On rare-earth element behavior in igneous rocks

On Rare-Earth Element Behavior in Igneous Rocks LARRY A. HASKIN Departments of Earth and Planetary Sciences and Chemistry and McDonnell Center for the Space Sciences, Washington University, St Louis, Mo. 63130, U.S.A.

Abstract Trends of the past decade in geochemicalinvestigations of trace elements are reflected in studies of rareearth elements (REE). Substantial improvements have been made in accuracy, sensitivity and convenience of REE analysis, mainly through improved techniques and equipment for neutron-activation analysis and mass spectrometric isotope dilution. The most detailed studies have been on marie rock systems, with a shift away from broad surveys to detailed examinations of single regions or formations. Ocean ridge tholeiitic basalts have been shown to derive from mantle sources partly depleted by earlier melting. Associated basalts deviating from typical ocean ridge tholeiite have been attributed to invading plumes of more primitive mantle matter. Lunar mare basalts and highland KREEP basalts have been characterized as having REE distributions roughly analogous to terrestrial ocean and continental distributions. Models for genesis ofmafic liquids as products of partial melting or fractional crystallization have been developed and applied. These models are dependent on values for distribution coefficients. Such values are not well known or the factors affecting them well characterized. Experimental techniques for estimating distribution coefficients are still being developed and tested. Present models cannot yet account quantitatively for the REE concentrations and distributions observed in even the best understood natural systems. The semiquantitative results on a variety of marie systemsneverthelessconstitute important gains in understanding of both trace element behavior and igneous rock petrogenesis.

Introduction The purpose o f this paper is to convey an impression o f the development o f our understanding o f trace-element behavior during igneous rock formation since the 1967 conference on the Origin and Distribution o f the Elements. It is impossible to review the progress broadly, so this discussion is restricted to rare-earth elements, mainly in mafic igneous rocks (REE). A more extensive review o f R E E geochemistry has been prepared by Haskin and Paster (1978). The R E E are especially valuable for geochemical study because o f their mutually similar behavior and, most recently, because o f data on S m - N d isotopic systematics. R E E a b u n d a n c e distributions have become an important tool, as part o f multielement analysis on single samples, to be used along with m a j o r element composition, field relationships, petrographic characterization, experimental petrology and isotopes, to name some o f the key pieces o f information needed to provide correct solutions to geochemical and petrological riddles. R E E studies in igneous rocks have benefitted strongly from improved analytical techniques, especially mass spectrometric isotope dilution and n e u t r o n activation analysis. It is n o w possible to obtain the same values for R E E , within stated analytical uncertainties, from several laboratories, as indicated by the results for two aliquants o f lunar basalt 14053, 175

L. A. Haskin

176

TABLE 1. ppm REE IN BASALT 14053

La Ce Nd Sm Eu Gd

A

B

12.8_+.2 36,4_+ .8 22+ 1 6.50+ ,06 1.23+.03 8.5_+.2

13.0 34.5 21.9 6.56 1.21 8.59

Tb Dy Ho Er Yb Lu

A

B

1.62_+.08 11.1 _+.4 2.1 + .2

....... 10.5 ...... 6.51 6.00

6.1 _+.3 0.89_+ .01

A. P. Helmke, neutron actwation analysis. N. J. Hubbard, Mass spectrometric isotope dilution.

one done by NAA, the other by MSID (Table 1). The improvements in analysis are a legacy of the Apollo lunar sample program. Along with accuracy, sensitivity and ease have also improved, as indicated, for example, by analyses of 155 l-2-mm fragments of Apollo 17 soils (Blanchard et aL, 1975). To discuss REE geochemistry, the now conventional comparison diagram is used, REE abundances in a sample are divided, element by element, by the abundances in some standard material, usually chondrites. As seen in Fig. 1, the relative REE abundances in chondritic meteorites are within error of measurement the same as those in the sun

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On Pare-Earth 30 -I

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(normalized here to La = 1) and perhaps therefore represent those of the parent material for planets. Most materials at Earth's surface are enriched in all REE compared with chotadrites, but more so in light REE (LREE), e.g. the North American Shales Composite (Fig. 1). Eu behaves anomalously when it is partially reduced from the typical lanthanide 3 + oxidation state to the 2 + state under common natural conditions. Mafic Igneous Rocks Much of the gain in knowledge of REE behavior has involved marie rocks. The lightlanthanide depleted rocks from the ocean ridges (Fig. 2) were interpreted by Gast (1968) and others to have been derived by partial melting of mantle sources that had previously lost a ~0

