W. B. BRYAN Woods Hole Oceanografhic Institution, Woods Hole, Massachusetts 02543

Cummingtonite-Dacite Drift Pumice Stranded at

West Palm Beach, Florida

The Atkntic Ocean contains rektively few In spite of the simikrities in refractive index active volcanoes and, unlike the Pacific, is not of the glass to these other pumice materials, the encircled by a volcanic "ring of fire." It is not West Palm Beach pumice is distinctive in its surprising then, that drift pumice is rarely mineralogy. Neither amphibole nor biotite observed on beaches along the eastern coast has been observed in the South Sandwich or of North America. In November 1969, K. O. Coral Sea pumice. According to Baker (1968), Emery collected three small pieces of pumice hydrous ferromagnesian minerals are rare in from the beach at West Palm Beach, Florida. kvas of the South Sandwich Isknds but are These and other similar fragments were lying common in those of the West Indies. The loosely on the surface near the high-tide line amphibole is a cummingtonite, pale green in and appeared to be recently deposited. Super- color with very faint pleochroism. The refrac- ficially, they are similar to the South Sandwich tive indices are in the range 1.630 to 1.650, but Islands pumice described by Cass and others, adhering glass and the fibrous habit prevented (1963). The three samples, arbitrarily kbelled precise determination of optical properties. A, B, and C, were studied to determine whether Crystal boundaries are sharp and clear, and or not the pumice could be traced to a closer there is no evidence of marginal reaction with source. the adjacent pumice glass. Although cum- All three pumice fragments are well rounded mingtonite has been reported from silicic and are about 3 cm in average diameter. They isknd-arc kvas (Kuno, 1938), it appears to be are composed of white frothy glass enclosing rare and there are few published analyses of widely scattered flakes of biotite. In fragment this mineral in volcanic occurrences. A micro- A, the gkss bubbles are drawn out into long probe analysis (Table 1) shows that the tubular cavities; in the B sample, the bubbles cummingtonite is near the magnesium-rich are flattened, giving the pumice a pkty cleav- limit of the naturally occurring minerals, age; and in the C sample, the vesicles are sub- spherical, giving the pumice a spongy texture. TABLE 1. CUMMINGTONITE FROM DRIFT PUMICE SAMPLE "B" There are also minor mineralogical differences, Oxide Wt I

the B sample containing a few dark fibrous S102 53.70 crystals of asbestiform amphibole, while a AUO, 2.23 T102 0.30 few phenocrysts of quartz and feldspar appear FeO 16.50 MnO 1.23 in sample C. CaO 1.80 Refractive indices of the pumice glass were MgO 21.40 Na20 0.41 measured at constant temperature using a K20 0.00 procedure similar to that described by Bryan Total 97.56 (1971). The refractive index of the A pumice Cations per 23 oxygens is 1.4973, and of the B sample, 1.4993; while S1 7.675 Al 0.325 8.000 the C sample is 1.5063; with a precision of about Al 0.052 ± .0002. The refractive index of sample A is T1 0.32 Fe 1.976 very close to the value of 1.496 ± .003 reported Mn 0.149 Ca 0.276 7.163 by Cass and others (1963) for the South Sand- Mg 4.563 wich Isknd pumice, while sample C is very Na 0.115 close to the values reported for the Coral Sea K 0.000 light pumice (Bryan, 1968, 1971). 100 Mg/(M9+Fe*Mn) = 68.2

Geological Society of America Bulletin, v. 83, p. 3745-3746, December 1972 3745

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which is at about 70 mol percent (OH)j- Grant GP 4384, and petrographic studies were MgySisC^. The summation is considered supported by the Office • of Naval Research satisfactory, as amphiboles typically contain under contract number N00014-66-C-0241. about 2 weight percent HzO. A reasonable source for this pumice may be REFERENCES CITED found in the West Indies on the southwest Baker, P. E., 1968, Comparative volcanology and coast of St. Lucia. Robson and Tomblin (1966, petrology of the Atlantic island arcs: Bull. p. 42-44) described the dacite of the domes of Volcanol., v. 32, p. 186-206. the Qualibou caldera. This dacite contains 5 Bryan, W. B., 1968, Low-potash dacite drift pumice from the Coral Sea: Geol. Mag., v. 105, p. to 15 percent quartz phenocrysts, accompanied 431-439. by biotite, amphibole, and ferrohypersthene 1971, Coral Sea drift pumice stranded on Eua phenocrysts. They stated that in the Belfond Island, Tonga, in 1969: Geol. Soc. America domes the amphibole is an iron-rich cumming- Bull., v. 82, p. 2799-2812. tonite, but no compositional data are given. Cass, I. G., Harris, P. G., and Holdgate, M. W., Their map (p. 41) shows the Belfond pumice 1963, Pumice eruption in the area of the South flow reaching the coast along a front of more Sandwich Isknds: Geol. Mag., v. 100, p. 321- than 2 mi, where small amounts of pumice 330. might readily be released periodically by storm Kuno, H., 1938, On the occurrence of a primary waves. Pumice so released would tend to be cummingtonitic hornblende in some dacites from Japan: Imp. Acad. Tokyo Proc., v. 14, p. carried into the Caribbean and then north 221-224. along the Florida coast by the Gulf Stream. Robson, G. R., and Tomblin, I. F., 1966, Catalogue of the active volcanoes of the world including Solfatara Fields, pt. 20, West Indies: Rome, ACKNOWLEDGMENTS Internat. Assoc. Volcanology, 56 p. Refractive index measurements and micro- probe analyses were done at the Geophysical MANUSCRIPT RECEIVED BY THE SOCIETY MAY 8, Laboratory, Carnegie Institution of Washing- 1972 ton. The microprobe facility was purchased CONTRIBUTION No. 2871 OF THE WOODS HOLE with the aid of National Science Foundation OCEANOGRAPHIC INSTITUTION

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