Zircon xenocryst evidence for crustal recycling at the Mid-Atlantic Ridge

The presence of zircon xenocrysts in Mid Atlantic Ridge (MAR) rocks poses essential questions concerning the path they have followed from their sources, how they have survived transport through the mantle, and why they are much more abundant than in any other ocean. Sixteen out of twenty-five seafloor samples collected between S 21 degrees to N 42 degrees contained xenocrysts, here defined as those grains that coexist with a significantly younger population or are significantly older than the band of the Atlantic seafloor age from which they were collected. The xenocrysts are of two types, young and old. Hf and O isotopes and trace elements indicate that the younger crystals (3.1 Ma to 25.6 Ma) are oceanic, derived from previous MAR rocks, whereas the older crystals (190 Ma to 32 Ga) are continental, derived from crust slivers located at the MAR or immediate vicinity. The high partial derivative O-18 (similar to 7 parts per thousand to 11 parts per thousand) of many older xenocrysts indicates ultrafast recycling through the mantle; otherwise, the fast diffusivity of oxygen would have driven the xenocrysts partial derivative O-18 to that of their mantle hosts, a partial derivative O-18 approximate to 5.3 parts per thousand. The incorporation of xenocrysts into MAR magmas mainly occurs after ridge jumps, when hot mafic magma pervades a new crustal section scavenging xenoliths, or locally melts felsic wall-rocks and mixes with the resulting zircon-laden low-T magmas. In either case, the fusible component of the xenocrysts source contaminates the upwelling mafic magma. When the source was oceanic, the resulting chemical and isotopic signatures of the contaminated magma would remain MORB-like, albeit enriched in incompatible elements. When the source was continental, the resulting magma would reflect a perceptible continental influence. Crustal contamination at the ridge explains the abundance of zircon xenocrysts and the distinctive chemistry of the MAR magmas compared with other MORBs. These features are ultimately related to the Atlantic slow-spreading because it promotes ridge jumping and, hence, active crustal recycling. (C) 2019 Elsevier B.V. All rights reserved.