Evidence of Alkali-Carbonate Melts in the Mantle

Academic Background

Carbonate melts play a crucial role in Earth’s deep carbon cycle, mantle melting, redox reactions, and the transport of highly incompatible elements. Although experimental studies, metasomatic transformations, and melt/fluid inclusions in mantle xenoliths, kimberlites, and diamonds suggest the presence of carbonate melts in the cratonic mantle, their exact compositions are difficult to determine due to their ephemeral nature and high reactivity. Once formed, these melts migrate away from their source and react with silicate mantle minerals, particularly orthopyroxene, leading to mantle metasomatism. Wehrlite, a product of the interaction between carbonate melts and peridotitic mantle, is an ideal candidate for studying in situ carbonate melts.

This study aims to reveal the presence and composition of alkali-carbonate melts in the cratonic mantle by analyzing a garnet wehrlite xenolith from the Majuagaa kimberlite in West Greenland. The research not only enhances our understanding of mantle metasomatism but also provides new evidence for Earth’s deep carbon cycle.

Source of the Paper

This paper was co-authored by Ekaterina S. Kiseeva (American Museum of Natural History, University College Cork), Vadim S. Kamenetsky (Institute of Oceanology, Chinese Academy of Sciences; Institute of Volcanology and Seismology, Russia), Ivan F. Chayka (D.S. Korzhinskii Institute of Experimental Mineralogy, Russia), Roland Maas (University of Melbourne), and Troels F.D. Nielsen (Geological Survey of Denmark and Greenland). The paper was published online on October 9, 2024, in the journal Geology, titled Alkali-carbonate melts in the cratonic mantle evidenced by a wehrlite xenolith from the Majuagaa kimberlite, West Greenland.

Research Process and Results

1. Research Object and Sample Processing

The primary object of study is a garnet wehrlite xenolith (sample GGU477406) from the Majuagaa kimberlite in West Greenland. This xenolith is an undeformed, coarse-grained peridotite lacking orthopyroxene, with unique mineral compositions, classified as garnet wehrlite. Researchers analyzed the major element compositions of olivine, clinopyroxene, and garnet using electron probe microanalysis (EPMA) and scanning electron microscopy (SEM).

2. Analysis of Melt Pools and Melt Inclusions

Abundant “melt pools” were discovered in the wehrlite xenolith, primarily composed of dolomite, calcite, serpentine, spinel, apatite, and phlogopite. Although the original magmatic mineralogy was largely destroyed by low-temperature alteration, crystallized carbonatitic melt was preserved as melt inclusions in spinel. These melt inclusions consist of carbonates, alkali carbonates, periclase/brucite, and minor halides, K-sulfides, apatite, and phlogopite.

3. Melt Inclusions in Spinel

Melt inclusions in spinel are key to the study. These inclusions are mainly composed of dolomite, Na-dolomite, alkali carbonates (e.g., Na-K carbonates), periclase, apatite, K-sulfides (e.g., K-Fe-Ni sulfides), and F-bearing brucite. Energy-dispersive X-ray spectroscopy (EDS) analysis revealed that their compositions are remarkably similar to Na-dolomite melts found in experiments and diamond fluid inclusions.

4. Isotopic Analysis

Rb-Sr and Sm-Nd isotopic analyses were conducted on the wehrlite xenolith and the host kimberlite. The results indicate that the kimberlite’s parental melt originated from a Mg-rich silicocarbonatitic melt in the cratonic mantle, while the wehrlite xenolith experienced carbonatization and low-degree partial melting. The isotopic data support a genetic link between wehrlite formation and the Neoproterozoic kimberlite event.

Research Conclusions and Significance

This study provides the first direct evidence of alkali-carbonate melts in the cratonic mantle. By analyzing melt pools in the wehrlite xenolith and melt inclusions in spinel, the researchers revealed the composition of these melts and their significance in mantle metasomatism. The results demonstrate that alkali-carbonate melts are widespread in the cratonic mantle and play a critical role in mantle metasomatism and kimberlite formation.

Scientific Value

1. Mechanism of Mantle Metasomatism: The study elucidates the interaction mechanisms between carbonate melts and mantle peridotite, particularly the formation of wehrlite.
2. Deep Carbon Cycle: The research provides new evidence for Earth’s deep carbon cycle, indicating that carbonate melts are important carriers of carbon.
3. Kimberlite Genesis: The findings support the view that kimberlite parental melts originate from carbonate melts in the cratonic mantle.

Application Value

1. Mineral Resource Exploration: The results aid in understanding the formation mechanisms of mineral resources (e.g., diamonds, rare earth elements) associated with carbonate melts.
2. Geodynamic Models: The study offers new constraints for geodynamic models, particularly the evolution of the cratonic mantle.

Research Highlights

1. First Discovery: The study provides the first direct evidence of alkali-carbonate melts in the cratonic mantle.
2. High-Precision Analysis: High-precision compositional analysis of melt inclusions was achieved using EPMA and SEM.
3. Isotopic Evidence: Rb-Sr and Sm-Nd isotopic data strongly support the genetic link between wehrlite and kimberlite.

Additional Valuable Information

The study also found that spinels in the melt pools have unique compositions and zoning patterns, further supporting their crystallization from carbonate melts. Additionally, the mineral assemblages in the melt inclusions are highly consistent with those found in experimental and natural carbonatites, providing additional evidence for the study.

This research not only deepens our understanding of mantle metasomatism and Earth’s deep carbon cycle but also provides important scientific insights for mineral resource exploration and geodynamic modeling.