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Experimental determination of rare earth fractionation patterns in partial melts from peridotite in the upper mantle
Earth and Planetary Science Letters, 34 (1977) 231-237 © Elsevier Scientific Publishing Company, Amsterdam - Printed in The Netherlands
231
[Sl
EXPERIMENTAL DETERMINATION OF RARE EARTH FRACTIONATION PATTERNS IN PARTIAL MELTS FROM PERIDOTITE IN THE UPPER MANTLE B J O R N O. MYSEN
Geophysical Laboratory, Carnegie Institution of Washington, Washington, D.C. 20008 (USA) and J O H N R. H O L L O W A Y
Chemistry Department, Arizona State University, Tempe, Ariz. 85281 (USA) Received October 28, 1976 Revised version received December 20, 1976
Rare earth element patterns of partial melts of a natural peridotite with a chondrite-like rare earth pattern as starting material have been experimentally determined at high pressure. The experimental data are combined with major element data on the same partial melts to test the applicability of models of volatile-free melting of peridotite to rock-forming processes in the upper mantle. Alkali basalts could not have been formed by partial melting of peridotite in the absence of volatiles because the major and trace element abundances produced experimentally are not compatible with those of natural alkali basalts. Abyssal tholeiite without light rare earth element depletion can be derived from volatile-free peridotite by direct partial melting. Suites of abyssal tholeiite and picritic basalt may have been formed by a two-stage melting process from a common peridotite source. The peridotite residue must have lost clinopyroxene and garnet at the time of formation of picritic melt. Abyssal tholeiites with depletion of light rare earth elements could have been derived only from a peridotite source that has been depleted of the same elements prior to the melting event yielding the tholeiite.
1. I n t r o d u c t i o n Models o f basalt genesis in a peridotitic upper mantle are c o m m o n l y based on trace e l e m e n t abundances in the basalts (e.g. [ 1 - 3 ] ) . Mysen and Kushiro [4] have discussed the melting behavior o f volatilefree peridotite mantle in the light o f phase equilibria in m o d e l and natural peridotite systems. T h e y found that the major element c o m p o s i t i o n s o f partial melts are controlled by the residual mineral assemblages and that melt compositions change little over extensive melting intervals provided the coexisting mineral assemblage does not change. Trace element abundances, however, such as those o f the rare earth elements ( R E E ) , may be sensitive to the degree o f melting even within the individual melting intervals. F u r t h e r m o r e ,
the applicability o f major element data f r o m melting experiments to the genesis o f basalts would rely heavily on the extent to which e x p e r i m e n t a l l y derived trace element patterns are compatible with those o f natural rocks. C o n s e q u e n t l y , an a t t e m p t has been made t 9 c o m b i n e experimental determinat!ons o f major and trace element data with partial melting o'f-upper mantle peridotite.
2. E x p e r i m e n t a l m e t h o d s Starting material was a sheared garnet p e r i d o t i t e nodule (1611) from a Lesotho kimberlite (donated by Dr. F.R. B o y d ) previously described by N i x o n and Boyd [5] and Shimizu [6]. This nodule was
.
232 chosen because it is relatively undepleted o f major elements [5] and because of its chondrite-like REE pattern [6]. Nodule 1611 has also been used to evaluate the melting behavior of volatile-free peridotite in the upper mantle by Mysen and Kushiro [4]. They found that at 20 kbar the mineral assemblage olivine + orthopyroxene + clinopyroxene + spinel coexists with alkali basalt melt from the solidus to about 2% melt: ing and with olivine tholeiite melt from about 2 to 25% melting. The assemblage olivine + orthopyroxene coexists with picritic melt between 25 and 45% melting, and olivine coexists with peridotitic komatiite melt with more than 45% melting. Their melting curve [4] has been reproduced in Fig. 1. Four points on the melting curve.were selected for this study to obtain REE patterns for each o f these four melt types. The points are at 1.3, 15, 35, and 50% melting. The starting material was crushed to less than 5 /am and fired at 1150°C a t f o 2 = 10 -9 atm overnight prior to addition of the radioactive spikes needed for determination of the REE contents o f the partial melts. This heat treatment did not alter the original mineral-
ogy of the sample [4]. Cerium (3.11 ppm spike) was chosen as representative of a light REE, samarium (0.585 ppm spike) as an intermediate REE, and thulium (0.0173 ppm spike) as a heavy REE. The REE contents of the melts were determined with beta-track counting [7] using 141Ce (energy = 475 keV, t]/2 = 32.5 days), 151Sm (energy = 76 keV, t]/2 = 93 years), and 17]Tm (energy = 97 keV, tl/2 = 1.9 years) as sources of beta particles. The REE pattern of nodule 1611 is chondrite-like and near unity [6]. Consequently, it was necessary to determine only the content of radioactive REE in the partial melts. This content divided by the total amount of spike added to the starting material results in an enrichment factor that equals the chondrite-normalized REE!pattern that could be derived from partial melts of this Nodule in the absence of any additional spike. The experiments were carried out in solid-media, high-pressure apparatus [8] using sealed Pt9sAu 5 sample containers. All aspects of the technique were identical with that used by Mysen and Kushiro [4] to ensure duplication of the degree of melting determined by them. Run durations were from 4 hours at 1450°C to 1 hour at 1650°C.
1700
3. Results •' ~ Y J 50%
/7
(.3 1 6 0 0 o D
1500 CL
E
t~kali olivine basalt
I.-1400
olivine tholeiite
XA
A
likoli olivine bosolt 130C
0
' 10
2'0
t 30
L
0
* 50
6~0
7'0
80
Per cent melt Fig. 1, Melting curves of two natural peridotites at 20 kbar under volatile-absent conditions [4 ]. A, sample 66SAL-1 ; B, sample 1611 (see [4]). Percentages on curve B denote conditions selected for the present experiments. Rock names are based on normative compositions of partial melts [4 ]. X A and X B, see text.
