OLR (1985) 32 (12) D. Submarine Geology and Geophysics 1031

10 kbar and the genesis of MORB's. Contr. Miner. Petrology, 90(1): 18-28.

Experiments performed at 10 kbar with peridotite- basalt 'sandwiches' to assess the effect of modal variations in source peridotite and the effect of temperature (degree of partial melting) on the composition of partial melts indicate that partial melting cannot be regarded as isobarically pseudoin- variant. Liquids obtained prescribe only an area on the OL-PL-CPX-SIL normative tetrahedron, and some have a composition similar to magnesian MORB's, supporting their derivation from partial melting of lherzolite at about l0 kbar. Earthquake Res. Inst., Univ. of Tokyo, Tokyo 113, Japan. (fcs)

85:7166 Humphris, S.E., Geoffrey Thompson, J.-G. Schilling

and R.H. Kingsley, 1985. Petr61ogical and geochemical variations along the Mid-Atlantic Ridge between 46°S and 32°S: influence of the Tristan da Cunha mantle plume. Geochim. cos- mochim. Acta, 49(6):1445-1464.

The influence of the Tristan da Cunha mantle plume is inferred from mineralogical and geochemical data on basalts dredged from the Mid-Atlantic Ridge. Major, trace, and REE composition demonstrate the existence of 2 basaltic types--one depleted in incompatible and LRE elements (N-type MORB) and one with 'enriched' geochemical characteristics (T-type MORB). Mixing hyperbolae for incompat- ible element and REE ratios support a composite model of evolution in which the ridgecentered hotspot of 100 to 80-60 mya migrated westward to become a near-ridge hotspot 60-80 mya, and would finally become totally intraplate, no longer affecting mid-ocean geochemistry. Dept. of Chem., WHOI, Woods Hole, MA 02543, USA. (hbf)

85:7167 Jambon, A., H.W. Weber and F. Begemann, 1985.

Helium and argon from an Atlantic MORB glass [30°41 "N, 41°49 "W]: concentration, distribution and isotopic composition. Earth planet. Sci. Letts, 73(2-4):255-267.

Gases from vesicles, released upon crushing, are characterized by a 4He/4°Ar ratio of 6_ + 1 and a 4°Ar/36Ar ratio of up to 22,600. In dissolved gases the 4He/4°Ar ratio was found to be 7.2 _+ 1.6 times higher and the 4°Ar/36Ar ratio about ten times lower than in vesicles. The difference of the elemental ratio H e / A t is as anticipated from the ratio of the solubilities in the investigated basalt of He and At. Thus, while elemental abundance ratios are compatible with equilibrium between vesicles and basalt the grossly different argon isotopic ratios are not. It is proposed

that two basalts, chemically very similar but with different 4°Ar/36Ar ratios, were mixed shortly before eruption. The 4He/a°Ar ratio in the primary magma is considerably higher than the radiogenic produc- tion ratio in all conceivable sources of MORB. Preferential removal from a melt of argon via vesicles is suggested; any metasomatic transfer seems unrealistic. GIS CNRS-BRGM, 1 A rue de la Ferollerie, 45045 Orleans, France.

85:7168 Poreda, Robert, 1985. Helium-3 and deuterium in

back-arc basalts: Lan Basin and the Mariana Trough. Earth planet. Sci. Letts, 73(2-4):244-254.

In Mariana Trough basalts, the concentration and D/H ratio of the water provide important evidence for a water-rich component from the subducting lithosphere. 3He/4He ratios are, in general, similar to MORB values and indicate that Mariana Trough basalts represent a mixture of a MORB-type mantle and an alkali and water-rich component from the descending slab. In contrast, the Lau Basin produces both hydrous and relatively anhydrous basalts. The D / H ratio in the hydrous Lau Basin basalt resembles those of Mariana Trough lavas. Low water contents and MORB-like D / H ratios in three of five Lau Basin samples show that the water-rich component, observed in all Mariana Trough lavas, is absent in some Lau Basin lavas and not essential for the production of back-arc basalts. The high 3He/4He ratio in the northern Lau Basin provides evidence for an enriched mantle source region perhaps related to the high-3He Samoan 'hotspot'. Scripps Inst. of Oceanogr., La Jolla, CA 92093, USA.

D290. Crust, mantle, core

85:7169 Chan, W.W. and B.J. MitcheLl, 1985. Intraplate

earthquakes in northern Svalbard. Tectonophysics, 114(1-4):181-191. Dept. of Earth and Atmos. Sci., Saint Louis Univ., St. Louis, MO 63156, USA.

85:7170 Jaupart, Claude and Ariel Provost, 1985. Heat

focussing, granite genesis and inverted meta- morphic gradients in continental collision zones. Earth planet, Sci. Letts, 73(2-4):385-397. Lab. de Geochim. et Cosmochim., Inst. de Phys. du Globe de Paris, 4, place Jussieu, 75230 Paris Cedex 05, France.