Partitioning of REE between calcite and carbonatitic melt containing P, S, Si at 900-650 °C and 100 MPa

Many carbonatites host deposits of REE and HFSE, and fractional crystallization might be a potentially powerful mechanism controlling magma enrichment by these metals to economically significant values. At present, information about the control of fractional crystallization by partition coefficients of ore-forming elements at magmatic stage is incomplete. Here we present an experimental study of REE partitioning between carbonatite melt and calcite in the system CaCO3-Na2CO3 with varying amounts of P2O5, F, Cl, SiO2, SO3 at 650–900°C and 100 MPa using cold-seal pressure vessels and LA-ICP-MS. The presence of phosphorus in the system generally increases the distribution coefficients but its effect decreases with increasing concentration. The influence of temperature is great: at 900 − 770°C DREE ≥1, while at lower temperatures the values are below unity. Silicon also promotes the fractionation of REE into calcite, while sulfur contributes to the retention of REE in the melt. Our results imply that calcite may impose significant control upon REE fractionation at the early stages of crystallization of carbonatite magma and can be a closest proxy for monitoring the REE content in initial melt.

Carbonatite-Related REE Deposits: An Overview

The rare earth elements (REEs) have unique and diverse properties that make them function as an “industrial vitamin” and thus, many countries consider them as strategically important resources. China, responsible for more than 60% of the world’s REE production, is one of the REE-rich countries in the world. Most REE (especially light rare earth elements (LREE)) deposits are closely related to carbonatite in China. Such a type of deposit may also contain appreciable amounts of industrially critical metals, such as Nb, Th and Sc. According to the genesis, the carbonatite-related REE deposits can be divided into three types: primary magmatic type, hydrothermal type and carbonatite weathering-crust type. This paper provides an overview of the carbonatite-related endogenetic REE deposits, i.e., primary magmatic type and hydrothermal type. The carbonatite-related endogenetic REE deposits are mainly distributed in continental margin depression or rift belts, e.g., Bayan Obo REE-Nb-Fe deposit, and orogenic belts on the margin of craton such as the Miaoya Nb-REE deposit. The genesis of carbonatite-related endogenetic REE deposits is still debated. It is generally believed that the carbonatite magma is originated from the low-degree partial melting of the mantle. During the evolution process, the carbonatite rocks or dykes rich in REE were formed through the immiscibility of carbonate-silicate magma and fractional crystallization of carbonate minerals from carbonatite magma. The ore-forming elements are mainly sourced from primitive mantle, with possible contribution of crustal materials that carry a large amount of REE. In the magmatic-hydrothermal system, REEs migrate in the form of complexes, and precipitate corresponding to changes of temperature, pressure, pH and composition of the fluids. A simple magmatic evolution process cannot ensure massive enrichment of REE to economic values. Fractional crystallization of carbonate minerals and immiscibility of melts and hydrothermal fluids in the hydrothermal evolution stage play an important role in upgrading the REE mineralization. Future work of experimental petrology will be fundamental to understand the partitioning behaviors of REE in magmatic-hydrothermal system through simulation of the metallogenic geological environment. Applying “comparative metallogeny” methods to investigate both REE fertile and barren carbonatites will enhance the understanding of factors controlling the fertility.

Forsterite reprecipitation and carbon dioxide entrapment in the lithospheric mantle during its interaction with carbonatitic melt: a case study from the Sung Valley ultramafic–alkaline–carbonatite complex, Meghalaya, NE India

Carbonatite melts derived from the mantle are enriched in CO2- and H2O-bearing fluids. This melt can metasomatize the peridotitic lithosphere and liberate a considerable amount of CO2. Experimental studies have also shown that a CO2–H2O-rich fluid can form Fe- and Mg-rich carbonate by reacting with olivine. The Sung Valley carbonatite of NE India is related to the Kerguelen plume and is characterized by rare occurrences of olivine. Our study shows that this olivine is resorbed forsterite of xenocrystic nature. This olivine bears inclusions of Fe-rich magnesite. Accessory apatite in the host carbonatite contains CO2–H2O fluid inclusions. Carbon and oxygen isotopic analyses indicate that the carbonatites are primary igneous carbonatites and are devoid of any alteration or fractionation. We envisage that the forsterite is a part of the lithospheric mantle that was reprecipitated in a carbonatite reservoir through dissolution–precipitation. Carbonation of this forsterite, during interaction between the lithospheric mantle and carbonatite melt, formed Fe-rich magnesite. CO2–H2O-rich fluid derived from the carbonatite magma and detected within accessory apatite caused this carbonation. Our study suggests that a significant amount of CO2 degassed from the mantle by carbonatitic magma can become entrapped in the lithosphere by forming Fe- and Mg-rich carbonates.