Analytical results are given in Table 1. The uncertainty is + 1 o, calculated according to the method of Mysen and Seitz [7]. The data are plotted for Fig. 2 and compared with REE patterns from various alkali basalts [3,9 ], abyssal tholeiites [2,10], and picrites [2,11 ]. The rock names associated with the experimental partial melts are based on normative compositions of the melts [4]. The experimental REE patterns from partial melts of 1.3 and 15% melting are different in bulk REE enrichment and also in geometry, despite their similar phase assemblages. The differences in bulk REE enrichment are mostly a function of degree of melting. The melt from 15% melting is also more depleted of the heavy REE than that from 1.3% melting (Fig. 3). If there were no relative fractionation of the REE as the degree of melting was increased from 1.3 to 15%, the enrichment factors o f Ce, Sm, and Tm in the melt from 15% melting would be about 9% of those of the 1.3% melt. The observed values are 9, 8, and 6%, res-
233 TABLE 1 Experimental results (values in ppm)
141Ce 151Sm 171Tm
1.3% melt
15% melt
35% melt
50% melt
264 +-6 47 -* 1 1.30 -+0.04
23.3 -+0.6 3.5 +-0.1 0.068 +-0.003
7.9 -+0.5 1.29 +-0.06 0.038 ± 0.0002
5.0 +-0.3 0.907 -+0.004 0.0251 -*0.0009
Coexisting mineral assemblages 1.3% melt: olivine + spinel + orthopyroxene + pigeonitic clinopyroxene; 15% melt: olivine + spinel + orthopyroxene + pigeonitic clinopyroxene; 35% melt: olivine + orthopyroxene; 50% melt: olivine. pectively, for Ce, Sm, and Tm. This depletion of the heavy REE may be caused by the reaction relation between partial melt and crystalline residue described
100 80 -<~
,0
~1.3%._
60 ~ o e
by Mysen and Kushiro [4]. The difference between the pattern of experimental olivine tholeiite melt (15% melting) and that of picrite (35% melting) is due to the absence of clinopyroxene in the residue of the latter inasmuch as clinopyroxene is somewhat enriched in light REE relative to heavy REE. Orthopyroxene and olivine will affect the relative variations between light and heavy REE in the partial melts only to a small degree because the partition coefficients, x-liq a n d / ~ El-li /~RP~E ~ , are quite small [12,13]. The larger total enrichment of REE in picrite melt compared with peridotitic komatiite melt is, therefore, simply the result of the larger proportion of crystals coexisting with picritic than with peridotitic komatiite melt.
~.8 1.7 1.6
// 1.5
0.?3:/0 "-+
_~j~per!dotitic komatiite0_59_Olo
-g_ o i
1.4
1.2
La Ce Pr Nd PmSmEu Gd TL) Dy Ho Er Tm Yb Lu Fig. 2. Experimentally determined REE fractionation patterns of experimentally produced partial melts. Alkali basalt (A), Hawaiian alkali basalt [9]. Alkali basalt (B), range of REE contents of alkali basalts from the Lesser Antilles [3]. Tholeiite (A), oceanic tholeiite from the Nazca plate [11]. Tholeiite (B), abyssal tholeiite, Baffin Bay [2]. Picrite (A), picrite basalt, Azores [10]. Picrite (B), picrite, Baff'm Bay [2].
Ce/Tm f.I I.(
Ce/Sm I ()
210 Per cent
310 melting
~
Fig. 3. Experimentally determined enrichment factors of light REE relative to heavy REE as a function of degree of melting.
5o
234 4. Discussion Alkali basalts are generally considered to be the results of only several percent melting owing to their enrichment in large ion lithophile elements and light REE. The REE data for 1.3% melting of volatile-free peridotite would, therefore, approximate those for alkali basalt, provided the phase equilibria [4] have application to the formation of natural alkali basalts and the REE contents of nodule 1611 are representative of the source rocks of such basalts. The ratios of the enrichment factors, Ce/Tm and Ce/Sm, respectively, are 1.3 and 1.1, at 1.3% melting (Fig. 3). In contrast, natural alkali basalts have chondrite-normalized Ce/Tm and Ce/Sm in the ranges 2 - 1 0 and 1.5-3.0, respectively [3,9,10]. Thus, even though the major element composition of experimentally produced partial melt at 1.3% melting of spinel peridotite resembles that of alkali basalt, the experimental REE data indicate that natural alkali basalts could not have been formed this way. Peridotite, generally considered to represent the composition of the source rock of basalt in the upper mantle, shows little enrichment in light REE [7,14,15]. A garnet peridotite source with a flat REE pattern may yield the appropriate REE pattern of a partial melt [1,3]. Mysen and Kushiro [4] showed, however, that the partial melt in equilibrium with garnet peridotite is not alkali basalt, but alkali picrite. It thus seems that alkali basalt could not have been formed by direct partial melting of any kind of volatile-free peridotite upper mantle source. An alternative explanation of their origin is melting of peridotite in the presence of CO2-rich volatiles [ 16, 17]. Possil~le effects on trace element contents of partial melts formed under such conditions have been discussed elsewhere [18]. Experimental data on vaporcrystal partitioning, however, are not yet available. A detailed discussion is, therefore, not warranted. Abyssal tholeiites have REE fractionation patterns with both light-REE enrichment and light-REE depletion relative to chondrite [2,9-11 ]. Basalt REE patterns from Baffin Bay tholeiite [2] show slight enrichment and are compatible with a model of magma formation by partial melting of volatile-free peridotite that initially had a flat REE pattern. Further melting of a source from which such tholeiite had been extracted would result in picrite (coexisting with harzburgite residue) with a REE pattern showing depletion of
light REE (Fig. 2). The chondrite-normalized REE pattern of picrite from Baffin Bay I2] shows such a depletion, indicating that the tholeiite-picrite association described by O'Nions and Clark [2] may have been formed by two-stage melting or successive tapping of a mantle-melt reservoir where the degree of melting had progressed to the stage of picrite partial melt. The REE patterns of tholeiites from the Nazca plate [11 ] indicate that the source of these basalts was depleted of light REE prior to the melting event leading to the formation of these tholeiites. Similar depletion on a regional scale of Icelandic basalts led to the suggestion by Schilling and Bonatti [ 11 ] and Schilling [19] of mantle plumes with varying trace element abundances. Evidence for heterogeneous distribution of REE in the mantle is also seen in the variation of REE patterns of peridotite nodules from kimberlite [6]. The question as to why there are regional differences of trace element concentrations in the upper mantle has not been resolved. One explanation is metasomatism through the action of a vapor or fluid phase. Such a hypothesis is supported by the comparison of REE data on sheared and granular nodules from kimberlite studied by Shimizu [6]. The granular nodules are generally depleted of major elements, whereas the sheared nodules are more fertile. Nevertheless, Shimizu [6] found that the granular nodules show an enrichment of light REE relative to heavy REE that is more pronounced than in the sheared nodules. In fact, many of the granular nodules contain less heavy REE than chondrite, whereast the chondrite-normalized light REE are higher than unity [6]. Even though their major element contents show strong evidence that these nodules are residual after a melting event, it was emphasized by Shimizu [6] that their REE patterns cannot be reconciled with such a process. Metasomatism in the continental upper mantle may, therefore, be important in controlling element composition. Because of the low concentrations of the trace elements, the effects of metasomatism on trace elements may be more readily detected than in the major element distribution. Eggler [20] and Newton and Sharp [21] have argued that volatiles are more likely to be bound in minerals than in a vapor phase in the continental upper mantle compared with the oceanic mantle. If the interpretation of the trace and major element
