Trace Element Discriminants for Mantle Processes - An Experimental Approach

Trevor H. Green, John Adam and Geoffrey T. Nichols, GEMOC, Macquarie

Conspicuous trace element differences are well documented for major types of igneous provinces in the earth's crust. Thus relative to primitive mantle, hot-spot plate centre volcanism (e.g. ocean island basalts) is characterized by large ion lithophile element (LILE), light rare earth element (LREE) and high-field-strength element (HFSE) enrichment . In contrast, normal mid-ocean ridge basalts are LILE and LREE depleted with HFSE unchanged, and most volcanics from subduction zone settings show LILE enrichment and similar or depleted HFSE relative to primitive mantle. These overall trace element features reflect mantle source regions that have been modified by metasomatic and/or melting processes, that in turn are controlled by trace element partitioning between fluid/melt/residual minerals.

Important residual minerals that may significantly affect LILE, REE and HFSE abundances include clinopyroxene (cpx), amphibole (amph) and garnet (gt), whereas rutile is a crucial accessory mineral that may influence HFSE abundances. The experimental database now available for the partitioning of trace elements between these minerals and fluid, various silicate melts (basaltic to high-SiO2 rhyolitic compositions) and carbonatite allows recognition of potential marker elements or ratios that characterize different metasomatic or melting events.

In general, mineral/fluid Ds are higher than mineral/melt values, but noteworthy exceptions occur. Thus for cpx mineral/fluid Ds for Pb and Ba are similar to mineral/melt values. Cpx/fluid fractionates U/Th more strongly than cpx/melt. Cpx/silicate and cpx/carbonate melt Ds appear similar, except for Zr and Hf which are fractionated in opposite direction and for HFSE/REE which are higher for cpx/carbonate pairs. For amph Rb and Pb mineral/fluid and mineral/melt Ds are close in value, but Rb/Ba behaviour is distinctly different for amph/fluid (<1) and each melt (>1 for carbonate melt, ~1 for silicate melt). Also Nb/Ta is fractionated more by amph/fluid and amph/carbonate melt than by amph/silicate melt, and HFSE/REE are higher for amph/carbonate pairs than for either amph/silicate melt or amph/fluid pairs. For gt/fluid, Ds for Ba and Sr are lower than mineral/melt values, and U/Nb and Pb/Sr will decrease in fluids but will increase in melts through gt fractionation. Gt/silicate or carbonate melt Ds show very similar behaviour. Although only a small number of rutile/fluid and melt D values is available, the very high D values for HFSE are striking, so that a small volume of rutile may have a major effect on these trace elements. Some results show that rutile/fluid or high-SiO2 melt fractionates Nb/Ta in the opposite direction to rutile/lower-SiO2 melt and to cpx, amph or gt/melt. Thus rutile/fluid or high-SiO2 melt fractionation will show a decrease in Nb/Ta compared with an increase in Nb/Ta for all the other mineral/melt fractionating cases. Also rutile/fluid DU>>DTh', that is opposite to cpx/fluid, but similar to (though much higher than) gt/fluid. Addition of F to the volatile component causes a marked drop in D values for HFSE, but less effect on LILE, for amph/melt pairs. Thus the relatively high HFSE/LILE ratios observed in alkaline magmas may indicate the presence of F in their petrogenesis, if amphibole is an important fractionating mineral.

Taken together these results indicate that relatively low Rb/Ba and HFSE/REE point to a carbonatitic melt acting as a mantle-modifying metasomatic agent, whereas high U/Th and Rb/Ba and lower U/Nb suggest a fluid role. If rutile/fluid or high-SiO2 melt partitioning behaviour exerts an important control on the geochemistry of a mantle source region, then derived magmas may have Nb/Ta < model mantle, whereas if rutile/lower-SiO2 melt control (together with cpx, amph or gt) is more significant then Nb/Ta will be > model mantle. Thus recently obtained Nb/Ta data for subduction zone volcanics of 11 to 33 may reflect this contrasting rutile/fluid or melt Nb and Ta partitioning behaviour. An important corollary is that a model continental crustal value of Nb/Ta ~ 11 would suggest that any major contribution to the growth of continental crust from subduction zone volcanism should come from magmas derived from a source region previously modified by a fluid or high-silica melt that had equilibrated with rutile, most likely in the subducted oceanic crust. Additional evidence for a fluid rather than a silicate melt role may come from careful assessment of Rb/La, U/Th and Pb/Sr relative to Nb/Ta. The fluid/melt partitioning data summarized here indicate that a negative correlation of these ratios would confirm fluid involvement. However, it is crucial to verify that rutile/fluid and rutile/melt Nb, Ta partitioning data show the contrasting behaviour that preliminary results suggest.

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