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

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• 作者：Sæ ; mundur A. Halldó ; rsson^a ; ^{saemiah@hi.is" class="auth_mail" title="E-mail the corresponding author} ; David R. Hilton^a ; Peter H. Barry^a ; ^b ; Evelyn Fü ; ri^a ; ^c ; Karl Grö ; nvold^d
• 刊名：Geochimica et Cosmochimica Acta
• 出版年：2016
• 出版时间：1 March 2016
• 年：2016
• 卷：176
• 期：Complete
• 页码：206-226
• 全文大小：2395 K

文摘

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 and 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 R_A), 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.