An Overview of the Geochemical Characteristics of Oceanic Carbonatites: New Insights from Fuerteventura Carbonatites (Canary Islands)

Gabriele Carnevale,Silvio G Rotolo,Laura Italiano,Alessandra Correale,Antonio Caracausi

doi:10.3390/min11020203

Gabriele Carnevale, Silvio G Rotolo + Show 3 more

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https://doi.org/10.3390/min11020203

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Abstract

The occurrence of carbonatites in oceanic settings is very rare if compared with their continental counterpart, having been reported only in Cape Verde and Canary Islands. This paper provides an overview of the main geochemical characteristics of oceanic carbonatites, around which many debates still exist regarding their petrogenesis. We present new data on trace elements in minerals and whole-rock, together with the first noble gases isotopic study (He, Ne, Ar) in apatite, calcite, and clinopyroxene from Fuerteventura carbonatites (Canary Islands). Trace elements show a similar trend as Cape Verde carbonatites, almost tracing the same patterns on multi-element and REE abundance diagrams. 3He/4He isotopic ratios of Fuerteventura carbonatites reflect a shallow (sub-continental lithospheric mantle, SCLM) He signature in their petrogenesis, and they clearly differ from Cape Verde carbonatites, i.e., fluids from a deep and low degassed mantle with a primitive plume-derived He signature are involved in their petrogenesis.

Highlights

Carbonatites are rare carbonate-rich rocks of igneous origin, with more than 50%modal carbonates [1]
Alkaline–carbonatite complexes are considered one of the main sources of critical metals for key economic sectors in heavily industrialized countries, and their occurrence is almost exclusive in continental regions [2], while in oceanic settings, carbonatites are very rare and they are limited to only two occurrences on the western side of the Atlantic Ocean just in front of the African coasts: at Cape Verde
Fuerteventura carbonatites are grouped into two main types: (i) calcio–carbonatites and (ii) silico–carbonatites, when the amount of silicate phases reach up to 50% of the rock volume

Summary

Introduction

Carbonatites are rare carbonate-rich rocks of igneous origin, with more than 50%modal carbonates [1]. Carbonatite petrogenesis is still a debated topic, and three petrogenetic processes have been variably proposed, all being related to the formation of a primary carbonate melt derived from a carbonated mantle [3]: (1) low-degree partial melting of a carbonated mantle source [4,5,6]; (2) immiscible separation from a CO2 -rich alkaline silicate melt [7,8], and (3) the late-stage result of a fractional crystallization of a carbonated alkaline silicate melt [6,9,10] Another important petrogenetic process is the (4) assimilation of carbonates in relatively shallow magma chambers (e.g., [11]). A thickened and metasomatized oceanic lithosphere is needed [14], and a relatively hot environment could favour both the survival of carbonate melt at shallow mantle depths and the emplacement of carbonatites at or near the surface [15], at the same time becoming more calcic with respect to primary dolomitic melts [15,16]

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