ANllMAlllUSl" HIGH 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
Geochemical Research Hungarian Academy of Sciences Budapest, Hungary 1 1 12
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 understand- ing 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 mag- mas that erupted as lavas to con- struct the islands did not pass through potentially contaminating material. Chemical compositions of Hawaiian lavas, therefore, com- pletely 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 impor- tance 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 charac- teristics 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 compo- nents that may have mixed before and during magma generation. An- other objective is to define the pro- cesses by which magmas change composition during storage in sub- volcano reservoirs before eruption.
Dur ing t h e 1980s numerous geochemists and igneous petrologists produced highly refined models for
Hawaiian magmatism. However, in- formation from new chemical studies of Hawaiian lavas is not always com- patible 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 ba- salt composition magmas produced by partial melting of the mantle. The melting occurred at depths of -60- 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 la- vas, 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 spat- ter that may mark the final erup- tions of a volcano.
Hundreds of analyses of Hawaiian volcanic rocks have established the detailed chemical makeup of the in-
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dividual volcanoes and helped to characterize their mantle source ma- terial. The rare-earth elements are an important part of these evalua- tions. 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 rare- earth elements (Yb and Lu) preferen- tially 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 el- ement 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 crystal- lized in basaltic magma during vol- cano development.
Rare-earth elements in lavas To decipher geochemical signatures and to improve petrogenetic (rock or- igin) models for ocean island volca- nism, rare-ear th element abun- 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 ba- saltic rock. Typically, the concentra- tions (in parts per million) are nor- malized to average values for chondritic meteorites and plotted as rare - earth element patterns (Figure 1). Normalizing the data to chon- drites provides reference to material that approximates the earths mantle composition. It also enables one to graphically compare rare-earth ele- ment concentrations among different rocks by eliminating the Oddo-Har- kins effect of higher concentrations for elements with even atomic num- bers.
The patterns for various composi- tional 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 geochemi- cal investigation of Kahoolawe Island would be fairly routine and the trace element compositions somewhat pre- dictable.
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 collec- tion of -200 samples was obtained with permission from U.S. Navy per- sonnel a t Pearl Harbor. Providing helicopter transportation and demo - lition experts who e,scorted us to en- sure 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 Ka- hoolawe 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 (Fig- ure la). For example, the concentra- tion of La, which would typically be 13-15 ppm in the basaltic rocks ex- amined, was 100-200 ppm. The
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.
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 la- vas 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 anom- alies (Figure la).
The anomalously high rare - earth elements and Y values were particu- larly 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 ex- plosives. However, a recent publica- tion (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 that is enriched with rare-earth elements and Y and occa- sionally is included in the partial melting tha t produces Hawaiian magmas. The second theory involved assimilation of some unusual rare- earth element/Y - bearing material (perhaps from the marine environ- ment) by some of the magmas as they ascended from the mantle to the sur- face. Finally, we considered second- ary processes such as surface weath- ering and exposure to hydtothermal solutions. We gave this idea lowest priority because the rocks in ques- tion 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 ele- ments vary systematically among themselves. This suggested that a single, special mantle phase that melted to yield rare-earth element/ Y-enriched magmas had not existed. Because the rocks were not enriched
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in elements likely to be abundant in a seawater environment, such as Sr and Mn, we discarded the idea of as- similated rare -ear th element/Y - bearing 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 enrich- ment 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 oc- cur 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 ex- amination 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 micro- probe analysis would be ideal for lo- cating 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 ele- ment/Y -rich phase would reveal the location of the phase in the rock. This method of examination could
not, however, be done using the stan- dard procedure for mineral analysis (using a 1-3-pm electron beam), be- cause the chances were small for lo- cating a phase that may be only micrometers in size. The investiga- tion required optimizing the spec- trometers for La and Y, enlarging the electron beam to a - 200-pm diame- ter, and scanning a polished speci- men of rare-earth element/Y-en- riched 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 sur- rounding fields of view of the speci- men as we saw them through the mi- croprobe optical system. These sketches would help us later locate and study the rare-earth element/Y- rich phases with the aid of a polariz- ing microscope.
Optical microscopy revealed irreg-
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ularly shaped, high-relief, somewhat fibrous-looking grains. We were un- familiar with the properties of this phase as viewed under polarized light, but the examination neverthe- less provided an understanding of how certain Kahoolawe basaltic la- vas served as hosts to high rare- earth element and Y levels. We could then place the lava sample back into the electron microprobe for quantita- tive 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 re- sults showed that the anomalous rare-earth element and Y abun- dances were not evenly distributed throughout the lava.
Examined from various places on
the island, the same lava flow con- tained rare-earth element and Y contents ranging from normal to highly enriched levels for basaltic rocks (Figure lb). This was signifi- cant because it indicated that the high rare-earth element and Y con- centrations were not part of the orig- inal magma system. Had they been, the concentrations would be homoge- neous in the lavas representing the magmas. Instead, the sporadic trace- element 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/Y- bearing material occurred in the rocks as especially small groundmass grains. Furthermore, the ground- mass sites support the mea...