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To link to full-text access for this article, visit this link: http://dx.doi.org/10.1016/j.earscirev.2009.11.001 Byline: Alan R. Hastie, Andrew C. Kerr Keywords: Caribbean oceanic plateau; mantle plume; slab window; primary magma; Curacao Abstract: The Caribbean oceanic plateau formed in the Pacific realm when it erupted onto the Farallon plate from the Galapagos hotspot at [approximately equal to]90Ma. The plateau was subsequently transported to the northeast and collided with the Great Arc of the Caribbean thus initiating subduction polarity reversal and the consequent tectonic emplacement of the Caribbean plate between the North and South American continents. The plateau represents a large outpouring of mafic volcanism, which has been interpreted as having formed by melting of a hot mantle plume. Conversely, some have suggested that a slab window could be involved in forming the plateau. However, the source regions of oceanic plateaus are distinct from N-MORB (the likely source composition for slab window mafic rocks). Furthermore, melt modelling using primitive (high MgO) Caribbean oceanic plateau lavas from Curacao, shows that the primary magmas of the plateau contained [approximately equal to]20wt.% MgO and were derived from 30 to 32% partial melting of a fertile peridotite source region which had a potential temperature (T.sub.p) of 1564-1614[degrees]C. Thus, the Caribbean oceanic plateau lavas are derived from decompression melting of a hot upwelling mantle plume with excess heat relative to ambient upper mantle. Extensional decompression partial melting of sub-slab asthenosphere in a slab window with an ambient mantle T.sub.p cannot produce enough melt to form a plateau. The formation of the Caribbean oceanic plateau by melting of ambient upper mantle in a slab window setting, is therefore, highly improbable. Author Affiliation: School of Earth and Ocean Sciences, Cardiff University, Main Building, Park Place, Cardiff, CF10 3YE, UK Article History: Received 22 December 2008; Accepted 10 November 2009

A critical aspect of the debate about the origin and conditions of metamorphism of the Alpe Arami (AA) peridotite is the disagreement over how much ilmenite is contained in the older generation of olivine and therefore how much Ti[O.sub.2] might have been dissolved at high pressure and temperature. We have now determined quantitatively the 3-dimensional distribution of ilmenite in AA olivine by confocal laser scanning microscopy (CLSM). The CLSM measurements show an average concentration of 0.31 vol% of ilmenite in olivine, with individual grains containing up to 1.2 vol%. This translates into average and maximum concentrations of 0.23 and 0.9 wt% Ti[O.sub.2] in olivine, respectively, and confirms the original estimation of maximum concentration of ~1 vol% Ti[O.sub.2]. The vast majority of ilmenite in AA olivine is distributed randomly (although topotactically oriented) and, in all cases, is accompanied by chromite in a ratio of ~4:1. These observations are consistent with an origin of the ilmenite (and chromite) by exsolution from an olivine solid solution at P = 9-12 GPa and temperatures above the stability field of titanian clinohumite, but are not consistent with suggested breakdown of titanian clinohumite. Combining these results with other recent findings suggests that exsolution followed deformation under relatively high fugacity of [H.sub.2]O, and that the high solubility of Ti[O.sub.2] is probably explained by pressure-induced accommodation of Ti in the tetrahedral site of silicates.

The authors discuss the changes within veined xenoliths, focusing on basaltic melt infiltration, reaction and crystallization processes. Topics include narrowing of alkali basaltic glass veins, and changes in the composition of trace elements.

Garnet- (and probably spinel-) bearing ultramafic rocks that form tectonic lenses within high-metamorphic-grade gneisses in mountain core zones may intrude by moving downward or laterally into deeply subducted continental crust from the overlying mantle wedge. This model is most credible for garnet peridotites associated with ultrahigh-pressure metamorphic terranes in mountains formed by continent-continent collision. The extremely high pressures implied by ultrahigh-pressure metamorphism indicate that continental crust was subducted to mantle depths >100-120 km, where the mantle is in the garnet peridotite facies. Exchange between mantle and crust seems likely, particularly the transfer of mantle from the hanging wall into the crustal footwall. The mantle wedge overlying the subducted crustal terrane creates a denser over lighter configuration that is gravitationally unstable. If the interface becomes hot or wet enough so that peridotite can deform by ductile flow, it would sink as mantle blobs into the underlying crustal rocks. If not, the exchange presumably occurs through a brittle mechanism. The intruded bodies will be transported to the surface passively as the host crustal terrane is exhumed buoyantly toward the surface. This mechanism is proposed in particular to explain the intrusion of garnetiferous peridotites in the ultrahigh-pressure Western Gneiss region of the Norwegian Caledonides during the Caledonian orogeny.

