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Partitioning of rare earth elements between CaSiO3 perovskite and coexisting phases: constraints on the formation of CaSiO3 inclusions in diamonds
Earth and Planetary Science Letters 181 (2000) 291^300 www.elsevier.com/locate/epsl
Partitioning of rare earth elements between CaSiO3 perovskite and coexisting phases: constraints on the formation of CaSiO3 inclusions in diamonds Wuyi Wang *, Tibor Gasparik, Robert P. Rapp Center for High Pressure Research, State University of New York at Stony Brook, Stony Brook, NY 11794, USA Received 10 December 1999; received in revised form 5 May 2000; accepted 7 June 2000
Abstract Minerals with CaSiO3 composition were found as inclusions in diamonds, and are considered to be originally of perovskite structure. To constrain their genesis and consequently the extent of circulation of mantle material, rare earth element (REE) partitioning between CaSiO3 perovskite and coexisting majoritic garnet (20 GPa, 1520³C) or MgSiO3 perovskite (25 GPa, 1600³C) was determined by combining the technologies of high-pressure synthesis and traceelement analysis using ion probe. It is consistent with previous experiments and confirms that CaSiO3 perovskite is the main REE depository, especially of the light-REE. KCe (CaPv/Gt) is V1900, and decreases gradually to 18.5 of KYb (CaPv/Gt). For CaPv/MgPv, it decreases gradually from 57.7 of KCe to about 10 of KYb . Estimated REE concentrations in the source lithology of the CaSiO3 inclusions according to these partitioning coefficients, either peridotitic or eclogitic paragenesis, show strong enrichment in light-REE (e.g. Cen 18^163), very different from normal mantle peridotite and subducted oceanic crust. It is proposed that interaction with carbonatic melt in the deep mantle may have played an important role in the formation of these CaSiO3 inclusions in diamonds, as well as in their ascending transportation. ß 2000 Elsevier Science B.V. All rights reserved. Keywords: perovskite; rare earths; partitioning; mantle; diamond; inclusions
1. Introduction High-pressure phase-equilibrium experiments have revealed that CaSiO3 with perovskite crystal structure (CaPv) is one of the main constitutional phases in the earth's transition zone and the lower mantle, for both the basaltic and the peridotitic
* Corresponding author. Fax: +1-631-632-8140; E-mail: [email protected]
compositions [1,2]. Melting experiments showed that REE strongly partition into CaPv [3^6]. This is consistent with crystal-chemical considerations, and suggests that CaPv is the main depository of REE in the deep mantle. Partitioning of REE between CaPv and other possible mantle phases at the conditions of their stable coexistence, such as majoritic garnet and MgSiO3 perovskite (MgPv), is fundamental in evaluating the plausible REE concentrations in CaPv and other high-pressure phases in the deep mantle. Minerals with CaSiO3 composition were found as inclusions in diamonds from Sa¬o Luiz, Brazil,
0012-821X / 00 / $ ^ see front matter ß 2000 Elsevier Science B.V. All rights reserved. PII: S 0 0 1 2 - 8 2 1 X ( 0 0 ) 0 0 2 0 8 - 9
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and Guinea. They are considered to have formed originally as perovskite [7^10]. The source-rock lithology of these inclusions is of primary importance in discussing their genesis and consequently the mantle dynamics. The major-element composition is of limited use in this respect, because CaPv can form in both the peridotitic and the eclogitic systems, and is basically very close to pure CaSiO3 composition [11]. Coexisting minerals in the same diamond hosts show that these CaSiO3 inclusions could be of either peridotitic or eclogitic paragenesis. Obviously, trace elements like REE can supply useful constraints concerning its formation. For this, the partitioning of REE among CaPv and other coexisting solid phases is essential. In this study, high-pressure experiments were performed to synthesize coexisting CaPv and
MgPv, or CaPv and majoritic garnet. The partitioning of REE between these phases was determined to constrain the formation of these CaSiO3 inclusions in diamonds at the deep mantle. 2. Inclusions of CaSiO3 composition from diamonds In total, ¢ve CaSiO3 inclusions were found in ¢ve diamonds from Sa¬o Luiz, Brazil [7^9]. Majorelement compositions are summarized in Table 1. These inclusions show simple major-element chemistry. Except for CaO and SiO2 , the contents of all other elements are less than 0.15 wt%. In diamond BZ116, one ferropericlase inclusion coexisted with the CaSiO3 inclusion in the same diamond. Ferropericlase had Mg/(Mg+Fe) ratio of
Table 1 Compositions of CaSiO3 inclusions from Sa¬o Luiz and Kankan diamondsa Sa¬o Luiz, Brazil
Kankan, Guinea
Sample Coexisting inclusionsb
BZ97 None
BZ115 Incl
BZ119 Incl
BZ252A None
BZ116B FPer
KK-32 Ca^silicates
KK-44g MgPv, ol, fPer, opx
SiO2 TiO2 Al2 O3 Cr2 O3 FeO NiO MnO MgO CaO Na2 O K2 O SrO Total
50.80 0.00 0.11 0.00 0.08 0.00 0.04 0.09 48.71 0.02 n.a.c n.a. 99.83
50.59 0.01 0.08 0.01 0.16 0.00 0.02 0.14 48.49 0.02 n.a. n.a. 99.51
51.03 0.02 0.07 0.01 0.04 n.a. 0.04 0.06 45.75 0.02 0.00 n.a. 97.04
51.18 0.06 0.07 0.03 0.16 0.02 0.06 0.14 48.43 0.01 0.02 n.a. 100.17
52.01 0.05 0.05 0.01 0.08 n.a. 0.00 0.46 46.94 0.04 0.00 n.a. 99.51
50.90 0.02 0.13 0.01 0.14 6 0.01 0.09 0.09 47.47 6 0.02 0.10 0.21 99.37
51.33 0.01 0.06 6 0.01 0.04 6 0.01 0.02 0.03 47.70 0.03 0.07 0.21 99.50
Ce Pr Nd Sm Eu Gd Dy Ho Yb Lu
281.0 37.5 194.5 55.0 24.50 63.0 46.9 7.27 9.83 2.50
156.0 22.1 126.5 41.7 21.15 49.9 40.6 6.69 9.84 1.92
a
Data from [7^10]. Major elements are in weight percent, and REE are in weight ppm. Incl, mall inclusions in the same diamond but not analyzed. CaPv , CaSiO3 perovskite; fPer, ferropericlase; opx, orthopyroxene; ol, olivine; Ca^silicates, Ca2 SiO4 and CaSi2 O5 . c Not analyzed. b
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Table 2 Compositions of starting material and run products, and REE partitioning Starting material wt%
a
TED-JH-1
Phase Points SiO2 TiO2 Al2 O3 Cr2 O3 Fe2 O3 FeO MnO MgO CaO Na2 O K2 O P2 O5 H2 O H2 O3 Total
49.36 0.69 6.00 0.97 8.15 0.18 16.45 14.51 0.86 0.54 0.11 1.75 0.22 99.78
High-pressure run product 20 GPa/1520³C/636 minb
25 GPa/1600³C/156 minb
Gt 5
CaPv 6
MgPv 4
CaPv 5
49.22 0.14 8.75 0.15
50.78 2.90 0.60 0.01
47.65 0.55 7.04 0.10
51.12 1.53 1.17 0.02
11.40 0.28 24.48 5.76 0.74 0.03
0.60 0.03 0.71 42.95 0.17 0.20
13.52 0.23 29.82 0.33 0.16 0.00
0.81 0.10 0.83 43.25 0.37 0.15
100.95
98.95
99.40
99.35
1.28 2.36 3.16 2.92 3.48 3.12 3.44
115.2 (0.65) 136.12 (0.71) 82.36 (0.49) 40.00 (0.31) 43.96 (0.35) 28.16 (0.25) 34.04 (0.32)
ppm La Ce Nd Sm Dy Er Yb
KCaPv=Gt 41.2 41.2 20.6 20.6 20.6 20.6 20.6
0.21 0.11 0.33 0.40 0.81 0.99 1.87
(0.02)c (0.02) (0.02) (0.02) (0.03) (0.03) (0.05)
125.87 (0.59) 210.46 (0.88) 110.24 (0.54) 66.08 (0.38) 54.82 (0.35) 32.20 (0.26) 34.54 (0.28)
602.8 1901.5 338.0 165.4 68.0 32.4 18.5
KCaPv=MgPv (0.16) (0.23) (0.17) (0.11) (0.16) (0.11) (0.15)
91.0 57.7 26.2 13.7 12.7 9.1 9.9
a
Original composition of JH-1 is from Itoh et al. [21]. Amounts of REE in the starting material are of those additionally doped. Experimental condition in P/T/t. Symbols: Gt, majoritic garnet; CaPv, CaSiO3 perovskite; MgPv, MgSiO3 perovskite. c Unit in parentheses represents one standard unit deviation in terms of the last digit cited. b
0.812, and contained signi¢cant amounts of NiO (1.33 wt%) and Cr2 O3 (0.75 wt%). Some other tiny inclusions were also observed in diamonds BZ115 and BZ119, but no compositional data are available since these tiny inclusions were lost during polishing. Analyses of rare earth elements (REE) have been performed only on two CaSiO3 inclusions [7^9], and both show considerable high contents and similarity in their distribution patterns (Table 1). The light-REE and middle-REE have abundances about 250^450U chondrite [12], while those of the heavy-REE decrease progressively with decreasing ionic radius to about 60 times of chondrite. Weak positive anomalies of Eu were detected for both CaSiO3 inclusions (Eu/Eu* of 1.26^1.41). Oxygen isotope compositions were analyzed at two points on one CaSiO3
