JOURNAL OF GEOPHYSICAL RESEARCH VOL. 68. No. 20 OCTOBEIt 15, 1963

Potassium and Rubidium Distribution in Hawaiian Lavas

PETER LESSING, ROBERT W. DECKER, AND ROBERT C. REYNOLDS, JR.

Dartmouth College, Hanover, New Hampshire

Abstract. Forty-five Hawaiian lavas were analyzed quantitatively for K and Rb by X-ray fluorescence. These rocks are members of the tholeiitic and alkali series. The authors have divided the alkali volcanic series into the low-potassium alkali lavas (less than 2 per cent K) and the high-potassium alkali lavas (more than 2 per cent K). The K/Rb ratio in the tholei­itic and low-potassium alkali lavas is 512 ± 11, whereas values as low as 260 have been meas­ured from high-potassium alkali lavas. The K/Rb ratio in the high-potassium alkali lavas decreases systematically with increasing K content. The K/Rb ratio of 260 is similar to the ratio in continental igneous rocks. The K/Rb ratio of 512 may be representative of the upper oceanic mantle. The K/Rb ratio remains fairly constant in normally differentiated rock suites. Therefore, the distinct change in the K/Rb ratio of the high-potassium alkali lavas suggests contamination of the Hawaiian magma with Rb-rich material or differential gaseous transfer of the alkalis. The results support a nonchondritic earth model.

Introduction. The volcanic rocks of the Ha­waiian Islands form two series: the tholeiitic series and the alkali series. The origin of these two volcanic series is a matter of debate. Three general hypot.heses have been presented: (1) one primary tholeiitic magm� with a magmati­cally differentiated alkali phase [MacDonald, 1949a, 1949b; Tilley, 1950; Powers, 1955], (2) one primary tholeiitic magma, which has been contaminated to produce the alkali phase [Daly,

1944J, and (3) two primary magmas (tholeiitic and alkali) derived by partial melting of the mantle at different depths [Kuno et al., 1957; Yoder and Tilley, 1962].

K and Rb are a coherent geochemical pair. Rb has a complete affinity for K, because it forms no mineral phases of its own and is chemically similar to K (Table 1).

Worldwide geochemical evidence indicates that the K/Rb ratio of continental igneous rocks is about 240 [Ta.ylor et al., 1956; Horstman, 1957; Gast, 1960J. Gast points out that an olivine ba­salt from Hawaii has a K/Rb ratio of 559.

Several investigators [TV ager and Mitchell, 1953; N ockolds and Allen, 1954, 1956] have also determined the K and Rb content of Hawaiian rocks. The K/Rb ratios from their data range from 120 to 1800.

The present study is an attempt to evaluate the distribution of K and Rb in a suite of Ha­waiian lavas representing different petrologic types.

P7'Ocedw·e. Potassium was determined quan-

titatively by a General Electric XRD-5 spec­trometer using a Cr tube, EDT crystal, and He atmosphere. Standards were prepared from a previously analyzed potassium feldspar, diluted with A1203• Calcium was used as an internal standard to eliminate matrix effects. All stand­ards and unknowns were mixed 1: 1 with CaCO •.

The standard deviat.ion determined by Reynolds [1963J is ±2.8 per cent.

Rubidium was quantitatively analyzed by X-ray fluorescence using rock W-l as a standard with 22 ppm Rb. Corrections for matrix effects were obtained by computing mass absorption coefficients from 13 published chemical analyses covering the entire range of Hawaiian lavas studied. Corrections for all other samples were obtained by extrapolation.

The per cent standard deviation of the Rb analyses is a function of the amount present (100 ppm Rb ±11 per cent; 10 ppm Rb ±23 per cent). A detection limit of 5 ppm Rb was

picked because the standard deviation at this level is ±50 per cent.

TABLE 1. Similarities of Potassium and Rubidium

Ionic size

Valence charge

Electronegativity

E: Rb

1.33A

+1

0.8

1.47A

+1

0.8

5851

5852 LESSING, DECKER, A ND REYNOLDS

TABLE 2. Analy ses of Ha wa iian Lavas

Sample K,% Rb, ppm K/Rb Rock Type and Location

H-1-61 0.37 >I< Quartz basalt, Palolo quarry, Oahu H-2-61 0.57 10.6 ± 2.7 538 1955 basalt flow, Puna, Hawaii H-3-61 0.41 6.7 ± 2.5 612 1960 olivine basalt flow, Kapoho,

Hawaii H-4-61 0.29 >I< 1840 picrite basalt flow, Nanawale

Bay, Hawaii H-5-61 0.59 11.0 ± 2.7 536 Ank aramite, Haleakala, Maui H-6-61 1.47 25.0 ± 3 . 6 588 Hawaiite, Mauna Kea, Hawaii H-7-61 2.45 60.0 ± 6.8 408 Mugearite, Mauna Kea, Hawai i H-8-61 3.65 117.0 ± 12.9 312 Trachyte, Puu Anahulu, Hualalai,

