Recycling of crustal material by the Iceland mantle plume: New evidence from nitrogen elemental and isotope systematics of subglacial basalts

Halldórsson, SA, Hilton DR, Barry PH, Füri E, Grönvold K.  2016.  Recycling of crustal material by the Iceland mantle plume: New evidence from nitrogen elemental and isotope systematics of subglacial basalts. Geochimica et Cosmochimica Acta. 176:206-226.

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We report new nitrogen (N2) abundance and isotope (δ15N) data for 43 subglacial basaltic glasses from the neovolcanic zones of Iceland, a key locality in studies of mantle plume geochemistry and crust–mantle processes. New helium and argon abundance and isotope data are also reported to supplement previous studies (Füri et al., 2010; Barry et al., 2014), allowing elemental ratios (e.g., N2/40Ar∗ where 40Ar∗ = radiogenic 40Ar) to be calculated. Subglacial basaltic glasses with N2 > 2 μcm3 STP/g show a wide range in δ15N values, from −2.91 to +11.96‰ (vs. Air), with values >6‰ only observed at one locality in the Eastern Rift Zone. Elemental ratios involving N2, i.e., N2/3He, and N2/40Ar∗, span several orders of magnitude from 2.5 × 105 to 9.0 × 107, and 32.8 to 1.46 × 106, respectively. In contrast, argon isotope ratios (40Ar/36Ar) are limited, ranging from air-like (∼298.6) values up to 1330. Glasses exhibit a wide range in helium isotope ratios (8–26 RA), with clear distinctions between individual rift segments. A number of processes have extensively modified original mantle source N isotope and relative abundance compositions – most significantly air interaction, crustal contamination in some instances, and possibly degassing-induced fractionation. Under the assumption that the starting 4He/40Ar∗ production ratio of Iceland mantle is identical to the depleted MORB mantle (DMM), a filtering protocol for the entire N dataset, based upon 40Ar/36Ar and 4He/40Ar∗ ratios, was adopted to identify samples with unmodified δ15N values. Consequently, we identify 22 samples that define the Icelandic mantle N-isotope distribution (δ15N = −2.29 to +5.71‰). Using the filtered dataset, we investigate simple binary mixing scenarios involving N2/3He–N2/40Ar∗–δ15N variations to identify mantle end-member compositions. Mixing scenarios are consistent with a recycled component in the Iceland mantle source, defined by a high and heterogeneous δ15N end-member. Moreover, this end-member is coupled to the high 3He/4He signature, and is characterized by He depletion and/or the presence of excess N2. These features strongly suggest the presence of recycled crustal N-component(s) integrated into and/or entrained by the Iceland plume source. These new results reveal the highly heterogeneous nature of nitrogen in the hybrid Iceland plume source, consistent with models based on trace elements and radiogenic isotopes that advocate for significant heterogeneity of recycled crustal component(s) sampled by the Iceland plume. A relatively young age of the recycled crustal material (possibly Phanerozoic) is consistent with the association of positive δ15N values and high N2/40Ar∗ ratios with constraints from radiogenic isotopes (e.g., Pb), thus indicating a relatively short time-interval (∼108 years) between subduction of crustal material and entrainment by the Iceland mantle plume.