Megacrysts of “bubbled” kaersutite in the Neogene-Quaternary of Western Syria: evidence of crystallization in a boiled melt/fluid?

The territory of Syria is a classic area of intraplate Neogene-Quaternary plateau basaltic magmatism (Ponikarov et al., 1969; Sharkov, 2000; Lustrino, Sharkov, 2006; Trifonov et al., 2011, etc.). These basalts belong to the Afro-Arabian large igneous province (LIP) (Ernst, 2014), whose origin, according to geophysical data, is related to the ascent of a mantle thermochemical plume that originated at the liquid iron core-silicate mantle boundary of (Hansen et al., 2012).

The basalt plateaus of Syria have a similar structure and are formed by numerous basaltic flows, as well as scoria and pyroclastic cones, often containing mantle xenoliths. Approximately 80% of them are represented by green spinel lherzolites and harzburgites, and subordinate amount (~20 %) of xenoliths belong to black series (hornblendite, hornblende clinopyroxenites, clinopyroxenites, phlogopitites, etc., as well as megacrysts of kaersutite, clinopyroxene, ilmenite, sanidine, etc.). Some of the kaersutite megacrysts have unusual “bubbled” structure, containing oval cavities up to 3-4 mm in diameter. We believe that these xenoliths are fragments of the upper cooled margin of the mantle plume above the adiabatic melting zone (Sharkov et al., 2017). Thus, they probe substance of mantle plume and bear important information about the processes within its interior.

As previously shown (Sharkov et al., 2017), the black series rocks were formed from a melt/fluid released fluid during the incongruent ("secondary") melting of the mantle plume head at the final stage of the magmatic system evolution. The crystallization of this fluid-supersaturated melt could be accompanied by its retrograde boiling, which led to the appearance of "bubbled" crystals.