A study was conducted to evaluate the effects of varying concentrations of dissolved water on the compositions of peridotite partial melts. The hydrous and anhydrous partial melts of similar silicate bulk composition were compared. The comparison of compositions of both melts may be useful in understanding the compositions of subduction-related lavas but it cannot explain the thermodynamic controls on the compositions of the water-bearing peridotite partial melts.

Spinel lherzolite and harzburgite xenoliths hosted in an alkali basalt dyke near Inver, Donegal, Ireland show abundant evidence of interaction between xenolith minerals and the host melt. Of particular interest are primary Cr-diopside and spinel with sieve-textured coronas. Coronas on primary Cr-diopside are up to 3 mm wide and are associated with veinlets of devitrified glass. The coronas comprise secondary Crdiopside with vermicular, interstitial alkali feldspar and chlorite grains up to 100 gm in size. The inclusion-free Cr-diopside cores are Al- and Na-rich whereas the coronas are Al- and Na-depleted and Ti-enriched. Sieve-textured spinels have similar texture to the clinopyroxene grains and are also associated with veinlets of infiltrated glass. However, the interstitial inclusions in the sieve-textured region are chlorite and nepheline. Inclusion-free spinel is part of a chromite-spinel solid solution and is Ti-poor. Spinel in the coronas has a greater chromite and ulvospinel component and falls close to a mixing line with spinel in the host alkali basalt, In addition to the sieve-textured grains, primary olivine in contact with infiltrated glass has Ferich rims, and orthopyroxene has broken down to form rims of olivine, clinopyroxene and a K-rich phase similar in composition to alkali-feldspar. Comparison of the compositions of the inclusion-free cores and sieve-textured rims shows that the rims have chemical signatures consistent with partial melting, that is, Al and Na depletion for clinopyroxene and Cr-enrichment for spinel. The textures of the coronas, particularly those around spinel and the reaction margins on orthopyroxene are identical to those produced during dissolution experiments. We suggest that silicate liquid from the host magma infiltrated the xenoliths during their ascent and since it was not in equilibrium with the xenolith minerals caused reaction. The occurrence of K-bearing interstitial minerals in the sieve-textured grains and reacted orthopyroxenes indicate that the coronas did not form by simple melting since none of the minerals that underwent breakdown are K-bearing. We suggest that the sieve-textured grains formed initially by partial melting and reaction associated with decompression and infiltration of liquid from the host magma. The melts included in the reacted phases were enriched in K by diffusion from the Si-poor infiltrated melt into the more Si-rich melt inclusions in the coronas.

The modal mineralogy, mineral chemistry and REE patterns of the anhydrous spinel peridotite xenoliths from the Ray Pic Quarternary volcano in Massif Central, France were analyzed to characterize plume-lithosphere interaction. Analysis of the mantle xenoliths from the Ray Pic Quarternary alkali basalt volcano indicated the presence of two contrasting types of peridotite exhibiting LREE-depleted patterns and LREE-enriched patterns. Furthermore, the Ray Pic peridotite xenoliths were formed by the interaction of asthenospheric mantle-derived melts/fluids and depleted lithospheric mantle.

The Atsagin-Dush volcano in Mongolia yielded spinel peridotite xenoliths ranging from lherzolites to clinopyroxene (cpx)-bearing harzburgites. An analysis of 15 peridotite xenoliths reveals that 13 of the xenoliths have whole-rock Sr isotope compositions and O isotope composition of all minerals. Trace element composition and Sr-Nd isotope compositions were also discovered in 11 clinopyroxene and melt pockets.

The REE pattern of clinopyroxene in peridotite ranges between highly LREE depleted to moderately enriched, while in Cr-diopside it ranges from moderately LREE depleted to enriched. MREE are always enriched relative to HREE. LREE and Sr increase in the clinopyroxene due to transition from peridotite to Cr-diopside websterite by a diffusion process. The modal cpx is elevated and the opx/cpx ratio is reduced in another type of peridotite to websterite dike transition. The study is conducted on Balmuccia massif, NW Italy.

The Ivrea-Verbano zone has been cited repeatedly as an exposed pre-Alpine crust-mantle transition on the basis, in part, of the presence of lenses of mantle peridotite and a possible petrologic Moho. A 10-km-thick mafic complex has been described as an intrusion at the contact between the mantle and a thick crustal sequence of metamorphosed wacke and pelitic rocks. The significance of these features is now challenged by new mapping that demonstrates that mantle peridotites in the southern Ivrea-Verbano zone were lenses tectonically interfingered with metasedimentary rocks prior to intrusion of the gabbroic complex. Although it remains possible that the Ivrea-Verbano section exposes part of a complex crust-mantle interface within which crustal and mantle rocks were interleaved, the association of peridotite and metasedimentary rocks is also consistent with assembly in an accretionary wedge. In either case, present exposures are from an unknown distance above the pre-Alpine contiguous mantle and reference to the section as a complete crust-mantle transition could be misleading.

