ISSN 0016-7029, Geochemistry International, 2009, Vol. 47, No. 9, pp. 857–881. © Pleiades Publishing, Ltd., 2009. Original Russian Text © V.A. Glebovitsky, L.P. Nikitina, A.B. Vrevskii, Yu.D. Pushkarev, M.S. Babushkina, A.G. Goncharov, 2009, published in Geokhimiya, 2009, No. 9, pp. 910– 936.
Nature of the Chemical Heterogeneity of the Continental Lithospheric Mantle V. A. Glebovitsky, L. P. Nikitina, A. B. Vrevskii, Yu. D. Pushkarev, M. S. Babushkina, and A. G. Goncharov Institute of Precambrian Geology and Geochronology, Russian Academy of Sciences, nab. Makarova 2, St. Petersburg, 199034 Russia; e-mail:
[email protected] Received in final form July 18, 2008
Abstract—The paper reports data on the chemical composition of mantle peridotite xenoliths from kimberlites and alkaline basalts that represent the continental lithospheric mantle (CLM) beneath Early Precambrian and Late Proterozoic–Cenozoic structures, respectively. In order to identify compositional trends during the melting of primitive material and propose the most reliable criteria for constraining the conditions of this process and its degree, we analyzed literature data on the melting of spinel and garnet peridotites within broad temperature and pressure ranges. It was determined that the degree of melting (F%) of pristine peridotite of composition close to that of the primitive mantle (PM) can be deduced from the Mg/Si and Al/Si ratios in the residue; an equation was proposed for evaluating F from the Mg/Si ratio. The Ca/Al ratio of residues at low (1–1.5 GPa) pressures and degrees of melting from 2–3 to 20–25% increases several times but decreases with increasing F at pressures higher than 3 GPa. The Na partition coefficient between melt and residue decreases at increasing pressure and approaches one at a pressure close to 20 GPa. Residues after low-degree melting are strongly depleted in Ti, Zr, Y, and Nb but are enriched in Cr. The application of these criteria to the composition of xenoliths brought to the surface from the mantle occurring beneath tectonic structures of various age led us to conclude that compositional heterogeneities of CLM (particularly the variations in the concentrations of major and certain siderophile elements) are controlled, first of all, by the melting of the mantle source material. These processes occurred under various thermodynamic conditions (T, P, and f O2 ) and differed in their intensity, and this predetermined the compositional diversity of the residual mantle material (its concentrations of Mg, Al, Si, Ca, Na, K, Ni, Co, V, and Cr). Our results are principally consistent with the hypothesis of the global magmatic ocean. It is thought that the early phases of its consolidation were variably controlled by the fractionation of minerals, for example, majorite. Moreover, heterogeneities in the distribution of siderophile elements could be partly predetermined by changes in the properties of these elements at ultrahigh temperatures and pressures. The processes of partial melting were the most intense during the early evolution of the mantle (perhaps, in the Early Precambrian), and hence, the mantle has different chemical composition beneath Archean cratons and Phanerozoic foldbelts. DOI: 10.1134/S001670290909002X
INTRODUCTION Compositional differences in the continental lithospheric mantle (CLM) beneath Archean, Proterozoic, and Phanerozoic structures (differences in the concentrations of major oxides, such as CaO, Al2O3, MgO, and FeO and the MgO/SiO2, CaO/SiO2, and Al2O3/SiO2 ratios) are documented and discussed in many reviews [1–7]. Nevertheless, the reasons for the chemical heterogeneity of the mantle remain largely uncertain as of yet, and this is one of the key problems in modern Earth sciences [8]. The pivoting aspects of this problem are as follows. (1) When did the chemical differentiation of the mantle occur in the geological history and how intense was this process? (2) How much was the Archean mantle different from the modern one?
