Identifying mantle carbonatite metasomatism through Os–Sr–Mg isotopes in Tibetan ultrapotassic rocks

Abstract

Mantle-derived magmas at convergent plate boundaries provide unique insights into the nature of materials subducted to and recycled from depths. Here we present a study of OsSrMg isotopes on the Oligocene-Miocene ultrapotassic rocks aimed at better understanding sediment subduction and recycling beneath southern Tibet. New isotopic data confirm that ultrapotassic rocks in southern Tibet are of mantle origin, but underwent crustal contamination as evidenced by the variably high 187 Os/ 188 Os that obviously deviates from normal mantle reservoir. Still some samples with mantle-like 187 Os/ 188 Os exhibit δ 26 Mg significantly lower than mantle and crustal lithologies, suggesting that the isotopically light Mg may not result from crustal contamination but retain specific fingerprint of carbonate-related metasomatism in mantle sources. Mantle carbonatite metasomatism is manifested by the inverse δ 26 Mg 87 Sr/ 86 Sr correlations, as well as the depletion of high field strength elements relative to rare earth elements and the enrichment of CaO in ultrapotassic rocks. The positive co-variations between δ 26 Mg and Hf/Sm defined by those low-187 Os/ 188 Os ultrapotassic rocks provide evidence for the potential of recycled dolomites to modify mantle Mg isotopic composition. The correlated spatial variations of δ 26 Mg and Hf/Sm are interpreted to reflect carbonatitic metasomatism associated with the northward subduction of the Neo-Tethyan oceanic slab and its profound influence on postcollisional ultrapotassic magmatism.

First Identification of Mafic Igneous Enclaves in Miocene Lavas of Southern Tibet With Implications for Indian Continental Subduction

“…However, numerous crustal xenoliths and zircon xenocrysts found in the ultrapotassic lavas may indicate significant crustal assimilation (Liu, Zhao, Zhu, Niu, Depaolo, et al, ; Liu, Zhao, Zhu, Niu, & Harrison, ; Miller et al, ). This scenario is consistent with the variably high 187 Os/ 188 Os values of the ultrapotassic lavas that were most likely caused by crustal assimilation (Liu et al, ). Given that the Konglong mafic enclaves likely originated from primary ultrapotassic magmas, they can provide key petrogenetic information concerning the origin and evolution of these magmas.…”

Magnesium Isotopic Evidence for Ancient Subducted Oceanic Crust in LOMU‐Like Potassium‐Rich Volcanic Rocks

“…Several ultrapotassic and potassic volcanic rocks related to the closure of the Tethys Ocean were considered to originate from the recycling of carbonate‐rich sediments within the lithospheric mantle (Ammannati et al, ; Conticelli et al, ; Liu et al, ). Both carbonated metasediments (e.g., carbonate‐bearing mudrocks, δ 26 Mg = −0.74 to −0.08) (Wang et al, ) and sedimentary carbonates (δ 26 Mg = −5.31 to −1.09) (Saenger & Wang, ) have light Mg isotopic composition compared to the upper mantle.…”

Marine Carbonate Component in the Mantle Beneath the Southeastern Tibetan Plateau: Evidence From Magnesium and Calcium Isotopes

“…For example, Huang et al () combined Ca and Sr isotopes to trace recycled carbonates and proposed that the addition of 4% ancient carbonates into the Hawaiian source could explain the variation of δ 44/40 Ca in Hawaiian shield lavas. D. Liu et al () identified carbonatite metasomatism using Mg–Sr–Os isotopes and suggested that the variably low δ 26 Mg of Tibetan ultrapotassic rocks could be attributed to carbonate‐bearing sediments. Mg–Sr–O isotopes were employed to study the origin of syenites from northwest Xinjiang, China, showing that syenites with low δ 26 Mg were caused by the incorporation of dolostone, limestone, and Indian sediment (Ke et al, ).…”