inclusion, using a wollastonite of known N18 O as the standard. The two measurements yielded values of N18 O of 4.92 and 6.95x with an estimated error of V1x [8]. These data are not distinguishable from the accepted mantle signature of 5.7 þ 0.5x [13]. More recently, some Ca^silicates (CaSi2 O5 , Ca2 SiO4 and CaSiO3 ) were found as inclusions in diamonds from Kankan district of Guinea [10]. The most striking feature is that the CaSiO3 phases contain substantial amounts of SrO (0.21 wt%). Contents of other oxides are similarly low as those from Sa¬o Luiz (Table 1). X-ray di¡raction was performed in situ on only one CaSiO3 grain from Guinea; it appeared to be triclinic and with walstromite structure [10]. Even though, the CaSiO3 inclusions in diamond are
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considered to represent primary CaSiO3 perovskite phase rather than wollastonite or walstromite for the following reasons. (1) High-pressure experiments con¢rmed that CaSiO3 wollastonite or walstromite are not stable phases in peridotite or eclogite compositions at relatively low pressures ( 6 15 GPa), instead Ca-rich pyroxene and garnet are stable [14^18]. A phase with the CaSiO3 composition can become stable only in the perovskite structure at the conditions of the earth's transition zone or the lower mantle [1,19,20]. (2) In diamonds from both Sa¬o Luiz and Kankan, inclusions of CaSiO3 were found to be associated with ferropericlase, which is very likely to be of the lower mantle origin; and more importantly: (3) Coexisting of CaSi2 O5 and Ca2 SiO4 in equal modal proportions indicates strongly that this was an originally single crystal of CaSiO3 perovskite. Retrograde transformation (3CaSiO3 = Ca2 SiO4 +CaSi2 O5 ) occurred during its ascending transportation [10]. 3. Experiment and analysis The starting material used in this study was powdered amphibolite JH-1 distributed by the Geological Survey of Japan [21]. High contents of CaO and MgO, and moderate contents of FeO and Al2 O3 make it suitable for synthesizing CaPv and garnet, or CaPv and MgPv. In order to facilitate measurement, REE were doped by adding 4.1 wt% of a tonalite powder, which was originally prepared for other experiments and contained 500^1000 ppm of REE. The mixture was ground in acetone in an agate mill to less than few micrometers in size to eliminate any possible heterogeneity, and then heated at 110³C to get rid of any possible absorbed acetone. Final composition of the starting material is summarized in Table 2. Concentrations of REE in the original JH-1 are not known, and only the doped amounts are listed in the table. High-pressure experiments were performed at 20 and 25 GPa and at temperatures expected along the mantle geotherm using a split-sphere anvil apparatus (USSA-2000). The 10/4 sample assembly (MgO octahedron edge length/trunca-
Fig. 1. Design of the high-pressure assembly 8/3 used for the experiment at 25 GPa.
tion edge length of WC cube in mm) was used for the experiment at 20 GPa. The design of this assembly and temperature and pressure calibration were presented elsewhere by Gasparik [22]. A new 8/3 assembly was developed for the experiment at 25 GPa, the design of which is shown in Fig. 1. Using this assembly, phase transformation of ringwoodite to MgPv plus periclase (23.1 GPa) [23] occurred at oil gauge pressure of 560 bar and 1600³C. Accordingly, a pressure of about 25 GPa could be reached at the oil pressure of 800 bar. In contrast to the LaCrO3 furnace of the 10/4 assembly, rhenium foil was used as the heater in the 8/3 assembly. The starting material was enclosed in a Pt capsule made by folding and squeezing Pt foil. Pt is e¤cient in sealing the water in the starting material, and expedites crystal growth. During an experiment, pressure was applied ¢rst at room temperature, after which temperature was increased at a rate of about 100³C/min to the desired value and maintained constant for the duration of the experiment. The charge was then quenched by turning o¡ the electric power supply to the furnace. Recovered sample capsules were mounted in epoxy and polished for examination. The analyses for major elements were performed using a Cameca electron microprobe. Operating conditions for î the wavelength-dispersive analyses were 10 nA beam current and 15 kV accelerating potential. Counting times ranged from 10 to 30 s, depending on the elements analyzed. Oxide ZAF correction method was employed to reduce the data. Stan-
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dards were natural silicate minerals and synthetic oxide phases. Concentrations of REE (La, Ce, Nd, Sm, Dy, Er, Yb) were obtained using a Secondary Ion Microscope (SIMS, Cameca IMS 3f) at the Woods Hole Oceanographic Institute. A well-calibrated augite megacryst (KH-1) from alkali basalt was used as the standard. Energy ¢ltering technique with a medium o¡set voltage of 360 V was applied to eliminate possible molecular interference. The primary ion beam 16 O3 was about 10 Wm in diameter. Details about trace-element measurement by SIMS were presented elsewhere [24,25]. The capsule material Pt has a strong tendency to absorb iron from the sample. No special attention was paid to the Mg/Fe variation of crystalline phases with spatial position, but all those close to capsule rim are used to determine element partitioning. 4. Results The experimental conditions and major-element compositions of the run products are summarized in Table 2. Due to the relatively long heating duration (2.5^10.5 h) and abundant of H2 O in the starting material (V2 wt%), which acted as £ux during the crystal growth, the run products were well crystallized and show equilibrium relation. As shown in Fig. 2, most of the grains were 20^30 Wm, but several close to the capsule wall reached 50^60 Wm. Large grain size is essential to assure precise trace-element measurement with ion probe. The run product at 20 GPa and 1520³C consisted of CaPv and garnet. Except for CaO and SiO2 , CaPv contains a signi¢cant amount of TiO2 (2.72 wt%), but all the other oxides are very low (i.e. 6 0.2 wt%). Garnet is rich in SiO2 , with about 50 mole% of majorite in solid solution, and contains a moderate amount of CaO (5.8 wt%). The Mg/(Mg+Fe) ratio of garnet is 0.797. Two coexisting perovskites, MgPv and CaPv, were synthesized at 25 GPa and 1600³C. CaPv is similar in composition to that formed at 20 GPa, while MgPv contains a signi¢cant amount of Al2 O3 (7.0 wt%) and has the Mg/(Mg+Fe) ratio of 0.801. None of these crystalline phases con-
295
Fig. 2. Backscattered electron image of run product at 20 GPa and 1520³C. It is composed of well-crystallized majoritic garnet (Gt) and CaSiO3 perovskite (CaPv).
tains water. Furthermore, the contents of K2 O are very low. From the mass balance, trace amounts of hydrous and K-rich melt should exist between these crystalline grains, but no segregation was observed in the run products. Concentrations of REE in synthesized phases and calculated partitioning coe¤cients for CaPv/ garnet and CaPv/MgPv are summarized in Table 2. Consistent with the predictions based on crystal-chemical considerations, all REE are strongly partitioned into CaPv, especially the light-REE. For both CaPv/Gt and CaPv/MgPv, the partitioning coe¤cient decreases gradually with the atomic number, showing smooth distribution patterns. KCe (CaPv/Gt) is as high as 1900, and decreases gradually to 18.5 of KYb (CaPv/Gt). For CaPv/ MgPv, it decreases gradually from 57.7 of KCe to about 10 of KYb . KLa (CaPv/Gt) of 602.8 is lower than the neighboring Ce, and the reason is not clear. For concentration of La in these CaSiO3 inclusions is unavailable, this value is not used for further calculation. Based on the partitioning coe¤cients determined in this study, MgPv may contain relatively more REE than majoritic garnet. The distribution coe¤cients of light-REE for CaPv/MgPv are much higher than for CaPv/Gt; however, the di¡erences in the heavy-REE are evidently smaller.
