OLR (1987) 34 (9) D. Submarine Geology and Geophysics 767

87:5099 Cullen, Andrew and A.R. McBirney, 1987. The

volcanic geology and petrology of Isla Pinta, Galapagos Archipelago. Geol. Soc. Am. Bull., 98(3):294-301.

The geologic development and petrogenesis of Pinta, on the northern edge of the Galapagos Archipelago, are consistent with models of the Galapagos which propose the transfer of enriched asthenosphere and melts from the hotspot to the ridge. The island itself marks the top of a shield volcano, emergent ~0.7 m.y. ago, which includes an older shield stage of tholeiitic basalts that underwent shallow-level frac- tionation in a periodically replenished magma chamber and a younger fissure stage of olivine tholeiites from the same mantle source that have a higher concentration of incompatible trace elements, probably due to a lower degree of partial melting. School of Geol. and Geophys., Univ. of Oklahoma, Norman, OK 73069, USA. (hbf)

87:5100 Dickin, A.P., 1987. Cerium isotope geochemistry of

ocean island basalts. Nature, Lond., 326(6110): 283-284.

The author reports cerium isotope ratios for eight ocean island basalts. When plotted against published neodymium isotope data these form a 'mantle array' of compositions which indicates coherent behaviour of the two elements during mantle differentiation. This precludes the recycling into the deep mantle of ocean-floor sediments with a large net cerium anomaly. Dept. of Geol., McMaster Univ., Ham- ilton, ON L8S 4Ml, Canada.

87:5101 Graham, D.W., W.J. Jenkins, M.D. Kurz and Rodey

Batiza, 1987. Helium isotope disequilibrium and geochronology of glassy submarine basalts. Na- ture, Lond., 326(611 I):384-386.

The ( U + T h ) / H e dating method is the oldest method of geochronology that involves radioactivity, but it has not been particularly successful. Isotope disequilibrium in the 3He/4He ratio between helium trapped in vesicles and that dissolved in the glass phase of some young seamount basalts can be used to determine (U +Th) /He ages for the basalts. The 3He/4He in vesicles (extracted by crushing in-vacuo) is used to correct the dissolved phase He (by fusion of the remaining powder) for the inherited com- ponent, and compute the radiogenic helium con- centration. The method is applicable to rocks containing phases with different (U +Th)/He, and the results have implications for dating lavas in the age range of 103 to 106 years (thus filling a gap

between the limits of the ~4C and K-Ar methods), and for reconstructing the geochemical history of young volcanic systems. Dept. of Chem., WHOI, Woods Hole, MA 02543, USA.

87:5102 Lanphere, M.A. and F.A. Frey, 1987. Geochemical

evolution of Kohala Volcano, Hawaii. Contr. Miner. Petrology, 95(1):100-113. USGS, 345 Middlefield Rd., Menlo Park, CA 94025, USA.

87:5103 Laverne, Christine, 1987. Unusual occurrences of

aegirine-auglte, fassaite and melanite in oceanic basalts (DSDP Hole 504B). Lithos, 20(2):135- 151.

A very unusual replacement of magmatic augite by secondary Ti- or Na-rich augite and aegirine-augite was found in five veins below 298 m sub-basement depth in Hole 504B on the south flank of the Costa Rica Rift. In addition, melanite was found in one of the veins and fassaite in another. The formation of these minerals and the fluids from which they were derived are discussed. The evidence suggests a deuteric rather than a hydrothermal origin. Ecole Natl. d'Ingenieurs de Sfax, B.P. W, 3038 Sfax, Tunisia. (hbf)

87:5104 le Roex, A.P., H.J.B. Dick, Levent Gulen, A.M. Reid

and A.J. Erlank, 1987. Local and regional heterogeneity in MORB from the Mid-Atlantic Ridge between 54.5°S and 51°S. Evidence for geochemical enrichment. Geochim. cosmochim. Acta, 51(3):541-555.

Basaltic lavas from the Bouvet triple junction range in texture from aphyric to highly olivine or plagi- oclase porphyritic. Olivine ___ Cr-spinel __+ plagio- clase are the dominant phenocryst phases, and clinopyroxene occurs as a phenocryst in many lavas. The range in differentiation can be attributed to crystal fractionation/accumulation. The geochemi- cal signature of enriched MORB from this region is distinct from that of the Bouvet mantle plume and the Southwest Indian and American-Antarctic ridges; it results from source region mixing achieved by low volume partial melts veining normal depleted mantle. Subsequent partial melting during upwelling has led to the enrichment observed in the MORB. Dept. of Geol., Univ. of Cape Town, Rondebosch 7700, South Africa.

g'/:5105 L~otot, Christophe and Robert Brousse, 1987.

Between the two Tahiti islands, the Taravao region is an autonomous volcano, disrupted by