235 composition of the continental upper mantle is correct, metasomatism must have occurred, and the transporting medium is most likely a vapor phase. But if there is a vapor phase in the continental upper mantle, the data of Eggler [20] and Newton and Sharp [21 ] would indicate an even stronger possibility of the presence of a vapor in the oceanic upper mantle. "Provided this vapor consists of two or more components, so that the solubility of trace elements may be varied as a function of the vapor composition, it may be possible to vary the trace element content and the relative enrichment of the rare earth elements in the upper mantle on a regional scale. Unfortunately, there is a complete lack of applicable data on partition coefficients between multicomponent vapors and condensed silicates (liquid and crystals). Thus, a quantitative evaluation of these effects must await further experimental data. Partial melting will also result in fractionation of trace elements. Such melting must, however, leave a residue of a bulk composition that can still be melted to yield a tholeiitic melt composition. Partial melting of the upper mantle may have occurred with or without volatiles. Aside from the effects on melt composition of melting in the presence of volatile components such as CO2 and H20 [16], melting of peridotite + H20 + CO 2 is at least divariant [22]. Melting in the absence of volatiles is nearly isobarically invariant [4]. Basalt can be formed in a peridotite mantle by melting in a region where the heat flux into the system is sufficient to transform a portion of the rock to a melt, and melting will continue until the heat transported out of the system exceeds the heat input necessary to sustain the melting reaction. Melting of the upper mantle under nearly invariant conditions will continue until, for example, one or more crystalline phases are exhausted and the melting temperature of the residue begins to increase. It is possible that at this point (e.g. point X A or X B in Fig. 1) the heat flux into the system will be sufficient to support further melting because heat is now also required to raise the temperature of the system. According to Yoder [23] this may be one reason why tholeiitic basalts are common, whereas picrites are rare. It was visualized [23] that, during melting at an invariant point, the trace elements could be fractionated by melt separating from the source before the melting reaction reached completion (point XA or XB in Fig. I). The first
magma will be enriched in light REE, according to the data in Fig. 2; it will probably also contain most of the LIL trace elements in the source region. The peridotite residue will become depleted in the same elements. As melting proceeds, more magma of similar major element composition will be produced and separated. This magma, however, will be depleted of light REE and LIL elements. If this is an accurate description of the melting process producing oceanic basalt and perhaps the large accumulations of plateau basalt, these particular rocks would show a record of the evolution of the trace elements in the partial melts [23]. To date such data have not been obtained. Trace element fractionation of the upper mantle by the above mechanism will be of temporary nature and will probably be quite localized geographically. Yoder [23] using the heat flow data of Lee [24] and elt an enthalpy of melting, AHnggarnetperidotite equal to 164 cal/g (see Yoder [23] for discussion of value of z~ga elt rnet peridotite ) calculated that it will take 82 m.y. to generate 5% melt from garnet peridotite. The enthalpy of melting will vary some with pressure (AVmelt is pressure-dependent) and the heat input to different segments of the mantle may differ from place to place. However, even allowing for such variations, the time period where a peridotite source could yield tholeiite melt (20-30% melting) is only several hundred million years. Thus, any source region of tholeiitic basalt could remain heterogeneous only with respect to LIL elements and at the same time have a major element composition that could still result in tholeiitic partial melt within such a period of time. Because of the constraints on the mantle geochemistry provided by this model, it could be tested and evaluated by studying the trace element evolution of basalt eruption cycles and the composition of peridotite nodules brought to the earth's surface in kimberlites and basalts themselves. Even though no systematic study of this type is available, the present evidence does not point in that direction [6,15,25]. The lack of systematic relations between trace and major element compositions was used above to argue for the presence of vapor in the upper mantle. Additional evidence for the presence of such a component has been summarized by Irving and Wyllie [26], Boettcher et al. [27], and Anderson [28]. Because of the multivariant nature of melting of rocks in the presence of a separate vapor phase with volatiles such
236 as CO2 and H20, it also seems more likely that such melting could account for at least some o f the trace element heterogeneity found in the upper mantle. Because o f the multivariancy o f the melting, the melting temperature o f the rock will increase during melting [22]. Thus, the heat flux into the system must be sufficient both to increase the temperature and to melt the rock. Because o f this heat requirement, such a melting reaction is more likely to halt before the peridotite source is exhausted in its basalt components than invariant melting where the temperature does not increase during melting. After a melt fraction thus derived is separated from the system, the residual peridotite is depleted o f its LIL trace elements, but could still be melted to generate more tholeiite. Neither o f the two melting models summarized above can explain the peculiar contrast between major and trace element composition of some of the granular peridotite nodules found in kimberlite [6]. Even though present-day oceanic volcanism may in large part be explained by simple partial melting of volatile-free peridotite upper mantle, the trace element patterns found in these rocks do require trace element fractionation of the source region. Some aspects o f this fractionation cannot be explained without resorting to the presence of volatiles. Partial melting cannot explain all the features, and it is proposed that metasomatism played an important role in distributing trace elements in the upper mantle.
