Anatomy of Intraplate Monogenetic Alkaline Basaltic Magmatism

Abstract

Intraplate basaltic systems, often occurring as fields of small monogenetic volcanoes, are dominated by eruption of alkaline basaltic rocks, ranging from nephelinite/basanite to transitional/subalkaline. Their generally primitive erupted compositions imply limited crustal modification, and hence they provide an important probe into deep, lithospheric mantle and partial melting processes. Partial melting and magmatic ascent processes can be investigated using the composition of crystals, glass and wholerock, although a combination of these is preferable. The whole-rock chemical variability within single eruptions or over the temporal and spatial extent of a volcanic field is controlled by the characteristics of the primary melting source, as well as near source percolative/reaction processes. Coupled crystaland -whole-rock detailed investigations are most promising to constrain the processes that modify primary melts into the primitive magmas that accumulate before ascent. Complex crystal textures and chemistry have so far demonstrated that basaltic magmas are principally processed and modified within the lithospheric mantle with minor modification en-route through the crust. Fractional crystallization and magma mixing modify melts throughout ascent, and can imprint secondary chemical intra-eruptive variability. Quantifiable temperature and pressure parameters based on crystalmelt compositions constrain the depth of formation, and hence provide information about the role of different mineral phases in deep versus shallow chemical evolution. Volatile components in the melt (e.g. H2O and CO2) can be quantified on glass and melt inclusions. These analyses, coupled with solubility models, may help to reconstruct initial dissolved volatile content to further constrain the source characteristics and magmatic ascent dynamics. Integrated studies of crystals and melt paint a picture of extended lithospheric mantle to minor crustal processing resulting from the complex deep plumbing of monogenetic basaltic systems. This highlights the need for improved resolution to characterize true primary signatures and hence elucidate the formation of intraplate alkaline basalts.

Editorial: Crystal Archives of Magmatic Processes

“…In alkaline continental systems, explosive monogenetic centers can erupt unusual cargoes of crystals and lithics (Irving and Frey, 1984), which may provide valuable constraints on magma source, ascent and eruption. In addition, the unpredictability and abundance of monogenetic volcanoes underline the importance of improving our understanding of their source to surface feeding mechanisms (Smith and Németh, 2017;Brenna et al, 2021 (Ubide et al, 2014;Jankovics et al, 2016). Results support the notion that alkaline monogenetic fields are fed by deep magmatic systems governed by melt accumulation, fractionation and contamination at variable mantle depths, and in which volatile saturation ultimately triggers mush disaggregation and rapid magma ascent.…”

Influence of deep magmatic source region in the growth of complex maar‐diatreme volcanoes

“…If we accept that the western adjacent maar is part of the overall history of the BMM, this will also suggest that eruptions started at the western part with the old adjacent maar, and the vent migrated towards the east to create the BMM. Because simple maars are thought to be fed by a single dike (e.g., Valentine & Connor, 2015), and considering that the plumbing system might be 'sealed' through solidification of this feeder dike after the cessation of the eruption due to lack of magma supply, and thus impeding the development of a central plumbing/ conduit (Brenna, Németh, et al, 2015), this migration of the vents and the large time gap between the events at BMM could also imply that each of the episodic magma batches rose as a newly separated dike (e.g., Brenna et al, 2021) and interacted with groundwater to create at each episode a small crater/diatreme that finally coalesced to form the amalgamated, compound maar-diatreme structure suggested by Chako-Tchamabé et al (2015). Thus, both the BMM and the adjacent maar crater could be considered as belonging to a complex magmatic plumbing system comprising several individuals nearby feeding dikes connected to a deep magma production zone in the mantle (Figure 13).…”

High-pressure homogenization of olivine-hosted CO<sub>2</sub>-rich melt inclusions in a piston cylinder: insight into the volatile content of primary mantle melts

“…It is now well accepted that low-silica magmas such as nephelinites and basanites cannot be produced directly from dry lherzolites and that the presence of volatiles and/or recycled components in the source region is required to explain their major element compositions, their high content of incompatible trace elements, and their isotopic signature (e.g., Pilet, 2015). The nature of the recycled components (oceanic crust, delaminated lower crust) is still a matter of debate, but the participation of a carbonate component is increasingly recognized as being critical (Dasgupta et al, 2007;Zeng et al, 2010;Mallik and Dasgupta, 2014;Hammouda and Keshav, 2015;Xu et al, 2020;Brenna et al, 2021). For example, Xu et al (2020) concluded that the Cenozoic low-silica alkaline lavas of eastern China were produced in two stages: first partial melting of recycled oceanic crust (eclogite) in the presence of pervasive carbonate melt that yielded relatively silica-rich melts and then reaction between these silica-rich melts and peridotite to generate silica-undersaturated alkaline magmas.…”