
20
2
23
26
38
30
30
30
18
49
40
11
23
26
34
22
15
0
0
0
35
30
30
30
42
37
36
10
22
0
0
5
2
4
3
4
4
3
2
35
3
16
38
23
24
38
1100
1100
1100
1100
1100
1100
1000
23
59
47
3
9
0
0
0
0
81
L+Ol+Cpx+Ga+Ap
8
22
30
1050
19
9
3
5
62
L+Ol+Cpx+Ap+V02
12
11
Temperature
Starting composition (wt %)
Interpreted phase
assemblage
L+Opx +Cpx +Ol+Sp+ Ap
L+Opx+Ol +Ga+ MsH Ap
L+Ol+Opx+Cpx+Ga+Dol
+Cpx+Ga+Dol+Mst+Ap
l7
13
0.7
L+Ol+Opx+Mst+ Dol
L+Ol+Opx+Ga+Ap
L+Ol+Opx +Ga+ Ap
L+Ol+Cpx+Dol
L+Ol+Opx+Cpx+Ga+Dol
L+Ol+Opx+Cpx+Mst
+Dol+Ap+U~
L = melt; 01 = olivine; Opx = orthopyroxene; Cpx = clinopyroxene; Sp = spinel; Ga = gamet, Ap = apatite; :v1st = magnesite; Dol = dolomite;
Lhz=lherzolite
S.AfrJ.Geol.,1993,96(3)
%, and all the volatile-free mineral compositions are
recalculated to 100%.
The products of two runs (2 and II) contained two pyroxene and two carbonate phases (dolomite and magnesite).
The conditions of run 2 (1000°C and 30 kbar) are very
close to the P-T coordinates of the respective monovariant
curve in the system CaO-MgO-Si~-C02 (Brey et ai.,
1983), while for run 11 (1100°C and 30 kbar) the experimental conditions are approximately 100° higher than this
curve. However, in our multicomponent system this fourphase assemblage is not monovariant, and it should not lie
strictly on the phase boundary for a simple system. The
complex composition of clinopyroxene in our runs results in
diopside, activity being less than one, and therefore clinopyroxene may coexist with othopyroxene and two crystalline carbonates under the P-T conditions corresponding to
the stability of dolomite, magnesite, and enstatite in the
simple system.
The substantial solubility of phosphorus in melts equilibrated with apatite revealed by previous studies (Ryabchikov et ai., 1991 a, 1991b; Gittins & Jago, 1991; Baker &
Wyllie, 1992) have been confinned by our new results. It
can be seen from Table 2 that apatite-saturated carbonaterich melts are characterized by P20 S contents close to 20 wt
%. This is higher than the results of Gittins & Jago (1991)
and Baker & Wyllie (1992), which may be attributed to the
fact that the compositions of our carbonate-rich melts are
different from those in the experiments of these authors.
Comparing the results of runs 11 (30 kbar, 1100°C) and
13 (20 kbar, 1050°C), we may conclude that Ca/(Mg+Fe)
ratios of melts co-existing with Iherzolitic mineral assemblages are close to one at these temperatures and pressures.
Run 20 (30 kbar, I I 50°C - no apatite added), where a
garnet lherzolite paragenesis is also present, yielded a liquid
with a Ca/(Mg+Fe) ratio of similar magnitude, which
implies that variations of P20 S contents in the liquid do not
affect this ratio drastically. However, a comparison of run
11 (30 kbar, 1100°C) with 2 (30 kbar, 10OO°C - both with
lherzolitic mineralogy) shows that with decreasing temperature lherzolite-buffered Ca/(Mg+Fe) ratios become significantly lower. The same conclusion may be drawn from a
comparison of run 13 of the present work (20 kbar, 1050°C)
with the results described in our previous publication for
20 kbar, 950°C. (Ryabchikov et al., 199Ib). From a
comparison of runs 13 (20 kbar), 11 (30 kbar), and 14
(35 kbar) one may conclude that with increasing pressure
Ca/(Mg+Fe) ratios of carbonate-phosphate melts tend to
become lower, as has been pointed out for carbonate-rich
melts by Brey et ai. (1983).
