Magmatic Degassing in the Crust Is Mantle Source Dependent

Abstract:

The ⁴He/⁴⁰Ar* ratio (where ⁴⁰Ar* is ⁴⁰Ar corrected for atmospheric contamination) is known to be sensitive to magmatic degassing due to the different solubilities of He and Ar in silicate melts: ⁴He/⁴⁰Ar* increases in the residual liquids because Ar is less soluble than He and therefore degasses more rapidly. Conversely, lithophile isotope ratios and incompatible trace element ratios (⁸⁷Sr/⁸⁶Sr, ¹⁴³Nd/¹⁴⁴Nd, La/Sm etc) are specifically chosen as these are largely insensitive to magmatic processes, including degassing (as far as mid-ocean ridges are concerned) but rather trace mantle heterogeneities. It is astonishing therefore that correlations between ⁴He/⁴⁰Ar* and lithophile isotope ratios (such as ⁸⁷Sr/⁸⁶Sr or ¹⁴³Nd/¹⁴⁴Nd) exist in South East Indian Ridge basalts and basaltic glasses [1]. These correlations appear to be the result of enhanced degassing of magmas produced from enriched mantle: enriched mantle probably has higher C contents relative to depleted mantle, therefore degassing of ‘enriched’ compositions will start at higher pressure and the proportion of volatiles lost will be greater than for ‘depleted’ lavas. The ⁴He/⁴⁰Ar* ratio of the erupted products depends on the proportion of volatiles lost, therefore ⁴He/⁴⁰Ar* is higher in lavas derived from enriched as opposed to depleted magmas. Naturally, enriched lavas are also distinct from depleted lavas in their lithophile isotopic composition (high ⁸⁷Sr/⁸⁶Sr, low ¹⁴³Nd/¹⁴⁴Nd) and thus the observed correlations between lithophile isotopes and degassing (⁴He/⁴⁰Ar*) is created.

 A simple degassing model suggests that, in order to generate the correlated variability in Sr and Nd isotopes and ⁴He/⁴⁰Ar*, the mantle C concentration likely varies by factor ~2 [1]. Thus it is possible to link mantle C variability – which is difficult to asses due to shallow level degassing – with Sr isotopic composition, which is very commonly measured in mid-ocean ridge basalts: Sr isotopes can be used as a proxy for mantle C concentration variation if the entire mantle Sr isotope variation is due to recycling of material with a near-constant Sr/C ratio. The validity of Sr isotopes as a mantle C heterogeneity proxy will be assessed using the PetDB database.

 References: Burnard, P., et al 2014. EPSL 395, 159-167.