Depleted components in the source of hotspot magmas: Evidence from the Ninetyeast Ridge (Kerguelen)

• Trace element and Hf isotopic ratios identify a depleted source for some hotspot basalt.
• Ninetyeast Ridge basalts were derived from a residue of ancient partial melting.
• Garnet-bearing residues of partial melting are a source of some hotspot basalt.

Although most ocean island basalts (OIB) are enriched in incompatible elements relative to mid-ocean ridge basalts, OIB depleted in these elements also occur on some islands. The Ninetyeast Ridge (NER) in the eastern Indian Ocean is a 5000 km long hotspot track defined by submarine basaltic volcanoes that were islands when they formed from 43 to 77 Ma. A subset of NER basalts, described as depleted, has high abundances of Sc, Y and Lu, which are relatively compatible in clinopyroxene and especially in garnet. It is unusual for magmas to have the trace element characteristics of a mineral. A likely explanation is that the depleted NER basalts were derived from a source that was created as a garnet- and clinopyroxene-bearing residue during partial melting. When this residue formed, the extent of melting must have been low as not all of the garnet and clinopyroxene was melted. To provide sufficient time for the relatively high Lu/Hf of the residue to develop the high 176Hf/177Hf that is characteristic of depleted NER basalts, this melting event must have been ancient. In the second much younger melting event that formed the NER, the extent of melting was sufficiently high to eliminate garnet and clinopyroxene from the ancient residue. Basalts erupted on a segment of the Mid-Atlantic Ridge near the Azores were also derived from an ancient garnet-bearing residue. Residues from ancient partial melting events involving low extents of melting are the dominant source of mid-ocean ridge basalts and depleted magmas associated with the Kerguelen and Azores hotspots. In contrast, a very different process has been inferred for creating the source of depleted Icelandic basalts. Their source was gabbro containing cumulate plagioclase and clinopyroxene. Such gabbros are common in the lower oceanic crust, and if recycled into the Icelandic hotspot they are a source of depleted Icelandic basalts.