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Journal of Petrology Volume 42 Number 1 Pages 3-4 2001
© Oxford University Press 2001
Orogenic Lherzolites and Mantle Processes: Editorial
1DEPARTMENT OF GEOLOGY, ROYAL HOLLOWAY, UNIVERSITY OF LONDON, EGHAM TW20 0EX, UK
2DIPARTAMENTO DI SCIENZE DELLA TERRA, UNIVERSITÀ DI PAVIA, PAVIA, ITALY
3INTITUT DES SCIENCES DE LA TERRE, DE L’EAU ET DE L’ESPACE DE MONTPELLIER, CNRS ET UNIVERSITÉ DE MONTPELLIER 2, MONTPELLIER, FRANCE
4DEPARTMENT OF EARTH, ATMOSPHERIC AND PLANETARY SCIENCES, MASSACHUSETTS INSTITUTE OF TECHNOLOGY, CAMBRIDGE, MA 02139, USA
5DEPARTMENT OF GEOLOGY AND GEOPHYSICS, WOODS HOLE OCEANOGRAPHIC INSTITUTION, WOODS HOLE, MA 02543, USA
6DIPARTIMENTO DI SCIENZE DELLA TERRA, UNIVERSITÀ DI MILANO, MILAN, ITALY
7DIPARTIMENTO DI SCIENZE DELLA TERRA, UNIVERSITÀ DI MODENA, MODENA, ITALY
8MAX-PLANCK INSTITUT FÜR CHEMIE, ABT. GEOCHEMIE, MAINZ, GERMANY
The papers in this Special Volume of the Journal of Petrology were presented at the Third International Lithosphere Conference held in Pavia, Italy, in September 1999. The meeting was attended by about 100 scientists who actively research mantle rocks and the processes of melt formation and extraction. The protoliths are garnet-, spinel- or plagioclase-facies peridotites, rocks brought to the surface during the eruption of alkali to potassic volcanoes, diatremes and maars. Mantle rocks are also exposed at the surface of the Earth as a result of tectonic processes (i.e. orogenic or alpine massifs) or mantle rocks are dredged or drilled on the ocean floor (i.e. abyssal peridotites). Whereas the two previous lithosphere meetings in Montpellier (1990) and Granada (1995) concentrated on orogenic massifs and xenoliths, the Pavia meeting dealt almost exclusively with the mineralogy, geochemistry and experimental petrology of mantle rocks and their partial melt products.
MORB MELT TRANSFER
Spinel- or plagioclase-facies lherzolites occur as abyssal peridotites in modern ocean basins and within ophiolites. They contain vital information about melt transfer processes related to the origin of mid-ocean ridge basalts. The Othris ophiolite, Greece, contains such spinel or plagioclase lherzolites and Dijkstra et al. propose that these shallow peridotites formed at a slow-spreading ridge. More significantly, they conclude that these rocks are the result of melt impregnation by mid-ocean ridge basalt (MORB) melts and that their ‘fertile’ compositions are not primary. Similarly, an influx of melts of MORB composition is proposed for the Horoman massif, Hokkaido, by Saal et al. on the basis of Re–Os isotopic data. Their data indicate a common petrogenesis for the layered peridotite–pyroxenite sequence. In contrast, the Nikanbetsu plagioclase lherzolite from Hokkaido, Japan, is believed by Takahashi to have experienced widespread partial melting because of the evidence for incipient melting, melt migration and crystallization.
ANCIENT AND MODERN ARCS
Subduction-zone processes are the second major producer of magma at the Earth’s surface and consequently melt generation and transfer processes are of considerable interest to scientists. Fluid and element recycling in subduction zones is reported by Scambelluri et al., who has utilized subducted serpentinites from ErroTobio, Italy. They conclude that the aqueous fluid, which equilibrated with such an assemblage, was enriched in chlorine and alkaline elements. Chlorine-rich slab-derived aqueous fluids are also invoked by Laurora et al. to explain the characteristics of carbonated mantle xenoliths from Patagonia. Older examples of mantle wedges may be found in the peridotites of Finero, which, according to Grieco et al., were metasomatized some 207 my ago by basaltic melts. This was responsible for the growth of secondary clinopyroxene–amphibole and for enrichments in sodium and the light rare earth elements.
