Origin Of Mid-ocean Ridge Basalts Research Articles

Transition from ultra-enriched to ultra-depleted primary MORB melts in a single volcanic suite (Macquarie Island, SW Pacific): Implications for mantle source, melting process and plumbing system

Compositional diversity of basalts forming the oceanic floor is attributed to a variety of factors such as mantle heterogeneities, melting conditions, mixing of individual melt batches, as well as fractionation and assimilation processes during magma ascent and emplacement. In this study the compositional range and origin of mid-ocean ridge basalts (MORB) is approached by petrological, mineralogical and geochemical studies of the Miocene Macquarie Island ophiolite, an uplifted part of the Macquarie Ridge at the boundary between the Australian and Pacific plates. In this study, earlier results on the enriched to ultra-enriched (La/Sm 1.4–7.9), isotopically homogeneous basaltic glasses are complemented by the compositions of olivine-phyric rocks, principal phenocrystic minerals and Cr-spinel hosted melt inclusions. Studied olivine, clinopyroxene and Cr-spinel phenocrysts are among the most primitive known for MORB (85–91mol% forsterite in olivine, 81–91 Mg# in clinopyroxene, and 66–77 Mg# and 34–60 Cr# in spinel) and represent primary and near-primary compositions of their parental melts. Geochemical characteristics of the liquids parental to clinopyroxene (La/Sm 0.8–6.3) and Cr-spinel (La/Sm 0.4–5) partly overlap with those of the basaltic glasses, but also strongly advocate the role of depleted to ultra-depleted primary melts in the origin of the Macquarie Island porphyritic rocks. The trace element composition of olivine phenocrysts and the systematics of rare-earth elements in glasses, melt inclusions, and clinopyroxene provide evidence for a peridotitic composition of the source mantle. Our data supports the mechanism of fractional “dynamic” melting of a single mantle peridotite producing individual partial melt batches with continuously changing compositions from ultra-enriched towards ultra-depleted. The incipient enriched melt batches, represented by basaltic glasses in this study, may erupt without significant modification, whereas consecutively derived less enriched and depleted melt fractions are affected by mixing and crystal fractionation. Continuous melt generation, extraction, replenishment of the plumbing system, mixing and eruption of “integrated” melts lead to obliteration of the initially enriched geochemical characteristics and ultimately result in dominantly “normal”, depleted MORB compositions.

Orogenic Lherzolites and Mantle Processes: Editorial

They contain vital information about melt transfer proThe papers in this Special Volume of the Journal of Petrology cesses related to the origin of mid-ocean ridge basalts. were presented at the Third International Lithosphere The Othris ophiolite, Greece, contains such spinel or Conference held in Pavia, Italy, in September 1999. The plagioclase lherzolites and Dijkstra et al. propose that meeting was attended by about 100 scientists who actively these shallow peridotites formed at a slow-spreading research mantle rocks and the processes of melt formation ridge. More significantly, they conclude that these rocks and extraction. The protoliths are garnet-, spinelor are the result of melt impregnation by mid-ocean ridge plagioclase-facies peridotites, rocks brought to the surface basalt (MORB) melts and that their ‘fertile’ compositions during the eruption of alkali to potassic volcanoes, diaare not primary. Similarly, an influx of melts of MORB tremes and maars. Mantle rocks are also exposed at the composition is proposed for the Horoman massif, Hoksurface of the Earth as a result of tectonic processes (i.e. kaido, by Saal et al. on the basis of Re–Os isotopic orogenic or alpine massifs) or mantle rocks are dredged data. Their data indicate a common petrogenesis for the or drilled on the ocean floor (i.e. abyssal peridotites). layered peridotite–pyroxenite sequence. In contrast, the Whereas the two previous lithosphere meetings in MontNikanbetsu plagioclase lherzolite from Hokkaido, Japan, pellier (1990) and Granada (1995) concentrated on orois believed by Takahashi to have experienced widespread genic massifs and xenoliths, the Pavia meeting dealt partial melting because of the evidence for incipient almost exclusively with the mineralogy, geochemistry and melting, melt migration and crystallization. experimental petrology of mantle rocks and their partial melt products.