4
5
6
7
8
9
10 11
12
13
14 Acknowledgements The authors are grateful to Drs. T.N. Arndt, T. Benjamin, D.H. Eggler, and H.S. Yoder, Jr., for critical reviews. This research was partially supported by National Science Foundation grant DES 73-00266A01.
References 1 P.W. Gast, Trace element fractionation and the origin of tholeiitic and alkaline magma types, Geochim. Cosmochim. Acta 32 (1968) 1057. 2 R.K. O'Nions and D.B. Clark, Comparative trace element geochemistry of Tertiary basalts from Baffin Bay, Earth Planet. Sci. Lett. 15 (1972)436. 3 N. Shimizu and R.J. Arculus, Rare earth element concentrations in a suite of basanitoids and alkali olivine basalts
from Grenada, Lesser Antilles, Contrib. Mineral. Petrol. 50 (1975) 231. B.O. Mysen and I. Kushiro, Compositional variations of coexisting phases with degree of melting of peridotite in the upper mantle, Am. Mineral. (1976) in press. P.H. Nixon and F.R. Boyd, Petrogenesis of the granular and sheared ultrabasic nodule suite in kimberlites, in: Lesotho Kimberlites, P.H. Nixon, ed. (Cape and Transvaal Printers Ltd., Cape Town, 1973) p. 48. N. Shimizu, Rare earth elements (REE) in garnets and clinopyroxenes from garnet lherzolite nodules in kimberlites, Carnegie Inst. Washington Yearb. 73 (1974) 954. B.O. Mysen and M.G. Seitz, Trace element partitioning determined by beta-track mapping - an experimental study using carbon and samarium as examples, J. Geophys. Res. 80 (1975) 2627. F.R. Boyd and J.L. England, Apparatus for phase equilibrium measurements at pressures up to 50 kilobars and temperatures up to 1750°C, J. Geophys. Res. 65 (1960) 741. J.-G. Schilling and J.W. Winchester, Rare earth contribution to the origin of Hawaiian lavas, Contrib. Mineral. Petrol. 23 (1969) 27. J.-G. Schilling, Azores mantle blob: rare earth evidence, Earth Planet. Sci. Lett. 25 (1975) 103. J.-G. Schilling and E. Bonatti, East Pacific Ridge (2° S 19° S) versus Nazca plate volcanism: rare earth evidence, Earth Planet. Sci. Lett. 25 (1975) 93. B.O. Mysen, Partitioning of samarium and nickel between olivine, orthopyroxene and liquid : preliminary data at 20 kbar and 1025°C, Earth Planet. Sci. Lett. 31 (1976) 1. C.C. Schnetzler and J.A. Philpotts, Partition coefficients of rare earth elements between igneous matrix material and rock forming mineral phenocrysts. Geochim. Cosmochim. Acta 34 (1970) 331. J.B. Reid and F.A. Frey, Rare earth distributions in lherzolite and garnet pyroxenite xenoliths and the constitution of the upper mantle, J. Geophys. Res. 76 (1971) 1184.
15 F.A. Frey, L.A. Haskin and M.A. Haskin, Rare earth abundances in some ultramafic rocks, J. Geophys. Res. 76 (1971) 2057. 16 B.O. Mysen and A.L. Boettcher, Melting in a hydrous mantle, II. Geochemistry of crystals and liquids formed by anatexis of mantle peridotite at high pressures and high temperatures as a function of controlled activities of water, hydrogen and carbon dioxide, J. Petrol. 16 (1975) 549. 17 B.O. Mysen, R.J. Arculus and D.H. Eggler, Solubility of carbon dioxide in natural nephelinite, tholeiite and andesite melts to 30 kbar pressure, Contrib. Mineral. Petrol. 53 (1975) 227. 18 B.O. Mysen, Melting in a hydrous mantle: phase relations of mantle peridotite with controlled water and oxygen fugacities, Carnegie Inst. Washington Yearb. 72 (1973) 467. 19 J.-G. Schilling, Iceland mantle plume, Nature 246 (1973) 141.
237 20 D.H. Eggler, Does CO 2 cause partial melting in the lowvelocity layer of the mantle?, Geology 2 (1976) 69. 21 R.C. Newton and W.E. Sharp, Stability of forsterite + CO 2 and its bearing on the role of CO 2 in the mantle, Earth Planet. Sci. Lett. 26 (1975) 239. 22 D.H. Eggler, CO 2 as a volatile component of the mantle: the system Mg2SiO4-SiO2-H20-CO2, Phys. Chem. Earth 9 (1975) 869. 23 H.S. Yoder, Jr., Generation of Basaltic Magma (National Academy of Sciences, Washington, D.C., 1976) 264 pp. 24 W.H.K. Lee, Thermal history of the Earth, Ph.D. Thesis, Univ. of California, Los Angeles, Calif. (1967) 344 pp. 25 J.A. Philpotts, C.C. Schnetzler and A.A. Thomas, Petro-
genetic implications of some new geochemical data on eclogitic and ultrabasic inclusions, Geochim. Cosmochim. Acta 36 (1972) 1131. 26 A.J. Irving and P.J. Wyllie, Subsolidus and melting relationships for calcite, magnesite on the join CaCO 3 MgCO 3 to 36 kbar, Geochim. Cosmochim. Acta 39 (1975) 35. 27 A.L. Boettcher, B.O. Mysen and P.J. Modreski, Phase relationships in natural and synthetic peridotite-H2 0 and peridotite-H20-CO2 systems at high pressures, Phys. Chem. Earth 9 (1975) 855. 28 A.T. Anderson, Some basaltic and andesitic gases, Rev. Geophys. 13 (1975) 37.