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L. A. Haskin

178

substantial fraction of their incompatible elements. Present evidence (e.g. Kay et al., 1970) indicates that such sources predominate for ocean floor igneous rocks, and that these sources are separate from those producing continental volcanics or oceanic islands. It is interesting to observe that basalts from lunar maria (Fig. 2) are similarly depleted in LREE and the best current interpretation is that their source regions are products of an earlier episode of melting that depleted them substantially in incompatible elements (e.g.Nava and Philpotts, 1973). The analogy between the two sets of rocks should not be pushed very far. It is nevertheless amusing to suggest that the naming of Moon's dark areas by the ancients was very clever after all, even if the lunar seas never contained water. Note the strong depletion in Eu, a consequence of Moon's very reducing condition; this depletion appears to be a characteristic of the source region for the lunar basalts with the missing Eu probably in the more feldspathic highlands (e.g. Shih et aL,1975). The transition from LREE-depleted basalts of the ocean floor igneous suite to the LREEenriched volcanics of continents is nicely illustrated by the basalts of Japan (e.g. Masuda, 1968), with the LREE-depleted basalts (Fig. 3) coming from the Pacific Ocean side, those most enriched in LREE from the Asian continent side. Schilling and coworkers (e.g. Schilling, 1975) have made a case for mantle plumes, bringing magma from sources not previously depleted in compatible elements, to account for lavas of islands well isolated from continents. They postulate mixing of such magmas with those of depleted sources to form the LREE-enriched basalts sometimes found along ocean ridges or to account for transitions lit basalt type such as those found in Iceland. Hermann and coworkers (1976) have recently ....

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On Rare.Earth Element Behavior in Igneous Rocks KEWEENAWAN LAVAS

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180

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shown that REE distributions in some basaltic komatiites from the ancient Barberton greenstone belt are similar to those of ocean floor igneous rocks, suggesting that some mantle sources may have been depleted very early in Earth's history, Petrographicalty clear cases of lunar highlands volcanic rocks cannot be found because of the intense meteoroid bombardment of the highlands; however, rocks such as poikilitic breccia 65015 (e.g. Haskin et al., 1973) may well be metamorphosed highland volcanics. (Such compositions are called KREEP because of their high concentrations of K, REE and P; Hubbard et al., 1971). There is some similarity with terrestrial continental volcanics in that the REE concentrations are rather high and the LREE are more enriched than the heavy ones. The present observed extremes in lunar highlands composition (Fig. 4) show evidence of processes that very strongly concentrated the REE and of the presence of considerable Eu in massive cumulate plagioclase feldspar. A series of Keweenawan lavas now under study (Haskin, Brannon and Green, to be published) is typical of continental volcanics (Fig. 5); these samples range from olivine tholeiite through alkali basalt and andesite to rhyolite. More extreme cases of relative LREE enrichment occur in alkali-rich basalts and kimberlites (Fig. 6); these data are from Loubet et al. (1972) (carbonatites), Fesq et al. (1975) (kimberlite), and for alkali-rich basalts (Kay and Gast, 1973, and Gerasimovskii et al., 1972). The important point here is the absence of any discontinuity between lightly differentiated tholeiites and strongly differentiated tholeiites and strongly differentiated alkali and volatile-rich lavas. This suggests that some volatile, highly mobile phase in the mantle greatly influences relative REE abundances in lavas; such phases have not generally been considered in quantitative theoretical models yet.

Mathematical Models for Trace-element Partitioning It should be possible to produce mathematical models that account quantitatively for observed concentrations and relative abundances of REE and other trace elements. The chemical basis for such models is straightforward (e,g. Mclntire, 1963); the difficulty is in recognizing all important petrological processes (Fig, 7) and applying the chemical aspects of the models properly to them. Such processes have, for example, yielded strong geochemical correlations (e.g. Sm and Hf, Fig. 8) among rocks of diverse types. This particul~ir correlation, not found universally among igneous rocks, supports field evidence for a genetic relationship among the diverse Keweenawan volcanics mentioned earlier. SOME PROCESSES OF IGNEOUS ROCK FORMATION PARTIAL MELTING COMPLETE MELTING FRACTIONAL CRYSTALLIZATION CRYSTAL SETTLING FILTER PRESSING

ASSIMILATION EXTRACTION MAGMA MIXING VAPOR TRANSPORT

CAN OCCUR SINGLY COMPLETE EQUILIBRIUM SEQUENTIALLY SURFACE EQUILIBRIUM IN PARALLEL KINETIC C O ~ R O L Fro. 7. Summaryof petrological processesto be considered in modellingof trace-elementpartitioning.