Origin and Petrogenesis of the Trinity Ophiolite Complex (California): New Constraints from REE and Nd Isotope Data

Sm-Nd isotopic results, as well as concentrations of major elements and REE, are reported for nine whole-rock samples of plagioclase lherzolite, lherzolite, harzburgite, clinopyroxene-rich dykes, layered and isotropic gabbros, and a plagiogranite dyke from the central part of the Silurian Trinity Ophiolite Complex (Toad Lake area) of northern California. The data provide new constraints on the origin and petrogenesis of this ophiolite complex and have potentially important implications with regard to the origin of mid-ocean ridge basalts (MORBs) and differentiation of the oceanic lithosphere. Although Sm-Nd isotopic data published earlier on samples from the northern (China Mountain) and southern (Gray Rock) parts of the complex yielded discordant ages and initial εNd values for peridotites [T=472 ± 32 Myr; εNd(T)=+ 10·4 ± 0·4], and crustal rocks from the overlying magma chambers or intrusive dykes [T∼440 Myr; εNd(T)∼ +7·0], all the samples investigated in this study define a single array in the Sm-Nd isochron diagram, allowing an age of 423 ± 48 Myr and an εNd(T) value of +6·4 ± 0·7 to be calculated. Furthermore, although the results published so far on peridotites and gabbroic dykes from Trinity indicated variable LREE depletion, all the basicultrabasic samples from Toad Lake are found to exhibit similar, strongly LREE-depleted patterns [0±17<(Ce/Sm)N<0·34]. As elsewhere in the Trinity Ophiolite Complex, peridotites in the Toad Lake area preserved evidence of partial melting (e.g., plagioclase lenses tracking the high-temperature flow plane) and melt extraction (e.g., depleted borders along cross-cutting dykes). Comparison of the new Sm-Nd results with U-Pb zircon ages for plagiogranite samples suggests that 423 ± 48 Myr is a geologically meaningful age, corresponding to the crystallization of plagioclase lenses in plagioclase lherzolites, as well as to the crystallization of dykes and rocks of the Toad Lake magma chamber. Because of the discrepancy in Sm-Nd ages, initial εNd(T) values, and REE patterns, peridotites and crustal rocks of the Trinity Ophiolite Complex have so far been regarded as genetically unrelated. The results from Toad Lake argue for partial reassessment of this view. As far as Sm-Nd isotopic data and REE patterns are concerned, peridotites, dykes, and gabbros from this area have features consistent with a cogenetic rock suite. Clinopyroxene-rich dykes near Toad Lake have been regarded as the remnants of the conduits which fed the magma chamber, a view which is fully confirmed by the present geochemical and isotopic results. Careful evaluation shows that these dykes have petrographic and geochemical features more representative of crystallized melts, rather than mineral segregates, hence allowing us to propose that their actual picritic composition (MgO∼ 16%) might reflect the composition of the magma which entered the Toad Lake magma chamber. Comparison of the present results with those published earlier shows the Trinity peridotite to be an isotopically heterogeneous body. Moreover, a difference in age may exist between peridotites from the northern and southern parts of the complex. It is possible that the Trinity Ophiolite Complex represents the remnant of the opening of an ocean, perhaps in an area similar to a modern back-arc or inter-arc basin.

Composition of basaltic liquids generated from a partially depleted lherzolite at 9 kbar pressure

Two contrasting views exist regarding the nature and origin of mid-ocean ridge basalts (MORB): first, that these olivine-tholeiites with Mg/(Mg = Fe) = 0.70–0.72 are primary and are generated by partial fusion of plagioclase + spinel lherzolite at around 30-km (≈9 kbar pressure) in the mantle1. Others2–8 suggest that even the most primitive MORBs are not primary but are derived by fractionation of olivine±pyroxene from picritic parental magmas that are generated at depths of 45 km or more. Duncan and Green2,3 argued for the existence of a second primary magma, a high-MgO quartz-normative or olivine-poor tholeiite, which is produced at depths ≤25 km beneath the ocean ridges. This problem, concerning the composition and depth of origin of primary MORBS, has been approached here by experimentally melting a plagioclase+spinel lherzolite xenolith from Hawaii at 9 kbar pressure. The experimental melts are very similar to some mid-ocean ridge basalts, which suggests that the latter are probably primary and are generated at shallow depths in the mantle.