Igneous rock associations 5. oceanic island volcanism II: mantle processes.(Series)

Publication: Geoscience Canada

Publication Date: 01-JUN-05

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COPYRIGHT 2005 Geological Association of Canada

SUMMARY

Oceanic island basalts (OIBs) have been central to understanding evolution of the Earth and mantle because their isolated positions in ocean basins limit the potential for magma contamination by continental crust. Melting processes (e.g., percentage melting) affect OIB chemistry but isotopic and trace-element ratios provide information on mantle-source compositions. They indicate that OIB mantle sources represent mixtures between mid-ocean ridge basalt (MORB) mantle and four other mantle components: EM1 (enriched mantle 1), EM2, HIMU (High U/Pb = Hi [mu]) and FOZO (FOcal ZOne). Mass-balance and noble-gas arguments indicate that most of the mantle is depleted but He and Ne isotopes, and convergence of Sr-Nd-Pb isotopic arrays suggest that FOZO is a somewhat primitive (unmelted) component common to all oceanic basalt sources. The other components contain "materials" such as basaltic ocean floor (HIMU), pelagic sediments (EM1), oceanic plateaus (EM1), subcontinental lithosphere (EM1, EM2), terrigenous sediments or subducted continental crust (EM2), which have been recycled by subduction processes, and mixed back into the depleted mantle. How these components cycle through the mantle is debated but heterogeneities occur on all length-scales. One school argues that oceanic islands develop above mantle plume convection cells that deliver recycled components and FOZO (lower mantle?) for mixing with depleted upper mantle. Others contend that propagating cracks in the lithosphere create oceanic islands, that plumes do not exist, that the upper and lower mantle are isolated and depleted, and that MORB and OIB form from the same upper-mantle reservoir. Small-scale melting allows OIB to sample local, low-melting-point heterogeneities that are averaged-out by the large-scale melting that forms MORB. These radically different views of mantle structure and composition indicate that OIB will continue to be a focal point in studies of Earth's evolution.

SUMMAIRE

L'etude des basaltes d'iles oceaniques (BIOs, ou OIBs en anglais) s'est avere essentielle pour la comprehension de l'evolution de la Terre et de son manteau, et cela, de par l'isolement de ces iles dans les bassins oceaniques, ce qui limite les possibilites de contamination par des materiaux de la croute continentale. Les mecanismes de fusion (le pourcentage de fusion par ex.) delimitent la composition chimique des BIOs, mais les ratios isotopiques et des elements traces permettent d'obtenir des indications sur la composition des sources mantelliques. Ils indiquent que les sources mantelliques des BIOs sont des melanges de basaltes de dorsales oceaniques (BDOs ou MORBs en anglais) de quatre autres composantes du manteau, soit des EM1 (enriched mantle), EM2, HIMU (ratio eleve de U/Pb = Hi [mu]), et FOZO (FOcal ZOne). Les etudes des bilans massiques et des gaz nobles indiquent que la plus grande partie du manteau a subit un appauvrissement, mais les isotopes He et Ne, ainsi que la convergence des ensembles isotopiques Sr-Nd-Pb portent a penser que la composante FOZO serait de composition a peu pros primitive (n'aurait pas subit de fusion) qui serait commune toutes les sources de basaltes oceaniques. Les autres composantes renferment des "materiaux" issus de plancher oceanique basaltique (HIMU), de sediments pelagiques (EM1), de plateaux oceaniques (EM1), de lithosphere sous-continentale (EM1 et EM2), de sediments terrigenes ou de croutes continentales enfouies (EM2) et qui ont ete recycles par des meanismes de subduction et reinjecte dans les materiaux appauvris du manteau. La facon dont ces composantes sont recyclees dans le manteau fait l'objet de discussions serrees et on observe la presence d'heterogeneite a toute echelle. Une des ecoles de pensee soutient que les iles oceaniques se forment au-dessus de cellules de convection de panaches mantelliques qui apportent des composantes recyclees et de la FOZO (manteau inferieur?) et les melangent avec les couches superieures appauvries du manteau. D'autres croient plutot que ce sont des fissures de la croute qui permettent la formation des iles oceaniques, qu'il n'y pas de panaches, que les couches inferieures et superieures du manteau sont isolees et appauvries et que les BIO et les BDO sont formes a partirr des materiaux des meme couches superieures. Les BIO seraient le reflet de fusions d'heterogeneites locales a faibles temperatures de fusion, alors que les BDO seraient le resultat de fusions a grande echelle expliquant une composition correspondant a la moyenne de toutes les heterogeneites. L'existence de points de vue si radicalement opposes sur la structure et la composition du manteau demontrent que les BIOs seront encore l'objet d'&udes sur l'evolution de la Terre.