THERMOBAROMETRY AND ULTRADEEP GARNETS OR GARNET PERIDOTITES
Accurate pressure and temperature estimates are crucial to our understanding of mantle rocks because these data constrain the depth of formation. Whether or not garnet peridotites from the Central Alps (e.g. Alpe Arami) are high pressure or not is a highly contentious issue, and Nimis & Trommsdorff use revised thermobarometry to demonstrate a maximum depth of subduction of l00 km (3·3 GPa) for these peridotites. This makes a high-pressure origin rather unlikely, and also ensures that the debate will continue. The recent discovery of majoritic garnets in the mountains of Norway is described by Von Roermund et al. Similarly, the interpretation of these rocks depends on accurate thermobarometry. They conclude that depths of 200–250 km (6·4–8·0 GPa) are appropriate for these rocks provided all the pyroxene in the garnets is produced by exsolution. The emplacement of ultradeep rocks in Norway and South Africa is tackled by Drury et al., who use numerical thermo-convection models to propose that asthenospheric diapirs were responsible for the incorporation of majoritic assemblages into the cratonic lithosphere (<200 km thick).
LITHOSPHERIC PROCESSES
Processes of lithospheric thinning may be better understood using an analogue approach to orogenic massifs. The recrystallization front of the Ronda massif is used by Lenoir et al. as an example of a thermal front to better understand erosion at the base of the lithosphere—a process facilitated by melt transfer. Melt ingress into the lithosphere and polybaric crystallization processes can lead to the formation of mantle megacrysts like those from Namibia studied by Davies et al. The presence of ‘Dupal’ chemistry leads them to propose that the melts came from the Discovery plume beneath the Gibeon region. Beccaluva et al. believe that alkaline basic melts caused metasomatic changes in xenoliths from Veneto, Italy, and that these melts may have originated in a similar source as late Cretaceous lamprophyres.
Eventually in thinned continental environments partial melts can pond at the crust–mantle boundary as described by Hermann et al. They demonstrate that in southern Europe during the Permian magmatic underplating led to fractionation of MORB tholeiitic melts. Rampone & Morten have investigated crustal metasomatism in garnet peridotites from the Ulten Zone of the Eastern Alps. These rocks record a complex metamorphic and deformation history where in situ melting of pelites or migmatization produced melts that infiltrated the surrounding peridotites contemporaneous with recrystallization to garnet + amphibole - facies.
Overall petrology and geochemistry play a vital role in understanding metasomatic processes, and the role of amphibole in the fractionation and incorporation of elements such as Nb, Ta, Zr and Hf is described by Tiepolo et al. They demonstrate that the behaviour of these elements depends on the major-element composition of amphibole and the presence or absence of dehydrogenation reactions. Using the extensive geochemical database for xenoliths and orogenic massifs, Downes concludes that the shallow mantle beneath Europe has similarities to spinel-facies mantle from elsewhere and that their geochemistry has been modified by an influx of plume-derived melts.
The Pavia meeting highlighted the need to integrate our scientific knowledge and to focus future research efforts on understanding unknown entities such as sub-arc mantle. To address some of the major issues related to convergent margins the next lithosphere meeting will be held on the Horoman peridotite, in Samani, Hokkaido, Japan, from August 26 to September 3, 2002. It is clear that to understand processes fully, an integrative approach must be adopted utilizing geological, geophysical, geochemical, experimental and theoretical data. Indeed, focused, multi-disciplinary studies on exposed sections of mantle lithosphere may hold the key to future advances in this field.
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