The genesis of deep-mantle xenocrystic zircon and baddeleyite megacrysts (Mbuji-Mayi kimberlite): Trace-element patterns

摘要

Fragments of five Cretaceous cm-size baddeleyite (ZrO_2) and two zircon (ZrSiO4) crystals from the Mbuji-Mayi kimberlite (DR Congo) were measured for trace elements by the LA-ICP-MS technique. EPMA was applied adjacent to the laser spots to control Hf, used as internal standard for all LA-ICP-MS measurements. The 22 analyses confirm perfectly identical Hf concentration on the intra and inter-grain scale. Trace elements also yield surprisingly similar over-all patterns, although individual megacrysts have crystallized within magmas generated from differently depleted mantle domains (εHfi + 5.1 to + 10.2). Examined in detail, the trace elements show differences in concentrations being correlated either on the inter or intra-grain scale. The five baddeleyite megacrysts have high Th, U, Ta, and Nb reaching up to 5000 times chondrite concentrations. In intermediate to heavy REE they are by 30-120 times chondrite enriched but strongly depleted in the lightest REE, except Ce showing a positive anomaly (Ce~(4+) substitutes Zr~(4+)). Corresponding (La/Yb)N range between 0.006 and 0.040. The two zircon megacrysts show trace element patterns similar to those of baddeleyite, also having high Th and U but low Ta and Nb. Intermediate to heavy REE are from 1 to 100 times chondrite enriched and they also show strong depletion in light REE yielding (La/Yb)N at 0.0001. Cerium is again an exception having about 20 times chondrite abundance. Highest trace element concentrations are correlated with the lowest εHfi of +5.1, and lowest concentrations with the highest εHfi of+ 10.2. This corroborates that not only εHfi but also trace elements, in particular HFSE of the megacrysts reflect the level of trace elements available at the time of partial mantle melting and megacryst growth. Such growth has occurred in the deep mantle at temperatures exceeding 2000 °C, to explain structural data indicative for the original presence of cubic ZrO _2. On their way to the surface cubic baddeleyites were successively transformed to tetragonal and then monoclinic symmetry. These transformations were not accompanied by measurable chemical changes. All megacrysts record (1) ancient timeintegrated Lu/Hf-fractionation of their mantle sources, with the differences in εHfi most likely reflecting the combined effect of different degrees of fractionation and differences in ages, and (2) the Cretaceous kimberlite event, during which the megacryst formed from highly trace element-HFSE enriched magmas. These highly enriched magmas were most likely produced by very small degrees of partial melting of the originally depleted mantle. The important correlation between εHfi and trace-element abundance of the individual megacrysts rules out any significant earlier re-enrichment of the individual mantle domains, which would have erased this correlation.