Haw aii H-8-61g 3.30 127.0 ± 13.8 260 Trachyte glass, Puu Waawaa,

Hualalai, Hawaii H-9-61b 0.57 13.3 ± 2.8 428 1801 alkali basalt flow (matrix),

Hualalai, Hawaii H-9-61c .. >I< 1801 alkali basalt flow (nodule),

Hualalai, Hawaii H-10-61 0.22 * Melilite nepheline basalt, Kauai H-1l-61 0.40 8.0 ± 2.5 500 1960 basalt flow, Kapoho, Hawaii H-12-61 0.30 6.7 ± 2.5 448 1959 basalt flow, Kilauea Iki lava

lake crust, Hawaii H-13-61 0.41 9.0 ± 2.6 456 1960 basalt flow, Kapoho, Hawaii H-14-61 0.29 >I< Ba salt, Kilauea Mil itary Camp, Hawaii H-15-61 0.32 6.7 ± 2.5 478 1959 basalt cinder, Kilauea Iki, Hawaii H-16-61 0.32 5.6 ± 2 . 5 572 1919 basalt flow, Kilauea caldera

floor, Hawaii H-17-61 0.37 * Basalt ejecta, summit of Mauna

Loa, Ha waii 13 2.49 60.0 ± 6.8 415 Hawa iite, Mauna Kea, Hawaii 14 1.54 30.0 ± 4.0 514 Hawai ite , Paau ilo quarry, Mauna

Kea, Hawaii 15 1.54 36.0 ± 4.5 428 Phlogopite hawaiite, Mauna Kea,

Hawaii 16 2.66 69.5 ± 7.8 383 Mugearite, Kohala, Hawaii 17 3.70 123.5 ± 13.3 299 Trachyte, Puu Anahulu, Hualala i,

Hawaii 18 3.49 120.0 ± 13.1 291 Trachyte glass, Puu Waawaa,

Hualalai, Hawaii PN >I< * Pyroxene nodule, Salt Lake crater,

Honolulu, Oahu 61-61 0.72 14.5 ± 2.9 496 Nepheline basanite, Black Point, Oahu 61-66 0 . 83 15.5 ± 2.9 536 Linos ai te (neph eli ne basanite),

Hanauma Bay, Oahu 61-96B 0.67 13.0 ± 2.8 515 Basalt, Hualalai, Hawaii 61-99A 3.53 121.0 ± 13.1 292 Trachyte glass, Puu Waawaa,

Hualalai, Hawaii 61-103 1.54 29.0 ± 3 . 9 531 Hawaiite, Mauna Kea, Hawaii 61-104 2.62 74.0 ± 8.2 354 Mugearite, Puu Makea, Kohala, Hawaii 61-106 1.43 10.5 ± 2.7 1360 Mugearite, Puu Lahikiola, Kohala,

Hawaii 61-109 1.29 22.0 ± 3.4 587 Hawaiite, Puu Kawaiwai, Kohala,

Hawaii 61-127 1.16 22.5 ± 3.4 516 Tuff, Di amond Head, Oahu 61-130 1.47 32.0 ± 4.2 460 Nepheline melilite basalt, Moiliili

quarry, Honolulu, Oahu 61-131 0.97 22.0 ± 3.4 441 Nepheline basalt, N uuanu stream,

Honolulu, Oahu 1-59 0.21 * Basalt, Halemaumau, Kilauea, Hawaii

POTASSIUM AND RUBIDIUM IN HAW AllAN LA V AS

TABLE 2. (Continued) 5853

Sample K,% Rb, ppm K/Rb Rock Type and Location

2-59 0.18 * 1924 olivine basalt ejecta, Halemaumau, Kilauea, Hawaii

3-59 0.27 6.0 ± 4-59 0.47 8.0 ± 8-59 0.39 8.0 ± 9-59 0.39 6.7 ± 11-59 0.47 9.0 ±

2.5 2.5 2.5 2.5 2.6

450 588 488 582 522

Basalt flow, Halina Pali, Hawaii 1955 flow clinker, Puna, Hawaii 1955 spatter cinder, Puna, Hawaii 1955 basalt flow, Puna, Hawaii 1955 olivine basalt, Puna, Hawaii Basalt, Sulfur Bankl:l, Kilauea, Hawaii Basalt, Mendocino ridge

12-59 0.21 *

MR-1 0.22 *

MR-2 0.38 * Basalt, 40023'N, 127°58'W

G-1 4.51

W-1 0.53 220.0

22.0 205 241

Granite, Westerly, R. I. Diabase, Centerville, Va.

• Not detected

Results. The results of this investigation are presented in Table 2. Data on standard rocks W-l and G-l are also included.