231
[Sl
EXPERIMENTAL DETERMINATION OF RARE EARTH FRACTIONATION PATTERNS IN PARTIAL MELTS FROM PERIDOTITE IN THE UPPER MANTLE B J O R N O. MYSEN
Geophysical Laboratory, Carnegie Institution of Washington, Washington, D.C. 20008 (USA) and J O H N R. H O L L O W A Y
Chemistry Department, Arizona State University, Tempe, Ariz. 85281 (USA) Received October 28, 1976 Revised version received December 20, 1976
Rare earth element patterns of partial melts of a natural peridotite with a chondrite-like rare earth pattern as starting material have been experimentally determined at high pressure. The experimental data are combined with major element data on the same partial melts to test the applicability of models of volatile-free melting of peridotite to rock-forming processes in the upper mantle. Alkali basalts could not have been formed by partial melting of peridotite in the absence of volatiles because the major and trace element abundances produced experimentally are not compatible with those of natural alkali basalts. Abyssal tholeiite without light rare earth element depletion can be derived from volatile-free peridotite by direct partial melting. Suites of abyssal tholeiite and picritic basalt may have been formed by a two-stage melting process from a common peridotite source. The peridotite residue must have lost clinopyroxene and garnet at the time of formation of picritic melt. Abyssal tholeiites with depletion of light rare earth elements could have been derived only from a peridotite source that has been depleted of the same elements prior to the melting event yielding the tholeiite.
1. I n t r o d u c t i o n Models o f basalt genesis in a peridotitic upper mantle are c o m m o n l y based on trace e l e m e n t abundances in the basalts (e.g. [ 1 - 3 ] ) . Mysen and Kushiro [4] have discussed the melting behavior o f volatilefree peridotite mantle in the light o f phase equilibria in m o d e l and natural peridotite systems. T h e y found that the major element c o m p o s i t i o n s o f partial melts are controlled by the residual mineral assemblages and that melt compositions change little over extensive melting intervals provided the coexisting mineral assemblage does not change. Trace element abundances, however, such as those o f the rare earth elements ( R E E ) , may be sensitive to the degree o f melting even within the individual melting intervals. F u r t h e r m o r e ,
the applicability o f major element data f r o m melting experiments to the genesis o f basalts would rely heavily on the extent to which e x p e r i m e n t a l l y derived trace element patterns are compatible with those o f natural rocks. C o n s e q u e n t l y , an a t t e m p t has been made t 9 c o m b i n e experimental determinat!ons o f major and trace element data with partial melting o'f-upper mantle peridotite.
2. E x p e r i m e n t a l m e t h o d s Starting material was a sheared garnet p e r i d o t i t e nodule (1611) from a Lesotho kimberlite (donated by Dr. F.R. B o y d ) previously described by N i x o n and Boyd [5] and Shimizu [6]. This nodule was
.
232 chosen because it is relatively undepleted o f major elements [5] and because of its chondrite-like REE pattern [6]. Nodule 1611 has also been used to evaluate the melting behavior of volatile-free peridotite in the upper mantle by Mysen and Kushiro [4]. They found that at 20 kbar the mineral assemblage olivine + orthopyroxene + clinopyroxene + spinel coexists with alkali basalt melt from the solidus to about 2% melt: ing and with olivine tholeiite melt from about 2 to 25% melting. The assemblage olivine + orthopyroxene coexists with picritic melt between 25 and 45% melting, and olivine coexists with peridotitic komatiite melt with more than 45% melting. Their melting curve [4] has been reproduced in Fig. 1. Four points on the melting curve.were selected for this study to obtain REE patterns for each o f these four melt types. The points are at 1.3, 15, 35, and 50% melting. The starting material was crushed to less than 5 /am and fired at 1150°C a t f o 2 = 10 -9 atm overnight prior to addition of the radioactive spikes needed for determination of the REE contents o f the partial melts. This heat treatment did not alter the original mineral-
ogy of the sample [4]. Cerium (3.11 ppm spike) was chosen as representative of a light REE, samarium (0.585 ppm spike) as an intermediate REE, and thulium (0.0173 ppm spike) as a heavy REE. The REE contents of the melts were determined with beta-track counting [7] using 141Ce (energy = 475 keV, t]/2 = 32.5 days), 151Sm (energy = 76 keV, t]/2 = 93 years), and 17]Tm (energy = 97 keV, tl/2 = 1.9 years) as sources of beta particles. The REE pattern of nodule 1611 is chondrite-like and near unity [6]. Consequently, it was necessary to determine only the content of radioactive REE in the partial melts. This content divided by the total amount of spike added to the starting material results in an enrichment factor that equals the chondrite-normalized REE!pattern that could be derived from partial melts of this Nodule in the absence of any additional spike. The experiments were carried out in solid-media, high-pressure apparatus [8] using sealed Pt9sAu 5 sample containers. All aspects of the technique were identical with that used by Mysen and Kushiro [4] to ensure duplication of the degree of melting determined by them. Run durations were from 4 hours at 1450°C to 1 hour at 1650°C.
1700
3. Results •' ~ Y J 50%
/7
(.3 1 6 0 0 o D
1500 CL
E
t~kali olivine basalt
I.-1400
olivine tholeiite
XA
A
likoli olivine bosolt 130C
0
' 10
2'0
t 30
L
0
* 50
6~0
7'0
80
Per cent melt Fig. 1, Melting curves of two natural peridotites at 20 kbar under volatile-absent conditions [4 ]. A, sample 66SAL-1 ; B, sample 1611 (see [4]). Percentages on curve B denote conditions selected for the present experiments. Rock names are based on normative compositions of partial melts [4 ]. X A and X B, see text.