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5. Discussion
5.2. Formation of the CaSiO3 inclusions in diamonds
5.1. General partitioning behavior The obtained partitioning coe¤cients con¢rmed the previous reports that CaPv is the main host of REE in the deep mantle [4^6,26], particularly of the light- to middle-REE. In di¡erent, large-dopant and electron microprobe analysis is the main experimental technique in previous studies [4^6], which probably su¡ers from a low detection limit and large analytical uncertainty. Shimizu et al. [26] determined these coe¤cients using ion probe analysis of natural systems, but no speci¢c data have been reported yet. The data obtained in this study are at least self-consistent, and well meet the requirement to discuss the formation of these CaSiO3 inclusions. The values and pattern of partitioning coe¤cients of CaPv/MgPv obtained here are similar to those calculated from the data published by Kato et al. [5], but slightly lower than other reports [6,26]. Gasparik and Drake [6] determined the partitioning of Sm between CaPv and MgPv in a hydrous and £uorine-bearing system at close temperatures (1500^1600³C) to this study; the values of KSm were 36^58. In a peridotite system at 25 GPa and 2450³C, KSm was reported to be higher than 100 [26]. All these data are larger than the KSm obtained in this study (V14). For CaPv/ Gt, main di¡erences are showing in light-REE. For example, KNd (CaPv/Gt) (338) is much larger than that of Kato et al.'s (54^112) [4]. All these variations among di¡erent studies may be attributed to the large di¡erences in system compositions, trace-element doping levels, as well as temperatures. To ¢gure out a speci¢c reason could be hard. It should be pointed out, however, that tiny CaPv inclusions if present in the analyzed MgPv grains may not be detectable and thus could a¡ect the analytical results, and be responsible partly for the variations of partitioning coe¤cients. This possibility can not be totally ruled out, although the MgPv locations for ion probe analysis were veri¢ed by both optical image and Ca ion image to avoid any possible overlap with CaPv. Even if this happened, it should not a¡ect the conclusions reached in this study.
The simple major-element composition of CaPv, observed in high-pressure experiments on both peridotitic and eclogitic systems, is consistent with the compositions of the CaSiO3 inclusions in diamonds. Because of this, it is impossible to constrain the lithological paragenesis of these CaSiO3 inclusions using the major-element compositions. Coexisting mineral inclusions with CaSiO3 in the same diamond hosts are important indicators; despite they are not in direct contact. The CaSiO3 inclusions may coexist with coesite, or ferropericlase [9,10]. This shows that the CaSiO3 inclusion can be of either peridotitic or eclogitic paragenesis [14,15,27,28]. Because the lithological paragenesis is unclear concerning the two CaSiO3 inclusions, in which the trace-element contents were determined (Table 1), both peridotitic and basaltic compositions are considered in the following discussion about the bulk trace-element concentrations. In a pyrolitic peridotite composition, CaPv appears at about 17 GPa by exsolving from majoritic garnet [14], and increases gradually with increasing pressure [29]. At the deep transition zone conditions, the mineralogical constitution of pyrolite consists of about 7 wt% CaPv, 39% majoritic garnet, and 54% ringwoodite. At higher P-T of the lower mantle conditions, it consists of about 8 wt% CaPv, 15% ferropericlase, and 77% MgPv [16]. Assuming that the CaPv inclusions in diamonds are of the peridotitic paragenesis, the bulk REE concentrations of the parent peridotite can be calculated from the REE contents of its constitutional minerals. Ringwoodite and ferropericlase can be neglected in this calculation, because their REE contents are negligible. From the partitioning coe¤cients determined in this study, the concentrations of REE in MgPv and majoritic garnet coexisting with CaPv can be obtained. The results are shown in Fig. 3, with consideration of two di¡erent pressure ranges. The light-REE in the estimated bulk peridotite is 20^60U chondrite, decreasing gradually to 6^10 times for the heavy-REE. These patterns are entirely di¡erent from the depleted or primitive peridotite mantle.
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Fig. 3. Estimated bulk REE concentrations in peridotite, assuming the CaSiO3 inclusions from diamonds are of the peridotitic paragenesis and the peridotite is pyrolitic in major-element composition. The results show stronger light-REE enrichment than in both depleted and primitive mantle peridotites [34].
In agreement with Ohtani et al. [30], this demonstrates that these CaSiO3 inclusions could not represent a simple chemical equilibrium relation. The most likely explanation for the cause of this enrichment is the in¢ltration of REE-rich carbonatic melt through the deep mantle, because of its low viscosity and ability to percolate through mantle rocks [31,32]. Sul¢de melt and metal melt may also exist or existed in the mantle. Very few is known about the trace-element features of these melts, but high concentrations of REE and other incompatible elements are less likely, and thus are not considered for further discussion. In experiments on model carbonated lherzolite in the system of CaO^MgO^Al2 O3 ^SiO2 ^CO2 , Dalton and Presnall [33] found that its solidus temperature at 7 GPa is still as low as V1400³C. Additionally, our recent experiments have shown that the melting temperature of a CO2 -rich primitive kimberlite is similarly low (V1450³C) up to 23 GPa [34]. It is apparent that carbonatic melt could easily form in the deep mantle if CO2 is present. In both peridotitic and eclogitic (basaltic) composi-
tions, CaPv is always the main host phase of TiO2 . In contrast, contents of TiO2 in all the reported CaSiO3 inclusions are very low (0.00^0.06 wt%). Contents of Sr (430^750 ppm from Sa¬o Luiz [9] and 0.21 wt% of SrO from Kankan [10]) are orders higher than basalt or pyrolitic peridotite. Such a high content of Sr or low content of TiO2 can never be a result of subsolidus phase transformation of any mantle common rocks. Carbonatic melt at the upper mantle is usually rich in Sr and may soak most of TiO2 from the surrounding mantle rocks during a close interaction. If the carbonatic melt at deeper mantle shares the same geochemical features, interaction with which may reasonably explain these features of CaSiO3 inclusions. It should be pointed out, however, that the extremely high concentrations of SrO in some CaSiO3 inclusions could not be a result of equilibrium reaction with any melt, and probably formed through rapid disequilibrium growth. Due to the observed coexistence of ferropericlase with CaPv in the same diamond host, this interaction is most likely occurring in
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Fig. 4. Estimated bulk REE concentrations in basalt (eclogite), assuming the CaSiO3 inclusions in diamonds are of the basaltic paragenesis. Large variation in the CaPv and garnet modes with increasing pressure is considered. The calculated results are very di¡erent from the mid-ocean ridge basalt for the evident enrichment in light-REE, but plot in the similar range of oceanic island basalts (shaded area, which includes both tholeiitic and alkali basalts) [35] at low CaPv modes. For every pair of estimations, the upper line is from sample BZ97, and the lower one from sample BZ115.
the lower mantle instead of the transition zone. It is not clear how these lower-mantle diamonds were transported to the earth's surface, but the carbonatic melt may have played an important role. While ascending and interacting with the surrounding mantle, the composition of the carbonatic melt may ¢nally change into kimberlitic by the time it reaches the earth's surface [34,35]. Retrograde transformation of CaSiO3 perovskite to CaSi2 O5 and Ca2 SiO4 phases, instead of lowerpressure walstromite [10], implies a rapid ascending transportation ; however, mantle convection is basically a slow process. Similar bulk REE calculation can also be performed assuming that the CaPv inclusions are of the eclogitic (basaltic) paragenesis. Because the phase relations for the basaltic composition at pressures higher than the stability of garnet are not well known, the assemblage of CaPv with MgPv will not be considered here. At the transition-zone and the uppermost lower-mantle conditions, stable phases in the basaltic composition are CaPv, garnet, stishovite, and an Al-rich phase
[15,18]. Stishovite and an Al-rich phase were not included in the bulk REE calculations, because stishovite contains almost no REE and the behavior of REE in the Al-rich phase is not known and is unlikely to have a signi¢cant e¡ect. The modes of CaPv and majoritic garnet at the transitionzone and lower-mantle conditions vary signi¢cantly with increasing pressure [15]. For example, the content of CaPv could change from several percent to 35 wt%. Three calculations were performed with the garnet mode of 80^35 wt%, and CaPv of 10^35 wt%. This corresponds to the pressure range from 22 to 28 GPa [15]. The calculated results are summarized in Fig. 4. Depending on the modes of CaPv, the light-REE in the supposed bulk eclogitic lithology are about 30^160U chondrite, decreasing gradually to 10^ 20U chondrite for the heavy-REE. These enriched patterns are di¡erent from the mid-ocean ridge basalt (MORB), and plot only partly in the range of oceanic island basalt (OIB) (Fig. 4), when the mode of CaPv is low. OIB usually contains a signi¢cant amount of TiO2 (0.7^3.8 wt%
