Petrogenesis of nephelinites and the deep carbon cycling
Nephelinite is a peralkaline(mostly sodic with high Na2O/K2O ratios)volcanic rock.It generally occurs as nephelinite-carbonatite complexes or small-volume lava flows in intraplate,both continental and oceanic environments,and is characterized by nephelite and clinopyroxene phenocrysts.Although rare,it spans the entire geological history from Archean to present.Overall,the output increases with the evolution of the Earth.It is generally believed that due to high ascent rate,nephelinitic magmas are less contaminated by crustal materials,and thus they can well indicate the nature of the mantle sources.However,the petrogenesis of nephelinite still remains controversial,and the debate focuses whether the mantle sources contain carbonate or whether carbonated mantle is prerequisite for the formation of nephelinite.In this paper,we compile the geological information and geochemical data of nephelinites from various locations worldwide and divided them into two groups.Group 1 nephelinites display a wide range of SiO2 and total alkali(Na2O+K2O)content,with negative Rb,K,Pb,Zr,Hf,Ti and Y anomalies,coupled with depleted Sr-Nd isotopic compositions.Group 2 nephelinites have higher SiO2 and total alkali(Na2O+K2O),lower MgO,TiO2,P2O5 content and Na2O/K2O ratio than Group 1,and exhibit positive U,Nb,Zr,Pb anomalies and negative Ti anomalies,as well as Nb-Ta and Zr-Hf fractionation.These distinct geochemical characteristics imply their different petrogenesis.The primary magmas of Group 1 nephelinites can be carbonate-rich silicate melt through interaction between carbonated eclogite-derived melt and peridotite.These melts may erupt directly,or experience limited fractional crystallization.The two groups of nephelinites may originate from a similar mantle source,but primary Group 2 nephelinite magmas have undergone significantly fractional crystallization and silicate-carbonatite liquid immiscibility.Thus,it can be inferred that carbonated mantle source is a prerequisite for nephelinite.Statistics on the number of outcrops of nephelinites worldwide have shown that they were first formed in the Archean but increased sharply at around 550Ma and 80Ma,which is possibly mainly attributed to a sudden increased flux of subducted crustal carbonates transporting to the mantle,while the occurrence of nephelinites in Archean suggests localized carbon enrichment or input in the Archean mantle.
万方数据
维普