Analytical results are given in Table 1. The uncertainty is + 1 o, calculated according to the method of Mysen and Seitz [7]. The data are plotted for Fig. 2 and compared with REE patterns from various alkali basalts [3,9 ], abyssal tholeiites [2,10], and picrites [2,11 ]. The rock names associated with the experimental partial melts are based on normative compositions of the melts [4]. The experimental REE patterns from partial melts of 1.3 and 15% melting are different in bulk REE enrichment and also in geometry, despite their similar phase assemblages. The differences in bulk REE enrichment are mostly a function of degree of melting. The melt from 15% melting is also more depleted of the heavy REE than that from 1.3% melting (Fig. 3). If there were no relative fractionation of the REE as the degree of melting was increased from 1.3 to 15%, the enrichment factors o f Ce, Sm, and Tm in the melt from 15% melting would be about 9% of those of the 1.3% melt. The observed values are 9, 8, and 6%, res-
233 TABLE 1 Experimental results (values in ppm)
141Ce 151Sm 171Tm
1.3% melt
15% melt
35% melt
50% melt
264 +-6 47 -* 1 1.30 -+0.04
23.3 -+0.6 3.5 +-0.1 0.068 +-0.003
7.9 -+0.5 1.29 +-0.06 0.038 ± 0.0002
5.0 +-0.3 0.907 -+0.004 0.0251 -*0.0009
Coexisting mineral assemblages 1.3% melt: olivine + spinel + orthopyroxene + pigeonitic clinopyroxene; 15% melt: olivine + spinel + orthopyroxene + pigeonitic clinopyroxene; 35% melt: olivine + orthopyroxene; 50% melt: olivine. pectively, for Ce, Sm, and Tm. This depletion of the heavy REE may be caused by the reaction relation between partial melt and crystalline residue described
100 80 -<~
,0
~1.3%._
60 ~ o e
by Mysen and Kushiro [4]. The difference between the pattern of experimental olivine tholeiite melt (15% melting) and that of picrite (35% melting) is due to the absence of clinopyroxene in the residue of the latter inasmuch as clinopyroxene is somewhat enriched in light REE relative to heavy REE. Orthopyroxene and olivine will affect the relative variations between light and heavy REE in the partial melts only to a small degree because the partition coefficients, x-liq a n d / ~ El-li /~RP~E ~ , are quite small [12,13]. The larger total enrichment of REE in picrite melt compared with peridotitic komatiite melt is, therefore, simply the result of the larger proportion of crystals coexisting with picritic than with peridotitic komatiite melt.
~.8 1.7 1.6
// 1.5
0.?3:/0 "-+
_~j~per!dotitic komatiite0_59_Olo
-g_ o i
1.4
1.2
La Ce Pr Nd PmSmEu Gd TL) Dy Ho Er Tm Yb Lu Fig. 2. Experimentally determined REE fractionation patterns of experimentally produced partial melts. Alkali basalt (A), Hawaiian alkali basalt [9]. Alkali basalt (B), range of REE contents of alkali basalts from the Lesser Antilles [3]. Tholeiite (A), oceanic tholeiite from the Nazca plate [11]. Tholeiite (B), abyssal tholeiite, Baffin Bay [2]. Picrite (A), picrite basalt, Azores [10]. Picrite (B), picrite, Baff'm Bay [2].
Ce/Tm f.I I.(
Ce/Sm I ()
210 Per cent
310 melting
~
Fig. 3. Experimentally determined enrichment factors of light REE relative to heavy REE as a function of degree of melting.
5o
234 4. Discussion Alkali basalts are generally considered to be the results of only several percent melting owing to their enrichment in large ion lithophile elements and light REE. The REE data for 1.3% melting of volatile-free peridotite would, therefore, approximate those for alkali basalt, provided the phase equilibria [4] have application to the formation of natural alkali basalts and the REE contents of nodule 1611 are representative of the source rocks of such basalts. The ratios of the enrichment factors, Ce/Tm and Ce/Sm, respectively, are 1.3 and 1.1, at 1.3% melting (Fig. 3). In contrast, natural alkali basalts have chondrite-normalized Ce/Tm and Ce/Sm in the ranges 2 - 1 0 and 1.5-3.0, respectively [3,9,10]. Thus, even though the major element composition of experimentally produced partial melt at 1.3% melting of spinel peridotite resembles that of alkali basalt, the experimental REE data indicate that natural alkali basalts could not have been formed this way. Peridotite, generally considered to represent the composition of the source rock of basalt in the upper mantle, shows little enrichment in light REE [7,14,15]. A garnet peridotite source with a flat REE pattern may yield the appropriate REE pattern of a partial melt [1,3]. Mysen and Kushiro [4] showed, however, that the partial melt in equilibrium with garnet peridotite is not alkali basalt, but alkali picrite. It thus seems that alkali basalt could not have been formed by direct partial melting of any kind of volatile-free peridotite upper mantle source. An alternative explanation of their origin is melting of peridotite in the presence of CO2-rich volatiles [ 16, 17]. Possil~le effects on trace element contents of partial melts formed under such conditions have been discussed elsewhere [18]. Experimental data on vaporcrystal partitioning, however, are not yet available. A detailed discussion is, therefore, not warranted. Abyssal tholeiites have REE fractionation patterns with both light-REE enrichment and light-REE depletion relative to chondrite [2,9-11 ]. Basalt REE patterns from Baffin Bay tholeiite [2] show slight enrichment and are compatible with a model of magma formation by partial melting of volatile-free peridotite that initially had a flat REE pattern. Further melting of a source from which such tholeiite had been extracted would result in picrite (coexisting with harzburgite residue) with a REE pattern showing depletion of
light REE (Fig. 2). The chondrite-normalized REE pattern of picrite from Baffin Bay I2] shows such a depletion, indicating that the tholeiite-picrite association described by O'Nions and Clark [2] may have been formed by two-stage melting or successive tapping of a mantle-melt reservoir where the degree of melting had progressed to the stage of picrite partial melt. The REE patterns of tholeiites from the Nazca plate [11 ] indicate that the source of these basalts was depleted of light REE prior to the melting event leading to the formation of these tholeiites. Similar depletion on a regional scale of Icelandic basalts led to the suggestion by Schilling and Bonatti [ 11 ] and Schilling [19] of mantle plumes with varying trace element abundances. Evidence for heterogeneous distribution of REE in the mantle is also seen in the variation of REE patterns of peridotite nodules from kimberlite [6]. The question as to why there are regional differences of trace element concentrations in the upper mantle has not been resolved. One explanation is metasomatism through the action of a vapor or fluid phase. Such a hypothesis is supported by the comparison of REE data on sheared and granular nodules from kimberlite studied by Shimizu [6]. The granular nodules are generally depleted of major elements, whereas the sheared nodules are more fertile. Nevertheless, Shimizu [6] found that the granular nodules show an enrichment of light REE relative to heavy REE that is more pronounced than in the sheared nodules. In fact, many of the granular nodules contain less heavy REE than chondrite, whereast the chondrite-normalized light REE are higher than unity [6]. Even though their major element contents show strong evidence that these nodules are residual after a melting event, it was emphasized by Shimizu [6] that their REE patterns cannot be reconciled with such a process. Metasomatism in the continental upper mantle may, therefore, be important in controlling element composition. Because of the low concentrations of the trace elements, the effects of metasomatism on trace elements may be more readily detected than in the major element distribution. Eggler [20] and Newton and Sharp [21] have argued that volatiles are more likely to be bound in minerals than in a vapor phase in the continental upper mantle compared with the oceanic mantle. If the interpretation of the trace and major element
