Evolution, degassing and deuteration of mafic alkaline magmas: A case study of the New England-Quebec lamprophyre suite

The wealth of information on mantle processes from investigations of hotspot melts erupting in the ocean basins is temporally limited to the young age of the oldest oceanic crust. If hotspot traces on the continental crust were preserved, identified and carefully characterized, we could have information on the composition and processes of the deep mantle going back billions of years. The New England - Quebec (NEQ) igneous province of eastern North America provides an ideal location for identifying the path of a hotspot on the continent, as this path is continuous from oceanic to continental crust, and represents a sustained point source of magmatism resulting in time progressive magmatic activity.; This study focuses on the role of volatiles in the generation and emplacement of alkaline magmas which may represent the surface expression of hotspots on the continents. It presents an integrated petrographic and analytical geochemical investigation of the NEQ lamprophyre suite, including measurement of the mineral chemistry of anhydrous, hydrous and C-bearing mineral phases, whole-rock CO ₂ and H₂O concentrations, whole-rock C and H isotopic compositions, and C and O isotopic compositions of carbonate separates. These data are combined to elucidate the evolution of volatiles in alkaline magmatic systems.; Several lines of evidence suggest whole-rock volatile contents represent concentrations of CO₂ and H₂O dissolved in the magma at depth, with minimal contribution from circulating crustal fluids. Various high-temperature processes (degassing, liquid immiscibility, crystallization of volatile-bearing magmatic phases) and low-temperature processes (vapor immiscibility, subsolidus reaction of exsolved volatiles with magmatic phases) subsequently operate upon initial volatile concentrations and isotopic compositions.; Whole-rock volatile concentrations correlate with degree of silica undersaturation, resulting from varying degrees of partial melting and/or fractional crystallization. CO₂ concentrations and isotopic composition are correlated, and are modeled by a degassing trend with a Δ_vapor-melt of +2‰ The correlation between CO₂ content and fraction of CO₂ is consistent with preferential release of CO₂ during mixed-volatile degassing. Low-temperature deuteration is evident to varying degrees in mineral chemistries of every dike. Solid-solution occurs in carbonates, which may suggest formation at higher temperatures and pressures. Localities with high CO₂/H₂O have Fe-rich carbonates. C and O isotopic compositions displaced from the mantle carbonatite box towards heavier values have Fe-rich carbonates and altered oxides, consistent with equilibration at decreasing temperatures with fluids associated with oxide alteration.