GEMOC ARC National Key Centre

TRACE ELEMENT CHARACTERISTICS OF MANTLE XENOLITHS FROM THE KERGUELEN ARCHIPELAGO (INDIAN OCEAN)

M. Grégoire and S.Y. O'Reilly, GEMOC, Macquarie

The Kerguelen islands are the third largest oceanic archipelago (6500 km2) after Iceland and Hawaii and magmatic activity has extended over 45 Ma. They have evolved from a location near the SEIR to a present-day intraplate setting. Therefore, the Kerguelen islands present a specific geological setting combining characteristics of the Iceland and Hawaiian regions. Their geodynamic evolution is related to a progressive change in composition of magmatism from tholeiitic to alkaline.

Alkali basalts from the Kerguelen islands have entrained many mantle peridotites (harzburgites and cpx-bearing dunites, type I xenoliths) in addition to various other ultramafic and mafic xenoliths (metamagmatic rocks). The harzburgites and the dunites were equilibrated in the spinel peridotite stability field (T = 850-1150 °C). To date no fertile mantle lherzolite has been found in the Kerguelen archipelago.

The harzburgites have been divided into protogranular Cr-diopside-bearing harzburgites and poikilitic harzburgites which contain an interstitial magnesian-augite, sometimes associated with phlogopite and amphibole. Some scarce samples of the two types of harzburgites show an unusual mineral association consisting of feldspar + olivine (2) + Ti-chromite + rutile + Mg-ilmenite + armalcolite + Ca-Cr armalcolite (FORIAC paragenesis). These minerals occur in reaction zones replacing opx and spinel or as thin veins or dykelets cross cutting olivines. Coarse grained dunites (ol * 90 wt%) always contain small amount of cpx and spinel. A lot of dunitic samples also contain phlogopite while amphibole or opx only appear in few of them. Dunites are sometimes observed as wall rocks of magmatic veins (websterites, hornblendites, clinopyroxenites).

Bulk rock trace element characteristics of type I Kerguelen xenoliths point out the fact that the whole samples have been affected at various degree by metasomatic processes. Most of the samples show LREE-enriched patterns and the few samples which display LREE-depleted patterns have a La/Sm ratio too high to be explain only by depletion processes. Clinopyroxenes from type I xenoliths show large trace-element variations, for example, Sr ranges from less than 0.5 to 371 ppm, Ti from 25 to 8500 ppm and Zr from less than 1 to 325 ppm. The amphiboles and phlogopites coexisting with clinopyroxene (cpx) in a few samples show high amounts of Nb (up to 100 ppm) and Ta (up to 10 ppm) especially in harzburgites. The feldspar of the FORIAC mineral paragenesis is variable in composition, but is alkali-rich (K2O: 1-10.5 wt%). Rutile and Ca-Cr-Zr armalcolite may contain significant amounts of Nb2O3 (0.30-1.85 wt%), ZrO2 (0.15-3.00 wt%) and Ce2O3 (0.07-1.45 wt%).

Trace-element characteristics emphasize the origin of the Kerguelen harzburgites as a result of two main processes: (1) high degree of partial melting (20-30 %), and (2) multiple metasomatic processes. The multi-metasomatic events are associated with the activity of the Kerguelen plume and explain the modification of this previously depleted mantle to varying degrees. The main metasomatic event resulting in trace element enrichment of the two types of harzburgite and the crystallization of Mg-augite ± phlogopite and amphibole is probably related to the percolation into the upper mantle of "liquids" ranging from basaltic melts to "carbonatitic melts" and may sometimes correspond to Ti- and alkali-rich, H2O-poor fluid. The two-stage process which explains the petrological and geochemical characteristics of Kerguelen harzburgites may be related to the origin and evolution of the Kerguelen archipelago: (i) partial melting is related to formation of the Kerguelen oceanic lithosphere in the vicinity of the South East Indian Ridge, (ii) reaction between the harzburgitic residue and melts related to the activity of the Kerguelen mantle plume in the within-plate setting of the islands.

Trace-element features of cpx-bearing dunites are consistent with a formation through reactions between formerly depleted harzburgites and basaltic melts of transitional to alkaline affinities propagating through cracks (reactions opx + liquid 1 ---> ol + liquid 2 and opx + liquid 1 --> cpx + liquid 2). As suggested for cpx-bearing dunites occurring in lherzolitic massifs, Kerguelen cpx-bearing dunites constitute wall-rocks of the veins and veinlets cross-cutting the upper mantle. The formation of Kerguelen dunites is related to the long-lived Kerguelen hotspot-activity.