Mineral chemistry investigations of the Quaternary alkaline basalts erupted at Dilo-Dukana and Mega volcanic fields (Ririba rift, South Ethiopia)

The Late Pleistocene-Holocene Dilo-Dukana and Mega volcanic fields (Ririba rift, South Ethiopia) formed through monogenetic eruptions of limited volumes of alkaline basalts containing abundant mantle and crustal xenoliths. This activity postdated the emplacement of voluminous Pliocene subalkaline basaltic lavas related to the main rifting phase. Several NE-SW aligned vents, that abruptly cut the rift-related structures, form the two volcanic fields, indicating the occurrence of magmatism disconnected from the main rift activity (Corti et al., 2019).

New detailed petrological and mineral chemistry data (major and trace elements) on these products allow us to investigate the ascent pathways and plumbing system architecture of the related magmas, as well as their magmatic evolution. Alkaline lavas are dominated by olivines and pyroxenes phenocrysts, with minor oxides. Olivines are commonly dendritic or show jagged rims. Feldspar is the main phase found in the groundmass, together with clinopyroxenes, olivines and oxides. Occurrence of feldspathoids in the groundmass is also observed in some samples.  

Two different olivine populations can be recognized according to their composition and zoning. Most olivines are normal zoned, with Fe-enriched rims. Minor reversed zoned crystals or complex banding zoning are also found. CaO contents in olivines increase from core to rim, regardless of the zoning pattern. This feature can be related to the dynamic of magmatic processes (i.e. heating due to recharge events, decompression). Orthopyroxenes are always in disequilibrium with the groundmass showing olivine coronas or clinopyroxene armored-rims, which supports silica-under saturation conditions in the host melt. Clinopyroxenes show complex sector zoning, especially in the microlites of the groundmass, indicating rapid crystallization rates. Some clinopyroxene phenocrysts and micro-phenocrysts show resorbed cores with Ti-, Al-, Fe-enriched (coupled with Si-, Cr-, Mg-, Na-depleted) rims suggesting rapid cooling and/or decompression during magma ascent, or an increase of the activity of oxygen.

All these features point to the important role of kinetic processes during magma crystallization and indicate the fundamental role of rapid decompression related to magma ascent. The occurrence of xenoliths agrees with this hypothesis, testifying the high “erosion and transport capacity” of these magmas. However, the presence of complex zoning or discordant trace elements behavior suggests the possibility of more complex processes in the feeding system.

Pliocene rift-related magmatic products were also investigated and compared with the Quaternary volcanics indicating a change in the magmatic system, especially in the upward melt transfer mode (ascent dynamic, storage levels) related to variations of the thermal and rheological state of the extensional lithosphere associated with the transitions from the main rift phase (Pliocene) to the later rift deactivation (Quaternary).

Reference: Corti, G., Cioni, R., Franceschini, Z., Sani, F., Scaillet, S., Molin, P., Isola, I., Mazzarini, F., Brune, S., Keir, D., Erbello, A., Muluneh, A., Illsley-Kemp, F., Glerum, A., (2019). Aborted propagation of the Ethiopian rift caused by linkage with the Kenyan rift. Nat. Commun. 10, 1309. https://doi.org/10.1038/s41467-019-09335-2.