Combining volatiles measurements in fluid inclusions with petrology of ultramafic xenoliths from the Massif d'Ambre: unravelling the nature and evolution of the northern Madagascar Sub-Continental Lithospheric Mantle

Integrating petrography and mineral chemistry data with the determination of volatiles concentration and isotopic fingerprint in fluid inclusions (FI) in ultramafic xenoliths is a novel approach which provides crucial information on the nature and evolution of the lithospheric mantle, together with important insight into how and where volatiles are stored and/or migrate through the lithosphere.

In this work, we investigated a new suite of ultramafic peridotite xenoliths from the Massif d’Ambre by integrating petrography, mineral and glass chemistry and the concentrations of volatiles [CO₂ and noble gases (He, Ne and Ar)] in fluid inclusions (FI) hosted in olivine (Ol), orthopyroxene (Opx) and clinopyroxene (Cpx). The Massif d’Ambre is a Cenozoic stratovolcano located in northern Madagascar originated upon intense volcanic activity from ~12 to ~0.85 Ma, and the area is characterized by the widespread occurrence of mantle xenoliths, mostly, but not restricted to, spinel lherzolites and subordinately pyroxenites, which are hosted in mafic volcanic rocks. The new suite comprises 18 lherzolites, 11 harzburgites, 2 dunites, 3 wehrlites and 1 Ol-clinopyroxenite. Based on their petrographic and textural features, the suite was divided into five distinct groups: group 1A (protogranular to porphyroclastic textures), group 1B (large and porphyroclastic olivines), group 2 (infiltrated dunites and wehrlites), group 3 (cumulate-textured wehrlites) and group 4 (Ol-clinopyroxenite). Xenoliths are modally and compositionally heterogeneous and a clear separation can be observed between groups 1A-1B and groups 2-3, as testified by the large forsterite range of olivine (Fo88.4 – 93.2 vs Fo78.7 – 89.1, respectively), the Mg# of orthopyroxene (89.5 – 93.2 vs 82.7 – 87.3, respectively) and clinopyroxene (90.9 – 95.2 vs 81.4 – 89.9, respectively). This systematics corroborates the distinct origin of the groups, with xenoliths belonging to 1A-1B having the most refractory character and reflecting high extents (up to 30%) of melt extraction, while groups 3-4 xenoliths reflecting less depleted or re-fertilized mantle portions. Based on glass analyses, we propose that a carbonatitic or carbonated alkaline agent may have interacted with some portion of the source mantle, in agreement with Coltorti et al. (2000). The noble gases in FI hosted in Ol, Opx and Cpx exhibit ³He/⁴He ratio corrected for air contamination (Rc/Ra values) ranging from 5.90 Ra to 7.05 Ra, which is below the typical MORB-like upper-mantle value (8 ± 1 Ra). Furthermore, the great majority of xenoliths exhibits ⁴He/⁴⁰Ar* ratios between ca. 0.2 to 0.8.

The major element distribution in mineral phases together with the systematic variations in FI composition will be used to place constraints on the origin and evolution (in terms of melting and metasomatism) of this portion of the mantle below the Massif d’Ambre and will be exploited to obtain a possible timeline for the petrological events that have characterized this lithospheric mantle portion.

Coltorti M., Beccaluva L., Bonadiman C., Salvini L. & Siena F. 2000. Glasses in mantle xenoliths as geochemical indicators of metasomatic agents. Earth Planet Sc. Lett., 183, 303–320.

Keywords: mantle xenoliths; lithospheric mantle; metasomatism; Massif d’Ambre