Identifying clinopyroxene-sulphide co-precipitation, rejuvenation mechanisms and platinum-group element fractionation processes in young continental mantle roots
Zusammenfassung der Projektergebnisse
In the present study we investigated the sulphide mineralogy of 11 well characterized mantle xenoliths from the Middle Atlas (Morocco). The sulphides were analysed for their major and trace element content. We find that: a) Sulphide populations are dominated by mss with pentlandite, isocubanite and pyrrhotite forming a subordinate fraction often too small for LA-ICP-MS analyses. Sulphides occur enclosed in primary silicates (type e) or silicate glass associated in small melt pocket (type m) and also interstitially within the silicate matrix (type i), but no trace element signature or mineral corresponds to a specific petrographic type. PGE signatures of Middle Atlas mss are reminiscent of residual and melt-like mss formed during melting experiments. Overall two groups (1 and 2) with residual PGE patterns (low [Pd/Ir]N) were identified, which differ in their Pt/Pd.These residual mss are present in all xenoliths with relevant sulphide population. Group 3 mss experienced little to no PGE fractionation, while group 4 mss resemble mss that precipitated from melt (high [Pd/Ir]N). Most xenoliths host both, residual and melt-like mss highlighting that mss do not equilibrate on thinsection scale. b) Melting models indicate that residual mss record melting in excess of 20%, although group 1 and 2 mss require different Pd compatibilities or Pd enrichment in the latter group. c) The origin of group 4 mss with melt-like PGE systematics may reside in the initial melting event that generated the residual mss or in metasomatic enrichment. d) Modal abundances of mss and overall [Pd/Ir]N variability correlate with whole rock Al2O3 and thus indicate that refractory peridotites do not host a substantial mss population and their PGE budget is likely hosted in PGE-rich alloys. In xenoliths with > 2.5 wt% Al2O3 the average mss [Pd/Ir]N appear to resemble that of the whole rock xenoliths suggesting that in these samples the mss dictate the PGE systematics of the lherzolites. In the same xenoliths, the lowest mss [Pd/Ir]N is found, thus these samples record the strongest depletion in mss further indicating that both Al and mss were introduced via metasomatism after the initial depletion. e) Other chalcophile / siderophile trace elements in mss appear to be more randomly distributed, although mss Pb, Au and Cd do vary more systematically with respect to mss grouping (i.e. PGE fractionation).
