GEMOC - Department of EPS - Faculty of Science

Trace elements and D/H in Phlogopite- and amphibole-bearing ultramafic xenoliths from Kerguelen Archipelago (TAAF). Nature of the melt fraction percolating oceanic lithospheric mantle.

A laser ablation-ICP-MS and ICP-MS study.

B. N. Moine^1,2, M. Grégoire¹, S.Y. O'Reilly¹, S.M.F. Sheppard² and J-Y. Cottin²

1. GEMOC Macquarie
2. St.Etienne University

The Kerguelen plume is remarkable among mantle plumes because its voluminous volcanic activity is long lived (~ 115 Ma) and occurred in diverse geotectonic environments related to the spreading of the Indian Ocean.

Numerous occurrences of ultramafic-mafic xenoliths (21 outcrops) have been described in Kerguelen archipelago (Leyrit et al., 1992 ; Grégoire et al., 1992 ; Mamias & Moine, 1995). Xenoliths occur in the youngest and the most alkaline volcanic dykes, lava flows and breccia pipes. Mantle harzburgites, lherzolites and dunites bear imprint of partial melting and multi-enrichment processes (mantle metasomatism) related to the activity of the Kerguelen mantle plume (Grégoire et al., 1997).

Some xenoliths outcrops display volatile-bearing ultramafic and mafic xenoliths. Amphibole and phlogopite (and associated clinopyroxene) are key indicators of modal mantle metasomatism related to the percolation of high alkaline and carbonatitic melts throught the previously depleted upper mantle.

Petrography:

Phlogopite-bearing harzburgites show a poikilitic texture related to the habit of clinopyroxene which encloses olivine, orthopyroxene and spinel grains.

Phlogopite occurs as millimetre interstitial crystals often associated with clinopyroxene, more rarely as crystals in thin veinlets. Amphibole appears in only a few samples, as small round grains, interstitial or sometimes included in clinopyroxene or phlogopite.

Phlogopite-bearing and phlogopite & amphibole-bearing dunites show coarse-grained textures. The spinel occurs as small globular or euhedral interstitial grains. The clinopyroxene is interstitial and associated with euhedral spinel and phlogopite. It often shows spongy rims and more rarely a totally spongy appearance. Amphibole displays the same habits as clinopyroxene but it is never associated with phlogopite.

Two petrographic groups of composite xenoliths rich in hydrous minerals have been distinguished :

*The first group consists of phlogopite-ilmenite-bearing hornblendite dykelets (<15mm) cross-cutting spinel-bearing dunites . Amphibole is also abundant as interstitial grains surrounding spinel in dunite in the vicinity of dykelets. At 3 or 4 centimetres from dykelets, amphibole disappears and clinopyroxene appears as interstitial grains.

*The second consists of phlogopite-rich clinopyroxenite dykelets cross-cutting clinopyroxene-rich dunite . The dunites contain few grains of spinel and are not enriched in phlogopite in the vicinity of dykelets.

Mineralogy :

Hydrous harzburgite clinopyroxenes are magnesian-augites and are similar to anhydrous poikilitic harzburgite clinopyroxenes. On the other hand, hydrous dunite clinopyroxenes are also magnesian-augites but with a large range of composition for Cr2O3 and Na2O beyond anhydrous dunite clinopyroxene field and a similtaneous enrichment in sodium and chromium.

Phlogopites are chromium-rich (1.5-2 wt% Cr2O3), sodium-rich (0.3-1.05 wt% Na2O), aluminium-rich (16-17.2wt% Al2O3) and moderately titaniferous (1.5-5.2 wt% TiO2). Dunite and harzburgite phlogopites lie in the "secondary phlogopite" field, associated with garnet (or spinel) peridotites after Delaney et al., 1980. In composite xenoliths, phlogopites are Ti-rich (7-8.2 wt% TiO2) and Al-rich (15.5-16.2 wt%Al2O3), but with a lower chromium content than in other hydrous peridotites.

Trace element geochemistry :

Clinopyroxenes in hydrous harzburgite are characterised by enrichment in REE (fig 1), particularly in LREE with a (La/Yb)n=10, a depletion in LILE and Nb, Ta, and negative anomalies in Pb, Ti, Sr, and Zr. They are similar to clinopyroxenes of poikilitic anhydrous harzburgite but display higher REE contents.

Amphibole displays higher LILE content than those of coexisting clinopyroxene (fig 2). It is characterised by large Zr, Hf, Pb and slight Ta negative anomalies.

Phlogopite displays very low REE, Th, U and Y and high Rb, Ba, Pb, Sr and Ti contents(fig 3). It shows very large Nb, Ta positive anomalies but only slight positive anomalies for Zr, Hf.

The theoritical liquids in equilibrium with clinopyroxenes have been calculated using alkaline melt/clinopyroxene partition coefficients from the compilation of Chalot-Prat & Boulier (1997). The calculated liquids for poikilitic harzburgites and phlogopite-bearing dunites display similarities with ultramafic and alkaline lamprophyric melts. On the other hand, the calculation for the amphibole & phlogopite-bearing dunite (MG91-143) gives a liquid very similar to a carbonatitic melt (fig 4 and 5).

The amphibole analysis on a 3 centimetres profil in a hornblendite composite xenolith dysplay interesting results (fig 6). Amphiboles are Ti-pargasite and Ti-magnesiohastingsite. Amphiboles show a large variation in Ti content from the dunitic wall-rock (Ti:1.2%) to the hornblendite dykelet (Ti:2.1%). On the other hand, chromium content displays an opposite evolution. Moreover, amphibole Ti-content is higher in the big dykelet (Ti: 2.1%) than in the small one (Ti: 0.7%).

Amphibole (fig 7) in the small vein commonly has much higher HFSE (Nb, Ta, Zr, Hf) contents than amphibole disseminated in mantle dunitic wall-rock. We observe a large fractionation of these trace elements in amphibole over a three centimetres distance, corresponding to an evolution of (Nb/Ta)n <1 and (Zr/Hf)n<1 in the vein to (Nb/Ta)n >1 and (Zr/Hf)n >1 in the wall-rock

The _D value of -65 to -96‰SMOW for mica and amphibole are within the accepted mantle range. However, Kerguelen micas are relatively depleted in comparison with kimberlitic micas. These may be explained by large differences in chemical compositions of micas (very High Ti-mica) and their influence on the partition coefficients of deuterium (Suzuoki & Epstein, 1976).

These Deuterium data are in agreement with Neon isotopic data (valbracht et al., 1995) which seem to indicate relative primitive and homogeneous composition. In opposite Sr, Nd, Pb and U radiogenic isotopes (Mattielli, 1996) display large heterogeneities in mantle source of the Kerguelen plume.

Conclusions

The study of amphibole and/or phlogopite-bearing mantle xenoliths from Kerguelen archipelago shows that :

*The hydrous poikilitic harzburgites are similar to the anhydrous ones.

*The phlogopite is a good reservoir for Nb, Ta, Rb, Sr, Ba and Pb but not so much for Zr and Hf.

*In composite xenoliths, we can observe important changes in amphibole HFSE concentrations and HFSE fractionation from the vein to the dunitic wall-rock (on a 3cm scale).

*It seems that the most important metasomatic agents affecting Kerguelen upper-mantle are high-alkaline melts (lamprophyres) and that the carbonatitic melts are less important than previously proposed.

*The D/H ratio of the percolating fluids is primitive and relatively homogeneous.