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for Hawaii basalts). If the CaSiO3 inclusions are crystallized from OIB composition, they should contain a large amount of TiO2 , because CaSiO3 perovskite is the main host phase of TiO2 in the deep mantle. Obviously, this high TiO2 content is not observed from these CaSiO3 inclusions. Additionally, considering the relatively rareness of REE-rich OIB in the subducted oceanic crust, it is more likely that the eclogitic CaSiO3 inclusions originally came from MORB and experienced strong reaction with carbonatic melt, in a similar way to that of peridotitic paragenesis. In an entirely di¡erent way, Hutchison [9] assumed that these CaSiO3 inclusions were crystallized directly from a silicate melt, and estimated its bulk REE concentrations by using experimentally determined CaPv/melt partitioning coe¤cients. For the resulting liquid has a £at REE trend and similar to MORB, Hutchison [9] pointed out that the CaSiO3 inclusions may represent the deep host rock associated with early earth magmatism. Generally, we believe the lower mantle temperature is far lower than the solidus temperature of mantle peridotite or eclogite (V2300^2400³C) [17,36], and thus subsolidus element partitioning is a more reasonable way to discuss its host rock composition. In summary, either peridotitic or eclogitic paragenesis of these reported CaSiO3 inclusions in diamonds, interactions of carbonatic melt with peridotitic or eclogitic lithology in the deep mantle may have played an important role in their formation and ascending transportation. This model could reasonably explain the observed compositional and isotopic features of these CaSiO3 inclusions. This would seem to indicate that the major activity of carbonatic melt is not limited only to the lithosphere or the upper mantle, and the carbonatic melt could be the source of carbon for the crystallization of diamond. Acknowledgements The authors thank Eiichi Takahashi, Lawrence A. Taylor, and Bernard J. Wood for their thoughtful and constructive reviews, which improved this paper signi¢cantly. Special thanks
299
are to Mark T. Hutchison for many helpful comments and suggestions. Trace-element analyses were performed at Woods Hole Oceanographic Institute in the Northeast National Ion Microprobe Facility, which was supported by grant EAR-9628749 from the National Science Foundation. We are indebted to Graham Layne and Nobu Shimizu for help. N. Imai at the Geological Survey of Japan is acknowledged for supplying the JH-1 amphibolite. This study was supported by the National Science Foundation grant EAR9710158 to T.G. The high-pressure experiments were performed in the Stony Brook High Pressure Laboratory, which is jointly supported by the National Science Foundation Science and Technology Center for High Pressure Research (EAR8920239) and the State University of New York at Stony Brook.[FA] References [1] H. Tamai, T. Yagi, High pressure and high-temperature phase-relations in CaSiO3 and CaMgSi2 O6 and elasticity of perovskite-type CaSiO3 , Phys. Earth Planet. Inter. 54 (1989) 370^377. [2] T. Irifune, J. Susaki, T. Yagi, H. Sawamoto, Phase-transformations in diopside CaMgSi2 O6 at pressure up to 25GPa, Geophys. Res. Lett. 16 (1989) 187^190. [3] T. Kato, A.E. Ringwood, T. Irifune, Experimental determination of element partitioning between silicate perovskites, garnets and liquids: constraints on early di¡erentiation of the mantle, Earth Planet. Sci. Lett. 89 (1988) 123^145. [4] T. Kato, A.E. Ringwood, T. Irifune, Constraints on element partition coe¤cients between MgSiO3 perovskite and liquid determined by direct measurement, Earth Planet. Sci. Lett. 90 (1988) 65^68. [5] T. Kato, E. Ohtani, Y. Ito, K. Onuma, Element partitioning between silicate perovskites and calcic ultrabasic melt, Phys. Earth Planet. Inter. 96 (1996) 201^207. [6] T. Gasparik, M.J. Drake, Partitioning of elements among two silicate perovskites, superphase B, and volatile-bearing melt at 23 GPa and 1500^1600³C, Earth Planet. Sci. Lett. 134 (1995) 307^318. [7] B. Harte, M.T. Hutchison, J.W. Harris, Trace element characteristics of the lower mantle: an ion probe study of inclusions in diamonds from Sa¬o Luiz, Brazil, Mineral. Mag. 58A (1994) 386^387. [8] B. Harte, J.W. Harris, M.T. Hutchison, G.R. Watt, M.C. Wilding, Lower mantle mineral associations in diamonds from Sa¬o Luiz, Brazil, in: Y. Fei, C.M. Bertka, B.O. Mysen (Eds.), Mantle Petrology: Field Observations and
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[9] [10]
[11] [12] [13] [14]
[15]
[16] [17] [18]
[19] [20] [21]
[22] [23]
W. Wang et al. / Earth and Planetary Science Letters 181 (2000) 291^300 High Pressure Experimentation: A Tribute to Francis R. (Joe) Boyd, 1999, pp. 125^153. M.T. Hutchison, Constitution of the deep transition zone and lower mantle shown by diamonds and their inclusions, Ph.D thesis, University of Edinburgh, 1997. W. Joswig, T. Stachel, J.W. Harris, W.H. Baur, G.P. Brey, New Ca-silicate inclusions in diamonds ^ traces from the lower mantle, Earth Planet. Sci. Lett. 173 (1999) 1^6. T. Gasparik, Transformation of enstatite-diopside-jadeite pyroxenes to garnet, Contrib. Mineral. Petrol. 102 (1989) 389^405. E. Anders, N. Grevesse, Abundances of the elements meteoritic and solar, Geochim. Cosmochim. Acta 53 (1989) 197^214. P. Deines, Stable isotope variations in carbonatites, in: K. Bell (Ed.), Carbonatites: Genesis and Evolution, Unwin, London, 1989, pp. 301^359. E. Takahashi, E. Ito, Mineralogy of mantle peridotite along a model geotherm up to 700 km depth, in: M.H. Manghnani, Y. Syono (Eds.), High-Pressure Research in Mineral Physics, American Geophysical Union, 1987, pp. 427^438. T. Irifune, A.E. Ringwood, Phase transformations in subducted oceanic crust and buoyancy relationships at depths of 600^800 km in the mantle, Earth Planet. Sci. Lett. 117 (1993) 101^110. T. Irifune, Absence of an aluminous phase in the upper part of the earth's lower mantle, Nature 370 (1994) 131^ 133. K. Hirose, Y. Fei, Y.Z. Ma, H.K. Mao, The fate of subducted basaltic crust in the earth's lower mantle, Nature 397 (1999) 53^56. W. Wang, E. Takahashi, Subsolidus and melting experiments of a K-rich basaltic composition to 27 GPa: Implication for the behavior of potassium in the mantle, Am. Mineral. 84 (1999) 357^361. T. Gasparik, A model for the layered upper mantle, Phys. Earth Planet. Inter. 100 (1997) 197^212. T. Gasparik, Diopside-jadeite join at 16^22 GPa, Phys. Chem. Miner. 23 (1996) 476^486. S. Itoh, S. Terashima, N. Imai, H. Kamioka, N. Mita, A. Ando, 1992 compilation of analytical data for rare earth elements, scandium, yttrium, zirconium and hafnium in 26 GSJ reference samples, Geostand. Newslett. 17 (1993) 5^ 78. T. Gasparik, Phase relations in the transition zone, J. Geophys. Res. 95 (1990) 15751^15769. E. Ito, E. Takahashi, Postspinel transformations in the system Mg2 SiO4 -Fe2 SiO4 and some geophysical implications, J. Geophys. Res. 94 (1989) 10637^10646.
[24] N. Shimizu, S.R. Hart, Applications of the ion probe to geochemistry and cosmochemistry, Annu. Rev. Earth Planet. Sci. 10 (1982) 483^526. [25] N. Shimizu, M.P. Semet, C.J. Allegre, Geochemical applications of quantitative ion microprobe analysis, Geochim. Cosmochim. Acta 42 (1978) 1321^1334. [26] N. Shimizu, K. Hirose, Y. Fei, Geochemical consequences of partial melting at the core-mantle boundary, in: Abstract of the ninth annual V.M. Goldschmidt conference, pp. 272^273. [27] W. Wang, E. Takahashi, Subsolidus and melting experiments of K-doped peridotite KLB-1 to 27 GPa: its geophysical and geochemical implications, J. Geophys. Res. 105 (2000) 2855^2868. [28] S.E. Kesson, J.D. Fitz Gerald, Partitioning of MgO, FeO, NiO, MnO and Cr2 O3 between magnesian silicate perovskite and magnesiowustite: implications for the origin of inclusions in diamond and the composition of the lower mantle, Earth Planet. Sci. Lett. 111 (1991) 229^240. [29] B.J. Wood, Phase transformations and partitioning relations in peridotite under lower mantle conditions, Earth Planet. Sci. Lett. 174 (2000) 341^354. [30] E. Ohtani, A. Suzuki, T. Kato, Flotation of olivine and diamond in mantle melt at high pressure: implications for fractionation in the deep mantle and ultradeep origin of diamond, in: M.H. Manghnani, T. Yagi (Eds.), Properties of Earth and Planetary Materials at High Pressure and Temperature, American Geophysical Union, 1998, pp. 227^240. [31] R.H. Hunter, D. MacKenzie, The equilibrium geometry of carbonate melts in rocks of mantle composition, Earth Planet. Sci. Lett. 92 (1989) 347^356. [32] E.B. Watson, J.M. Brennan, D.R. Baker, Distribution of £uids in the continental mantle, in: M.A. Menzies (Ed.), Continental Lithosphere, Clarendon Press, Oxford, 1990, pp. 111^125. [33] J.A. Dalton, D.C. Presnall, Carbonatitic melts along the solidus of model lherzolite in the system CaO-MgOAl2 O3 -SiO2 -CO2 from 3 to 7 GPa, Contrib. Mineral. Petrol. 131 (1998) 123^135. [34] W. Wang, T. Gasparik, Melting of a primitive aphanitic kimberlite at 5^23 GPa: implications for the origin of kimberlitic melts, Am. Mineral. (2000), submitted. [35] A.V. Girnis, G.P. Brey, I.D. Ryabchikov, Origin of group IA kimberlite: £uid-saturated melting experiments at 45^ 55 kbar, Earth Planet. Sci. Lett. 134 (1995) 283^296. [36] J. Zhang, C. Herzberg, Melting experiments on an anhydrous peridotite KLB-1 from 5.0 to 22.5 GPa, J. Geophys. Res. 99 (1994) 17729^17742.
