Metasomatic processes in the central Siberian cratonic mantle: Evidence from garnet xenocrysts from the Zagadochnaya kimberlite

Garnet xenocrysts (n=386) and associated mineral inclusions and intergrowths from the diamond-free Zagadochnaya kimberlite (Daldyn field, Yakutia) were studied to provide new insights into mantle processes beneath the central Siberian Craton. Electron microprobe analyses indicate that the great majority of the garnets follow the 'lherzolitic' trend in the CaO vs Cr2O3 diagram (Cr2O3=0·5-9·3wt %, CaO=3·2-10·3wt %), with 510% falling in the wehrlitic field and 53% falling in a transitional region between the lherzolitic and harzburgitic fields. A representative subset of the garnets (n=28) was further analysed for trace elements by laser ablation inductively coupled plasma mass spectrometry. Based on both major and trace element data, three main compositional groups are distinguished: Group A garnets (Cr2O3=1·3-5·2 wt%) may contain inclusions of chromian diopside and are characterized by progressively increasing, chondrite (CI)-normalized rare earth element (REE) abundances from La to Lu; Group B garnets (Cr2O3=5·4-8·6 wt%) are less depleted in light REE (LREE) and show nearly flat patterns from Sm to Lu; Group C garnets (Cr2O3=7·3-8·4 wt %) are characterized by humped to strongly sinusoidal REE patterns, with Yb between 0·5 and 3·0×CI. Numerical simulations of melt-rock interactions show that the wide spectrum of garnet REE compositions can be produced by a unique episode of melt injection and percolation through a refractory mantle column, whereby the melt progressively changes its composition owing to chromatographic ion exchange, fractional crystallization, and assimilation of peridotitic minerals, under decreasing melt/rock ratios.The calculated composition of the metasomatizing melt has a kimberlitic affinity, but is distinct from the composition of the host Zagadochnaya kimberlite. Most of the Group B and C garnets show replacement by a secondary mineral assemblage of (Ca, Cr)-poor garnet, chromian diopside, and chromite (± phlogopite±amphibole).The (Ca, Cr)-poor garnets are enriched in almost all incompatible trace elements, and often show humped CI-normalized REE patterns.The textures and mineralogy of these secondary mineral assemblages, the calculated compositions of the melts in equilibrium with the secondary garnets and clinopyroxenes, and Ca concentration profiles across garnet zoning indicate deep-seated (100-130 km) pervasive reaction with melts related to the host kimberlite.The lack of mantle materials from depths greater than 130 km, the absence of diamond, the abundance of secondary mineral assemblages, and the high-Mg composition of the kimberlite are consistent with a relatively slow ascent of the melts to intermediate and shallow mantle depths (c. 130 km) and extensive melt-mantle interactions before eruption.