ANALYTICAL

ANOMALOUSLY NIGO RARE - EARTH ELEMENT ABUNDANCES IN HAWAIIAN LAVAS

R. V. Fodor Department of Marine, Earth, and

Atmospheric Sciences North Carolina State University Raleigh, NC 27695-8208

Gábor 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 char­acteristics 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 con­s t ruc t the i s l ands did not pa s s through potentially contaminating material. Chemical compositions of Hawai i an 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

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 mant le compo­nents tha t may have mixed before and during magma generation. An­other objective is to define the pro­cesses by which magmas change composition during storage in sub-volcano 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-ear th 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 - 6 0 -90 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 s t ra t igraphie 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-

0003-2700/92/0364-639A/$02.50/0 © 1992 American Chemical Society

ANALYTICAL CHEMISTRY, VOL. 64, NO. 11, JUNE 1, 1992 · 639 A

Niihau

Kauai

Oahu

Lanai

Molokai Maui

Hawaii mmrm

ANALYTICAL APPROACH

dividual volcanoes and helped to characterize their mantle source ma­terial. The rare-ear th elements are an important part of these evalua­tions. Part icularly valuable is the geochemical behavior of ra re-ear th 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 ra re -earth 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 ra re -ear th 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 volca-n ism, r a r e - e a r t h e lement a b u n ­dances 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 nor­m a l i z e d to 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-Har-kins effect of higher concentrations for elements with even atomic num­bers.

The patterns for various composi­tional types of Hawaiian lavas have been characterized (1) and can be reasonably well predicted for a par­ticular 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

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 at Pearl Harbor. Providing helicopter transportation and demo­lition experts who escorted 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-ear th element abundances that were dramat ica l ly 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-ear th 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 1 0 0 - 2 0 0 ppm. The

Figure 1. Patterns obtained using 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.

rocks with high rare-ear th 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 ra re -ea r th elements studied. Such occurrences of comparatively low Ce are referred to as negative Ce anom­alies (Figure la).

The anomalously high rare-ear th elements and Y values were particu­larly intriguing because Kahoolawe had been a bombing target for the U.S. Navy since World War II. 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 r a r e -ea r th element and Y concentrat ions 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 r a re phase t h a t is enriched wi th rare-ear th elements and Y and occa­sionally is included in the part ia l me l t ing t h a t produces Hawa i i an magmas. The second theory involved assimilation of some unusual rare-ear th e lement /Y-bear ing mater ia l (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 hydrothermal 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-ear th elements and Y are not necessarily mobilized in rocks and concentrated elsewhere during the commonly observed geologic pro­cesses.

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 r a re -ea r th ele­ments vary systematically among themselves. This suggested tha t a single, special mant le phase t ha t melted to yield rare-ear th element/ Y-enriched magmas had not existed. Because the rocks were not enriched

640 A · ANALYTICAL CHEMISTRY, VOL. 64, NO. 11, JUNE 1, 1992

(a) Enriched lavas

Typical shield lava

Atomic number

(b) Highly enriched

Normal

Slightly enriched

Atomic number

in elements likely to be abundant in a seawater environment, such as Sr and Mn, we discarded the idea of as­s imi la ted r a r e - e a r t h e l emen t /Y-bearing marine material before erup­tion.

Identifying the rare-earth element/ Y-bearing phase The key to finding the origin of the ra re -ea r th 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-ear th 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-ear th element/ Y-rich samples had revealed nothing special about their mineral assem­blages. All observed phases had been expected to be present in these rocks.

We decided tha t electron micro-probe 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 ra re -ear th ele­ments and Y. By viewing the sample during investigation, the slightest hint of La or Y X-rays created by the beam falling on a r a r e -ea r th 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 l -3-μπι electron beam), be­cause the chances were small for lo­cating a phase t ha t may be only micrometers in size. The investiga­tion required optimizing the spec­trometers for La and Y, enlarging the electron beam to a - 200-μπι diame­ter, and scanning a polished speci­men of r a r e - e a r t h e lement /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^m-s ized grains hidden amid the normal assemblage of pyroxene, plagioclase, and Fe-Ti oxides.

From t h a t po in t 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 op t ica l s y s t e m . T h e s e sketches would help us later locate and study the rare-earth element/Y-rich 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 · 641 A

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 unders tanding of how certain Kahoolawe basaltic la­vas served as hosts to high ra re -earth 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 Hawai ian volcanic rocks.

Enrichment by secondary processes Concurrent with our research, F. A. Frey of the Massachusetts Institute of Technology performed neutron ac­tivation rare-earth element analysis of additional samples from the same lava that we were examining with an electron microprobe. The NAA re­sults showed t ha t the anomalous r a r e - e a r t h e lement and Y abun­dances were not evenly distributed throughout the lava.