Runs II and 20 also show that the presence or absence of
large amounts of P20 S does not appreciably affect the partition coefficient for Na between clinopyroxene and liquid.
This value (close to 0.3) measured in the present work is
similar to the previous result for the system wehrlitecarbonate-apatite (Ryabchikov et al., 1991b).
Runs 2 and 8 in which uraninite was present among the
primary crystalline phases, show that the U02 saturation
concentration in the carbonate-phosphate melt at 30 kbar
and lOOO-1050°C is just above 1 wt %. Concentrations of
U~ in apatites co-existing with uraninite were found to be
more variable (0.13 - 0.25% for run 2 and 0.34 - 0.41 % for
145
run 8). These data indicate that the apatite-melt partition
coefficient is likely to be in the range 0.2 - 0.3. These
values are consistent with previously published partition
coefficients for an apatite-silicate melt assemblage although
they were measured at a much higher temperature (Benjamin et ai., 1980). Uranium concentrations in clinopyroxene,
orthopyroxene, and olivine co-existing with uraninite are all
below the sensitivity limit of electron microprobe analysis.
This means that partition coefficients for these rock-fonning
silicates are less than 0.01.
Discussion
In runs 2 and II carbonate-phosphate liquids co-exist with
apatite, dolomite, and magnesite in addition to silicate solid
phases. With changing temperature these carbonates and
apatite would be dissolved in liquid or precipitated from it,
adjusting Na2C03 content to the equilibrium level which
increases with decreasing temperature. The sodium content
in the melt in tum buffers the Na20 concentration in equilibrium clinopyroxene. At 10OO°C and 30 kbar (run 2) clinopyroxene interacting with liquid contains more than 5 wt %
Na20, which is much higher than the sodium concentrations
in clinopyroxenes from mantle Iherzolites. If clinopyroxene
has Na20 in amounts typical for mantle peridotites, then the
amount of sodium carbonate in the equilibrium melt would
be present in much lower concentration than in the liquid of
run 2, and thus this lherzolite-buffered melt would be below
its solidus temperature and should solidify. On the other
hand, clinopyroxene equilibrated with ljquid in run II
(1 100°C, 30 kbar) contains 1.9 wt % Na20 which is
well within the range of sodium concentrations in clinopyroxenes from fertile mantle Iherzolites. From comparison of
these data we may conclude that the lherzolite-buffered
solidus, of carbonate-phosphate melts lies at 30 kbar between 1000 and 1100°C. This is consistent with the position
of the solidus of carbonated fertile peridotite at the same
pressure measured by Falloon & Green (1989), which shows
that, although the presence of an additional component
(phosphate) should lower solidus temperatures in apatitebearing carbonated lherzolite, this reduction is not very
subs tan tial.
The estimated position of the solidus implies that
carbonate-phosphate melts representing the initial stages of
partial melting of mantle peridotites may appear at temperatures somewhat higher than the conductive geotherm of
stable ancient cratons (~800°C at 30 kbar). However, in the
presence of H20 this temperature may be significantly
lowered, thus initiating melting.
A low fraction volatile-rich melt such as investigated in
the present work would extract many incompatible elements,
including rare-earths (Ryabchikov et al., 1991 b) and
uranium (this work), from rock-forming minerals. The
migration of carbonate-phosphate magmas may result,
therefore, in the creation within the upper mantle of zones
enriched in REE, uranium, phosphorus, and other incompatible clements. The solidification of such primary carbonatitic magmas may be brought about by cooling or decompression which has been discussed in detail in a number of
papers (Green & Wallace, 1988; Ryabchikov et ai., 1989;
Yaxley et al., 1991). The migration of melt into rocks with a
S.-Afr.Tydskr.Geol.,1993,96(3)
146
Table 3 Microprobe analyses of phases
products
in
run
Table 3
cont.
17
Run
G1
Cpx
13
Opx
01
Sp
1.56
n.d.
0.39
2.42
16.12
25.19
8.74
n.d.
n.d.
23.83
21.75
92.3
53.00
0.14
7.52
2.51
16.12
15.96
2.51
0.83
n.d.
n.d.
n.d.
91.6
58.26
n.d.
2.5
4.28
33.30
1.02
n.d.