On Rare-Earth Element Behavior in Igneous Rocks 20

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182

L. A. Haskin

Strong correlations between major and trace elements have been discovered (e.g. Sc and FeO in lunar highland rocks, Fig. 9) and the absence of universal correlation between any pair of major and trace elements has been reconfirmed (e.g. Sc and FeO, Keweenawan basalts, Fig. 10). Proper models must describe both correlations and lack of correlations, Present quantitative models emphasize partitioning according to a Nernst distribution, which states simply that the ratio of concentrations between two equilibrium phases is constant (for constant T, P, composition). The Nernst distribution is used directly for complete equilibrium partitioning or incrementally for surface-equilibrium partitioning, when the interior of a crystallizing solid becomes quickly isolated from the cooling melt, or in some other manner depending on the process being modelled (e.g. Shaw, 1970). REE tend to prefer liquid rather than solid phases; different minerals accept them selectively (Fig. 11). Garnet eagerly accepts HREE, but not LREE, apatite accepts all REE; feldspars accept Eu better than other REE; Ca-rich pyroxene accepts REE more readily than Ca-poor pyroxene or olivines, etc. For fractional crystallization, REE usually become enriched in the residual liquid. To separate two trace elements from each other effectively in a simple partitioning process, a substantial difference in distribution coefficients is required; in fractional crystallization all trace elements with D < 0.1 .effectively remain in the liquid phase; those with D > 10 are all quickly extracted into the crystallizing solid. (1n this case, the value of D is the weighted average of D values for all crystallizing minerals).

Applications of Trace-element Partitioning Models Partitioning models have been applied to a number of igneous systems with useful results; they have been applied in poor and misleading ways in other systems, mainly when obvious constraints of petrology have been ignored. Zielinski (1975) has shown that REE distributions in a suite of volcanic rocks from Reunion Island, ranging from olivine-rich alkali basalt to trachyte, correspond to fractional crystallization of common minerals in amounts required to produce the observed major element compositions. Severe fractional crystallization is required (~90~o). Arth and Hanson (1975) have argued that strongly fractlonated REE distributions in some early Precambrian acidic volcanic and intrusive rocks might be explained by partitioning an approximately chondritic REE distribution against garnet. If the mantle is presumed chondritic and only simple silicate liquid-mineral partitioning is allowed, garnet appears to be required to produce any REE distribution substantially enriched in LREE. Other kinds of processes may well be important, however. Helmke and Haskin (1973) showed how groups of successive lavas from Steens Mountain, Oregon, might be related by fractional crystallization. Rather extensive crystallization (up to 60~o) was required to produce cumulative solids with slightly positive Eu anomalies and corresponding residual liquids with small negative ones. They also showed how mixtures of crystallized solid and parent liquid yielded the compositional characteristics often found in gabbros, with REE relative abundances showing a broad peak with a maximum in the vicinity of Gd. Allegre et al. (1977) have developed a mathematical formalism for optimizing the parameters in a simple fractional crystallization model to provide the best fit to data on a rock system. They discuss the information that can be obtained from a variety of trace elements with differing geochemical behaviors. Nesbitt and Sun (1976) have emphasized the importance of trace-element ratios in understanding igneous rock petrogenesis.

