How Hot and Wet are Mantle Derived Magmas and Their Sources?

Abstract:

The compositional and thermal heterogeneity of convecting mantle critically affect magma production and compositions and cannot be easily distinguished from each other. The way to resolve this ambiguity is an independent estimation of temperature and composition of mantle sources of various types of magma.

Here we report application of olivine-spinel-melt geothermometers based on partition of Al, Cr, Sc, Y, Fe and Mg as well as direct measurement of H₂O concentrations in melt for olivine hosted melt and spinel inclusions from different primitive lavas of MORB, OIB, LIP, Archean komatiites and SSZ. The results suggest significant variations of crystallization temperature for the same Fo of high magnesium olivines from different types of mantle-derived magmas: from the lowest (down to 1220^oC) for MORB and SSZ to the highest (up to over 1500^oC) for komatiites and Siberian meimechites. These results confirm the relatively low temperature of the mantle sources of MORB and SSZ magmas, low to moderate amount of H₂O (0.2-1 wt%) in komatiite primary melts, high H₂O contents (over 2 wt%) of SSZ primary magmas and higher temperatures in the mantle plumes.

The established liquidus temperatures and compositions of primary melts allow estimating potential temperatures of their mantle sources. The highest potential temperatures over 1800 ^oC are characteristic for Archean komatiites. For Phanerozoic age the highest potential temperatures (1650^oC) are found for the largest LIPs: Siberian, North Atlantic and Caribbean. The sources of OIBs yield significant range of potential temperatures: 1400-1600^oC, positively correlated with magma production rate. MORBs yield potential temperature between 1350-1400^oC except those from ultra slow spreading ridges (e.g. Knipovich ridge), which display potential temperatures down to 1250 ^oC. Potential temperatures of SSZ mantle sources are typically within the range for MORB, suggesting origin of SSZ primary melts by H₂O fluxing of convecting mantle wedge. Exceptions are some boninites, which require higher temperature and plume related sources.

The results strongly confirm mantle plume theory and external source of H₂O in SSZ mantle. The H₂O in komatiites could be incorporated in the transitional zone.