235 composition of the continental upper mantle is correct, metasomatism must have occurred, and the transporting medium is most likely a vapor phase. But if there is a vapor phase in the continental upper mantle, the data of Eggler [20] and Newton and Sharp [21 ] would indicate an even stronger possibility of the presence of a vapor in the oceanic upper mantle. "Provided this vapor consists of two or more components, so that the solubility of trace elements may be varied as a function of the vapor composition, it may be possible to vary the trace element content and the relative enrichment of the rare earth elements in the upper mantle on a regional scale. Unfortunately, there is a complete lack of applicable data on partition coefficients between multicomponent vapors and condensed silicates (liquid and crystals). Thus, a quantitative evaluation of these effects must await further experimental data. Partial melting will also result in fractionation of trace elements. Such melting must, however, leave a residue of a bulk composition that can still be melted to yield a tholeiitic melt composition. Partial melting of the upper mantle may have occurred with or without volatiles. Aside from the effects on melt composition of melting in the presence of volatile components such as CO2 and H20 [16], melting of peridotite + H20 + CO 2 is at least divariant [22]. Melting in the absence of volatiles is nearly isobarically invariant [4]. Basalt can be formed in a peridotite mantle by melting in a region where the heat flux into the system is sufficient to transform a portion of the rock to a melt, and melting will continue until the heat transported out of the system exceeds the heat input necessary to sustain the melting reaction. Melting of the upper mantle under nearly invariant conditions will continue until, for example, one or more crystalline phases are exhausted and the melting temperature of the residue begins to increase. It is possible that at this point (e.g. point X A or X B in Fig. 1) the heat flux into the system will be sufficient to support further melting because heat is now also required to raise the temperature of the system. According to Yoder [23] this may be one reason why tholeiitic basalts are common, whereas picrites are rare. It was visualized [23] that, during melting at an invariant point, the trace elements could be fractionated by melt separating from the source before the melting reaction reached completion (point XA or XB in Fig. I). The first
magma will be enriched in light REE, according to the data in Fig. 2; it will probably also contain most of the LIL trace elements in the source region. The peridotite residue will become depleted in the same elements. As melting proceeds, more magma of similar major element composition will be produced and separated. This magma, however, will be depleted of light REE and LIL elements. If this is an accurate description of the melting process producing oceanic basalt and perhaps the large accumulations of plateau basalt, these particular rocks would show a record of the evolution of the trace elements in the partial melts [23]. To date such data have not been obtained. Trace element fractionation of the upper mantle by the above mechanism will be of temporary nature and will probably be quite localized geographically. Yoder [23] using the heat flow data of Lee [24] and elt an enthalpy of melting, AHnggarnetperidotite equal to 164 cal/g (see Yoder [23] for discussion of value of z~ga elt rnet peridotite ) calculated that it will take 82 m.y. to generate 5% melt from garnet peridotite. The enthalpy of melting will vary some with pressure (AVmelt is pressure-dependent) and the heat input to different segments of the mantle may differ from place to place. However, even allowing for such variations, the time period where a peridotite source could yield tholeiite melt (20-30% melting) is only several hundred million years. Thus, any source region of tholeiitic basalt could remain heterogeneous only with respect to LIL elements and at the same time have a major element composition that could still result in tholeiitic partial melt within such a period of time. Because of the constraints on the mantle geochemistry provided by this model, it could be tested and evaluated by studying the trace element evolution of basalt eruption cycles and the composition of peridotite nodules brought to the earth's surface in kimberlites and basalts themselves. Even though no systematic study of this type is available, the present evidence does not point in that direction [6,15,25]. The lack of systematic relations between trace and major element compositions was used above to argue for the presence of vapor in the upper mantle. Additional evidence for the presence of such a component has been summarized by Irving and Wyllie [26], Boettcher et al. [27], and Anderson [28]. Because of the multivariant nature of melting of rocks in the presence of a separate vapor phase with volatiles such
236 as CO2 and H20, it also seems more likely that such melting could account for at least some o f the trace element heterogeneity found in the upper mantle. Because o f the multivariancy o f the melting, the melting temperature o f the rock will increase during melting [22]. Thus, the heat flux into the system must be sufficient both to increase the temperature and to melt the rock. Because o f this heat requirement, such a melting reaction is more likely to halt before the peridotite source is exhausted in its basalt components than invariant melting where the temperature does not increase during melting. After a melt fraction thus derived is separated from the system, the residual peridotite is depleted o f its LIL trace elements, but could still be melted to generate more tholeiite. Neither o f the two melting models summarized above can explain the peculiar contrast between major and trace element composition of some of the granular peridotite nodules found in kimberlite [6]. Even though present-day oceanic volcanism may in large part be explained by simple partial melting of volatile-free peridotite upper mantle, the trace element patterns found in these rocks do require trace element fractionation of the source region. Some aspects o f this fractionation cannot be explained without resorting to the presence of volatiles. Partial melting cannot explain all the features, and it is proposed that metasomatism played an important role in distributing trace elements in the upper mantle.