On Rare-Earth Element Behavior in Igneous Rocks

183

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184

L, A. Haskin

Proper application of partitioning models can be tricky. The simplest application of the model to a system undergoing fractional crystallization assigns a distribution coefficient to the crystallizing solid phase, then shows an exponential increase of concentration for a REE in the residual liquid and a corresponding increase for the crystallized solid, but beginning at a lower concentration. This does not always occur as seen for Sm in the Kiglapait intrusion (Fig. 12) or in the Skaergaard intrusion (Paster et aL, 1974). When minerals from different zones, or levels of fractionation, are analyzed, it appears as if the D values were changing strongly. In fact, the bulk D values for REE mainly reflect changing mineralogy (e.g. onset of crystallization of clinopyroxene and apatite) and the quantity of parent liquid trapped in each zone. When the trapped liquid solidifies as a closed system, the final growth of crystals incorporates more REE from trapped liquid than is in the cumulate minerals. The model for the Skaergaard layered series, when these effects were properly taken into account, gave a reasonable description of observed REE concentrations for constant values of D and demonstrated that the unexposed portion of the intrusion could not be nearly as large as the 70°,0 to 80';,0 previously believed. The much smaller hidden zone was independently confirmed by gravity studies. Proper attention to presence of trapped liquid enabled Haskin et al. (1974) to show that a lunar highlands cumulate of plagioclase and olivine had a parent liquid similar to that of average highlands matter rather than one similar to KREEP. The importance of closed system crystallization (e.g. Helmke et aL, 1972) is too often overlooked. As a closed system crystallizes, the incompatible elements become highly concentrated in residual liquid. These elements are forced into the major crystals least reluctant to accept them. This occurs until the concentrations of the formerly incompatible elements become high enough to precipitate exotic mineral phases in which they are essential, or at least welcome, constituents. Consider the hypothetical crystallization of a mafic liquid with a chondritic REE distribution to produce an idealized ecologite consisting

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On Rare-Earth Element Behavior in Igneous Rocks

185

only of garnet and clinopyroxene (no mesostasis) (Fig. 13). The HREE are in garnet, the LREE in pyroxene. A comparison of the pyroxene concentrations with those of the whole rock might suggest that pyroxene preferred the LREE, but as the distribution coefficients indicate, neither mineral prefers LREE, they are concentrated in the clinopyroxene because that mineral can accept them more easily than garnet can. It is important to test models, however sound they may seem, on systems whose petrological nature is well understood. Haskin and Korotev (1977) have made such a test of the closed-system model on a single sample of lunar basalt. They analyzed separated minerals and mesostasis and showed by a mixing model that they could match the composition of the whole rock very closely in terms of the minerals and mesostasis. They used the closed-system description in conjunction with D values from the literature on terrestrial rocks to test whether the mineral compositions could be described by the model. Because the agreement was reasonable, they then reversed the procedure, assumed validity of the model, and calculated average distribution coefficients for the minerals during solidification of the rock. Current models for trace-element partitioning as applied so far have worked well in some cases but by no means all cases; indicating that all important processes have not yet been properly described. Frey and Green (1974) have studied peridotite nodules as possible sources for basalts. One class of nodules studied had low REE concentrations and a distribution that might serve as a parent to ocean floor basalts. The other type had lower concentrations of heavy REE, but higher concentrations of light REE as if parental to alkali basalts. Surprisingly, the more refractory, alkali and calcium-poor, residue-like nodules, were the most enriched in the light REE, just the opposite of what would be required to make the two basalt types. In this and other works, Frey and coworkers have demonstrated that major and trace elements are not closely coupled in the basalt source regions. I

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FIG. 14. Comparisondiagram for REE in five samplesof a single Cascadesmesa basalt flow (lateral sampling) (Brannon, Haskin and McBirney.unpublisheddata). Furthermore, observed variations in trace element concentrations in terrestrial lavas themselves are not entirely compatible with fractionation models as presently applied: A horizontal segment of a single flow of Cascades basalt (Fig. 14) (Brannon, McBirney and Haskin, to be published) shows substantial differences in REE distribution. The change in ratio of La to Sm would correspond to ~60% fractional crystallization of pure clinopyroxene, clearly impossible, especially since major element concentrations a n d mineralogy hardly vary. In some igneous systems, alteration can be a complicating factor (e.g. Hallman and Henderson, 1977). Distribution Coefficients An important contribution to understanding REE behavior comes from studies of synthetic silicate minerals and liquids. One example is the determination of values for distribution coefficients. Values can be obtained by several methods (Fig. 15) from natural materials and by several methods from synthetic materials. Most data in the literature were obtained by analyses of phenocrysts and their host matrices, treated as equilibrium TECHNIQUES FOR MEASURING DREE N A T U R A L MA T E R I A L S

PHENOCRYST--MATRIX--VOLCANICS, SCHNETZLER and PHILPOTTS PARTIAL MELT--RESIDUE--LIZARD, FREY SEQUENTIAL CUMULATES--SKAERGAARD, PASTER et al. APHYRIC BASALT--MARE BASALT, HASKIN and KOROTEV SYNTHETIC MA TERIALS

AQUEOUS EQUILIBRATION--CULLERS et al. DRY MELT--ELECTRON PROBE--WEILL and coworkers --CRYSTAL SEPARATION--MASUDA and KUSHIRO --BETA TRACK--MYSEN ZONE MELTING--BALASHOV et al. FIG. 15. Summaryof methods used for estimatingor determiningD values.

