ANALYTICAL.
ANllMAlllUSl" HG I H RARE-EARTH ELEMENT ABUNUANCES IN HAWAIIAN LAVAS R. V. Fodor Department of Marine, Earth, and Atmospheric Sciences North Carolina State University Raleigh, NC 27695-8208
GBbor Dobosi
Geochemical Research Hungarian Academy of Sciences Budapest, Hungary 1112
G. R. Bauer
Department of Land and Natural Resources Division of Water Resource Management Honolulu, HI 96813
The compositions of the lavas that comprise the Hawaiian Islands are often used as models for understanding volcanism and the chemical char acteristics of oceanic volcanoes. From a chemical viewpoint, Hawaii is an ideal paradigm because of its location far from continental crust. Hawaii's oceanic setting tells us that the magmas that erupted as lavas to construct t h e islands did not pass through potentially contaminating material. Chemical compositions of Hawaiian lavas, therefore, completely represent the source from which they were derived: peridotite rock of the upper mantle. Chemical and isotopic analyses of Hawaiian volcanic rocks have become routine laboratory procedures. In the interpretation and modeling of data for a suite of rocks, we often focus on 0003-2700/92/0364-639N$02.50/0 0 1992 American Chemical Society
the significance of the abundance of certain trace elements, such as Zr, Nb, Th, and Ce, and on the importance of isotope ratios, such as 87Sr/ 86Sr, which may vary by < 0.00005 from one sample to another. The objectives of these analytical studies include the identification of some geologically important characteristics of the Earth, such as the chemical nature of the upper mantle beneath each volcano, compositional changes in the mantle during volcano construction, and the chemically and isotopically varied mantle components that may have mixed before and during magma generation. Another objective is to define the processes by which magmas change composition during storage in subvolcano reservoirs before eruption. D u r i n g t h e 1980s n u m e r o u s geochemists and igneous petrologists produced highly refined models for
Hawaiian magmatism. However, information from new chemical studies of Hawaiian lavas is not always compatible with existing models. In this article we will discuss one such study of a suite of lavas from the island of Kahoolawe that contain rare-earth element abundances unlike those ever expected for Hawaiian volcanic rocks.
Chemical characteristics of lavas The lavas of the 15 volcanoes of the eight major islands (Niihau, Kauai, Oahu, Molokai, Lanai, Kahoolawe, Maui, and Hawaii) originated as basalt composition magmas produced by partial melting of the mantle. The melting occurred at depths of -6090 km. Over the past 5 million years, the magmas erupted to construct broad, shield-like volcanoes reaching far above the sea floor to form the Hawaiian Islands. In their sampling of the island lavas, geologists and geochemists note the stratigraphic positions of the samples on the volcanoes-whether they represent the main body of the volcano, the shield edifice, lavas that filled a caldera at the summit of the volcano, or other features such as small cones made of cinders or spatter that may mark the final eruptions of a volcano. Hundreds of analyses of Hawaiian volcanic rocks have established the detailed chemical makeup of the in-
ANALYTICAL CHEMISTRY, VOL. 64, NO. 11, JUNE 1,1992
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ANALYTICAL APPROACH dividual volcanoes and helped to characterize their mantle source material. The rare-earth elements are an important part of these evaluations. Particularly valuable is the geochemical behavior of rare-earth elements during the partial melting of peridotite rock to form basaltic magmas and during crystallization of those magmas to form basaltic rocks. For example, because heavy rareearth elements (Yb and Lu) preferentially partition into certain minerals, the role of a mantle phase such as garnet in the production of basalt can be evaluated from the rare-earth element content in lavas. Also, because light rare-earth elements (La and Ce) have different compatibility with crystallizing basaltic minerals than do middle (Eu)and heavy rare-earth elements, we can determine the type and amount of minerals that crystallized in basaltic magma during volcano development.
