Melt extraction pathways and stagnation depths beneath the Madeira and Desertas rift zones (NE Atlantic) inferred from barometric studies

Abstract

The Madeira and Desertas Islands (eastern North Atlantic) show well-developed rift zones which intersect near the eastern tip of Madeira (São Lourenço peninsula). We applied fluid inclusion barometry and clinopyroxene-melt thermobarometry to reconstruct levels of magma stagnation beneath the two adjacent rifts and to examine a possible genetic relationship during their evolution. Densities of CO₂-dominated fluid inclusions in basanitic to basaltic samples from São Lourenço yielded frequency maxima at pressures of 0.57–0.87 GPa (23–29 km depth) and 0.25–0.32 GPa (8–10 km), whereas basanites, basalts and xenoliths from the Desertas indicate 0.3–0.72 GPa (10–24 km) and 0.07–0.12 GPa (2–3 km). Clinopyroxene-melt thermobarometry applied to Ti-augite phenocryst rim and glass/groundmass compositions indicates pressures of 0.45–1.06 GPa (15–35 km; São Lourenço) and 0.53–0.89 GPa (17–28 km; Desertas Islands) which partly overlap with pressures indicated by fluid inclusions. We interpret our data to suggest a multi-stage magma ascent beneath the Madeira Archipelago: main fractionation occurs at multiple levels within the mantle (>15 km depth) and is followed by temporary stagnation within the crust prior to eruption. Depths of crustal magma stagnation beneath São Lourenço and the Desertas differ significantly, and there is no evidence for a common shallow magma reservoir feeding both rift arms. We discuss two models to explain the relations between the two adjacent rift systems: Madeira and the Desertas may represent either a two-armed rift system or two volcanic centres with separate magma supply systems. For petrological and volcanological reasons, we favour the second model and suggest that Madeira and the Desertas root in distinct regions of melt extraction. Magma focusing into the Desertas system off the hotspot axis may result from lithospheric bending caused by the load of the Madeira and Porto Santo shields, combined with regional variations in melt production due to an irregularly shaped plume.

Appendix

Xenolith petrography

DES4

Spinel wehrlite consisting to about 80% of early-formed olivine up to 3 mm with some kink bands, and 20% of interstitial clinopyroxene with abundant melt inclusions. Cpx is associated with late-formed and smaller, euhedral to subhedral olivines (<0.5 mm) and small spinels (<250 µm). Fine-grained spinels (about 20 µm) occur throughout the xenolith. Low intracrystalline deformation and the occurrence of interstitial clinopyroxene with olivine inclusions is evidence for a cumulate origin of the xenolith.

DGR132

Spinel dunite with coarse-grained olivine up to 8 mm in size showing abundant kink bands, melt and fluid inclusions, and euhedral to subrounded spinel up to 0.6 mm in size. Basaltic matrix of fine-grained clinopyroxene, olivine and plagioclase locally occurs in cracks and pockets and is interpreted as host melt which penetrated along grain boundaries and cracks. The dunite xenoliths are interpreted as cumulates.

M51/1-437DR-1

Harzburgite from the submarine Funchal ridge with abundant fluid inclusions. Olivine porphyroclasts (about 80%) from 1 to 6 mm in size show curvilinear grain boundaries and abundant kink bands. Orthopyroxenes porphyroclasts (20%) with clinopyroxene exsolution lamellae show embayments at the xenolith surface, demonstrating that they have reacted with the surrounding melt. No oxide minerals were found. Because of the deformation indicated by kink bands and the resorbed pyroxenes rims, the xenolith is interpreted to represent a fragment of the refractory mantle beneath Madeira.