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Partitioning of rare earth elements between CaSiO3 perovskite and coexisting phases: constraints on the formation of CaSiO3 inclusions in diamonds Wuyi Wang *, Tibor Gasparik, Robert P. Rapp Center for High Pressure Research, State University of New York at Stony Brook, Stony Brook, NY 11794, USA Received 10 December 1999; received in revised form 5 May 2000; accepted 7 June 2000
Abstract Minerals with CaSiO3 composition were found as inclusions in diamonds, and are considered to be originally of perovskite structure. To constrain their genesis and consequently the extent of circulation of mantle material, rare earth element (REE) partitioning between CaSiO3 perovskite and coexisting majoritic garnet (20 GPa, 1520³C) or MgSiO3 perovskite (25 GPa, 1600³C) was determined by combining the technologies of high-pressure synthesis and traceelement analysis using ion probe. It is consistent with previous experiments and confirms that CaSiO3 perovskite is the main REE depository, especially of the light-REE. KCe (CaPv/Gt) is V1900, and decreases gradually to 18.5 of KYb (CaPv/Gt). For CaPv/MgPv, it decreases gradually from 57.7 of KCe to about 10 of KYb . Estimated REE concentrations in the source lithology of the CaSiO3 inclusions according to these partitioning coefficients, either peridotitic or eclogitic paragenesis, show strong enrichment in light-REE (e.g. Cen 18^163), very different from normal mantle peridotite and subducted oceanic crust. It is proposed that interaction with carbonatic melt in the deep mantle may have played an important role in the formation of these CaSiO3 inclusions in diamonds, as well as in their ascending transportation. ß 2000 Elsevier Science B.V. All rights reserved. Keywords: perovskite; rare earths; partitioning; mantle; diamond; inclusions
1. Introduction High-pressure phase-equilibrium experiments have revealed that CaSiO3 with perovskite crystal structure (CaPv) is one of the main constitutional phases in the earth's transition zone and the lower mantle, for both the basaltic and the peridotitic
* Corresponding author. Fax: +1-631-632-8140; E-mail: [email protected]
compositions [1,2]. Melting experiments showed that REE strongly partition into CaPv [3^6]. This is consistent with crystal-chemical considerations, and suggests that CaPv is the main depository of REE in the deep mantle. Partitioning of REE between CaPv and other possible mantle phases at the conditions of their stable coexistence, such as majoritic garnet and MgSiO3 perovskite (MgPv), is fundamental in evaluating the plausible REE concentrations in CaPv and other high-pressure phases in the deep mantle. Minerals with CaSiO3 composition were found as inclusions in diamonds from Sa¬o Luiz, Brazil,
0012-821X / 00 / $ ^ see front matter ß 2000 Elsevier Science B.V. All rights reserved. PII: S 0 0 1 2 - 8 2 1 X ( 0 0 ) 0 0 2 0 8 - 9
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and Guinea. They are considered to have formed originally as perovskite [7^10]. The source-rock lithology of these inclusions is of primary importance in discussing their genesis and consequently the mantle dynamics. The major-element composition is of limited use in this respect, because CaPv can form in both the peridotitic and the eclogitic systems, and is basically very close to pure CaSiO3 composition [11]. Coexisting minerals in the same diamond hosts show that these CaSiO3 inclusions could be of either peridotitic or eclogitic paragenesis. Obviously, trace elements like REE can supply useful constraints concerning its formation. For this, the partitioning of REE among CaPv and other coexisting solid phases is essential. In this study, high-pressure experiments were performed to synthesize coexisting CaPv and
MgPv, or CaPv and majoritic garnet. The partitioning of REE between these phases was determined to constrain the formation of these CaSiO3 inclusions in diamonds at the deep mantle. 2. Inclusions of CaSiO3 composition from diamonds In total, ¢ve CaSiO3 inclusions were found in ¢ve diamonds from Sa¬o Luiz, Brazil [7^9]. Majorelement compositions are summarized in Table 1. These inclusions show simple major-element chemistry. Except for CaO and SiO2 , the contents of all other elements are less than 0.15 wt%. In diamond BZ116, one ferropericlase inclusion coexisted with the CaSiO3 inclusion in the same diamond. Ferropericlase had Mg/(Mg+Fe) ratio of
Table 1 Compositions of CaSiO3 inclusions from Sa¬o Luiz and Kankan diamondsa Sa¬o Luiz, Brazil
Kankan, Guinea
Sample Coexisting inclusionsb
BZ97 None
BZ115 Incl
BZ119 Incl
BZ252A None
BZ116B FPer
KK-32 Ca^silicates
KK-44g MgPv, ol, fPer, opx
SiO2 TiO2 Al2 O3 Cr2 O3 FeO NiO MnO MgO CaO Na2 O K2 O SrO Total
50.80 0.00 0.11 0.00 0.08 0.00 0.04 0.09 48.71 0.02 n.a.c n.a. 99.83
50.59 0.01 0.08 0.01 0.16 0.00 0.02 0.14 48.49 0.02 n.a. n.a. 99.51
51.03 0.02 0.07 0.01 0.04 n.a. 0.04 0.06 45.75 0.02 0.00 n.a. 97.04
51.18 0.06 0.07 0.03 0.16 0.02 0.06 0.14 48.43 0.01 0.02 n.a. 100.17
52.01 0.05 0.05 0.01 0.08 n.a. 0.00 0.46 46.94 0.04 0.00 n.a. 99.51
50.90 0.02 0.13 0.01 0.14 6 0.01 0.09 0.09 47.47 6 0.02 0.10 0.21 99.37
51.33 0.01 0.06 6 0.01 0.04 6 0.01 0.02 0.03 47.70 0.03 0.07 0.21 99.50
Ce Pr Nd Sm Eu Gd Dy Ho Yb Lu
281.0 37.5 194.5 55.0 24.50 63.0 46.9 7.27 9.83 2.50
156.0 22.1 126.5 41.7 21.15 49.9 40.6 6.69 9.84 1.92
a
Data from [7^10]. Major elements are in weight percent, and REE are in weight ppm. Incl, mall inclusions in the same diamond but not analyzed. CaPv , CaSiO3 perovskite; fPer, ferropericlase; opx, orthopyroxene; ol, olivine; Ca^silicates, Ca2 SiO4 and CaSi2 O5 . c Not analyzed. b
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293
Table 2 Compositions of starting material and run products, and REE partitioning Starting material wt%
a
TED-JH-1
Phase Points SiO2 TiO2 Al2 O3 Cr2 O3 Fe2 O3 FeO MnO MgO CaO Na2 O K2 O P2 O5 H2 O H2 O3 Total
49.36 0.69 6.00 0.97 8.15 0.18 16.45 14.51 0.86 0.54 0.11 1.75 0.22 99.78
High-pressure run product 20 GPa/1520³C/636 minb
25 GPa/1600³C/156 minb
Gt 5
CaPv 6
MgPv 4
CaPv 5
49.22 0.14 8.75 0.15
50.78 2.90 0.60 0.01
47.65 0.55 7.04 0.10
51.12 1.53 1.17 0.02
11.40 0.28 24.48 5.76 0.74 0.03
0.60 0.03 0.71 42.95 0.17 0.20
13.52 0.23 29.82 0.33 0.16 0.00
0.81 0.10 0.83 43.25 0.37 0.15
100.95
98.95
99.40
99.35
1.28 2.36 3.16 2.92 3.48 3.12 3.44
115.2 (0.65) 136.12 (0.71) 82.36 (0.49) 40.00 (0.31) 43.96 (0.35) 28.16 (0.25) 34.04 (0.32)
ppm La Ce Nd Sm Dy Er Yb
KCaPv=Gt 41.2 41.2 20.6 20.6 20.6 20.6 20.6
0.21 0.11 0.33 0.40 0.81 0.99 1.87
(0.02)c (0.02) (0.02) (0.02) (0.03) (0.03) (0.05)
125.87 (0.59) 210.46 (0.88) 110.24 (0.54) 66.08 (0.38) 54.82 (0.35) 32.20 (0.26) 34.54 (0.28)
602.8 1901.5 338.0 165.4 68.0 32.4 18.5
KCaPv=MgPv (0.16) (0.23) (0.17) (0.11) (0.16) (0.11) (0.15)
91.0 57.7 26.2 13.7 12.7 9.1 9.9
a
Original composition of JH-1 is from Itoh et al. [21]. Amounts of REE in the starting material are of those additionally doped. Experimental condition in P/T/t. Symbols: Gt, majoritic garnet; CaPv, CaSiO3 perovskite; MgPv, MgSiO3 perovskite. c Unit in parentheses represents one standard unit deviation in terms of the last digit cited. b
0.812, and contained signi¢cant amounts of NiO (1.33 wt%) and Cr2 O3 (0.75 wt%). Some other tiny inclusions were also observed in diamonds BZ115 and BZ119, but no compositional data are available since these tiny inclusions were lost during polishing. Analyses of rare earth elements (REE) have been performed only on two CaSiO3 inclusions [7^9], and both show considerable high contents and similarity in their distribution patterns (Table 1). The light-REE and middle-REE have abundances about 250^450U chondrite [12], while those of the heavy-REE decrease progressively with decreasing ionic radius to about 60 times of chondrite. Weak positive anomalies of Eu were detected for both CaSiO3 inclusions (Eu/Eu* of 1.26^1.41). Oxygen isotope compositions were analyzed at two points on one CaSiO3
inclusion, using a wollastonite of known N18 O as the standard. The two measurements yielded values of N18 O of 4.92 and 6.95x with an estimated error of V1x [8]. These data are not distinguishable from the accepted mantle signature of 5.7 þ 0.5x [13]. More recently, some Ca^silicates (CaSi2 O5 , Ca2 SiO4 and CaSiO3 ) were found as inclusions in diamonds from Kankan district of Guinea [10]. The most striking feature is that the CaSiO3 phases contain substantial amounts of SrO (0.21 wt%). Contents of other oxides are similarly low as those from Sa¬o Luiz (Table 1). X-ray di¡raction was performed in situ on only one CaSiO3 grain from Guinea; it appeared to be triclinic and with walstromite structure [10]. Even though, the CaSiO3 inclusions in diamond are