Rare-earth elements in lavas To decipher geochemical signatures and to improve petrogenetic (rock origin) models for ocean island volcanism, r a r e - e a r t h element abundances are determined in all newly collected suites of Hawaiian lavas. Data are acquired primarily by using neutron activation analysis (NAA). The eight rare-earth elements La, Ce, Nd, Sm, Eu, Tb, Yb, and Lu are usually determined as a set for basaltic rock. Typically, the concentrations (in parts per million) are normalized t o a v e r a g e v a l u e s for chondritic meteorites and plotted as rare - earth element patterns (Figure 1).Normalizing the data to chondrites provides reference to material that approximates the earth‘s mantle composition. It also enables one to graphically compare rare-earth element concentrations among different rocks by eliminating the Oddo-Harkins effect of higher concentrations for elements with even atomic numbers. The patterns for various compositional types of Hawaiian lavas have been characterized (I) and can be reasonably well predicted for a par ticular rock after its major element composition is determined. In light of this, we believed that our geochemical investigation of Kahoolawe Island would be fairly routine and the trace element compositions somewhat predictable. Kahoolawe Island Prior to our work, Kahoolawe Island was the only Hawaiian volcano that 640 A
had not been studied geochemically. It is managed by the U.S. military, and access is restricted. Our collection of -200 samples was obtained with permission from U.S. Navy personnel a t Pearl Harbor. Providing helicopter transportation and demolition experts who e,scorted us to ensure our safety, the Navy allowed us to comb the island. Our first assessment of Kahoolawe basaltic rocks (2) yielded some lava rare - earth element abundances that were dramatically different from those expected. These rock samples were collected from lava vents that represent the last episodes of Kahoolawe volcanism. Many rocks were enriched with a greater rare-earth element content than that found in other Hawaiian lavas, including the main shield lavas of Kahoolawe (Figure la). For example, the concentration of La, which would typically be 13-15 ppm in the basaltic rocks examined, was 100-200 ppm. The
At
Figure 1. Patterns obtained usir., NAA. (a) Rare-earth element patterns for a typical shield lava and for two rare-earth element- and Y-enriched lavas of Kahoolawe Island. Star indicates a negative Ce anomaly. (b) Rare-earth element patterns for four samples from different parts of the same lava flow on Kahoolawe. The pattern labeled “normal” is for a portion of lava believed to be free of rare-earth element and Y enrichment. The great differences in rare-earth element abundances (from “normal” to “highly enriched”) indicate that the rare-earth element abundances are not evenly distributed throughout the lava. Patterns are constructed by dividing part-per-million values for the lavas by the average rare-earth element abundances of chondritic meteorites.
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rocks with high rare-earth element abundances also had unusually high Y concentrations-in one case, up to five times greater than expected. Additionally, some Kahoolawe lavas had low Ce abundances relative to the high amounts of the remaining rare-earth elements studied. Such occurrences of comparatively low Ce are referred to as negative Ce anomalies (Figure la). The anomalously high rare - earth elements and Y values were particularly intriguing because Kahoolawe had been a bombing target for the U.S. Navy since World War 11. The lengthy history of bombing made us wonder whether the unusual rare earth element and Y concentrations were related to trace elements in explosives. However, a recent publication (3) citing a lava on Oahu with similar rare-earth element and Y concentrations dissuaded us from pursuing this hypothesis.
Possible explanations for rare-earth element and Y enrichments We were left with three alternative geochemical hypotheses. The first was that the upper mantle contains a rare phase t h a t is enriched with rare-earth elements and Y and occasionally is included in the partial melting t h a t produces Hawaiian magmas. The second theory involved assimilation of some unusual rareearth element/Y - bearing material (perhaps from the marine environment) by some of the magmas as they ascended from the mantle to the surface. Finally, we considered secondary processes such as surface weathering and exposure to hydtothermal solutions. We gave this idea lowest priority because the rocks in question did not appear to be weathered or altered, or to have been exposed to hydrothermal activity. Furthermore, rare-earth elements and Y are not necessarily mobilized in rocks and concentrated elsewhere during the commonly observed geologic pro cesses. We also found problems with the first two hypotheses. The rare-earth element and Y abundances of the anomalously enriched rocks did not vary systematically with other trace elements in geochemically similar rocks, nor did the rare-earth elements vary systematically among themselves. This suggested that a single, special mantle phase t h a t melted to yield rare-earth element/ Y-enriched magmas had not existed. Because the rocks were not enriched
SWAGEIDR Tube Fittings in elements likely to be abundant in a seawater environment, such as Sr and Mn, we discarded the idea of assimilated r a r e - e a r t h element/Ybearing marine material before erup tion.
