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The oceanic sector of the Cameroon line consists of three volcanic islands: Principe, Silo Tome and Pagalu. New 4°Ar-39Ar data for Pagalu basalts, combined with published K-Ar ages for Principe and S~to TomE, indicate that all three islands have been active in the past 5 Ma. They have similar petrogenetic histories, with basements of basaltic flows capped by more evolved rocks. However, the age of the earliest exposed volcanic rocks decreases occanward from Principe (31 Ma) to Sao Tome (13 Ma) to Pagalu (4.8 Ma). This age progression is consistent with the suggested motion of the African plate over this period of time. The average incompatible trace element compositions of < 10 Ma lavas with >/4 wt% MgO on each island are very similar. However, (87Sr/S6Sr)t increases from 0.7029 to 0.7037 and (2°6pb/2°4pb) t decreases from 20.2 to 18.9 from Principe through Silo Tome to Pagalu for all samples younger than 10 Ma. In addition to the overall spatial isotopic variations, Principe and Silo Tome display temporal isotopic variations, with Pb isotopic ratios becoming progressively more radiogenic. Pagalu shows no temporal geochemical Or isotopic differences and the island has the least radiogenic Pb but most radiogenic Sr. These distinctive Pb, Sr and Nd isotopic compositions are also found in the early tholeiitic hyaloclastite breccia from Principe (31 Ma). Similarly, the Nd and Sr isotopic compositions are identical to those of the earliest S'ao Tome lavas (13 Ma) and the Pb isotopic compositions of early Sao Tome samples are only slightly radiogenic relative to Pagalu. Therelk)re, it is probable that all these islands were initiated from a common source, similar to that of Pagalu, that migrated relative to the melt zone of each island with time. Since their initiation, the magma conduits at Principe and Silo Tome have been gradually modified by the introduction of a HIMU component. The common source from which the islands were initially derived probably represents ambient upper mantle, entrained with the plume head during ascent. This entrained component is like 'PREMA', but the Nd and Sr isotopic data indicate that it represents variably mixed depleted and enriched components, such as DMM and EMI. The HIMU component is probably representative of a lower mantle source from which the plume head was derived. The long-lived episodic magmatism on Principe provides evidence that the initial melt migration paths from the upper mantle form a hot zone that can be re-activated after long periods (107 yr) of apparent quiescence. The progression to HIMU characteristics within each island probably reflects the gradual flattening of the contaminated plume head within this hot zone, near the base of the lithosphere, and the melting of a stem composed of relatively uncontaminated HIMU mantle. 0012-821X/94/$07.00

The concentrations of incompatible trace elements in ocean island basalts (OIB) from the central Atlantic extend to relatively enriched and fractionated compositions in regions of older oceanic lithosphere. Certain trace element ratios normally considered to be uniform in the mantle, such as Ce/Pb, are particularly variable. However, other trace element ratios that are expected to be variable because of differences in bulk distribution coefficient, such as Cc/U, are relatively uniform. The Ce/Pb ratios in enriched OIB are correlated with unusually high U/PI, and low K/U. These U/Pb ratios would have generated excessively radiogenic Pb if they were long-term (lo9 yr) features of the source such as might result from core formation or recycling of h&rothermally altered ocean floor basalts. However, volcanic centers with high U/W do have high 206Pb/ 204Pb for their Pb/ 204Pb, a feature that is most easily modelled by enrichment in U relative to Pb about 10' yr prior to melting, a time similar to the age of the lithosphere. We propose that the source regions of these magmas are enriched by the introduction of small degree partial melts soon after the formation of the oceanic lithosphere. Metasomatism of the uppermost mantle by small degree partial melts produced in equilibrium with a combination of residual upper mantle major silicate phases, together with minor amphibole ( 5 2%), sulfide (< 0.2%) and phlogopite (< 0.2%) at about the time of formation of the lithosphere, would generate a 'near-surface fractionated' (NSF) source with low K/U and high U/Pb, AzosPb/*Pb and Ce/Pb, while maintaining Cc/U, Nb/U, Ba/C!e and Ba/Nb that are only slightly fractionated relative to other OIB. An important feature of the modelling of NSF mantle is that U is more incompatible than Ba or Rb. This is confirmed by the variability in incompatible trace element ratios with U concentration for enriched OIB. However, this contrasts with the relative incompatibility deduced from U-Th-Ra disequilibrium data for MORB and OIB, endorsing the view that the variability in highly incompatible trace element ratios in enriched OIB is dominated by source enrichment effects that are distinct from the fractionation that takes place during the production of the erupted magmas.The Cc/U, Ba/Ce and U/Pb ratios of all OIB, including enriched OIB from regions of old lithosphere, are uniform relative to data for MORB. This appears inconsistent with the degree of isotopic variability in OIB relative to MORB and is difficult to explain unless the variations in incompatible trace element ratios in MORB are dominated by effects other than melting. Ratio-element plots provide evidence that the incompatible element ratios of MORB are affected by OIB-component contamination in the source or in transit to the surface and this is consistent with covariation between trace element ratios and some isotopic compositions in MORB. The ratios and concentrations of highly incompatible trace elements in MORB vary as a consequence of this contamination, as well as deg...

In microgravity, crystals of semiconductors and proteins can be grown with improved crystallinity, offering the prospect of improved structural analyses (for proteins) and better electronic properties (for semiconductors) 1-3. Here we study the effect of a microgravity environment on the crystallization of a class of materials-layered microporous tin(IV) sulphides 4-11-whose crystal structure is determined by weak interlayer interactions (electrostatic, hydrogen-bonding and van der Waals) as well as strong intralayer covalent bonds. We find that the crystals grown in microgravity (on board the Space Shuttle Endeavour) show improved crystal habits, smoother faces, greater crystallinity, better optical quality and larger void volumes than the materials grown on Earth. These differences are due at least in part to the