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considered to represent primary CaSiO3 perovskite phase rather than wollastonite or walstromite for the following reasons. (1) High-pressure experiments con¢rmed that CaSiO3 wollastonite or walstromite are not stable phases in peridotite or eclogite compositions at relatively low pressures ( 6 15 GPa), instead Ca-rich pyroxene and garnet are stable [14^18]. A phase with the CaSiO3 composition can become stable only in the perovskite structure at the conditions of the earth's transition zone or the lower mantle [1,19,20]. (2) In diamonds from both Sa¬o Luiz and Kankan, inclusions of CaSiO3 were found to be associated with ferropericlase, which is very likely to be of the lower mantle origin; and more importantly: (3) Coexisting of CaSi2 O5 and Ca2 SiO4 in equal modal proportions indicates strongly that this was an originally single crystal of CaSiO3 perovskite. Retrograde transformation (3CaSiO3 = Ca2 SiO4 +CaSi2 O5 ) occurred during its ascending transportation [10]. 3. Experiment and analysis The starting material used in this study was powdered amphibolite JH-1 distributed by the Geological Survey of Japan [21]. High contents of CaO and MgO, and moderate contents of FeO and Al2 O3 make it suitable for synthesizing CaPv and garnet, or CaPv and MgPv. In order to facilitate measurement, REE were doped by adding 4.1 wt% of a tonalite powder, which was originally prepared for other experiments and contained 500^1000 ppm of REE. The mixture was ground in acetone in an agate mill to less than few micrometers in size to eliminate any possible heterogeneity, and then heated at 110³C to get rid of any possible absorbed acetone. Final composition of the starting material is summarized in Table 2. Concentrations of REE in the original JH-1 are not known, and only the doped amounts are listed in the table. High-pressure experiments were performed at 20 and 25 GPa and at temperatures expected along the mantle geotherm using a split-sphere anvil apparatus (USSA-2000). The 10/4 sample assembly (MgO octahedron edge length/trunca-
Fig. 1. Design of the high-pressure assembly 8/3 used for the experiment at 25 GPa.
tion edge length of WC cube in mm) was used for the experiment at 20 GPa. The design of this assembly and temperature and pressure calibration were presented elsewhere by Gasparik [22]. A new 8/3 assembly was developed for the experiment at 25 GPa, the design of which is shown in Fig. 1. Using this assembly, phase transformation of ringwoodite to MgPv plus periclase (23.1 GPa) [23] occurred at oil gauge pressure of 560 bar and 1600³C. Accordingly, a pressure of about 25 GPa could be reached at the oil pressure of 800 bar. In contrast to the LaCrO3 furnace of the 10/4 assembly, rhenium foil was used as the heater in the 8/3 assembly. The starting material was enclosed in a Pt capsule made by folding and squeezing Pt foil. Pt is e¤cient in sealing the water in the starting material, and expedites crystal growth. During an experiment, pressure was applied ¢rst at room temperature, after which temperature was increased at a rate of about 100³C/min to the desired value and maintained constant for the duration of the experiment. The charge was then quenched by turning o¡ the electric power supply to the furnace. Recovered sample capsules were mounted in epoxy and polished for examination. The analyses for major elements were performed using a Cameca electron microprobe. Operating conditions for î the wavelength-dispersive analyses were 10 nA beam current and 15 kV accelerating potential. Counting times ranged from 10 to 30 s, depending on the elements analyzed. Oxide ZAF correction method was employed to reduce the data. Stan-
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dards were natural silicate minerals and synthetic oxide phases. Concentrations of REE (La, Ce, Nd, Sm, Dy, Er, Yb) were obtained using a Secondary Ion Microscope (SIMS, Cameca IMS 3f) at the Woods Hole Oceanographic Institute. A well-calibrated augite megacryst (KH-1) from alkali basalt was used as the standard. Energy ¢ltering technique with a medium o¡set voltage of 360 V was applied to eliminate possible molecular interference. The primary ion beam 16 O3 was about 10 Wm in diameter. Details about trace-element measurement by SIMS were presented elsewhere [24,25]. The capsule material Pt has a strong tendency to absorb iron from the sample. No special attention was paid to the Mg/Fe variation of crystalline phases with spatial position, but all those close to capsule rim are used to determine element partitioning. 4. Results The experimental conditions and major-element compositions of the run products are summarized in Table 2. Due to the relatively long heating duration (2.5^10.5 h) and abundant of H2 O in the starting material (V2 wt%), which acted as £ux during the crystal growth, the run products were well crystallized and show equilibrium relation. As shown in Fig. 2, most of the grains were 20^30 Wm, but several close to the capsule wall reached 50^60 Wm. Large grain size is essential to assure precise trace-element measurement with ion probe. The run product at 20 GPa and 1520³C consisted of CaPv and garnet. Except for CaO and SiO2 , CaPv contains a signi¢cant amount of TiO2 (2.72 wt%), but all the other oxides are very low (i.e. 6 0.2 wt%). Garnet is rich in SiO2 , with about 50 mole% of majorite in solid solution, and contains a moderate amount of CaO (5.8 wt%). The Mg/(Mg+Fe) ratio of garnet is 0.797. Two coexisting perovskites, MgPv and CaPv, were synthesized at 25 GPa and 1600³C. CaPv is similar in composition to that formed at 20 GPa, while MgPv contains a signi¢cant amount of Al2 O3 (7.0 wt%) and has the Mg/(Mg+Fe) ratio of 0.801. None of these crystalline phases con-
295
Fig. 2. Backscattered electron image of run product at 20 GPa and 1520³C. It is composed of well-crystallized majoritic garnet (Gt) and CaSiO3 perovskite (CaPv).
tains water. Furthermore, the contents of K2 O are very low. From the mass balance, trace amounts of hydrous and K-rich melt should exist between these crystalline grains, but no segregation was observed in the run products. Concentrations of REE in synthesized phases and calculated partitioning coe¤cients for CaPv/ garnet and CaPv/MgPv are summarized in Table 2. Consistent with the predictions based on crystal-chemical considerations, all REE are strongly partitioned into CaPv, especially the light-REE. For both CaPv/Gt and CaPv/MgPv, the partitioning coe¤cient decreases gradually with the atomic number, showing smooth distribution patterns. KCe (CaPv/Gt) is as high as 1900, and decreases gradually to 18.5 of KYb (CaPv/Gt). For CaPv/ MgPv, it decreases gradually from 57.7 of KCe to about 10 of KYb . KLa (CaPv/Gt) of 602.8 is lower than the neighboring Ce, and the reason is not clear. For concentration of La in these CaSiO3 inclusions is unavailable, this value is not used for further calculation. Based on the partitioning coe¤cients determined in this study, MgPv may contain relatively more REE than majoritic garnet. The distribution coe¤cients of light-REE for CaPv/MgPv are much higher than for CaPv/Gt; however, the di¡erences in the heavy-REE are evidently smaller.
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5. Discussion
5.2. Formation of the CaSiO3 inclusions in diamonds
5.1. General partitioning behavior The obtained partitioning coe¤cients con¢rmed the previous reports that CaPv is the main host of REE in the deep mantle [4^6,26], particularly of the light- to middle-REE. In di¡erent, large-dopant and electron microprobe analysis is the main experimental technique in previous studies [4^6], which probably su¡ers from a low detection limit and large analytical uncertainty. Shimizu et al. [26] determined these coe¤cients using ion probe analysis of natural systems, but no speci¢c data have been reported yet. The data obtained in this study are at least self-consistent, and well meet the requirement to discuss the formation of these CaSiO3 inclusions. The values and pattern of partitioning coe¤cients of CaPv/MgPv obtained here are similar to those calculated from the data published by Kato et al. [5], but slightly lower than other reports [6,26]. Gasparik and Drake [6] determined the partitioning of Sm between CaPv and MgPv in a hydrous and £uorine-bearing system at close temperatures (1500^1600³C) to this study; the values of KSm were 36^58. In a peridotite system at 25 GPa and 2450³C, KSm was reported to be higher than 100 [26]. All these data are larger than the KSm obtained in this study (V14). For CaPv/ Gt, main di¡erences are showing in light-REE. For example, KNd (CaPv/Gt) (338) is much larger than that of Kato et al.'s (54^112) [4]. All these variations among di¡erent studies may be attributed to the large di¡erences in system compositions, trace-element doping levels, as well as temperatures. To ¢gure out a speci¢c reason could be hard. It should be pointed out, however, that tiny CaPv inclusions if present in the analyzed MgPv grains may not be detectable and thus could a¡ect the analytical results, and be responsible partly for the variations of partitioning coe¤cients. This possibility can not be totally ruled out, although the MgPv locations for ion probe analysis were veri¢ed by both optical image and Ca ion image to avoid any possible overlap with CaPv. Even if this happened, it should not a¡ect the conclusions reached in this study.