Mineralogic and petrographic differences between an upper zone and lower zone of the Horoman peridotite complex in Hokkaido, Japan, indicate that the complex preserves evidence of temperature variations from its formation as an upwelling upper mantle diapir. Aluminum and calcium concentrations in orthopyroxine indicate a higher temperature path of decompression path for the upper zones, 1100-1150 degrees C at about 20 kbar, than the lower zone, 900-950 degrees C at about 20 kbar.

Low-stress deformation experiments conducted on partially molten peridotite at mantle temperatures and pressures reveal that dynamic wetting in the earth causes melt separation at low melt fractions and melt focussing at ridges by virtue of the low viscosities of mantle rock melts and selected analogue materials. Melt in triple-functions, termed static microstructure, is stronger than the melt in grain boundaries, termed dynamic microstructure. The two are similar in low-stress deformation experiments and experiments on salt-brine mixtures.

Earth's primitive mantle, comprising the modern mantle and the crust, is represented by Best Bulk Silicate Earth, BBSE, a weighted average composition, which is a theory based on the study of continental ocean basalts and oceanic theolites. The theory also involves the study ultramafic ophiolites, Archean komatiites, oceanic and continental alkali basalts and spinet- and garnet-bearing xenoliths from basalts and kimberlites. Total effect of core formation and volatility is also revealed. BBSE also explains the condensation and sublimation in solar nebula, and loss of volatility during the formation of earth. It indicates that Earth's accreted matter is condensed over a broad range of temperature.

In a continuing effort to understand crust-mantle dynamics, we have determined the S and Pb isotopic compositions of mantle sulfides encapsulated within diamonds from under the Siberian craton and compared these results to those of previously investigated African counterparts. Because diamond inclusions are isolated from exchange with surrounding mantle, they may preserve the history of diamond growth and act as direct tracers of the origins of mantle materials. Study of these inclusions may thus offer the best chance of recognizing global-scale interaction between Earth's crust and mantle. Although delta(sup 34)S values of the Siberian sulfides do not deviate significantly from the mantle value of 0 0/00 plus or minus 3 0/00, Pb isotopic compositions are highly variable. Pb isotopic compositions of sulfides from peridotitic suite diamonds generally plot near the terrestrial Pb growth curve, with model ages ranging between 0 and 2 Ga, whereas sulfides from eclogitic suite diamonds have radiogenic compositions, plotting beyond the growth curve. These results, which are similar to those for sulfides in African diamonds, suggest that the sulfides from eclogitic suite diamonds were derived from a source with an unusually high U/Pb ratio and may indicate a common process (such as subduction of crustal materials into the mantle) operating beneath Africa and Siberia. The absence of extremely radiogenic Pb in sulfides from eclogite xenoliths suggests that the radiogenic material from which eclogitic suite diamonds grew was a transient feature of the mantle, associated with diamond growth. The ultimate origin of this high U/Pb signature, however, remains enigmatic. Large variations in Pb isotopic composition of sulfides from different zones in a single peridotitic suite diamond document (1) crystallization of the diamond's core near 2.0 Ga, (2) growth of its outer zone in an environment with a high U/Pb ratio similar to the growth environment of eclogitic suite diamonds, and (3) growth of the diamond rim near the time of kimberlite emplacement.

The prograde metamorphic fluids at the site of dehydration reactions of serpentinite are analyzed. The regional metamorphism of the Malenco serpentinized peridotite gives rise to the formation of the mineral assemblage diopside-antigorite-olivine-brucite and the opaque assemblage magnetite-native copper-chalcocite-pentlandite-heazlewoodite-awaruite. Moreover, primary fluid inclusions are trapped in diopside overgrowth, concurrent with the opaque assemblage.

The Variscan orogen in the Moldanubian Zone of the Bohemian Massif, Czechoslovakia, featured high-temperature Group A eclogites that originated from high-pressure crystallization and massive formation of garnet and clinopyroxene obtained from very deep basaltic sources. Chemical and isotopic analyses such as Rb-Sr and Sm-Nd were used to determine eclogite equilibration conditions. Results showed that the eclogites exhibited average minimum temperature and pressure indicative of a heterogenous mantle source which formed garnets and clinopyroxenes showing retrograde compositional natures.

No dramatic variation was noted in pyroxenite dykes and ambient peridotite. Insignificant variation was observed since the two were characterized by powerful enrichment in a compatible element and the same incompatibility diagrams. Pyroxenite dykes and ambient peridotite observed also exhibited high refractory major element composition. Metasediment-derived components were found to be capable of affecting the carbonatite-like geochemical features.