Carbonatite metasomatism in the mantle : sources and roles of carbonate in metasomatic enrichment processes in the lithosphere

Recent high pressure experimental work in the Hawaiian pyrolite+H20+C02 system (Wallace and Green, 1988; Falloon and Green, 1990) has revealed the presence of a stability field for high Mg/(Mg+LFe), sodic dolomitic carbonatite melt in equilibrium with pargasitic lherzolite at pressures greater than the decarbonation reaction opx + dol = ol + cpx + C02. Carbonatite melts could therefore be formed as low degree primary melts of carbonated peridotite at depths greater than -60km, or by crystallization and reaction of amphibole+clinopyroxene from carbonated, undersaturated silicate melts within the upper mantle at depths of 60-90 km. Such carbonatite melts could segregate and ascend from their source regions at very low melt fractions ( <0.02%; Hunter and MacKenzie, 1988). Analogy with natural eruptive carbonatites suggests that primary carbonatites would possess extreme LILE enrichment, and may therefore have an important role in metasomatism of the lithosphere at pressures of 15-20 kbars (Green and Wallace, 1988). A suite of mantle xenoliths (apatite+amphibole bearing magnesian wehrlites, lherzolites and harzburgites) from western Victoria, Australia, display the predicted (Green and Wallace, 1988) petrographic and geochemical signatures of carbonatite metasomatism, namely, an unusual assemblage of magnesian olivine and diopsidic clinopyroxene in the absence of orthopyroxene (wehrlites), textural evidence of replacement of primary orthopyroxene by jadeitic clinopyroxene and forsteritic olivine (lherzolites and harzburgites), the almost ubiquitous presence of significant accessory apatite,.unusually high whole-rock CaO/Al203 and Na20/Al203 values~ and extreme LILE enrichment without concomitant TiO2 enrichment. Comparison of the trace element and isotopic characteristics of this carbonatite metasomatised suite of apatite+amphibole-bearing magnesian spinel wehrlites with other, previously reported (O'Reilly and Griffin, 1988; Stolz and Davies, 1988), southeastern Australian spinel peridotite xenoliths, suggests that those containing apatite+low-Ti pargasite have also been metasomatised by ephemeral, dolomitic carbonatite melts. Low Ti/Eu, high Zr/Hf, and restricted bulk earth-like Sr-Nd isotopic characteristics are characteristic of t~is metasomatic style within the southeastern Australian lithosphere. This con~rasts with metasomatism related to hydration or basaltic intrusion of the lithosphere, in which Ti/Eu and Zr/Hf remain unfractionated at primitive mantle values of around 7740 and 36 respectively. A model for carbonatite metasomatism is proposed in which undersaturated mafic silicate melts, derived from asthenospheric upwelling or diapirism, crystallize clinopyroxene + amphibole ± phlogopite as they ascend into the stability field of hydrous garnet peridotite+carbonatite melt at P~30 kbars. The residual liquid evolves to sodic dolomitic carbonatite with low Ti/Eu, high Zr/Hf, and extreme enrichment in Sr and REE. This carbonatite melt segregates at low melt fractions and ascends rapidly, finally being absorbed by the lithosphere as it intersects a series of end-member decarbonation reactions at around 15-20 kbars. The nett effect of these is to react the enstatite and Mg-Tschermak's components of primary orthopyroxene, and MgAl204 component in spinel, with sodium carbonate and dolomitic components of the carbonatite, producing jadeite-ureiite solid solution in clinopyroxene, forsteritic olivine and C02-rich fluid. Evolution of the reacting carbonatite leads to deposition of pargasitic amphibole and apatite. In some cases, replacement of primary orthopyroxene by sodic Cr-diopsidic clinopyroxene and forsteritic olivine was complete. Subsequent annealing produced the texturally equilibrated wehrlites. Interruption of the metasomatic process by entrainment in the host magma produced apatite+amphibole-bearing spinel lherzolites and harzburgites which retain direct textural evidence of the decarbonation reactions, namely rims and veins of olivine+clinopyroxene on primary orthopyroxene grains. The extreme LILE enrichment of the carbonatite, and the ephemeral style of metasomatism resulted in imposition of its distinctive trace element and isotopic signature onto refractory spinel peridotite at this level. This model was tested experimentally by reversing the decarbonation .. reactions. Synthetic amphibole±apatite bearing wehrlite compositions with 1, 5 or 7 wt% C02 were run at PT conditions within the carbonatite melt stability field of Wallace and Green (1988). This produced lherzolite or harzburgite residue in equilibrium with sodic dolomitic melt. The model implies the release of large C02 fluxes of the order of several wt% in the lithosphere in regions of carbonatite metasomatism (<15-20 kbars). This may provide a mechanism for incorporating a carbonatite component into some intra-plate volcanic products as has recently been proposed by Nelson et al. (1988) and Dupuy et al. (1992). A second major theme of this study relates to the fate of subducted carbonate. Many DSDP reports allude to the presence of calcitic carbonate in veins and vugs in oceanfloor basalts, related to low temperature alteration processes. A series of high pressure experiments was conducted at 1525 kbar and Tg50°C. Reconnaissance experiments using magnesite instead of calcite revealed the presence of a bulk compositional control on the equilibrium carbonate composition. A shift to more magnesian bulk compositions resulted in stabilization of Ca-bearing magnesite, whereas runs conducted at identical PT conditions using calcite contained Mg-bearing calcite as the equilibrium carbonate composition. The results of the hydrous basalt+carbonate experiments are supported by a study of a suite of natural carbonate-bearing eclogites from the Krusne hory Mountains in the Bohemian part of the Czechoslovakian segment of the Saxothuringicum (Klapova, 1990). The suite contains textural and mineralogical evidence of the above garnet+carbonate reactions in both the prograde and retrograde senses. For example, some samples show patches of calcite, surrounded by magnesite, surrounded in tum ,by thin rims of dolomite, in contact with garnet. The garnet is often markedly pyroperich on the rim, compared with the core. Textures and compositions imply prograde conversion of primary calcite to magnesite, with retrograde conversion to dolomite. Analysis of garnet and carbonate compositions and textures in these samples, and comparison with the basalt+carbonate experimental data suggest that the rocks achieved peak metamorphic conditions of 25 kbar and 650-700°C, followed by a near-isobaric heating event, and subsequent uplift and retrogression. The study suggests that at least some carbon involved in the Earth's carbon budget can be recycled through subduction related processes. The ultimate fate of subducted carbon may depend on oxygen fugacity conditions in the deeper mantle. Recycled carbonate may result in diamond formation at low f02 conditions, in particular those diamonds included in eclogitic xenoliths hosted by kimberlites, or containing garnet±omphacite inclusions. Alternatively, if f02 is sufficiently high, carbon could be fixed in the mantle as dolomite or magnesite, and may be involved in carbonatite or fluid metasomatism, or in petrogenesis of undersaturated silicate melts (melilitites, nephelinites) or carbonatites (Brey and Green, 1976).