The simple major-element composition of CaPv, observed in high-pressure experiments on both peridotitic and eclogitic systems, is consistent with the compositions of the CaSiO3 inclusions in diamonds. Because of this, it is impossible to constrain the lithological paragenesis of these CaSiO3 inclusions using the major-element compositions. Coexisting mineral inclusions with CaSiO3 in the same diamond hosts are important indicators; despite they are not in direct contact. The CaSiO3 inclusions may coexist with coesite, or ferropericlase [9,10]. This shows that the CaSiO3 inclusion can be of either peridotitic or eclogitic paragenesis [14,15,27,28]. Because the lithological paragenesis is unclear concerning the two CaSiO3 inclusions, in which the trace-element contents were determined (Table 1), both peridotitic and basaltic compositions are considered in the following discussion about the bulk trace-element concentrations. In a pyrolitic peridotite composition, CaPv appears at about 17 GPa by exsolving from majoritic garnet [14], and increases gradually with increasing pressure [29]. At the deep transition zone conditions, the mineralogical constitution of pyrolite consists of about 7 wt% CaPv, 39% majoritic garnet, and 54% ringwoodite. At higher P-T of the lower mantle conditions, it consists of about 8 wt% CaPv, 15% ferropericlase, and 77% MgPv [16]. Assuming that the CaPv inclusions in diamonds are of the peridotitic paragenesis, the bulk REE concentrations of the parent peridotite can be calculated from the REE contents of its constitutional minerals. Ringwoodite and ferropericlase can be neglected in this calculation, because their REE contents are negligible. From the partitioning coe¤cients determined in this study, the concentrations of REE in MgPv and majoritic garnet coexisting with CaPv can be obtained. The results are shown in Fig. 3, with consideration of two di¡erent pressure ranges. The light-REE in the estimated bulk peridotite is 20^60U chondrite, decreasing gradually to 6^10 times for the heavy-REE. These patterns are entirely di¡erent from the depleted or primitive peridotite mantle.
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Fig. 3. Estimated bulk REE concentrations in peridotite, assuming the CaSiO3 inclusions from diamonds are of the peridotitic paragenesis and the peridotite is pyrolitic in major-element composition. The results show stronger light-REE enrichment than in both depleted and primitive mantle peridotites [34].
In agreement with Ohtani et al. [30], this demonstrates that these CaSiO3 inclusions could not represent a simple chemical equilibrium relation. The most likely explanation for the cause of this enrichment is the in¢ltration of REE-rich carbonatic melt through the deep mantle, because of its low viscosity and ability to percolate through mantle rocks [31,32]. Sul¢de melt and metal melt may also exist or existed in the mantle. Very few is known about the trace-element features of these melts, but high concentrations of REE and other incompatible elements are less likely, and thus are not considered for further discussion. In experiments on model carbonated lherzolite in the system of CaO^MgO^Al2 O3 ^SiO2 ^CO2 , Dalton and Presnall [33] found that its solidus temperature at 7 GPa is still as low as V1400³C. Additionally, our recent experiments have shown that the melting temperature of a CO2 -rich primitive kimberlite is similarly low (V1450³C) up to 23 GPa [34]. It is apparent that carbonatic melt could easily form in the deep mantle if CO2 is present. In both peridotitic and eclogitic (basaltic) composi-
tions, CaPv is always the main host phase of TiO2 . In contrast, contents of TiO2 in all the reported CaSiO3 inclusions are very low (0.00^0.06 wt%). Contents of Sr (430^750 ppm from Sa¬o Luiz [9] and 0.21 wt% of SrO from Kankan [10]) are orders higher than basalt or pyrolitic peridotite. Such a high content of Sr or low content of TiO2 can never be a result of subsolidus phase transformation of any mantle common rocks. Carbonatic melt at the upper mantle is usually rich in Sr and may soak most of TiO2 from the surrounding mantle rocks during a close interaction. If the carbonatic melt at deeper mantle shares the same geochemical features, interaction with which may reasonably explain these features of CaSiO3 inclusions. It should be pointed out, however, that the extremely high concentrations of SrO in some CaSiO3 inclusions could not be a result of equilibrium reaction with any melt, and probably formed through rapid disequilibrium growth. Due to the observed coexistence of ferropericlase with CaPv in the same diamond host, this interaction is most likely occurring in
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Fig. 4. Estimated bulk REE concentrations in basalt (eclogite), assuming the CaSiO3 inclusions in diamonds are of the basaltic paragenesis. Large variation in the CaPv and garnet modes with increasing pressure is considered. The calculated results are very di¡erent from the mid-ocean ridge basalt for the evident enrichment in light-REE, but plot in the similar range of oceanic island basalts (shaded area, which includes both tholeiitic and alkali basalts) [35] at low CaPv modes. For every pair of estimations, the upper line is from sample BZ97, and the lower one from sample BZ115.
the lower mantle instead of the transition zone. It is not clear how these lower-mantle diamonds were transported to the earth's surface, but the carbonatic melt may have played an important role. While ascending and interacting with the surrounding mantle, the composition of the carbonatic melt may ¢nally change into kimberlitic by the time it reaches the earth's surface [34,35]. Retrograde transformation of CaSiO3 perovskite to CaSi2 O5 and Ca2 SiO4 phases, instead of lowerpressure walstromite [10], implies a rapid ascending transportation ; however, mantle convection is basically a slow process. Similar bulk REE calculation can also be performed assuming that the CaPv inclusions are of the eclogitic (basaltic) paragenesis. Because the phase relations for the basaltic composition at pressures higher than the stability of garnet are not well known, the assemblage of CaPv with MgPv will not be considered here. At the transition-zone and the uppermost lower-mantle conditions, stable phases in the basaltic composition are CaPv, garnet, stishovite, and an Al-rich phase
[15,18]. Stishovite and an Al-rich phase were not included in the bulk REE calculations, because stishovite contains almost no REE and the behavior of REE in the Al-rich phase is not known and is unlikely to have a signi¢cant e¡ect. The modes of CaPv and majoritic garnet at the transitionzone and lower-mantle conditions vary signi¢cantly with increasing pressure [15]. For example, the content of CaPv could change from several percent to 35 wt%. Three calculations were performed with the garnet mode of 80^35 wt%, and CaPv of 10^35 wt%. This corresponds to the pressure range from 22 to 28 GPa [15]. The calculated results are summarized in Fig. 4. Depending on the modes of CaPv, the light-REE in the supposed bulk eclogitic lithology are about 30^160U chondrite, decreasing gradually to 10^ 20U chondrite for the heavy-REE. These enriched patterns are di¡erent from the mid-ocean ridge basalt (MORB), and plot only partly in the range of oceanic island basalt (OIB) (Fig. 4), when the mode of CaPv is low. OIB usually contains a signi¢cant amount of TiO2 (0.7^3.8 wt%