INTRODUCTION

Oceanic islands represent a small proportion of the Earth's surface yet their volcanic rocks are amongst the most studied; this is because they occur far from continental crust that might contaminate rising magma, and they are unaffected by orogenic, metamorphic and tectonic processes (Basaltic Volcanism Study Project = BVSP, 1981, p. 161). Thus, oceanic island basalts (OIBs) provide important chemical evidence regarding mantle composition, magma formation processes and magma evolution. Perhaps reflecting the availability and accuracy of major element, trace element and isotopic analyses, the emphasis of studies on OIB has shifted over 40 years from the effects of differentiation, to the impact of melting processes on magmas, to studying what magma chemistry can tell us about mantle-source compositions. Today, based mostly on isotopic data from OIB and mid-ocean ridge basalt (MORB), five types of mantle-source regions, or components, are recognized (Zindler and Hart, 1986; Hofmann, 2003). These appear to reflect variable melt-extraction and subduction-related recycling of materials back into the mantle over Earth history, although the specifics of how each "component" formed are debated.

Models for the distribution, scale and melting behaviour of these components are even more contentious and lead to very different hypotheses for chemical structure and convection patterns in the mantle. One school sees some amount of plume-related(?) exchange between primitive (relatively unmelted) lower mantle and "depleted" (previously melted) upper mantle (e.g., Allegre, 2002; Hofmann, 2003). The other sees the entire mantle as convecting and depleted; the lower mantle is separated from the upper mantle that is locally enriched in subduction-recycled components (Anderson, 1999; Hamilton, 2003). Mantle plumes may not exist (Anderson, 2000). Thus a healthy, and at present unresolved, debate has emerged that reflects fundamental views of how planet Earth works, and OIBs are front and centre in the discussion.

This paper looks at how OIB compositions (Table 1) reflect source region compositions and mantle melting processes (e.g., pressure effects, impact of fluids, melting percentage). Hypotheses for the origin of mantle heterogeneity (the mantle components), evidence for the survival of primitive mantie, and models for chemical structure in the Earth are then reviewed. This manuscript complements Greenough et al., (2005) that covers the mineralogy, petrology and differentiation of oceanic island basalts.

SOURCE-REGION MELTING

Controls on Major-Element Compositions

A recurring theme in petrology is whether a basalt has experienced differentiation or is "primary" and derived directly from the mantle. Although debated, primary magmas are thought to have Mg# values of about 0.72 as a result of equilibration with [Fo.sub.92] olivine that is found in most mantle lherzolite xenoliths (Roeder and Emslie, 1970). At least a few aphyric basalts on most oceanic islands have Mg# values around 0.72, which is consistent with this hypothesis. Olivine partitioning data indicate that primary basaltic magmas should have Ni/MgO ratios (ppm/wt.% oxide) between 22 and 50 with Ni = 200 to 1000 ppm (Basaltic Volcanism Study Project, 1981, p. 424). A review of the Ni, Cr, Co and Sc contents in approximately 200 "primary" basalts (Mg# values of 0.72) from French Polynesia, using regression analysis, gives concentration ranges of 350-550, 650-930, 70-80 and 20-40 ppm, respectively.

Figure 1a illustrates the pressure effects on the beginning-of-melting point in a synthetic ternary system modelling lherzolite. Points [I.sub.0], [I.sub.1], [I.sub.2][, [I.sub.3] show that magmas become increasingly silica-undersaturated, and Ne normative, with increasing pressure (1 atmosphere, 10, 20, and 30 kbar, respectively). Experiments on an anhydrous spinel lherzolite indicate that magmas change from Ne normative (alkaline) to Hy normative (tholeiitic) as melting percentages increase, and the Ne component increases as pressure increases (Fig. 1a). As Haase (1996) emphasized, the silica content of magmas is very sensitive to pressure. Melting experiments also show that C[O.sub.2] leads to undersaturated magmas, whereas [H.sub.2]0 produces more silica-rich magmas (Fig. 1b). These generalizations suggest that relative to tholeiites, oceanic island alkaline magmas reflect small percentages of melting of C[O.sub.2]-rich lherzolite at high pressures.

[FIGURE 1 OMITTED]

Metasomatism

Metasomatism is the process whereby migration of C[O.sub.2]- and [H.sub.2]0-rich fluids (or melts), through the mantle, leads to local incompatible element enrichment prior to magma formation. Alkaline OIBs commonly contain too much Rb for their observed [sup.87]Sr/[sup.86]Sr ratios. This suggests that metasomatic fluids or melts carry large ion lithophile elements to their sources a short time prior to melt extraction. Metasomatism may also occur long before magma genesis. Many experiments suggest that ancient subduction-related metasomatism affected OIB mantle sources long before island magmatism. Clearly, metasomatism coincident with magmatism is important, but it cannot yield the large variations in daughter...

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