0.33
n.d.
n.d.
n.d.
93.3
42.94
n.d.
0.31
8.93
47.27
0.11
n.d.
n.d.
0.44
n.d.
n.d.
90.4
0.47
0.19
59.05
8.81
22.00
0.39
n.d.
8.81
0.27
n.d.
n.d.
Run
Phases
Si02
Ti02
AI20 3
FeO*
MgO
CaO
Na20
Cr203
NiO
P2 0 S
C~
M
Run
58.71
n.d.
42.84
n.d.
0.17
0.13
n.d.
n.d.
Ah 0 3
FeO*
1.52
4.92
1.58
3.67
n.d.
5.14
n.d.
3.03
n.d.
1.56
MgO
20.44
51.23
41.27
18.63
CaO
20.59
5.14
n.d.
0.73
n.d.
0.4
0.17
4.67
n.d.
n.d.
n.d.
n.d.
32.14
n.d.
n.d.
n.d.
n.d.
n.d.
0.41
n.d.
n.d.
n.d.
n.d.
50.58
96.0
46.52
95.5
Na20
Cr203
NiO
P20 5
CO 2
34.9
15.90
27.45
88.1
n.d.
94.4
n.d.
n.d.
94.7
18
01
Ap
Si~
42.72
n.d.
21.81
6.30
0.62
n.d.
n.d.
1.84
42.29
n.d.
0.31
6.38
50.35
0.09
n.d.
n.d.
0.57
2.79
51.09
0.44
n.d.
0.64
n.d.
n.d.
93.4
n.d.
42.35
n.d.
n.d.
Ap
2.47
n.d.
0.34
2.39
16.09
29.55
4.15
n.d.
n.d.
23.45
21.56
92.3
49.93
0.16
21.70
5.18
20.06
7.30
n.d.
2.08
n.d.
0.37
n.d.
87.3
n.d.
n.d.
n.d.
4.54
35.69
Na20
Cr203
NiO
P2 0 S
CO2
M
57.81
n.d.
3.18
3.88
33.16
1.40
0.22
0.35
n.d.
n.d.
n.d.
93.8
n.d.
n.d.
0.50
n.d.
0.12
0.60
2.34
50.39
1.34
n.d.
n.d.
n.d.
48.84
93.3
n.d.
42.70
n.d.
n.d.
9.04
A1 2 0 3
1.71
n.d.
1.17
FeO*
4.10
MgO
CaO
18.77
25.00
3.89
n.d.
57.52
n.d.
3.88
3.50
33.50
1.05
n.d.
0.51
n.d.
17.39
27.97
89.1
n.d.
n.d.
n.d.
94.5
n.d.
0.26
n.d.
85.9
Phases
GI
Opx
Ga
01
Si~
2.18
n.d.
58.14
n.d.
42.64
n.d.
42.33
n.d.
n.d.
n.d.
0.45
3.72
18.25
25.80
21.93
5.88
21.34
5.72
n.d.
7.92
49.30
0.15
4.23
n.d.
2.39
4.16
33.85
1.06
n.d.
0.41
0.19
1.94
n.d.
n.d.
n.d.
0.71
2.62
59.49
0.68
n.d.
n.d.
21.80
23.57
89.7
n.d.
n.d.
n.d.
93.5
n.d.
0.35
n.d.
86.6
0.29
n.d.
Ti0 2
Na20
Cr203
NiO
P2 0 5
CO 2
M
21.45
5.44
Run
11
Phases
G1
Cpx
Opx
01
Ga
Dol
Ytst
Ap
Si02
Ti0 2
AI 20 3
FeO
MgO
CaO
1.29
n.d.
0.24
2.18
16.78
27.07
53.38
0.67
6.86
2.52
14.74
20.02
55.90
n.d.
1.21
3.87
37.25
1.19
42.62
n.d.
n.d.
7.55
49.14
0.42
43.35
0.21
22.32
4.85
19.97
6.92
0.41
n.d.
0.16
1.61
20.96
29.47
0.46
n.d.
0.22
2.90
39.54
6.38
0.53
n.d.
n.d.
0.55
2.92
50.87
Na20
Cr203
NiO
P2 0 5
CO2
M
6.48
n.d.