Examined from various places on

the island, the same lava flow con­ta ined r a r e - e a r t h element and Y contents rang ing from normal to highly enriched levels for basaltic rocks (Figure lb). This was signifi­cant because it indicated tha t the high rare-ear th 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 trace-element enr ichments 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-ear th element/Y -bear ing ma te r i a l occurred in the rocks as especially small groundmass gra ins . Fur thermore , the ground-mass sites support the means of orig­ination by secondary processes such as weathering.

Our documentation of this unusual Hawaiian geochemical feature neces­s i ta ted quan t i t a t i ve analyses for identification and visual character­izations of the phases. To acquire

these, we used an electron micro-probe 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-ear th element/Y -rich phase was texturally in terre­lated with the groundmass mineral assemblages (Figure 2).

To our surprise, the electron mi­croprobe analyses revealed a phos­phate with 32.4 wt % P 2 0 5 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-ear th elements in this phase compared with that of the host lava (2). Although no published analyses of phosphates were identi­cal to our values, some values for the rare-earth element/Y-bearing phos­phate phase known as rhabdophane were close to the composition we de­t e r m i n e d for our p h a s e . R h a b ­dophane has seldom been reported in the literature, but the available in­formation pointed to its origin by secondary mineralization. This indi­cated that surface weathering pro­cesses most likely accounted for the r a re -ea r th element and Y enrich­ments in Kahoolawe rocks.

There was one remaining problem. Why did the Kahoolawe basalts ap-

Figure 2. Backscattered electron scanning photomicrograph of rare-earth element/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-μιη scale bar.

Composition (wt %) of a rare-earth element/ Y -bearing phosphate grain

642 A · ANALYTICAL CHEMISTRY, VOL. 64, NO. 11, JUNE 1, 1992

Backscattered electron image Ρ Nd

Ce Y Si

P2Os 32.4 FeO 0.75 CaO 0.93 La203 5.9 Ce203 16.7 Pr203 3.4 Nd203 14.8 Sm203 4.3 EuO 1.5 Gd203 3.4 Tb203 0.28 Dy203 2.4 Er203 0.92 Yb203 0.92 Y203 8.4 H20, F, CI ND

Total 97.0

Analysis by electron microprobe; ND indicates not determined.

Sample preparation for XRF-AA-ICP-CHEM.

pear fresh yet contain a weathering product previously unknown in Ha­waiian rocks? While we were ponder­ing this problem, several papers about r a r e - e a r t h e lement and Y transport in geologic systems were pub l i shed (4). Add i t i ona l l y , 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 documenta t ion of the Kahoolawe rare-earth element/Y-bearing phos­phate and the results of the more re­cent studies, we concluded that in­cipient wea the r ing of Kahoolawe rocks was one of two different but re­lated processes responsible for con­centrating rare-ear th elements and Y in the basaltic rocks. The other was soil formation.

Kahoolawe volcano had undergone substantial surface weathering and soil development, and rare-ear th ele­ments and Y can be transported to a weathering front and concentrated during soil formation under certain climatic conditions. The rare-ear th 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 r a r e -ea r th elements are mobile in aqueous systems, and because rare-earth element solubil­ity is very sensitive to pH, there are opportunities during surficial alter­ation 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 wea ther ing-produced clay minerals in otherwise fresh-

m

dr

10b i e

Cho

104ΐ Rare-earth element

e,

phosphate Q. Ε i o 3 i __ Mnct lawa

CO m2 5758 60 62 63 65 7071

Atomic number

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).

looking Kahoolawe rocks have served as sinks for rare-ear th elements and Y mobilized during soil formation over the past million years.

Although our study explains the occur rence of anoma lous ly h igh t r a c e - e l e m e n t co n cen t r a t i o n s in some Hawaiian basalts, it does not explain the apparent absence of rare-earth element and Y enrichment in the lavas of the main body of the vol­cano. We can only speculate t ha t eruptions of shield lavas were too rapid to allow weathering and soil formation to occur to the extent of mobilizing rare-ear th elements and Y. We will continue our efforts to characterize and compare the rare-earth element/Y-bearing phases in Kahoolawe lavas and the phases that must be present in the other reported rare-ear th 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 ra re -ear th 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.

References (1) Frey, F. Α.; Roden, M. F. In Mantle

Metasomatism; Menzies, M.; Hawkes-worth, C, Eds.; Academic Press: Lon­don, 1987; pp. 423-63.

(2) Fodor, R. V.; Frey, F. Α.; Bauer, G. R.; Clague, D. A. Contrib. Mineral. Petrol., in press.

(3) Roden, M. F.; Frey, F. Α.; 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. Α.; Taylor, S. R. Chem. Geol. 1991, 93, 245-65.

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.

Gabor Dobosi received a Ph.D. in geology from Kossuth Lojos University, Hungary, in 1980. He is a research geologist spe­cializing in microprobe analyses of min­eral phases in basalt and peridotite rocks.

G. R. Bauer received an M.S. degree from the University of Hawaii in 1970. He was trained as a petrologist studying igneous rocks and L· now a specialist in ground­water geology.

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