(3) How many characteristics of the modern mantle were predetermined by early planetary differentiation? (4) Did the degree of the chemical differentiation of the mantle increase with time? Theoretically, the following processes could bring about the chemical heterogeneity of mantle material: (1) heterogeneous accretion, which controlled heterogeneities at the scale of geospheres; (2) crystallization differentiation of the magmatic ocean (this process produced cumulates); (3) partial melting of the primitive mantle (PM), as well as any other varieties of mantle material; (4) refertilization; (5) mantle metasomatism; (6) subduction of crustal material and the interaction of molten crustal rocks with mantle material; and
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Table 1. Chemical and mineral composition of lherzolites [12, 13] Sample
Oxide, wt % SiO2
Al2O3
MgO
Elemental ratio CaO
FeOtot
INTA INTB
46.19 45.65
2.58 2.60
38.43 34.64
4.00 8.87
6.85 6.06
FERB FERC FERD FERE
44.97 46.03 41.95 45.40
4.55 3.85 7.48 4.26
33.77 40.32 38.06 36.52
7.88 0.63 2.25 4.82
6.55 7.81 7.60 7.08
Mg/Si
1.0 GPa [13] 1.07 0.98 1.0 GPa [12] 0.97 1.30 1.17 1.10
(7) supply of elements expelled from the core to the mantle. These processes could overlap and affect one another, and this can give rise to polygenic mantle heterogeneities. Our research was centered on the evaluation of the contributions made by the partial melting and crystallization differentiation of the hypothetical magmatic ocean to the development of the chemical heterogeneity of CLM. Traces of the crystallization differentiation of the magmatic ocean were identified on the Moon and, strictly speaking, there are no reasons to rule out this process for the Earth, although data on zircons dated at 4.4 Ga from Australian quartzites [9] seem to be at variance with this hypothesis. Information on mantle melting can be derived from mantle xenoliths and volcanic rocks (komatiites, MORB, OIB, IAB, and basalt from continental magmatic provinces). This paper reports results of our analysis of the chemical composition of peridotite xenoliths from kimberlites and alkaline basalts that represent, respectively, the upper mantle beneath Early Precambrian and Late Proterozoic–Cenozoic structures. We also analyze published experimental data on the melting of spinel and garnet peridotites (having a composition close to that of the primitive mantle) within a broad P–T range, which is necessary to identify principal compositional trends of the primitive material and propose criteria for evaluating the conditions under which the residues were formed. The application of these criteria to the composition of the upper mantle under tectonic structures of various age makes it possible to quantify the role of melting processes in the development of the compositional heterogeneity of CLM after the segregation of the Earth’s core and its silicate shell. This also enabled us to elucidate whether the character of this heterogeneity varied with time. A very informative and fruitful approach to clarifying the nature of the mantle heterogeneity is also the comparison of concentrations of certain siderophile elements in xenoliths from the upper mantle underlying Archean cratons and in volcanic rocks derived from mantle sources.
Mineral composition
Al/Si
Ca/Al
Ol
Cpx
Opx
Sp
0.06 0.06
2.10 4.61
0.50 0.50
0.17 0.40
0.30 0.07
0.03 0.03
0.12 0.11 0.20 0.11
2.41 0.22 0.41 1.53
0.50 0.50 0.50 0.50
0.40 0.01 0.10 0.24
0.07 0.46 0.30 0.24
0.03 0.03 0.10 0.03
ANALYSIS OF EXPERIMENTAL DATA ON THE MELTING OF SPINEL AND GARNET PERIDOTITES This section presents a review of experimental data [10–14] on the composition of melts and residues produced at various degrees of melting (F%) of spinel and garnet peridotites within a broad range of P–T conditions. Melting of Spinel Peridotites at 1.0–1.5 GPa The chemical and mineral compositions of lherzolites that were melted at 1270–1390°C and 1.0 GPa [12, 13] are summarized in Table 1. Among them, samples INTA and FERE most closely approximate the composition of the primitive mantle. 1 The other samples differed from them in having an elevated concentration of Al2O3 (FERD) or a lower concentration of CaO (FERC). The table also reports the Al/Si, Mg/Si, and Ca/Al ratios of the rocks. The melting temperatures, percentage of melt (F), its composition, and the composition of the residue (we calculated it from mass balance considerations) are reported in Table 2. The changes in the Al/Si, Mg/Si, and Ca/Al ratios in the residues with increasing temperature and degree of melting are shown in Fig. 1. In the Al/Si–Mg/Si diagram, the data points of the residues for each of the pristine rocks define individual trends that can be approximated by polynomials of various degrees. As the degree of melting increases, the Al/Si ratio of the residues decreases and the Mg/Si ratio increases. The character of the trends and their arrangement in the diagram depend on the CaO and Al2O3 concentrations and, hence, on the Ca/Al ratio and on the mineralogy of the original rock. As the Ca/Al ratio decreases (from 4.6 to 0.4) and the Mg/Si ratio increases in the original composition, the com1 Here
we use the authors' [12, 13] numbers of samples of spinel peridotites (INTA, INTB, FERB, FERC, FERD, and FERE) that were melted.
