Carbonate magmas

• Published in: Journal of the Geological Society Vol. 150; no. 4; pp. 637 - 651
• Main Author: Bailey, D. K
• Format: Journal Article
• Language: English
• Published: London Geological Society of London 01-07-1993; The Geological Society of London; Geological Society
• Subjects: bimodal composition; carbonaceous composition; chemical composition; Crystalline rocks; Earth sciences; Earth, ocean, space; Exact sciences and technology; experimental studies; Geochemistry; igneous and metamorphic rocks; Igneous and metamorphic rocks petrology, volcanic processes, magmas; magmas; melting; Petrology; Soil and rock geochemistry; temperature; ultramafic composition; Africa; magmatic associations; pyroxenite; volcanic rocks; eruptions; phonolites; phase equilibria; igneous rocks; lamprophyres; xenoliths; genesis; carbonatites; immiscibility; kimberlite; Gregory Rift; P-T conditions; nephelinite; East African Rift; alkali gabbros; chemical composition; ultramafics; magmatic differentiation; metasomatism; peridotites
• ISSN: 0016-7649; 2041-479X
• DOI: 10.1144/gsjgs.150.4.0637

In this review it is noted that a nephelinite/phonolite volcano in Tanzania (Oldoinyo Lengai) is currently erupting Na-Ca-K carbonate magma (at approx 600 degrees C) but that this is unlike all other intrusive and effusive carbonatites (350 examples worldwide) which are dominantly composed of Ca, Mg, Fe carbonates, and have negligible alkali contents. Although a number of effusive calcio- carbonatites are considered to be degraded alkali carbonatites, there are several examples (including one magnesio-carbonatite Rufunsa) which are close to their erupted composition and substantiate the existence of high-T carbonate magmas lacking essential alkalis at the time of eruption. In these associations silicate magmas are generally absent, and in most the effusive carbonatites have been erupted directly from the mantle (with entrained peridotite debris and minerals). They provide a link with the ultramafic association (peridotite and pyroxenite), seen in some carbonatite intrusions, with the commonly associated ultramafic lamprophyres and with carbonate-rich kimberlites. Most large carbonatite intrusions seem to have been emplaced at lower T than effusives, probably as a near-solidus mush, with the interstitial fluid metasomatizing the country rocks. A wider perspective of carbonate magma genesis is needed to encompass various kinds of differentiation from alkaline silicate magmas, and primary carbonate magmas from various depths in the mantle (+- silicate melts). The strongly bimodal composition distribution of calcic and dolomitic carbonatites also awaits explanation. Half of the known carbonatites are in Africa; their timing and distribution indicate that the activity is a response to lateral forces acting across the plate.