9:00 AM - 9:15 AM
[SIT15-01] Low-degree melts as an effective tool of material transport in the upper mantle
★Invited Papers
Keywords:cratonic mantle, wehrlite xenolith, Greenland kimberlite, primary carbonatitic melts
The transport processes in the subcontinental lithospheric mantle (SCLM) are usually governed by a few types of melts, generated by low-degree melting of peridotite or eclogite. Such melts are considered to be parental to kimberlites, carbonatites, lamproites and their high volatile and, in particular, CO2 content may be responsible for their low wetting angles, low viscosity and extreme high reactivity (e.g. Yaxley et al., 2019).
Compositionally, the overprint of such melts is frequently found in mantle xenoliths, manifested by veins of secondary material, kelyphitic rims around garnets, the presence of secondary spongy clinopyroxene and strong enrichment of primary minerals, such as garnet and pyroxenes, in incompatible elements (LILe, REE, HSFE).
Texturally, the presence of highly reactive lithospheric low-degree melts is evidenced by melt pools and immiscible droplets or round segregations (e.g. Frezzotti et al., 2002; Pyle and Haggerty, 1994 ) usually attributed to initial silicate and carbonate compositions.
Given their major importance for the properties and material transport in the cratonic mantle, there has been a significant effort in an attempt to describe their composition. In-situ characterisation of low-degree partial melts, however, is hampered due to their modification during the ascent as well as rapid alteration and weathering at the surface. Some attempts to decipher the compositions of low-degree mantle melts were done by studying melt inclusions in kimberlite megacrysts and kimberlite-hosted mantle xenoliths. Melt pools enriched in carbonates, alkali-carbonates and alkali-chlorides were observed in clinopyroxene and olivine megacrysts (Abersteiner et al., 2018), in multiphase inclusions in monticellite (Araujo et al., 2009) and in ilmenite and zircon megacrysts (Kamenetsky et al., 2014).
Despite multiple attempts to capture them in the natural samples along with experimental results, the melts circulating in the upper mantle are still elusive with scattered evidence for their composition and types.
In this study we report a petrological and geochemical evidence of primary low-degree melts observed in a wehrlite xenolith from Majuagaa kimberlite in West Greenland. We show, for the first time, alkali-carbonatitic-chloride melt pools and veins with clear evidence of liquid immiscibility, responsible for the formation of pure carbonatitic melts and likely silicate melts. Immiscible silicate melts were strongly altered and serpentinised. Immiscible carbonatitic melts were modified into dolomite and calcite. The unmodified primary carbonatitic melts were preserved as inclusions in liquidus spinel crystals that crystallised from such melts. These inclusions contain a mineral aggregate of dolomite, alkali-carbonate, ferropericlase, sylvite and sulphide and overall have no or almost no SiO2.
We argue that carbonatitic melts have infiltrated the initial peridotite and caused its wehrlitisation (removal of orthopyroxene) accompanied by incipient melting with the formation of two immiscible melts. Both melts are obliterated by the secondary processes except small pockets of carbonatitic melt that remained preserved in spinel crystals. These carbonatitic melts are direct evidence for how the material is transported and how highly incompatible magmas are generated in SCLM.
References
Abersteiner et al. 2018, Chemical Geology, v. 478, p. 76-88.
Araujo, D. P., Griffin, W. L., and O'Reilly, S. Y., 2009, Lithos, v. 112, p. 675-682.
Frezzotti, M. L., Touret, J. L. R., and Neumann, E. R., 2002, European Journal of Mineralogy, v. 14, no. 5, p. 891-904.
Kamenetsky et al., 2014, Chemical Geology, v. 383, p. 76-85.
Pyle, J. M., and Haggerty, S. E., 1994, Geochimica Et Cosmochimica Acta, v. 58, no. 14, p. 2997-3011.
Yaxley, G. M., et al., 2019, CO2-Rich Melts in Earth, in Orcutt, B. N., Daniel, I., and Dasgupta, R., eds., Deep Carbon: Past to Present: Cambridge, Cambridge University Press, p. 129-162.
Compositionally, the overprint of such melts is frequently found in mantle xenoliths, manifested by veins of secondary material, kelyphitic rims around garnets, the presence of secondary spongy clinopyroxene and strong enrichment of primary minerals, such as garnet and pyroxenes, in incompatible elements (LILe, REE, HSFE).
Texturally, the presence of highly reactive lithospheric low-degree melts is evidenced by melt pools and immiscible droplets or round segregations (e.g. Frezzotti et al., 2002; Pyle and Haggerty, 1994 ) usually attributed to initial silicate and carbonate compositions.
Given their major importance for the properties and material transport in the cratonic mantle, there has been a significant effort in an attempt to describe their composition. In-situ characterisation of low-degree partial melts, however, is hampered due to their modification during the ascent as well as rapid alteration and weathering at the surface. Some attempts to decipher the compositions of low-degree mantle melts were done by studying melt inclusions in kimberlite megacrysts and kimberlite-hosted mantle xenoliths. Melt pools enriched in carbonates, alkali-carbonates and alkali-chlorides were observed in clinopyroxene and olivine megacrysts (Abersteiner et al., 2018), in multiphase inclusions in monticellite (Araujo et al., 2009) and in ilmenite and zircon megacrysts (Kamenetsky et al., 2014).
Despite multiple attempts to capture them in the natural samples along with experimental results, the melts circulating in the upper mantle are still elusive with scattered evidence for their composition and types.
In this study we report a petrological and geochemical evidence of primary low-degree melts observed in a wehrlite xenolith from Majuagaa kimberlite in West Greenland. We show, for the first time, alkali-carbonatitic-chloride melt pools and veins with clear evidence of liquid immiscibility, responsible for the formation of pure carbonatitic melts and likely silicate melts. Immiscible silicate melts were strongly altered and serpentinised. Immiscible carbonatitic melts were modified into dolomite and calcite. The unmodified primary carbonatitic melts were preserved as inclusions in liquidus spinel crystals that crystallised from such melts. These inclusions contain a mineral aggregate of dolomite, alkali-carbonate, ferropericlase, sylvite and sulphide and overall have no or almost no SiO2.
We argue that carbonatitic melts have infiltrated the initial peridotite and caused its wehrlitisation (removal of orthopyroxene) accompanied by incipient melting with the formation of two immiscible melts. Both melts are obliterated by the secondary processes except small pockets of carbonatitic melt that remained preserved in spinel crystals. These carbonatitic melts are direct evidence for how the material is transported and how highly incompatible magmas are generated in SCLM.
References
Abersteiner et al. 2018, Chemical Geology, v. 478, p. 76-88.
Araujo, D. P., Griffin, W. L., and O'Reilly, S. Y., 2009, Lithos, v. 112, p. 675-682.
Frezzotti, M. L., Touret, J. L. R., and Neumann, E. R., 2002, European Journal of Mineralogy, v. 14, no. 5, p. 891-904.
Kamenetsky et al., 2014, Chemical Geology, v. 383, p. 76-85.
Pyle, J. M., and Haggerty, S. E., 1994, Geochimica Et Cosmochimica Acta, v. 58, no. 14, p. 2997-3011.
Yaxley, G. M., et al., 2019, CO2-Rich Melts in Earth, in Orcutt, B. N., Daniel, I., and Dasgupta, R., eds., Deep Carbon: Past to Present: Cambridge, Cambridge University Press, p. 129-162.