Light elements in oceanic and ophiolitic serpentinities

Despite the key importance of altered oceanic mantle as a repository and carrier of light elements (B, Li, and Be) to depth, its inventory of these elements has hardly been explored and quantified. In order to constrain the systematics and budgets of these elements in the oceanic lithosphere, this thesis has studied samples of highly serpentinized peridotites from the mid-Atlantic ridge (ODP Leg 209) and progressively metamorphosed alpine ophiolites (Totalp, Platta and Malenco). Light element concentrations in minerals and whole rock samples within serpentinized peridotites were thus acquired to understand these processes. Moreover the isotopic ratios (δ⁷Li, δ¹¹B and ⁸⁷Sr/⁸⁶Sr) of whole rock samples of oceanic serpentinites have been determined. With the aim of developing a new analytical protocol to analyse efficiently Li, Be and B whole rock data, we combined fusion of sample powders using an Ir-strip heater with in-situ Li, Be and B measurements on SIMS. To assure that no light element loss or gain occurs during heating on the Ir-strip, experiments were conducted at different heating times and temperatures. Li and Be concentrations are within analytical uncertainties. B, on the contrary, is highly volatile on the Ir-strip and high loss occurs at 1800°C. During exhumation of the mantle, late stage melt impregnation can influence the light element whole rock signal. Once exposed or emplaced near the ocean floor, alteration is the major process influencing the primary light element mantle signature of the oceanic mantle. At ODP Leg 209, late-stage cpx have distinctly higher Li concentrations than depleted mantle, thus the latter dominates Li in whole rock sample. B, on the contrary, is enriched in serpentine and the latter hence controls the B content of the serpentinized mantle. Calculation of the B and Li budget of an idealized oceanic plate showed that at slow and fast spreading ridges, B content is controlled by serpentinites, while the oceanic crust controls the Li budget. Thus, serpentinization of the oceanic plate is an important process in the understanding of the element recycling within the subduction zone factory. Isotope measurements on whole rock samples from ODP Leg 209 revealed the highest δ¹¹B (+40.66‰) and the lowest δ⁷Li (-28.46‰) reported so far for oceanic serpentinites. Serpentinization is the major process enriching B and leaching Li out of the rock. Hence, also the isotopic signal must be related to serpentinization. Moderate water-rock interaction, as calculated on the basis of ⁸⁷Sr/⁸⁶Sr ratios, very likely produced the δ⁷Li and δ¹¹B values found at ODP Leg 209. Seawater alteration would lead to an isotopic signal of δ¹¹B +21‰ in serpentinized peridotite, which is not in line with our data set. Therefore, a different process must have led to such high δ¹¹B values, if equilibrium conditions are assumed. Our modelling results, based on the isotopic signature of a batch of fluid interacting with rocks during penetration into the oceanic plate, show that with increasing volume passed continuously evolution occurs. Such an evolved fluid could account for the high δ¹¹B and low δ⁷Li signal reported at ODP Leg 209. The Totalp-Platta-Malenco transect shows prograde alpine metamorphism from prehnite-pumpellyite to epidote-amphibolite facies from north to south. In the Malenco massif, the Be concentrations within primary mantle phases and metamorphic minerals are similar. Thus, Be mobility during prograde metamorphism is very low or Be is immobile. B and Li, on the contrary, show a mobile behaviour. The polymorphic change between chrysotile and antigorite expels B, while Li concentration stays in the same range, but decreases slightly. Further on, chlorites behave similar to serpentine with respect to B and Li loss with increasing metamorphism. As serpentine and chlorite are phyllosilicates, an explanation for the observed B and Li change might be the internal structural order, as internal order increases with increasing P-T.

Thèse de doctorat : Université de Neuchâtel, 2009 ; Th.2064