1.91
0.65
0.56
n.d.
n.d.
n.d.
0.28
0.64
0.24
n.d.
0.29
n.d.
1.25
n.d.
n.d.
19.19
26.77
93.2
n.d.
n.d.
n.d.
91.1
n.d.
n.d.
n.d.
94.5
0.26
n.d.
n.d.
92.1
n.d.
1.24
n.d.
88
n.d.
n.d.
47.00
95.9
n.d.
n.d.
49.95
96.0
n.d.
41.87
n.d.
n.d.
15
Ti0 2
A1 2 0 3
FeO*
MgO
CaO
Na20
Cr203
NiO
P2 0 5
CO 2
M
Cpx
01
Dol
5.21
n.d.
0.86
2.18
14.90
22.88
43.34
n.d.
n.d.
6.48
49.8
0.16
n.d.
n.d.
0.84
n.d.
n.d.
1.43
25.01
24.74
n.d.
59.69
0.66
7.19
2.89
14.51
19.22
2.25
0.59
n.d.
0.24
40.73
92.4
n.d.
n.d.
n.d.
89.9
n.d.
n.d.
n.d.
93
n.d.
n.d.
47.60
96.9
0.37
n.d.
0.13
19
Run
Q1
13.00
3.96
0.17
Ga
Si02
Ti02
A12 0 3
FeO*
MgO
CaO
Na20
Cr203
NiO
P2 0 5
CO2
M
Si~
Ti02
Opx
Ap
A1 2 0 3
Dol
G1
Mst
FeO*
MgO
CaO
Mst
Phases
Ga
Si02
Ti02
01
Run
Opx
Phases
Opx
12
G1
Run
G1
M
Phases
Run
Phases
n.d.
91.7
n.d.
41.52
n.d.
n.d.
20
Phases
QL
Si~
Ti0 2
Al 2 0 3
FeO*
Cpx
Opx
01
Ga
Dol
2.45
n.d.
0.33
3.70
52.25
0.65
6.97
3.27
58.19
n.d.
1.99
5.45
42.63
n.d.
n.d.
6.82
43.32
n.d.
22.23
7.05
n.d.
n.d.
n.d.
2.68
MgO
CaO
16.95
26.64
14.17
19.94
50.11
0.15
20.82
4.82
21.32
28.53
Na20
Cr203
NiO
P2 0 5
CO 2
5.87
n.d.
1.76
0.68
32.69
1.39
n.d.
0.28
n.d.
n.d.
n.d.
1.48
0.18
n.d.
n.d.
0.15
43.91
89.1
0.31
n.d.
n.d.
88.5
n.d.
n.d.
n.d.
91
0.29
n.d.
n.d.
92.9
n.d.
n.d.
n.d.
84
n.d.
n.d.
47.30
93.4
M
S.Afr.J.Geol.,1993,96(3)
147
Table 3 cont.
14
Run
Phases
Gl
Ga
Cpx
01
Ap
Si02
Ti02
2.18
43.25
55.15
42.23
0.59
n.d.
0.17
n.d.
n.d.
n.d.
A120 3
FeO*
0.48
22.48
5.08
n.d.
0.11
2.66
5.52
2.23
8.40
0.61
MgO
17.17
20.26
15.98
48.69
2.83
CaO
25.76
6.53
18.51
0.30
51.00
Na20
6.47
n.d.
2.39
n.d.
0.39
Cr203
n.d.
1.15
0.65
n.d.
n.d.
NiO
n.d.
n.d.
n.d.
0.38
n.d.
P20 5
CO2
24.32
0.42
n.d.
n.d.
42.47
20.96
n.d.
n.d.
n.d.
M
92
86.8
92.7
91.2
n.d.
n.d.
result in an increase in the solidus temperature. Thus
carbonate-phosphate-rich magmas would transfer their load
of incompatible elements from the less refractory peridotites
to more refractory ones, and they would be immobilized
there. The correlation between the relative enrichment in the
most incompatible elements of mantle rocks, and the degree
of their depletion with respect to Na, Ca, and other 'basaltic'
components has been noted by many authors (e.g. Nickel &
Green, 1984; Stosch, 1980), and this is consistent with the
mechanism outlined here.