On Rare-Earth Element Behavior in Igneous Rocks

187

solid-liquid pairs. Results from this method were first reported at the last Paris conference by Schnetzler and Philpotts (1967). Frey (1969) obtained values for distribution coefficients by treating the Lizard high-temperature peridotite as a residue and nearby gabbro as the liquid from partial melting. Paster et al. (1974) obtained values from Skaergaard minerals and estimates of residual liquid concentrations. Haskin and Korotev (1977) obtained them from an aphyric lunar basalt as shown earlier. Results from natural materials show consistently the preferences of rock-forming minerals for certain of the REE but offer little information for explaining the spread of values. In addition to equilibrium effects of T, P, and composition, there may be kinetic effects in natural systems as shown, for example, by Albarede and Bottinga (1972). Laboratory experiments offer the chance to study the effects of these parameters. An equilibration of REE tracers between an aqueous fluid and silicate crystals or silicate liquid was tried by Cullers et al. (1973). Values obtained fell within the range found for natural systems, but Zielinski and Frey (1974) were unable to match the results and there are other limitations to the method, which has not yet been adequately tested. The most trustworthy experimental values obtained so far have come from water-free systems. Weill, Drake and coworkers (e.g. Drake and Weill, 1975) used the electron probe to determine concentrations in crystals and glass in REE-doped silicates; this method requires substantially larger concentrations of REE than are present in natural systems, inviting deviations from dilute solution behavior. Some workers have tried lower REE concentrations and analysis of crystals and glass by mass spectrometric isotope dilution; failure to remove last traces of glass from crystals can seriously affect results (e.g. M asuda and Kushiro, 1970; Shimizu and Kushiro, 1975). The most recent technique, developed by Mysen and Seitz (e.g. Mysen, 1976) uses beta tracks from radioactive tracers for determining concentrations, thus allowing very low concentrations of dopant. Early results by this technique indicated that levels of REE (~0.1 wt.Yo) in synthetic silicates made for microprobe and EPR measurements do indeed violate dilute solution conditions. However, the results of Cullers et al. (1970) indicated that mafic minerals equilibrated with aqueous fluid accept REE to concentrations of parts-per-thousand without observable change in behavior, so more studies are needed to clarify this problem.

EPR Spectroscopy Electron paramagnetic resonance (EPR) spectroscopy was used by Morris (1975) to indicate the nature of the sites occupied in synthetic silicate crystals by Gd 3 ÷ and Eu 2÷ ions. Except in feldspar, Gd 3 + occupied normal lattice sites whenever Ca was a major constituent of the mineral. Charge compensation apparently occurred remote from the site of the ion, as it had no effect on the spectrum. In feldspar the spectrum for Gd 3 ÷ was the same as that found for Gd in silicate glass, apparently because the 3 + ion was very small for the site or because the high charge disrupted the crystal structure at that site. Eu 2 ÷ occupied Ca sites in all minerals containing them. Both Gd 3÷ and Eu 2 ÷ tended to cluster in pure Mg ortho- or metasilicates. Morris and coworkers (1974a, b) also studied effects of oxygen fugacity and bulk composition on the relative amounts of Eu 2 ÷ and Eu 3+ in silicate glasses. They found a strong effect of bulk composition on the mole fraction ofEu 2 + in the melt at constant oxygen fugacity. Fraser (1975) has suggested that this results from amphotoeric behavior of REE 3÷ oxides.

188

L. A, Haskin

It is a general characteristic of the past t0 years' progress in understanding the behavior of REE and other trace elements in igneous systems that new experimental and analytical chemical techniques were important. These techniques are dependent upon expensive, sophisticated, complex electronic instrumentation. Advances in knowledge during the next decade will surely also depend on the further use and development of such instruments. Acknowledgement The author thanks the New Holland Publishing Company for permission to reprint several of the figures from the Handbook of Rare Earths, volume 2 (Haskin and Paster, 1978).

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On Rare-Earth Element Behavior in Igneous Rocks

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