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for Hawaii basalts). If the CaSiO3 inclusions are crystallized from OIB composition, they should contain a large amount of TiO2 , because CaSiO3 perovskite is the main host phase of TiO2 in the deep mantle. Obviously, this high TiO2 content is not observed from these CaSiO3 inclusions. Additionally, considering the relatively rareness of REE-rich OIB in the subducted oceanic crust, it is more likely that the eclogitic CaSiO3 inclusions originally came from MORB and experienced strong reaction with carbonatic melt, in a similar way to that of peridotitic paragenesis. In an entirely di¡erent way, Hutchison [9] assumed that these CaSiO3 inclusions were crystallized directly from a silicate melt, and estimated its bulk REE concentrations by using experimentally determined CaPv/melt partitioning coe¤cients. For the resulting liquid has a £at REE trend and similar to MORB, Hutchison [9] pointed out that the CaSiO3 inclusions may represent the deep host rock associated with early earth magmatism. Generally, we believe the lower mantle temperature is far lower than the solidus temperature of mantle peridotite or eclogite (V2300^2400³C) [17,36], and thus subsolidus element partitioning is a more reasonable way to discuss its host rock composition. In summary, either peridotitic or eclogitic paragenesis of these reported CaSiO3 inclusions in diamonds, interactions of carbonatic melt with peridotitic or eclogitic lithology in the deep mantle may have played an important role in their formation and ascending transportation. This model could reasonably explain the observed compositional and isotopic features of these CaSiO3 inclusions. This would seem to indicate that the major activity of carbonatic melt is not limited only to the lithosphere or the upper mantle, and the carbonatic melt could be the source of carbon for the crystallization of diamond. Acknowledgements The authors thank Eiichi Takahashi, Lawrence A. Taylor, and Bernard J. Wood for their thoughtful and constructive reviews, which improved this paper signi¢cantly. Special thanks
299
are to Mark T. Hutchison for many helpful comments and suggestions. Trace-element analyses were performed at Woods Hole Oceanographic Institute in the Northeast National Ion Microprobe Facility, which was supported by grant EAR-9628749 from the National Science Foundation. We are indebted to Graham Layne and Nobu Shimizu for help. N. Imai at the Geological Survey of Japan is acknowledged for supplying the JH-1 amphibolite. This study was supported by the National Science Foundation grant EAR9710158 to T.G. The high-pressure experiments were performed in the Stony Brook High Pressure Laboratory, which is jointly supported by the National Science Foundation Science and Technology Center for High Pressure Research (EAR8920239) and the State University of New York at Stony Brook.[FA] References [1] H. Tamai, T. Yagi, High pressure and high-temperature phase-relations in CaSiO3 and CaMgSi2 O6 and elasticity of perovskite-type CaSiO3 , Phys. Earth Planet. Inter. 54 (1989) 370^377. [2] T. Irifune, J. Susaki, T. Yagi, H. Sawamoto, Phase-transformations in diopside CaMgSi2 O6 at pressure up to 25GPa, Geophys. Res. Lett. 16 (1989) 187^190. [3] T. Kato, A.E. Ringwood, T. Irifune, Experimental determination of element partitioning between silicate perovskites, garnets and liquids: constraints on early di¡erentiation of the mantle, Earth Planet. Sci. Lett. 89 (1988) 123^145. [4] T. Kato, A.E. Ringwood, T. Irifune, Constraints on element partition coe¤cients between MgSiO3 perovskite and liquid determined by direct measurement, Earth Planet. Sci. Lett. 90 (1988) 65^68. [5] T. Kato, E. Ohtani, Y. Ito, K. Onuma, Element partitioning between silicate perovskites and calcic ultrabasic melt, Phys. Earth Planet. Inter. 96 (1996) 201^207. [6] T. Gasparik, M.J. Drake, Partitioning of elements among two silicate perovskites, superphase B, and volatile-bearing melt at 23 GPa and 1500^1600³C, Earth Planet. Sci. Lett. 134 (1995) 307^318. [7] B. Harte, M.T. Hutchison, J.W. Harris, Trace element characteristics of the lower mantle: an ion probe study of inclusions in diamonds from Sa¬o Luiz, Brazil, Mineral. Mag. 58A (1994) 386^387. [8] B. Harte, J.W. Harris, M.T. Hutchison, G.R. Watt, M.C. Wilding, Lower mantle mineral associations in diamonds from Sa¬o Luiz, Brazil, in: Y. Fei, C.M. Bertka, B.O. Mysen (Eds.), Mantle Petrology: Field Observations and
EPSL 5564 16-8-00
300
[9] [10]
[11] [12] [13] [14]
[15]
[16] [17] [18]
[19] [20] [21]
[22] [23]
W. Wang et al. / Earth and Planetary Science Letters 181 (2000) 291^300 High Pressure Experimentation: A Tribute to Francis R. (Joe) Boyd, 1999, pp. 125^153. M.T. Hutchison, Constitution of the deep transition zone and lower mantle shown by diamonds and their inclusions, Ph.D thesis, University of Edinburgh, 1997. W. Joswig, T. Stachel, J.W. Harris, W.H. Baur, G.P. Brey, New Ca-silicate inclusions in diamonds ^ traces from the lower mantle, Earth Planet. Sci. Lett. 173 (1999) 1^6. T. Gasparik, Transformation of enstatite-diopside-jadeite pyroxenes to garnet, Contrib. Mineral. Petrol. 102 (1989) 389^405. E. Anders, N. Grevesse, Abundances of the elements meteoritic and solar, Geochim. Cosmochim. Acta 53 (1989) 197^214. P. Deines, Stable isotope variations in carbonatites, in: K. Bell (Ed.), Carbonatites: Genesis and Evolution, Unwin, London, 1989, pp. 301^359. E. Takahashi, E. Ito, Mineralogy of mantle peridotite along a model geotherm up to 700 km depth, in: M.H. Manghnani, Y. Syono (Eds.), High-Pressure Research in Mineral Physics, American Geophysical Union, 1987, pp. 427^438. T. Irifune, A.E. Ringwood, Phase transformations in subducted oceanic crust and buoyancy relationships at depths of 600^800 km in the mantle, Earth Planet. Sci. Lett. 117 (1993) 101^110. T. Irifune, Absence of an aluminous phase in the upper part of the earth's lower mantle, Nature 370 (1994) 131^ 133. K. Hirose, Y. Fei, Y.Z. Ma, H.K. Mao, The fate of subducted basaltic crust in the earth's lower mantle, Nature 397 (1999) 53^56. W. Wang, E. Takahashi, Subsolidus and melting experiments of a K-rich basaltic composition to 27 GPa: Implication for the behavior of potassium in the mantle, Am. Mineral. 84 (1999) 357^361. T. Gasparik, A model for the layered upper mantle, Phys. Earth Planet. Inter. 100 (1997) 197^212. T. Gasparik, Diopside-jadeite join at 16^22 GPa, Phys. Chem. Miner. 23 (1996) 476^486. S. Itoh, S. Terashima, N. Imai, H. Kamioka, N. Mita, A. Ando, 1992 compilation of analytical data for rare earth elements, scandium, yttrium, zirconium and hafnium in 26 GSJ reference samples, Geostand. Newslett. 17 (1993) 5^ 78. T. Gasparik, Phase relations in the transition zone, J. Geophys. Res. 95 (1990) 15751^15769. E. Ito, E. Takahashi, Postspinel transformations in the system Mg2 SiO4 -Fe2 SiO4 and some geophysical implications, J. Geophys. Res. 94 (1989) 10637^10646.
[24] N. Shimizu, S.R. Hart, Applications of the ion probe to geochemistry and cosmochemistry, Annu. Rev. Earth Planet. Sci. 10 (1982) 483^526. [25] N. Shimizu, M.P. Semet, C.J. Allegre, Geochemical applications of quantitative ion microprobe analysis, Geochim. Cosmochim. Acta 42 (1978) 1321^1334. [26] N. Shimizu, K. Hirose, Y. Fei, Geochemical consequences of partial melting at the core-mantle boundary, in: Abstract of the ninth annual V.M. Goldschmidt conference, pp. 272^273. [27] W. Wang, E. Takahashi, Subsolidus and melting experiments of K-doped peridotite KLB-1 to 27 GPa: its geophysical and geochemical implications, J. Geophys. Res. 105 (2000) 2855^2868. [28] S.E. Kesson, J.D. Fitz Gerald, Partitioning of MgO, FeO, NiO, MnO and Cr2 O3 between magnesian silicate perovskite and magnesiowustite: implications for the origin of inclusions in diamond and the composition of the lower mantle, Earth Planet. Sci. Lett. 111 (1991) 229^240. [29] B.J. Wood, Phase transformations and partitioning relations in peridotite under lower mantle conditions, Earth Planet. Sci. Lett. 174 (2000) 341^354. [30] E. Ohtani, A. Suzuki, T. Kato, Flotation of olivine and diamond in mantle melt at high pressure: implications for fractionation in the deep mantle and ultradeep origin of diamond, in: M.H. Manghnani, T. Yagi (Eds.), Properties of Earth and Planetary Materials at High Pressure and Temperature, American Geophysical Union, 1998, pp. 227^240. [31] R.H. Hunter, D. MacKenzie, The equilibrium geometry of carbonate melts in rocks of mantle composition, Earth Planet. Sci. Lett. 92 (1989) 347^356. [32] E.B. Watson, J.M. Brennan, D.R. Baker, Distribution of £uids in the continental mantle, in: M.A. Menzies (Ed.), Continental Lithosphere, Clarendon Press, Oxford, 1990, pp. 111^125. [33] J.A. Dalton, D.C. Presnall, Carbonatitic melts along the solidus of model lherzolite in the system CaO-MgOAl2 O3 -SiO2 -CO2 from 3 to 7 GPa, Contrib. Mineral. Petrol. 131 (1998) 123^135. [34] W. Wang, T. Gasparik, Melting of a primitive aphanitic kimberlite at 5^23 GPa: implications for the origin of kimberlitic melts, Am. Mineral. (2000), submitted. [35] A.V. Girnis, G.P. Brey, I.D. Ryabchikov, Origin of group IA kimberlite: £uid-saturated melting experiments at 45^ 55 kbar, Earth Planet. Sci. Lett. 134 (1995) 283^296. [36] J. Zhang, C. Herzberg, Melting experiments on an anhydrous peridotite KLB-1 from 5.0 to 22.5 GPa, J. Geophys. Res. 99 (1994) 17729^17742.
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