New insights into deep carbon recycling and formation of nepheline-bearing alkaline rocks from Sr-Nd-Mg isotope compositions

Abstract

Magnesium isotopes are widely used to trace recycled carbonates in the mantle source. We recently recognized extremely light Mg isotope values (δ26Mg = −0.50 to −0.62‰) in nepheline syenites in the Tarim large igneous province (TLIP), NW China. To evaluate the significance of the light Mg isotopes, we conducted petrological, mineral chemical, zircon U-Pb ages, and geochemical and isotopes (Sr-Nd-Mg) analyses on the nepheline syenite to understand its petrogenesis. Laser ablation−inductively coupled plasma−mass spectrometry zircon U-Pb dating yields an age of 272.5 ± 1.4 Ma for the nepheline syenite. Petrographic and geochemical studies show that the nepheline syenite and nephelinite in the TLIP display similar mineral assemblages, clinopyroxene Sr isotope compositions and bulk-rock Sr and Nd isotope compositions (87Sr/86Sr(i) = 0.70364−0.70399, εNd(t) = +3.51 to +4.49 versus 87Sr/86Sr(i) = 0.70348−0.70371, εNd(t) = +3.28 to +3.88 for nepheline syenite and nephelinite, respectively), indicating they are possibly co-magmatic. Rhyolite-MELTS modeling shows that the nepheline syenite formed from nephelinite by fractional crystallization of spinel, olivine, clinopyroxene, apatite, and biotite. In combination with information from previous studies, we correlated the extremely light magnesium isotopes of nepheline syenite to “genetic genes” of nephelinite (δ26Mg = −0.35 to −0.55‰) which were produced by the reaction between peridotite and carbonated silicate melt derived from the carbonated eclogite. We invoke a three-stage model for the genesis of the nepheline syenite in the TLIP. Initially, the subduction of oceanic crust delivered the sedimentary carbonate rocks into the deep mantle and formed carbonated eclogite. The carbonated silicate melt derived by the melting of the carbonated eclogite reacted with ambient peridotite to form primary nephelinitic magma. Finally, fractional crystallization of nephelinitic melt during ascent produced the nepheline syenite. Our study provides insights into the implication of light magnesium isotopes for deep carbon recycling in the origin of alkaline rocks.