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Table 2. Temperatures, percentage of melt F, and ranges of the major-element concentrations (wt %) in the melts and residues after the melting of peridotites Parameter
INTA
Tmin÷Tmax Fmin÷Fmax
1285÷1390 2.9÷21.3
(SiO2)min÷(SiO2)max (Al2O3)max÷(Al2O3)min (MgO)min÷(MgO)max (CaO)max÷(CaO)min
50.30÷51.29 17.42÷10.79 11.51÷16.22 12.4÷12.9
(SiO2)max÷(SiO2)min (Al2O3)max÷( Al2O3)min (MgO)min÷(MgO)max (CaO)max÷(CaO)min
46.07÷44.82 2.14÷0.36 39.23÷44.45 3.75÷1.59
INTB
FERB
FERD
1290÷1390 1270÷1390 1270÷1390 2.2÷19.5 5÷47.9 3.7÷30.1 Melt [12, 13] 52.27÷50.11 53.13÷49.5 50.43÷48.69 16.65÷9.74 19.79÷9.91 19.19÷17.22 10.48÷15.93 7.81÷15.25 9.57÷17.50 11.69÷15.46 8.69÷15.49 10.87÷6.89 Residue (mass balance calculation) 45.65÷44.57 44.54÷40.8 41.62÷39.05 2.28÷0.78 3.75÷0.0 7.03÷3.29 34.67÷39.17 35.13÷50.8 39.2÷47.69 8.87÷7.28 7.84÷0.88 1.92÷0.25
FERC
FERE
1270÷1390 2.3÷11.7
1300÷1390 10.3÷35.9
54.69÷48.23 19.15÷16.46 7.93÷19.16 8.59÷3.97
50.88÷50.66 19.09÷11.57 8.52÷15.84 9.8÷12.17
45.83÷45.74 3.49÷2.18 41.08÷43.12 0.44÷0.19
50.66÷50.09 2.72÷0.17 39.47÷48.1 4.02÷0.7
Note: The min and max subscript indices correspond to the minimum and maximum degree of melting, respectively
positional trends of the residues shift toward higher Mg/Si ratios. The Al/Si–Ca/Al diagram (Fig. 1b) shows an important feature of the melting products of lherzolites. The composition of the residues strongly depends on the original composition of the samples. The data points of the residues define individual trends in the diagram, and these trends shift toward higher Ca/Al ratios (at the same Al/Si ratios) in response to an increase in this ratio in the original sample. Melting of Peridotites under High Pressures Experiments on the melting of garnet peridotite WKR (of composition close to the composition of PM) at pressures of 3.0–7.0 GPa and temperatures of 1515–1950°C were carried out by Walter [11], and experiments on the melting of peridotite KLB-1 (pyrolite) at pressures of 22.0– 24.5 GPa and temperatures of 2100–2400°C were conducted by Tronnes and Frost [14]. Garnet peridotite WKR has the following composition (wt %): SiO2 = 44.9, Al2O3 = 4.26, MgO = 37.3, CaO = 3.45, Na2O = 0.22, K2O = 0.09, and TiO2 = 0.16; Al/Si = 0.107, Mg/Si = 1.07, Ca/Al = 1.09, and Na/K = 2.19. The changes in the Al/Si, Mg/Si, and Ca/Al ratios in the melt and residue depending on the degree of melting are shown in Fig. 2. The residues after the melting of garnet peridotite WKR at pressures of 3–7 GPa and temperatures of 1515–1800°C have Al/Si ratios close to those of residues after the melting of spinel peridotites FERB and FERC at lower temperatures (1270–1390°C) and pressure (1.0 GPa) but have slightly higher Mg/Si ratios (Fig. 2a). The Ca/Al ratio of the residues decreases with increasing temperature and degree of melting (Fig. 2b). The dependence of the Mg/Si ratio of the residues on the degree of melting (9÷86%) of garnet peridotite WKR at GEOCHEMISTRY INTERNATIONAL
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temperatures of 1515–1950°C and pressures of 3–7 GPa led us to derive the following equation for calculating the degree of melting F (%) on the Mg/Si ratio: F ( ± 3 ) = ( –244.4 ± 97.3 ) + ( 305.9 ± 144.3 )Mg/Si (1) 2 + ( –71.0 ± 52.6 ) ( Mg/Si ) . The melting of fertile peridotite of the composition KLB-1 and pyrolite at ultrahigh temperatures and pressures (22.0–24.5 GPa and 2100–2400°ë) generates residue of composition close to that of the original peridotite (Table 3, Fig. 2a). Melts produced by the melting of peridotites under a low pressure (
