Abstract To provide new insights into the type and extent of mantle metasomatism in the subcratonic lithospheric mantle, we examined the Sr–Nd–O isotopic compositions of orthopyroxene, clinopyroxene, garnet, ilmenite and phlogopite from sheared garnet lherzolite, granular garnet harzburgites and lherzolites and clinopyroxene-phlogopite rocks from the Grib kimberlite in the Arkhangelsk diamond province in northwestern Russia. Clinopyroxene and orthopyroxene from sheared garnet lherzolite initially have a close value of 87Sr/86Sr(t) (~0.7034) and close weak positive eNd. Orthopyroxene and clinopyroxene are in oxygen isotope equilibrium with coexisting olivine. Clinopyroxene from a garnet harzburgite has a low 87Sr/86Sr(t) isotope ratio of 0.70266. Clinopyroxene from granular garnet lherzolites has a relatively narrow variation in 87Sr/86Sr(t) (0.70456–0.70582) and considerably larger variations in eNd (−4.3 − +1.0) isotope ratios. Garnet displays elevated initial 87Sr/86Sr(t) values (0.70540–0.70633). Ilmenite shows a narrow range in 87Sr/86Sr(t) (0.70497–0.70522) coupled with eNd values of +0.4 and +3.5. These isotopic data suggest granular garnet lherzolite of mantle metasomatism took place during the interaction of kimberlite melts with SCLM that contained mica-amphibole-rutile-ilmenite-diopside (MARID)-type metasomes. Clinopyroxenes from clinopyroxene-phlogopite (phlogopite wehrlite) xenoliths display a broader range in 87Sr/86Sr(t) (0.70486–0.70813) that is significantly higher than the kimberlite values and a circa-chondritic eNd (−0.1 − +1.3) with a restricted δ18O range (5.11‰–5.33‰). More radiogenic Sr isotopic composition decoupled from Nd isotopes could have been induced by metasomatic melt/fluid related to a subducted material. The isotopic compositions of mantle minerals preserve Sr–Nd isotopic evidence of pre-kimberlite metasomatic events that were probably due to incomplete reequilibration with ultramafic carbonated melt. Based on mineral pairs Rb–Sr isochrons and a clinopyroxene-based Sm–Nd errochron, these mantle metasomatic events correspond to ~550–600 Ma and ~1200 Ma episodes of magmatic–thermal activity.

中文翻译：

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俄罗斯西北部格里布金伯利岩下亚克拉通岩石圈地幔中地幔交代作用可变来源的 Sr Nd O 同位素证据

摘要 为了对亚克拉通岩石圈地幔中地幔交代作用的类型和程度提供新的见解，我们研究了来自剪切石榴石二辉橄榄岩、粒状石榴石斜辉石和二沸石的斜辉石、斜辉石、石榴石、钛铁矿和金云母的 Sr-Nd-O 同位素组成，以及来自俄罗斯西北部阿尔汉格尔斯克钻石省 Grib 金伯利岩的单斜辉石-金云母岩。来自剪切石榴石二辉石的单斜辉石和斜方辉石最初具有接近的值 87Sr/86Sr(t) (~0.7034) 和接近弱的正 eNd。正辉石和斜辉石与共存橄榄石处于氧同位素平衡。来自石榴石方辉石的单斜辉石具有 0.70266 的低 87Sr/86Sr(t) 同位素比。来自粒状石榴石锂辉石的单斜辉石在 87Sr/86Sr(t) (0. 70456–0.70582) 和相当大的 eNd (−4.3 − +1.0) 同位素比率变化。石榴石显示出升高的初始 87Sr/86Sr(t) 值 (0.70540–0.70633)。钛铁矿显示出 87Sr/86Sr(t) (0.70497–0.70522) 的窄范围以及 +0.4 和 +3.5 的 eNd 值。这些同位素数据表明，在金伯利岩熔体与含有云母-角闪石-金红石-钛铁矿-透辉石 (MARID) 型交代体的 SCLM 相互作用期间，发生了地幔交代作用的粒状石榴石二镁橄榄石。来自单斜辉石-金云母（金云母 wehrlite）捕虏体的单斜辉石在 87Sr/86Sr(t) (0.70486–0.70813) 中显示出更广泛的范围，明显高于金伯利岩值和球粒陨石 eNd (-0.1)，a - +1。 δ18O 范围 (5.11‰–5.33‰)。从 Nd 同位素中解耦出来的更多放射性 Sr 同位素组成可能是由与俯冲物质相关的交代熔体/流体引起的。地幔矿物的同位素组成保留了金伯利岩前交代事件的 Sr-Nd 同位素证据，这可能是由于与超基性碳酸盐熔体的不完全再平衡。基于矿物对 Rb-Sr 等时线和基于斜辉石的 Sm-Nd 测线，这些地幔交代事件对应于约 550-600 Ma 和约 1200 Ma 的岩浆-热活动事件。