Monique Seyler


Abyssal peridotites equilibrated in the spinel lherzolite facies and sampled along ultra-slow, slow and intermediate spreading ridges, at transform faults and on-axis, in the Atlantic and Indian oceans, are compared for available mineralogical / geochemical data, with special emphasis on modal compositions and textures.
Large mantle regions (100->1000 km) below the Atlantic and Indian ridges have been highly depleted by partial melting and melt extraction, not always in relation with known nearby hotspots. In those regions, residual harzburgites locally underwent secondary crystallization of clinopyroxene, spinel and olivine as a result of late-stage melt / rock reactions involving dissolution and/or incongruent (re)melting of residual orthopyroxene. In extreme cases, these reactions lead to the formation of secondary lherzolites and/or wehrlites closely associated with dunites. Trace element compositions of the secondary clinopyroxenes, as well of residual clinopyroxenes where some are still present, suggest that reacting melts derived from primary melts generated by partial melting of already depleted upper mantle.
Residual peridotites sampled along transform faults are in the whole less depleted than on-axis peridotites. However, they are also characterized by a wide compositional range that includes highly depleted harzburgites. Thus, offaxis more fertile abyssal peridotite compositions probably not reflect a colder thermal regime of the ridge near the transforms. Instead, peridotite compositional variability along the transforms likely results from episodic magmatic activity at slow spreading mid-oceanic ridges, with temporal variations in the degree of melting. Pyroxenes dissolution textures and intergranular igneous clinopyroxenes can also be observed locally in less depleted, residual harzburgites and lherzolites although in much lower amounts than in the highly depleted harzburgites.
In all peridotite samples, igneous clinopyroxenes and other minerals produced by melt precipitation and last-stage melt-rock reactions crystallized at the convective – conductive thermal regime transition, such that high temperatures are responsible for near-homogeneous compositions of all textural types of minerals. At a global scale, when regional averages of a large number of samples are considered, modal compositions and mineral chemistry (except sodium and the most incompatible trace elements) are relatively well correlated, and consistent with variable degrees of partial melting (~5-25%) of a lherzolitic source. Nevertheless, at a local scale, each subaxial mantle domain has its own compositional trend indicating variable melting parameters and/or source composition. Furthermore, orthopyroxene contents for a given olivine content allow to define a subgroup of orthopyroxene- rich harzburgites which is characteristically found in the most refractory subaxial mantle regions and whose clinopyroxenes are, in average, richer in sodium and chromium than those from orthopyroxene-poor harzburgites and lherzolites. Such differences in compositions might result from partial melting occurring at higher pressures in the more depleted regions. However, even in taking in account various late magmatic / metamorphic events which could have modified the primary mineral proportions, the present variations in the olivine / orthopyroxene ratios of all abyssal peridotites cannot be explained by any partial melting processes, at a global and regional / local scales.
In the eastern region of the Southwest Indian ridge, along-axis compositional variability is as high as in transform zones. The scale of this compositional heterogeneity is correlated with the roughness and irregularity of the rift floor. In contrast with transform zone peridotites, however, mineral compositions and modal compositions do not correlate with each other and classical partial melting indicators are not internally coherent. The most refractory harzburgites clearly underwent metasomatism by fertile, alkali-rich, hydrous, asthenospheric melts, before subridge partial melting and high temperature annealing, while lherzolites appear to be well equilibrated hybrid rocks resulting from refertilization of refractory peridotites by basaltic melts mostly generated in the garnet stability field. Such subaxial mantle may represent heterogeneous upper mantle that upwelled beneath the present ridge and has been preserved because of very low degrees of melting. Alternatively, it may possible that in this very slow spreading / cold environment, thick subaxial lithosphere formed during the previous magmatic episode, was thermochemically eroded during the subsequent (present-day) magmatic event.

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DOI: https://doi.org/10.4454/ofioliti.v30i2.307