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   This project “Melt-rock rection and melt migration in the MORB mantle through combined natural and experimental studies” is funded in the frame of the PRIN project (Progetti di Ricerca di rilevante Interesse Nazionale) Prot.2015C5LN35 by the Italian Ministry of Education and Research.

     This project aims at exploring the heterogeneity of the MORB mantle by studying the relationships between melt genesis, migration and extraction from the source region. We propose a multidisciplinary approach, which integrates field-based, geochemical and isotopic studies and experimental investigations.

     Our selected geologic objects are mantle samples from fossil and modern oceanic lithosphere, namely from the Jurassic ophiolites exposed along the Alpine-Apennine belt, and from slow- to ultra-slow spreading ridge segments.

    The project has two main objectives:
1) unravel the origin and chemical-isotopic fingerprints introduced by pyroxene-rich components in the MORB mantle;
2) explore the consequence of a heterogeneous mantle source on the chemistry of MORBs and the migration of primary melts through channelized flow.

 

The research project is articulated in five Work Packages (WP):

Objective 1: Role of pyroxenites in creating mantle source heterogeneity

    WP1: Multiple origin of pyroxenites from fertile MORB mantle

    WP2: Origin of pyroxenites from depleted oceanic mantle

    WP3: Peridotite modifications by interaction with pyroxenite-derived melts

Objective 2: Consequence of a heterogeneous mantle source on the chemistry of MORBs and the channelized melt migration

    WP4: Role of pyroxenites in MORB generation

    WP5: The chemical variability of melts migrating through replacive dunites

 

Involvement of the research units in the Work Packages:

 

Detailed objectives of the Work Packages:

WP1: Multiple origin of pyroxenites from fertile MORB mantle

   Major goal of WP1 is to define the origin, extent and scale of lithologic, chemical and isotopic variability introduced in fertile peridotites (proxies of MORB mantle sources) by pyroxenite components.

We will examine “aged” pyroxenite layers from fertile ophiolitic mantle sections (External Ligurides, Erro-Tobbio, Totalp). We will develop field-based geochemical and isotopic studies (by radiogenic Nd-Hf-Os systematics and Fe, Ni, Cu stable isotopes) on pyroxenites, on both well characterized samples and new occurrences, with the aim to unravel their origin, and trace potential recycling of crustal material into the mantle.

Studies on natural samples will provide the chemical data set to plan experimental investigations, focused on the origin of secondary pyroxenites by melt-peridotite reaction. We plan to perform in-situ crystallization-reaction experiments at 2.0-2.5 GPa and over a wide range of T (1200-1450°C), with the twofold aim to unravel i) mineralogy and composition of secondary pyroxenite, ii) mineralogical and chemical modification of wall-rock peridotite by reaction with pyroxenite-derived melt. We expect to constrain the extent of reactivity of pyroxenite-derived melts as a function of depth, melt composition and host peridotite composition, in order to evaluate their capability to migrate through the upper mantle.

 

WP2: Origin of pyroxenites from depleted oceanic mantle

   WP2 will be dedicated to define the origin of pyroxenites found as layers and/or veins in depleted peridotites, representing the MORB-melt residues, with the specific target to determine if they derive from MORB melt-peridotite interaction or they represent preserved relics of previous pyroxenites.

The sample set will consist of pyroxenites (spinel websterites and clinopyroxenites) in depleted peridotites, representing mantle residues after MORB production, from both ophiolitic (Internal Ligurides, Chenaillet, Alpine Corsica) and modern oceanic settings (SWIR). We will perform microstructural and in-situ mineral geochemical investigations, combined with Nd-Hf-Os isotopic studies. We will have a twofold target: i) to shed light on the origin of pyroxenites in the oceanic depleted mantle, ii) to acquire timing information about pyroxenite emplacement. Integrated results of WP1 and WP2 are expected to provide a unique physical and geochemical portrayal of small-scale pyroxenite-related heterogeneities in the MORB mantle.

 

WP3: Peridotite modifications by interaction with pyroxenite-derived melts

   In WP3, we aim to discuss the extent and length-scale of chemical and isotopic changes caused in host mantle peridotite by interaction with pyroxenite-derived melts, and to unravel the process driving the interaction. We will focus on peridotite-pyroxenite profiles from two settings, the External Liguride ophiolites and the Smoothseafloor mantle from SWIR.

Previous work by Borghini et al. (2013, Geology) revealed cm-scale chemical and Nd isotopic changes in wall-rock peridotites of pyroxenite layers, covering the global Nd isotopic variation seen in abyssal peridotites. Combined Lu-Hf, Sm-Nd isotope data will enable to test the existence of Nd-Hf isotopic decoupling related to porous melt flow.

Parallel experimental work will be devoted to study the trace element changes in mantle clinopyroxene during reactive melt infiltration. The results of this set of experiments will improve the understanding of grain scale processes that control the distribution of trace element between reactive melt and minerals during melt-rock reaction and melt transport.

 

WP4: Role of pyroxenites in MORB generation

    WP4 will integrate melting experiments and petrologic-geochemical studies of specific abyssal peridotite – MORB suites, to discuss the contribution of pyroxenite components in the genesis of MORBs.

Experiments will be focused on the partial melting behaviour of secondary pyroxenite lithologies. This issue is still poorly constrained experimentally, although secondary pyroxenites have been largely invoked as heterogeneities in the MORB mantle source. Starting materials will be homogeneous glasses obtained by complete melting of selected natural rock powders. We plan to perform a systematic experimental investigation at P-T conditions of MORB production (1-1.5 GPa, 1200-1400°C), in order to follow the chemical evolution of melts produced by secondary pyroxenite as a function of temperature. Results will be used to evaluate their contribution in models of basalt generation from a peridotite-pyroxenite mixed source.

Further insights on the contribution of pyroxenites to MORBs will be derived by the study of oceanic mantle-crust temporal series. We aim to develop a new method to reveal the presence of pyroxenites in the source based on their thermal effect on the melting process. This approach can be applied only on temporal series where the effects of source and temperature variability can be constrained. We will focus on two sampling series: i) the Vema lithosperic section (MAR), covering 0-26 My seafloor extension, ii) the Smoothseafloor region, covering 0-6 My.

 

WP5: The chemical variability of melts migrating through replacive dunites

   This WP is aimed to explore the compositional variability of melts migrating through replacive dunite channels, in order to verify whether the process of channelized melt migration may be responsible for the extraction of enriched components from the source region.

We will examine replacive dunite samples in key study areas of the Alpine-Apennine ophiolites (Lanzo, Eastern Liguria, Northern Tuscany, Alpine Corsica), focusing on the geochemistry of olivine, to derive the composition of equilibrium melts.

FEG-SEM analyses on olivine will detect chemical heterogeneity in relation to growth zoning, intracrystalline diffusion and dissolution/reprecipitation processes. Li and B analyses by SIMS will be critical to detect enriched components in the mantle source. Also, we plan to develop a new analytical protocol (in collaboration with M. Hamada, JAMSTEC, Japan) to obtain H concentrations by SIMS, through a combined SIMS-FTIR approach. Determination of H contents in olivine will allow direct comparison between H, Li and B abundances, with implications on the mechanism of water entrainment in the olivine lattice. Final target will be to discuss the relationships between mantle heterogeneity and the channelized melt migration, through numerical modeling.