Trace element distribution in calcite-dolomite carbonatites from Spitskop: inferences for differentiation of carbonatite magmas and the origin of carbonates in mantle xenoliths

摘要

We have studied trace element distribution in intrusive calcite-dolomite carbonatites from South Africa, in which primary igneous textures are remarkably well preserved. These rocks contain few non-carbonate minerals and show no effects of crustal contamination. The trace element compositions of carbonates and fine-grained interstitial aggregates were determined in situ by laser ablation inductively coupled plasma mass spectrometry while whole rock samples were analyzed by solution inductively coupled plasma mass spectrometry. A range of rare earth element (REE) contents and various distribution patterns have been found for minerals in each rock. Calcite phenocrysts typically have low REE abundances (1-20*primitive mantle) and nearly flat primitive mantle-normalized distribution patterns with no or minor enrichments in light REE. In comparison, interstitial calcite in the same rocks is strongly enriched in light over heavy REE (La/Yb_N ≈ 200). Fine-grained interstitial material (apatite-bearing) has high contents of REE and of highly incompatible trace elements. These in situ analyses have documented significant chemical differences between distinct textural groups of carbonate minerals (phenocrysts and interstitial grains) which must be related to their crystallization sequence. These results indicate incompatible behavior of REE during fractional crystallization of carbonatite liquids and suggest strong enrichments of residual liquids in highly incompatible elements. Sr abundances show relatively little variation, consistent with carbonate/melt partition coefficients higher than for REE but close to or less than unity. The REE abundances in calcite phenocrysts are nearly as low as those reported for individual carbonate grains from mantle peridotite xenoliths and are lower than in composite carbonate-rich pockets found in some xenoliths. These results strongly indicate that the carbonates in mantle xenoliths are crystal cumulates from primitich melts rather than quenched carbonatite liquids. Furthermore, relatively low REE abundances (and high Sr) may be common in primary mantle-derived carbonate liquids. Strong enrichments in highly incompatible elements found in many carbonatites may require complex fractionation of initial liquids during ascent as well as derivation from highly enriched mantle domains.