Considering the relatively high partition coefficients of
REE (Ryabchikov et al., 1991b) and U02 (this work) between apatite and carbonate-rich liquid the authors conclude
that the solidification of these melts would be followed by
the fixation of uranium and REE in apatite. The investigation of metasomatized mantle rocks revealed that uranium is
indeed concentrated in them in the form of fine-grained
apatite (Kleeman et al., 1969).
2
Run
Phases
Gl
Cpx
Opx
01
Ap
Conclusions
Si02
1.04
53.01
57.74
42.12
0.91
Ti0 2
n.d.
0.05
n.d.
n.d.
n.d.
Al 20 3
FeO*
0.53
7.82
1.04
n.d.
0.23
1. The solidus temperature at 30 kbar for fertile mantle
lherzolite in the presence of crystalline carbonates and
apatite lies between 1000 and 1100c C.
5.29
6.19
5.68
7.74
0.55
MgO
20.57
20.52
34.12
49.26
2.29
CaO
12.76
5.88
0.35
n.d.
54.14
Na20
13.15
5.34
n.d.
n.d.
0.21
Cr203
n.d.
0.87
0.63
n.d.
n.d.
NiO
n.d.
0.29
0.31
0.89
n.d.
P20 5
CO2
17.98
n.d.
n.d.
n.d.
38.89
27.58
n.d.
n.d.
n.d.
n.d.
1.10
n.d.
n.d.
n.d.
0.19
U02
M
87.0
91.5
85.5
Run
91.9
n.d.
8
Phases
Gl
Cpx
01
Ap
Si02
2.86
54.28
41.89
n.d.
Ti0 2
n.d.
n.d.
n.d.
n.d.
A1 20 3
FeO*
0.91
6.22
n.d.
0.13
4.49
4.81
8.94
0.59
MgO
17.80
14.05
48.66
2.09
CaO
19.06
15.73
n.d.
54.49
Na20
10.31
3.88
n.d.
0.69
Cr203
n.d.
1.03
n.d.
n.d.
NiO
n.d.
n.d.
0.51
n.d.
P20 5
15.42
n.d.
n.d.
39.62
CO2
28.03
n.d.
n.d.
n.d.
U02
1.12
n.d.
n.d.
0.37
M
n.d.
87.6
= not
detected; FeO*
83.9
90.7
n.d.
= total iron recalculated as FeO; M = 100*Mg/
= glass; other abbreviations the same as in
(Mg+Fe) (atomic ratio); GI
Table 2
lower chemical potential of calcium and sodium, such as
more refractory peridotites (harzburgites) depleted in 'basaltic' components, may also be a reason for its solidification
(Ryabchikovet al., 1991a; 1991b). In the latter case Na2C02
concentrations in the liquid should be lowered, which would
2. The near-solidus melts in this system are characterized
by Ca/(Mg+Fe) ratios close to 1 at 30 kbar. This ratio
becomes lower with decreasing temperature or increasing
pressure.
3. Near-solidus melts of carbonate-phosphate-bearing lherzolites dissolve substantial amounts of phosphorus (approximately 20 wt % P20S) and Na2C03.
4. Uranium, together with REE's and some other incompatible elements, is intensely extracted by carbonatephosphate melts from silicate mantle minerals.
5. The crystallization of carbonate-phosphate liquids may
result in the fixation of uranium in apatite precipitating from
these melts.
References
Baker, M.B. & Wyllie, PJ. (1992). High-pressure apatite solubility in
carbonate-rich liquids: Implications for mantle metasomatism.
Ceochim. Cosmochim. Acta, 56, 3409-3422.
Benjamin, T., Heuser, W.R., Burnett, D.S. & Seitz, M.G. (1980). Actinide
crystal-liquid partitioning for clinopyroxene and Ca3(P04h. Ceochim.
Cosmochim. Acta, 44, 1251-1264.
Brey, G.B., Brice, W.R., Ellis, D.1., Green, D.H., Harris, K.L. &
Ryabchikov, J.D. (1983). Pyroxene-carbonate reactions in the upper
mantle. Earth Planet. Sci. Lett., 62, 63-74.