4
5
6
7
8
9
10 11
12
13
14 Acknowledgements The authors are grateful to Drs. T.N. Arndt, T. Benjamin, D.H. Eggler, and H.S. Yoder, Jr., for critical reviews. This research was partially supported by National Science Foundation grant DES 73-00266A01.
References 1 P.W. Gast, Trace element fractionation and the origin of tholeiitic and alkaline magma types, Geochim. Cosmochim. Acta 32 (1968) 1057. 2 R.K. O'Nions and D.B. Clark, Comparative trace element geochemistry of Tertiary basalts from Baffin Bay, Earth Planet. Sci. Lett. 15 (1972)436. 3 N. Shimizu and R.J. Arculus, Rare earth element concentrations in a suite of basanitoids and alkali olivine basalts
from Grenada, Lesser Antilles, Contrib. Mineral. Petrol. 50 (1975) 231. B.O. Mysen and I. Kushiro, Compositional variations of coexisting phases with degree of melting of peridotite in the upper mantle, Am. Mineral. (1976) in press. P.H. Nixon and F.R. Boyd, Petrogenesis of the granular and sheared ultrabasic nodule suite in kimberlites, in: Lesotho Kimberlites, P.H. Nixon, ed. (Cape and Transvaal Printers Ltd., Cape Town, 1973) p. 48. N. Shimizu, Rare earth elements (REE) in garnets and clinopyroxenes from garnet lherzolite nodules in kimberlites, Carnegie Inst. Washington Yearb. 73 (1974) 954. B.O. Mysen and M.G. Seitz, Trace element partitioning determined by beta-track mapping - an experimental study using carbon and samarium as examples, J. Geophys. Res. 80 (1975) 2627. F.R. Boyd and J.L. England, Apparatus for phase equilibrium measurements at pressures up to 50 kilobars and temperatures up to 1750°C, J. Geophys. Res. 65 (1960) 741. J.-G. Schilling and J.W. Winchester, Rare earth contribution to the origin of Hawaiian lavas, Contrib. Mineral. Petrol. 23 (1969) 27. J.-G. Schilling, Azores mantle blob: rare earth evidence, Earth Planet. Sci. Lett. 25 (1975) 103. J.-G. Schilling and E. Bonatti, East Pacific Ridge (2° S 19° S) versus Nazca plate volcanism: rare earth evidence, Earth Planet. Sci. Lett. 25 (1975) 93. B.O. Mysen, Partitioning of samarium and nickel between olivine, orthopyroxene and liquid : preliminary data at 20 kbar and 1025°C, Earth Planet. Sci. Lett. 31 (1976) 1. C.C. Schnetzler and J.A. Philpotts, Partition coefficients of rare earth elements between igneous matrix material and rock forming mineral phenocrysts. Geochim. Cosmochim. Acta 34 (1970) 331. J.B. Reid and F.A. Frey, Rare earth distributions in lherzolite and garnet pyroxenite xenoliths and the constitution of the upper mantle, J. Geophys. Res. 76 (1971) 1184.
15 F.A. Frey, L.A. Haskin and M.A. Haskin, Rare earth abundances in some ultramafic rocks, J. Geophys. Res. 76 (1971) 2057. 16 B.O. Mysen and A.L. Boettcher, Melting in a hydrous mantle, II. Geochemistry of crystals and liquids formed by anatexis of mantle peridotite at high pressures and high temperatures as a function of controlled activities of water, hydrogen and carbon dioxide, J. Petrol. 16 (1975) 549. 17 B.O. Mysen, R.J. Arculus and D.H. Eggler, Solubility of carbon dioxide in natural nephelinite, tholeiite and andesite melts to 30 kbar pressure, Contrib. Mineral. Petrol. 53 (1975) 227. 18 B.O. Mysen, Melting in a hydrous mantle: phase relations of mantle peridotite with controlled water and oxygen fugacities, Carnegie Inst. Washington Yearb. 72 (1973) 467. 19 J.-G. Schilling, Iceland mantle plume, Nature 246 (1973) 141.
237 20 D.H. Eggler, Does CO 2 cause partial melting in the lowvelocity layer of the mantle?, Geology 2 (1976) 69. 21 R.C. Newton and W.E. Sharp, Stability of forsterite + CO 2 and its bearing on the role of CO 2 in the mantle, Earth Planet. Sci. Lett. 26 (1975) 239. 22 D.H. Eggler, CO 2 as a volatile component of the mantle: the system Mg2SiO4-SiO2-H20-CO2, Phys. Chem. Earth 9 (1975) 869. 23 H.S. Yoder, Jr., Generation of Basaltic Magma (National Academy of Sciences, Washington, D.C., 1976) 264 pp. 24 W.H.K. Lee, Thermal history of the Earth, Ph.D. Thesis, Univ. of California, Los Angeles, Calif. (1967) 344 pp. 25 J.A. Philpotts, C.C. Schnetzler and A.A. Thomas, Petro-
genetic implications of some new geochemical data on eclogitic and ultrabasic inclusions, Geochim. Cosmochim. Acta 36 (1972) 1131. 26 A.J. Irving and P.J. Wyllie, Subsolidus and melting relationships for calcite, magnesite on the join CaCO 3 MgCO 3 to 36 kbar, Geochim. Cosmochim. Acta 39 (1975) 35. 27 A.L. Boettcher, B.O. Mysen and P.J. Modreski, Phase relationships in natural and synthetic peridotite-H2 0 and peridotite-H20-CO2 systems at high pressures, Phys. Chem. Earth 9 (1975) 855. 28 A.T. Anderson, Some basaltic and andesitic gases, Rev. Geophys. 13 (1975) 37.