The Jacupiranga carbonatite (Brazil) was investigated to better understand how modal proportions and mineral chemistry control the fO2 recorded by a natural carbonatite rock.
Mineral compositions were determined using SEM-EDS, EPMA-WDS and Raman microspectroscopy building upon detailed petrography.
Based on magnetite-ilmenite oxythermobarometry, rock-forming temperatures range from 505 to 732 °C, with a wide span of fO2 values (∆FMQ –1.21 to +4.56). In each sample, ilmenite grains and lamellae are distinguishable, with the latter recording higher temperature–fO2 conditions. Consistently, independently determined homogenization temperatures of apatite-hosted fluid inclusions show a similar range between 550 and 685 °C. From the wall-rock contact toward the carbonatite body, samples exhibit a decrease in the modal abundance of olivine, phlogopite, magnetite, ilmenite, and dolomite, alongside an increase in Mg-calcite. Over the same spatial gradient, temperature, fO2, and the magnesioferrite (in magnetite) and geikielite (in ilmenite) components decrease. During the antiskarn reaction, silica and mafic components (Mg, FeT, Al, and Ti) derived from the ultramafic wall-rock were incorporated into olivine, phlogopite, magnetite-ilmenite, and dolomite. Calcite-rich samples, which were less influenced by the wall-rock, preserved ephemeral alkali components of the carbonatite melts within burbankite. Apatite-hosted fluid inclusions represent coeval fluid modified by antiskarn reactions. Adjacent to equilibrium magnetite-ilmenite pairs, olivine and phlogopite exhibit high mg# (93–97 and 88–95, respectively).
Despite the broad span of ∆FMQ values recorded by the oxide pairs, we found only a weak positive correlation between the recorded fO2 and mg#. Consequently, we suggest that the high mg# (>88–90) of carbonatitic olivine is primarily controlled by the low (<0.2) 〖K_D〗_(Fe^(2+)-Mg)^(olivine-carbonatite melt) (Fe2+/Mg exchange partition coefficient between olivine and the carbonatite melt) at crustal conditions. The exceptionally high mg# of olivine and phlogopite, compared to the adjacent dunite cumulates (mg# 84–88), combined with their negligible Ni and Cr contents, indicate an in situ antiskarn process.
This study demonstrates that carbonatite systems can be characterized by dynamic redox conditions, spanning the range from Fe3O4 ̶ Fe2O3 to Fe2SiO4 ̶ Fe3O4 ̶ SiO2 during their evolution and emplacement.
The study of textural and chemical characteristics of mafic minerals from mantle xenoliths is essential to investigate the nature of the upper mantle in a continental geodynamic context, melts generation and their effects related to mantle metasomatism. Particular textures in mantle minerals, inclusions or secondary veins of different nature (silicates vs carbonates), bubbles, represent petrographic tools to investigate these processes within the mantle. Petrographic 2D thin sections might overlook these mineralogical features, and 3D textural analysis through X-ray computed microtomography (micro-CT) are crucial to overcome these limitations.
We focused on the Mt. Vulture volcano (southern Italy) rare mantle xenoliths, brought to the surface by a melilitite-carbonatite magma (141 ka), with particular emphasis to spinel-wehrlite xenoliths and wehrlitization processes that is located close to an area of intense CO2 degassing associated to catastrophic earthquakes.
Preliminary results showed interesting 3D textural distributions within the studied xenoliths-forming mantle minerals. In particular, the micro-CT allowed to furnish new constrains on the relationship between fluids entrapment and migration, and structural discontinuities. Indeed, some minerals (especially those from the wehrlite xenoliths) showed a well-correlated distribution of fluid inclusions along the secondary fracturing planes.
This publication results from work conducted under the transnational access/national open access action at Istituto Nazionale di Geofisica e Vulcanologia – Osservatorio Vesuviano (INGV-OV) and supported by WP3 ILGE - MEET project, PNRR - EU Next Generation Europe program, MUR grant number D53C22001400005. The author thanks also Gianmarco Buono and Lucia Pappalardo for their support during the analyses and the post processing process.
OIB localities (e.g., Tristan, Samoa) have been considered ideal natural laboratories for studying mantle heterogeneity. Indeed, Sr, Nd, and Pb isotopes of lavas collected from OIB systems have provided insights into the existence of distinct mantle reservoirs, the origins of which are closely related to local tectonic processes: DMM, HIMU, EM1, and EM2. In this context, we aim to investigate the isotopic composition of noble gases in fluid inclusions trapped in xenoliths and lavas from Samoa and Tristan islands, two well-known enriched mantle (EM) localities.
Our goal is to evaluate the role of noble gas cycling and active tectonic processes on the composition of the upper mantle. Our results show that CO2 is the most abundant volatile in all samples (lavas and xenoliths) from both localities. The 4He/20Ne ratio in most samples is lower than 150, suggesting the presence of atmospheric components in the fluid inclusions. This is further confirmed by the relatively low 40Ar/36Ar ratios, particularly in Tristan samples, which show values below 360. It is worth noting that the Samoa sample exhibits a 40Ar/36Ar ratio of 1000.4, the highest of the dataset. The Rc/Ra values (3He/4He corrected for atmospheric contamination) observed in the Samoa samples align with the Ar ratios mentioned above, as the 3He/4He ratio is the highest reported (13.32Ra). This is above the MORB range, indicating a contribution from lower mantle fluids, likely derived from the Samoan hotspot. In contrast, Tristan samples exhibit low Rc/Ra values, with an average of 5.12Ra. These low helium ratios suggest the presence of a more radiogenic, 4He-rich mantle. The low helium ratios may be related to the EM nature of the mantle. Previous studies in the Canary Islands have shown a decrease in 3He/4He ratios in the eastern part of the archipelago, where EM components have been identified (Hoernle et al., 1993; Simonsen et al., 2001; Day and Hilton, 2011, 2021; Sandoval-Velasquez et al., 2021). However, it is confirmed that an EM component can show a wide range of variation for the 3He/4He ratio, ranging from low values of 5-6Ra to values beyond the typical MORB range, which overlaps (and complicates the distinction) with other OIB contexts with HIMU signature.
This publication results from work conducted under the transnational access/national open access action at INGV-Palermo- Noble gas laboratory supported by WP3 ILGE - MEET project, PNRR - EU Next Generation Europe program, MUR grant number D53C22001400005.