Eggler, D.H. (1978). The effect of C~ upon partial melting of peridotite
in the system Na20-CaO-Ah03-MgO-SiOrC02 to 35 kbar with an
analysis of melting in a peridotite-H2 0-C02 system. Amer. 1. Sci.,
278, 305-343.
Falloon, T.1. & Green, D.H. (1989). The solidus of carbonated, fertile
peridotite. Earth Planet. Sci. Lett., 94, 364-370.
Gittins, J. & Jago, B.C. (1991). Extrusive carbonatites: their origin
reappraised in the light of new experimental data. Ceol. Mag., 128,
301-305.
Green, D.H. & Wallace, M.E. (1988). Mantle metasomatism by ephemeral
carbonatite melts. Nature, 336, 459-462.
Jones, K.P. & Wyllie, PJ. (1983). Low-temperature glass quenched from
a synthetic rare earth carbonatite: implication for the origin of the
Mountain Pass deposit, California. Econ. Ceol., 78, 1721-1723.
Kleeman J.D., Green, D.H. & Lovering, J. (1969). Uranium distribution
in ultramafic inclusions from Victorian basalts. Earth Planet. Sci.
Lett., 5, 449-458.
148
Nickel, K.G. & Green, D.H. (1984). The nature of the upper-most mantle
beneath Victoria, Australia, as deduced from ultramafic xenoliths. In:
Komprobst, l. (Ed.), Kimberlites II: The Mantle and Crust-mantle
Relationships. Proc. 3d Intl. Kimberlite Conf., Elsevier, Amsterdam, 2,
330-338.
Press, S., Witt, G., Seck, B.A., Ionov, D. & Kovalenko, V.l. (1989).
Spinel peridotite xenoliths from the Tariat Depression, Mongolia. I:
Major element chemistry and mineralogy of a primitive mantle
xenolith suite. Geochim. Cosmochim. Acta, 50,2587-2599.
Ryabchikov, l.D. (1987). Geochemical Evolution of the Earth's Mantle.
Nauka Publishers, Moscow, 37 pp (in Russian).
----, Brey, G., Kogarko, L.N. & Bulatov, V.K. (1989). Partial melting of
carbonated peridotite at 50 kbar. Geokhimiya, 1, 3-9 (in Russian).
----, Edgar, A.D., Wyllie, P.l. (1991a). Partial melting in the system
carbonate-phosphate-peridotite at 30 kbar. Geokhimiya, 2, 163-168
(in Russian).
----, Orlova, G.P., Sen in, V.G. & Trubkin, N.V. (1991b). Interphase
distribution of rare earth elements during partial melting in the system
peridotite-carbonate-phosphate. Geologhiya Rudnykh Mestorozhdeniy,
S.-Afr .Tydskr .Geol.. 1993,96(3)
3, 78-86 (in Russian).
Stosch, H.G. (1980). Zur Geochemie der ultrabasischen Auswurflinge des
Dreiser Weihers in der Westeifel: Hinweise aUf die Evolution des
Iwntinentalen oberen Erdmantels. Inaugural-Dissertation zur
Doktorgrades, Universitiit zu Koln, 233 pp.
Sun, S.-S. (1982). Chemical composition and the origin of the Earth's
primitive mantle. Geochim. Cosmochim. Acta, 46, 179-192.
Wallace, M.E. & Green, D.H. (1988). An experimental determination of
primary carbonatite magma com~sition;. Nature, 335, 343-346.
Wanke, H. (1981). Constitution of terrestrial planets. Phil. Trans. Roy.
Soc. Lond., A393, 287-302.
Wyllie, P.l., Baker, M.B. & White, B.S. (1990). Experimental boundaries
for the origin and evolution of carbonatites. Lithos, 26, 3-19.
Wyllie, P.J. & Huang, W.L. (1975). Peridotite, kimberlite, and carbonatite
explained in the system CaO-MgO-Si02-C02 • Geology, 3, 621-{)24.
Yaxley, G.M., Crawford, A.1. & Green, D.H. (1991). Evidence for
carbonatite metasomatism in spinel peridotite xenoliths from western
Victoria, Australia. Earth Planet. Sci. Lett., 107,305-317.