Identifying the rare-earth element/ Y-bearing phase The key to finding the origin of the rare-earth element and Y enrichment in the basalts was to determine how these elements were contained. That is, which basalt mineral phases housed high amounts of rare-earth elements and Y, and how did they occur in the rocks? Was it a phenocryst phase (mineral grain visible without magnification), or was it microscopic and hidden in the groundmass of the basalts? Our detailed microscopic examination of the rare-earth element/ Y -rich samples had revealed nothing special about their mineral assem blages. All observed phases had been expected to be present in these rocks. We decided that electron microprobe analysis would be ideal for locating the phase because we would be able to scan the sample using an electron beam while the detectors were optimized for rare -earth ele ments and Y. By viewing the sample during investigation, the slightest hint of La or Y X-rays created by the beam falling on a rare-earth element/Y -rich phase would reveal the location of the phase in the rock. This method of examination could not, however, be done using the standard procedure for mineral analysis (using a 1-3-pm electron beam), because the chances were small for locating a phase that may be only micrometers in size. The investigation required optimizing the spectrometers for La and Y, enlarging the electron beam to a - 200-pm diameter, and scanning a polished specimen of rare-earth element/Y-enriched lava until a La or Y signal was detected. The search was completed in seconds. The groundmass of the lava sample examined was rich with 10-30 -pm - sized grains hidden amid the normal assemblage of pyroxene, plagioclase, and Fe-Ti oxides. From t h a t point on, we drew sketches of all the located rare-earth element/Y-rich grains and the surrounding fields of view of the specimen as we saw them through the microprobe optical system. These sketches would help us later locate and study the rare-earth element/Yrich phases with the aid of a polarizing microscope. Optical microscopy revealed irreg-
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ANALYTICAL CHEMISTRY, VOL. 64, NO. 11, JUNE 1,1992
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ANALYTICAL APPROACH ularly shaped, high-relief, somewhat fibrous-looking grains. We were unfamiliar with the properties of this phase as viewed under polarized light, but the examination nevertheless provided an understanding of how certain Kahoolawe basaltic lavas served as hosts to high rareearth element and Y levels. We could then place the lava sample back into the electron microprobe for quantitative determination of the material previously unknown in Hawaiian volcanic rocks.
Enrichment by secondary processes Concurrent with our research, F. A. Frey of the Massachusetts Institute of Technology performed neutron ac tivation rare - earth element analysis of additional samples from the same lava that we were examining with an electron microprobe. The NAA results showed that the anomalous rare-earth element and Y abundances were not evenly distributed throughout the lava. Examined from various places on
the island, the same lava flow contained rare-earth element and Y contents ranging from normal to highly enriched levels for basaltic rocks (Figure lb). This was significant because it indicated that the high rare-earth element and Y concentrations were not part of the original magma system. Had they been, the concentrations would be homogeneous in the lavas representing the magmas. Instead, the sporadic traceelement enrichments in the lavas most likely developed after the lavas had cooled to rock (-1.15 million years ago). This observation was consistent with how the rare-earth element/Ybearing material occurred in the rocks as especially small groundmass grains. Furthermore, the groundmass sites support the means of origination by secondary processes such as weathering. Our documentation of this unusual Hawaiian geochemical feature necessitated quantitative analyses for identification and visual character izations of the phases. To acquire
II
these, we used an electron microprobe with wavelength- dispersive spectrometers and the capability to do backscattered electron scanning imaging and X-ray mapping. These techniques provided grain size measurements and grain shapes along with element distribution maps of the groundmasses. We thus understood how the rare-earth element/Yrich phase was texturally interrelated with the groundmass mineral assemblages (Figure 2). To our surprise, the electron microprobe analyses revealed a phosphate with 32.4 wt % P,O, and several percent each of La, Ce, Nd, and Y oxides (see box below). The pattern in Figure 3 illustrates the high concentration of rare-earth elements in this phase compared with that of the host lava (2).Although no published analyses of phosphates were identi cal to our values, some values for the rare - earth element/Y- bearing phos phate phase known as rhabdophane were close to the composition we determined for our phase. Rhabdophane has seldom been reported in the literature, but the available information pointed to its origin by secondary mineralization. This indi cated that surface weathering processes most likely accounted for the rare-earth element and Y enrichments in Kahoolawe rocks. There was one remaining problem. Why did the Kahoolawe basalts ap-
Eomposition (wt %) of a rare-earth element/Ybearing phosphate grain
Figure 2. Backscattered electron scanning photomicrograph of rare-eaelement/Y- bearing phosphate grains (brightest areas) and photos showing distributions of selected elements as determined by X-ray mapping with an electron microprobe. The small white line at bottom right of each photo is a 10-pm scale bar.
