Metasomatic Processes in the Lithospheric Mantle Beneath the No. 30 Kimberlite (Wafangdian Region, North China Craton)

Abstract

This paper presents the first major and trace element compositions of mantle-derived garnet xenocrysts from the diamondiferous No. 30 kimberlite pipe in the Wafangdian region, and these are used to constrain the nature and evolution of mantle metasomatism beneath the North China Craton (NCC). The major element data were acquired using an electron probe micro-analyzer and the trace element data were obtained using laser ablation inductively coupled plasma-mass spectrometry. Based on Ni-in-garnet thermometry, equilibrium temperatures of 1107–1365 °C were estimated for peridotitic garnets xenocrysts from the No. 30 kimberlite, with an average temperature of 1258 °C, and pressures calculated to be between 5.0 and 7.4 GPa. In a CaO versus Cr2O3 diagram, 52% of the garnets fall in the lherzolite field and 28% in the harzburgite field; a few of the garnets are eclogitic. Based on rare earth element patterns, the lherzolitic garnets are further divided into three groups. The compositional variations in garnet xenocrysts reflect two stages of metasomatism: early carbonatite melt/fluid metasomatism and late kimberlite metasomatism. The carbonatite melt/fluids are effective at introducing Sr and the light rare earth elements, but ineffective at transporting much Zr, Ti, Y, or heavy rare earth elements. The kimberlite metasomatic agent is highly effective at element transport, introducing, e.g., Ti, Zr, Y, and the rare earth elements. Combined with compositional data for garnet inclusions in diamonds and megacrysts from the Mengyin and Wafangdian kimberlites, we suggest that these signatures reflect a two-stage evolution of the sub-continental lithospheric mantle (SCLM) beneath the NCC: (1) early-stage carbonatite melt/fluid metasomatism resulting in metasomatic modification of the SCLM and likely associated with diamond crystallization; (2) late-stage kimberlite metasomatism related to the eruption of the 465 Ma kimberlite.

Development of a Dense Cratonic Keel Prior to the Destruction of the North China Craton: Constraints From Sedimentary Records and Numerical Simulation

“…The above inferred compositional change of the cratonic keel seems to be also supported by the studies of mantle xenoliths carried by magmas erupted prior to the destruction of NCC: the whole‐rock/olivine Mg# shifts towards that of the asthenosphere (88–89), no longer as refractory as that expected (~92–93) for stable cratons (Gao et al, ; Lee et al, ; Zheng, ; Zhu et al, ).…”

Geochemistry of Magmatic and Xenocrystic Spinel in the No.30 Kimberlite Pipe (Liaoning Province, North China Craton): Constraints on Diamond Potential

“…The equilibrium temperatures in the garnet xenocrysts from the No.30 kimberlite pipe calculated using single grain Ni-in-garnet thermometer [53] ranges from 1100 to 1350 • C, with a peak at about 1150 • C ( Figure 5; [27]). However, the calculated equilibrium temperatures of xenocrystic chromites from the No.30 kimberlite using single grain Zn-in-chromite [15] vary from 828 to 1394 • C, with a peak at about 950 • C ( Figure 5, Table 2).…”

“…Thus even though the No.30 kimberlite sampled the garnet/chromite-bearing mantle rock from the diamond stability field, the diamond grade in the kimberlite is low because the sampled mantle rocks are diamond-barren [16] and the diamond window is narrow. [27] are calculated using the Ni-in-garnet thermometer [53], and equilibrium temperatures with the cores of xenocrystic spinels are estimated using Zn-in-chromite thermometer [15].…”

“…As previously suggested by Zhu et al [26], the oxidizing environment also contributes to low-grade diamonds in the No.30 kimberlite. The estimated fO2 of the No.30 kimberlite magma is [27] are calculated using the Ni-in-garnet thermometer [53], and equilibrium temperatures with the cores of xenocrystic spinels are estimated using Zn-in-chromite thermometer [15].…”

“…The temperature and oxygen fugacity of the No.30 kimberlite magma are estimated in order to discuss the diamond resorption. The diamond potential of the No.30 kimberlite is discussed on the basis of the trace elements signature and temperature estimates of the xenocrystic garnet xenocrysts [27] and spinel (this paper).…”