Protolith Oceanic Island Arc dari Granitoid Tipe M dan I di Karangsambung, Kebumen, Jawa Tengah

Granitoid rocks which found at Luk Ulo melange complex as rock fragments with pale gray colour and faneritic texture. Petrogenesis and geotectonic of the granitoid is under debate. Some geologists consider as plagiogranite, which is formed from the Mid Ocean-ridge (MOR); or leucogranite which is formed from continental collision, and others argue as arc-related granitoid type.The field studies ware carried out on 5 (five) tracks around Luk Ulo River and 1 (one) track at Lokidang River. The pale grey Karangsambung granitoid is composed of the mainly K-feldspar (34-55%), plagioclase (10-25%) and quartz (25-35%), and chemically contains SiO2 (61.25 - 66.06%); Al2O3 (13.94 – 14.61%), K2O (2.53 - 4.00%), Na2O (3.42 - 4.10%), CaO (2.32 - 4.76%), Fe2O3 total (5.85 – 8.71%), MgO (0.98 – 1.97%). The granitoid is M- and I-type that were formed at 760o - 800o C with a depth of about 20-30 km, resulting from the differentiation of magma from a fragment origin of the K-enriched oceanic island arc originating from drifting of the IAB fragment. The sample of basalt 17D has a relatively high of Nb/Ta ratio (20), low Rb (<2 ppm), low Ba (17 ppm), and is interpreted as interacting with MORB mantle magma containing rutile-melt;whereas quartz monzonite (17A) has a relatively low of Zr/Sm ratio (3.86), which is indicated to have been contaminated by a carbonatite magma. The spidergram pattern of mantle metagabbro (sample no. 13) similar with the basalt from IAB-Bransfield Strait (Antarctica). Results of a comprehensive geochemical study proposes that the current condition of the Karangsambung zone is part of geotectonic of ACM-Eurasia, that composed of a combination of four rock fragments, i.e. (a) the rocks which sourced from IAB fragments, (b) mantle MORB, (c) continental crust from the origin of ACM-Eurasia, (d) the origin fragment from carbonatite magma.Keyword: Luk Ulo Melange Complex, pale grey granitoid, Island-arc granitoid, M and I-type granitoid

Lamprophyre-Carbonatite Magma Mingling and Subsolidus Processes as Key Controls on Critical Element Concentration in Carbonatites—The Bonga Complex (Angola)

The Bonga complex is composed of a central carbonatite plug (with a ferrocarbonatite core) surrounded by carbonatite cone sheets and igneous breccias of carbonatitic, fenitic, phoscoritic and lamprophyric xenoliths set in a carbonatitic, lamprophyric or mingled mesostase. To reconstruct the dynamics of the complex, the pyrochlore composition and distribution have been used as a proxy of magmatic-hydrothermal evolution of the complex. An early Na-, F-rich pyrochlore is disseminated throughout the carbonatite plug and in some concentric dykes. Crystal accumulation led to enrichment of pyrochlore crystals in the plug margins, phoscoritic units producing high-grade concentric dykes. Degassing of the carbonatite magma and fenitization reduced F and Na activity, leading to the crystallization of magmatic Na-, F- poor pyrochlore but progressively enriched in LILE and HFSE. Mingling of lamprophyric and carbonatite magmas produced explosive processes and the formation of carbonatite breccia. Pyrochlore is the main Nb carrier in mingled carbonatites and phoscorites, whereas Nb is concentrated in perovskite within mingled lamprophyres. During subsolidus processes, hydrothermal fluids produced dolomitization, ankeritization and silicification. At least three pyrochlore generations are associated with late processes, progressively enriched in HFSE, LILE and REE. In the lamprophyric units, perovskite is replaced by secondary Nb-rich perovskite and Nb-rich rutile. REE-bearing carbonates and phosphates formed only in subsolidus stages, along with late quartz; they may have been deposited due to the release of the REE from magmatic carbonates during the hydrothermal processes.

New forms of abundant carbonatite–silicate volcanism: recognition criteria and further target locations

In the Calatrava province of central Spain, numerous Quaternary pyroclastic vents have erupted carbonatite magmas carrying silicate melt fragments, mantle debris and megacrysts. Lava flows are rare. Maar and scoria deposits have carbonate matrices and pass into tuff sheets with carbonate contents >50%, which are spread widely away from the eruptive centres and constitute the most abundant form of effusive carbonate. Immense quantities of mantle debris are present in the erupted material. The tuffs have a distinctive fabric, which consists of a pale matrix carrying black silicate glass clasts that contain globules of immiscible carbonate and carbonate phenocrysts. There is evidence of similar volcanism in the Limagne province of central France and in other intra-continental provinces in Europe and Africa. About 500 vents have been identified in France and Spain: all the vents examined to date have erupted carbonatite magma. Such eruptions are not generally recognized in classical volcanology. As pyroclastic carbonatite was not previously recognized in Spain and France, a detailed examination of other mafic and ultramafic alkaline provinces, where research has traditionally concentrated on lava flows, is vital. For any search to be successful, evidence from the pyroclastic rocks will be required.