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pear fresh yet contain a weathering product previously unknown in Hawaiian rocks? While we were ponder ing this problem, several papers about rare-earth element and Y transport in geologic systems were published ( 4 ) . Additionally, we learned of an ongoing study of Australian basalts that were also seemingly fresh but contained anomalously high rare-earth element and Y contents (5). After considering our analytical documentation of the Kahoolawe rare - earth element/Y- bearing phos phate and the results of the more recent studies, we concluded that incipient weathering of Kahoolawe rocks was one of two different but related processes responsible for concentrating rare - earth elements and Y in the basaltic rocks. The other was soil formation. Kahoolawe volcano had undergone substantial surface weathering and soil development, and rare-earth elements and Y can be transported to a weathering front and concentrated during soil formation under certain climatic conditions. The rare-earth elements and Y adhere to clays produced in rocks during the earliest breakdown of primary material, such as groundmass glass, and form secondary phases such as rhabdophane. Also, because rare - earth elements are mobile in aqueous systems, and because rare - earth element solubility is very sensitive to pH, there are opportunities during surficial alter ation for them to fractionate. In addition, Ce solubility is sensitive to oxidation potential, which can lead to a range of Ce anomalies in geologic material. Apparently, small amounts of weathering-produced clay minerals in otherwise fresh-
looking Kahoolawe rocks have served as sinks for rare-earth elements and Y mobilized during soil formation over the past million years. Although our study explains the occurrence of anomalously high trace -element concentrations in some Hawaiian basalts, it does not explain the apparent absence of rareearth element and Y enrichment in the lavas of the main body of the volcano. We can only speculate that eruptions of shield lavas were too rapid to allow weathering and soil formation to occur to the extent of mobilizing rare-earth elements and Y. We will continue our efforts to characterize and compare the rareearth element/Y - bearing phases in Kahoolawe lavas and the phases that must be present in the other reported rare-earth element/Y-enriched Hawaiian rocks, such as on Oahu. This marks a departure from traditional studies of Hawaiian geochemistry, but it will increase our understanding of rare-earth element mobility and phases in geologic systems. The National Science Foundation helped fund this work through grant EAR-8903704. We thank the personnel of the U.S.Navy's Pacific Third Fleet, Pearl Harbor, for their cooperation.
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References (1) Frey, F. A.; Roden, M. F. In Mantle Metasomatism; Menzies, M.; Hawkesworth, C., Eds.; Academic Press: London, 1987; pp. 423-63. (2) Fodor, R. V.;Frey, F. A.; Bauer, G. R.; Clague, D. A. Contrib. Mineral. Petrol., in press. ( 3 ) Roden, M. F.; Frey, F. A,; Clague, D. A. Earth Planet. Sci. Lett. 1984, 69, 141-58. (4) a. Lottermoser, B.G. Lithos 1990,24, 151-67. b. Ponader, C.W.; Brown, G. E. Geochim. Cosmochim. Acta 1989, 53, 2893-2903. c. Wood, S.A. Chem. Geol. 1990,82,159-86. (5) Price, R. C.; Gray, C. M.; Wilson, R. E.; Frey, F. A.; Taylor, S. R. Chem. Geol. 1991, 93, 245-65.
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R. V: Fodor received a Ph.D. in geology from the University of New Mexico in 1972. He is a professor of geology studying the geochemical and mineralogical compositions of igneous rocks. Gdbor Dobosi received a Ph.D. in geology from Kossuth b j o s University, Hungary, in 1980. He is a research geologist specializing in microprobe analyses of mineral phases in basalt and peridotite rocks. Figure 3. Rare-earth element pattern for the composition of the phosphate in the groundmass of Kahoolawe lavas, compared with the pattern for a lava that contains it (host lava).
G. R. Bauer received an M.S. degreefrom the University of Hawaii in 1970. He was trained as a petrologist studying igneous rocks and is now a specialist in groundwater geology.
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