Sodium Inverse Relationships During Melting in Ultraslow Spreading Regions: Insights from SWIR-Smoothseafloor Peridotites

Abstract:

Ultraslow spreading ridges are key regions to unravel mantle processes. Low potential temperatures and reduced melting allow decrypting early melting processes and shad lights on the source short-scale heterogeneities and their interactions with transient melts. Mantle-derived peridotites from the Smoothseafloor region of the eastern Southwest Indian Ridge reveal countertrending Na-Ti relationships. Na apparently behaves as a compatible element during partial melting similarly to light REEs. Heavy REEs, however, follow a normal relationship with the other melting indicators (e.g. Cr#), a behaviour that results in pattern rotation around a pivot element when looking to REE systematic. These relationships can be explained by percolation of relatively enriched, grt-field derived, melts in the spinel-field melting mantle ¹. A feature that also explains the inverse Na-Cr# correlation, frequently observed in abyssal mantle rocks. Experimental relationships constraint the grt-field derived melts to be produced by low-melting paragenesis that experience a garnet to spinel phase transition shallower than mantle peridotites for a given temperature. Based on potential mantle temperatures estimated by Cannat et al., 1999², the grt-sp transition can be set at ca. 2.0 and 1.5 GPa for mantle peridotites and Mg pyroxenites respectively with the onset of mantle melting at 1.2 GPa. Mass balance calculations based on the amount of produced melt constrains the pyroxenitic fraction < 10% by mass of the mantle source. The contemporaneous presence of lithologies too depleted with respect to the described process suggests that some portions of the mantle source are inherited from more sustained ancient depletion events not related to present-day processes beneath this ridge portion.

PNRA funding : PdR 2013/B1.02

1. Brunelli, D., et al., 2104. Percolation of enriched melts during incremental open-system melting in the spinel field : A REE approach to abyssal peridotites from the Southwest Indian Ridge. Geochim. Cosmochim. Acta 127,190–203.

2. Cannat, M., et al., 1999. Formation of the axial relief at the very slow spreading Southwest Indian Ridge (49° to 69°E). J. Geophys. Res. 104, 22825–22843.