Trace-element composition and zoning in clinopyroxene- and amphibole-group minerals: Implications for element partitioning and evolution of carbonatites

摘要: article i nfo The present work is a first comprehensive study of the trace-element composition and zoning in clinopyroxene- amphibole-group minerals from carbonatites, incorporating samples 14 localities worldwide (Afrikanda, Aley, Alno, Blue River, Eden Lake, Huayangchuan, Murun, Oka, Ozernaya Varaka, Ozer- nyi, Paint Pinghe, Prairie Turiy Mys). new electron-microprobe data presented here signifi- cantly extend known compositional range clinopyroxenes amphiboles carbonatites. These confirm that calcic sodic carbonatites are not separated by gap, instead forming an arcuate trend nearly pure diopside through intermediate aegirine-augite com- positions confined to limited CaFeSi2O6 contents (15-45 mol%) aegirine with b25 mol% CaMgSi2O6 negligible proportion CaFeSi2O6. A large set LA-ICPMS shows clinopyrox- enes different characterized relatively low levels Cr, Co Ni (≤40 ppm) man- ifold variations concentration trivalent lithophile some incompatible elements (1-150 ppm Sc, 26-6870 V, 5-550 Sr, 90-2360 Zr, nil 150 REE), recorded cases within single crystal. relative contribution whole-rock Rb, Nb, Ta, Th U budget negligible. major-element spans alkali- Al-poor members (tremolite) Na-Al-rich Mg- or, less commonly, Fe-dominant (magnesiohastingsite, hastingsite pargasite), calcic-sodic, potassic-sodic compositions between magnesio- ferrikatophorite, richterite, magnesioriebeckite, ferri-nyboite (potassic-)magnesio-arfvedsonite. In parison clinopyroxenes, contain similar tetravalent high-field-strength (Ti, Zr Hf) compatible transition (Cr, Ni), but capable much higher concentrations Sc (up 500 43 1470 1230 Ba, 80 Pb, 1070 REE, 140 Y, 180 Nb). contribute as 25% Zr+Hf, 15% Sr 35% Rb+Ba budget. Both clin- opyroxenes may also host significant share (~10%) bulk heavy-REE content. Our show partitioning REE clinopyroxene (and, samples, amphi- bole) melt clearly bimodal requires revision existing models assuming single-site partitioning. Clinopyroxenes exhibit diversity patterns cannot be explained exclusively on basis crystal chemistry compatibility these minerals. Paragenetic analysis indicates most cases, observed pat- terns develop response removal selected trace phases co-precipitating ene amphibole (especially magnetite, fluorapatite, phlogopite pyrochlore). With exception magnesiohastingsite-richterite sample Afrikanda, invariability ratios major- ity zoned crystals implies fluids involved development implications for mineral exploration targeting Nb other types carbonatite-hosted rare-metal mineralization discussed.