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

Magnesium isotopes are widely used to trace recycled carbonates in the mantle source. We recently recognized extremely light Mg isotope values (626Mg = -0.50 to -0.62%0) 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 geochemi-cal and isotopes (Sr-Nd-Mg) analyses on the nepheline syenite to understand its petro-genesis. Laser ablation-inductively coupled plasma-mass spectrometry zircon U-Pb dat-ing yields an age of 272.5 +/- 1.4 Ma for the nepheline syenite. Petrographic and geochem-ical studies show that the nepheline syenite and nephelinite in the TLIP display similar mineral assemblages, clinopyroxene Sr iso-tope compositions and bulk-rock Sr and Nd isotope compositions (87Sr/86Sr(i) = 0.70364- 0.70399, epsilon Nd(t) = +3.51 to +4.49 versus 87Sr/86Sr(i) = 0.70348-0.70371, epsilon Nd(t) = +3.28 to +3.88 for nepheline syenite and nephelin-ite, respectively), indicating they are possibly co-magmatic. Rhyolite-MELTS modeling shows that the nepheline syenite formed from nephelinite by fractional crystalliza-tion of spinel, olivine, clinopyroxene, apatite, and biotite. In combination with information from previous studies, we correlated the ex-tremely light magnesium isotopes of neph-eline syenite to "genetic genes" of nephelinite (626Mg = -0.35 to -0.55%0) which were pro-duced 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 oce-anic crust delivered the sedimentary carbon-ate rocks into the deep mantle and formed carbonated eclogite. The carbonated silicate melt derived by the melting of the carbon-ated eclogite reacted with ambient peridotite to form primary nephelinitic magma. Finally, fractional crystallization of nephelinitic melt during ascent produced the nepheline sy-enite. Our study provides insights into the implication of light magnesium isotopes for deep carbon recycling in the origin of alka-line rocks.