CARBONATE MAGMAS

For 40 years, the case for the existence of carbonate magmas rested on field observations of carbonatite intrusions, in which the lack of th ermal effects raised an apparent conflict with the high melting temper atures of pure carbonates. Since 1960, the position has changed, with the growth of experimental studies and increasing observations of effu sive carbonate rocks. A nephelinite/phonolite volcano in Tanzania is c urrently erupting Na-Ca-K carbonate magma (around 600-degrees-C). This is unlike all other intrusive and effusive carbonatites (350 examples worldwide) which are dominantly composed of Ca, Mg, Fe carbonates, an d have negligible alkali contents. Although a number of effusive calci o-carbonatites are considered to be degraded alkali carbonatites, ther e are several examples (including one magnesio-carbonatite) which are close to their erupted composition, and substantiate the existence of high T carbonate magmas lacking essential alkalis at the time of erupt ion. In these associations silicate magmas are absent (or minor), 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), see n in some carbonatite intrusions, with the commonly associated ultrama fic lamprophyres (which may also carry mantle xenoliths), and with car bonate-rich kimberlites. Many carbonatite intrusions also have little or no associated silicate magmas, putting in question a popular view t hat carbonatites normally form only minor parts of alkaline igneous co mplexes of nephelinite/phonolite type. The corollary, that the carbona tites are normally differentiates is even less sound, because in alkal ine complexes the carbonatite is always last in the eruption sequence. Here the carbonatite may represent the final residua expelled from th e source region. Most large carbonatite intrusions seem to have been e mplaced at lower T than effusives, probably as a near-solidus mush, wi th the interstitial fluid metasomatizing the country rocks. A wider pe rspective of carbonate magma genesis is called for, to encompass vario us kinds of differentiation from alkaline silicate magmas, and primary carbonate magmas from various depths in the mantle (with or without s ilicate melts). The strongly bimodal composition distribution of calci c and dolomitic carbonatites is a further factor awaiting explanation. Half of the known carbonatites are in Africa, and their timing and di stribution indicate that the activity is a response to lateral forces acting across the plate. Carbonate magmatism is waiting to be unleashe d. This activity demands attention because it is now clear that carbon ate magmatism is a crucial surface expression of deep mantle processes .