Discussion. Ahrens et al. [1952J analyzed continental igneous rocks from worldwide sources for K and Rb. These rocks show little variation about a mean K/Rb ratio of 240 [Taylor et al.} 1956J. Similar analyses of rock suites from the literature [Ahrens et al.} 1952; Nockolds and Allen, 1954, 1956; Taylor et al.} 1956; Horstman, 1957J show fairly constant K/Rb ratios. It is believed from the work cited above that the K/Rb ratio remains nearly con­stant (at about 240) in magmatic ally differen­

tiated rock suites. The Hawaiian lavas analyzed in the present

study illustrate the coherence of Rb and K. Furthermore, the KjRb ratio of 512 remains

1000

600 .

. . 400

200

0.2

. . . . ..

• W-I

0.4 0.6

fairly constant throughout most of the series and then decreases to as low as 260 (Figure 1). It is interesting that the decrease in the KjRb ratio occurs within the alkali volcanic series. Such a decrease in the K/Rb ratio has not been found in other magmatic ally differentiated rock suites . Therefore, the high-potassium alkali lavas have apparently been modified by some process other than crystal settling. It is con­venient to define two types of lavas within the alkali series: those alkali lavas with an average KjRb ratio of 512 and containing less than 2 per cent K will be called the low-potassium alkali lavas, and those alkali lavas with a K/Rb ratio lower than 512 and containing more than 2 per cent K will be called the high-potassium alkali lavas.

Proponents of tke hypothesis of two primary

1.0

%K

S

2.0

0 G-I

4.0 6.0

512

240

10.0

Fig. 1. Potassium versus the K/Rb ratio in Hawaiian lavas. Solid circles, tholeiitic lavas; open circles, alkali lavas.

5854 LESSING, DECKER, AND REYNOLDS

magmas [Kuno et al., 1957; Yoder and Tilley, 1962] agree that the magmas would be tholeiitic and alkali. The present data indicate that the KjRb ratio is identical for both the tholeiitic and low-potassium alkali lavas. A modification

by the mixing of these two magmas could not

possibly result in a decrease of the K/Rb ratio in the high-potassium alkali lavas.

Daly [1944] presented an argument for con­

tamination of the alkali series by marine car­

bonates. This has been criticized by MacDonald [1949a] and Winchell [1947] on the basis that

there would not be enough marine carbonates

available. The alkali series represents only a

few per cent of the Hawaiian lavas, and only a

small portion of the alkali series is the high­

potassium type. It seems possible, however, that

contamination of the high-potassium alkali lavas

by marine argillaceous sediments may be a con­

trolling factor. Furthermore, it is known that

marine argillaceous material is enriched in Rb.

A K/Rb ratio of 205 has been recorded for red

clay from the Pacific Ocean [Horstman, 1957]. .MacDonald [1949a, p. 1567] suggests 'small parasitic magma chambers' and 'small residual

magma reservoirs' to account for some of the late-stage alkali series. These separate chambers may exist and it might be here that the high­potassium alkali lavas incorporate previously deposited argillaceous sediments, thus decreas­ing the K/Rb ratio. An alternative hypothesis

of gaseous transfer could possibly explain the decreased KjRb ratio in the high-potassium alkali lavas. Two submarine basalts from the Mendocino ridge (40023'N, 127°58'W) were also analyzed. The K and Rb are very low, thus excluding the possibility that these submarine basalts are representative of the contaminating material.

In the present study we have found a value of 512 for the K/Rb ratio of Hawaiian tholeiitic basalts and low-potassium alkali lavas. The K/Rb ratio of 512 may be representative of the upper oceanic mantle and (following Gast [1960]) would support a nonchondritic earth model. (Gast finds that chondritic meteorites have a K/Rb ratio of 280, which is similar to that of continental rocks.)

Conclusions. 1. There is a systematic co­herence of Rb and K in Hawaiian lavas.

2. The results show that the KjRb ratio of Hawaiian tholeiitic basalts and low-potassium

alkali lavas (less than 2 per cent K) is 512 ± �l 3. There is a systematic decrease in th�

KjRb ratio from 512 to 260 with increasing 1\ in the high-potassium alkali lavas.

4. The results suggest contamination of the high-potassium alkali lavas with Rb-rich mate­rial (e.g., argillaceous sediments).

5. A K/Rb ratio of 512 is suggested for the upper oceanic mantle.

6. The results support a nonchondritic earth model.

Acknowledgments. We wish to express Our Bin. cere thanks to James Moore, Charles Thornton and Y. R. Nayudu, who very kindly supplied manl: of the samples for this study. John Lyons and Richard Stoiber critically read the manuscript and offered valuable suggestions.

Financial assistance of the Dartmouth College Geology Department is greatly appreciated.

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Daly, R. A., Volcanism and petrogenesis as illus­trated in the Hawaiian Islands, Bull. Geol. Soc. Am., 55, 1363-1399, 1944.

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POTASSIUM AND RUBIDIUM IN HAWAIIAN LAVAS 5855

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(Manuscript received May 16, 1963; revised July 29, 1963.)