Transient mantle cooling linked to regional volcanic shut-down and early rifting in the North Atlantic Igneous Province

Abstract

The Paleocene to Early Eocene Faroe Islands Basalt Group (FIBG) comprises a c. 6.5-km-thick lava flow-dominated sequence located within the centre of the North Atlantic Igneous Province (NAIP). The currently defined pre-breakup and syn-breakup sequences of the FIBG are separated by a significant volcanic hiatus, during which time the coal-bearing Prestfjall Formation was deposited. This major volcanic hiatus is identified across large parts of the NAIP and was preceded on the Faroe Islands by a reduction in eruption rate evidenced by an increased number and thickness of inter-lava sedimentary beds between the simple lava flows of the pre-breakup Beinisvørd Formation. High tempo eruptions resumed after this hiatus with the development of the compound lava flow fields of the Malinstindur Formation which reveal limited evidence for inter-lava breaks. In order to investigate this key transition, flow by flow geochemical sampling of a composite c.1.1-km-thick lava flow sequence spanning this transition were collected and analysed. Three chemically distinct groups are defined based on rare earth elements (REEs) and incompatible trace element signatures. Two high-Ti groups (TiO 2 > 2 wt%), B2 and B3, dominate the sampled Beinisvørd Formation and display light REE-enriched signatures (La/Yb N c. 2.9-5.9) and evidence for garnet in the source melting region (Dy/Yb N c. 1.5-1.6). At the very top of the Beinisvørd Formation, a distinct group of lava flows, B1, displaying lower TiO 2 for a given MgO wt% (TiO 2 c. 1-2 wt%), weakly light REE-enriched profiles (La/Yb N c. 1.7-2.4) and a spinel-dominated mantle melting signature (Dy/Yb N c. 1.1-1.2) is identified. Sr, Nd and Pb isotopic signatures for the three groups overlap, revealing limited evidence of crustal contamination, and therefore supporting a mantle melting origin for inter-group variations, rather than source composition or contamination. The group B1 lava flows form a unique stratigraphic occurrence on the islands and provide clear evidence for both a reduction in the initial pressure of melting, alongside an increase in the overall degree of partial melting relative to groups B2 and B3. Increased partial melting is interpreted as evidence for the early onset of rifting and lithospheric thinning to the north of the Faroe Islands. The accompanying reduction in initial pressure of melting provides the first petrological evidence that a transient reduction in mantle temperature leads to the province-wide volcanic hiatus. Our study demonstrates an intimate linkage between rifting history and fluctuations in mantle temperature highlighting that any over-arching model for the evolution of the NAIP must take both into equal account.

Expedition 396 Preliminary Report: Mid-Norwegian Margin Magmatism and Paleoclimate Implications

“…Similarly, geochemical enrichment of volcanic rocks of the East Greenland Margin (chondrite-normalized [Ce/Y]N and isotopes; Fitton et al, 1998;Tegner et al, 1998;Brown and Lesher, 2014) increases from south to north. In addition to scientific drilling results, evidence from the thick onshore Faroe Islands and East Greenland exposures reveal a temporal evolution that supports fluctuations in mantle potential temperature with time (e.g., Tegner et al, 1998;Millett et al, 2020a). Temporal fluctuations in mantle potential temperature are also invoked from independent geophysical observations, which reveal major uplift and subsidence events during the North Atlantic Igneous Province (NAIP) history that have been invoked as evidence for plume pulsing within the province (e.g.…”

The Rosebank Field, NE Atlantic: Volcanic characterisation of an inter‐lava hydrocarbon discovery

“…(a) TiO 2 versus MgO highlighting the division of the Rosebank samples into High‐Ti and Low‐Ti groups divided at 3 wt.% TiO 2 . (b) Rare Earth Element (REE) multi‐element plots normalised to Chondrite (Sun & McDonough, 1989), highlighting the range of Low‐Ti and High‐Ti Rosebank samples compared to samples from the Beinisvørð Formation of the Faroe Islands (Millett et al., 2020)…”

Methodology for Evaluating the North Atlantic Igneous Province Reveals Multiple Hotspots at Breakup

“…Although the basalts on Iceland, versus Central East Greenland and Faroe Islands, originate from different depths (e.g., Waight and Baker, 2012; Matthews et al., 2016), they can be divided into at least two separate mantle sources; a depleted low TiO 2 (<1.5%) and a more fusible enriched high TiO 2 (≥1.5%), source (Søager and Holm, 2009, 2011; Waight and Baker, 2012). The low TiO 2 (LoTi) melts have trace element ratio's Dy/Yb N ∼1–1.1 and La/Sm N < 1, suggesting a shallow, high degree of melting, while the high TiO 2 (HiTi) yield Dy/Yb N > 1.1; La/Sm N > 1, reflects a deeper, lower degree of melting (Fram and Lesher, 1993; Millett et al., 2017, 2020; Tegner et al., 1998a; Figures 3a and 3b). However, the LoTi basalts frequently re‐occur throughout the stratigraphic record (Figure 2a), often coincident with HiTi basalts (Jolley et al., 2012; Tegner et al., 1998a).…”

“…The base of the syn breakup basalts (Figure 2a) in both CEG and FI, comprise of high‐MgO olivine xenoliths (Larsen et al., 1999; Millett et al., 2020), that could petrologically mark destruction of deep magma chambers and continental breakup (Figure 2a). The cumulates are stratigraphically closely followed by sequences of LoTi MORB‐like flows (Jolley et al., 2012; Tegner et al., 1998a; Figure 2a), confirming continental breakup (Brooks, 2011).…”

“…A decreasing, positive correlation between La/Sm N and Dy/Yb N in the pre-breakup section (green symbols, Figure 3a) suggest an increase in the mean extent of melting (F) and decline in pressure (P), indicative of a lithospheric thinning trend from L2 ∼ 75 km to L3 ∼ 60 km in ∼3 Ma, along the proto-Reykjanes ridge (Figures 3a and 3b). However, when continental lithospheric thickness is reduced to L3 ∼ 60 km in the top 200 m of the Beinisvørd Fm ∼57 Ma, the La/Sm N -Dy/Yb N diagram suggest an additional drop in to T p ∼ 1,400°C (ΔT p ∼ 100°C; Millett et al, 2020), deviating slightly from the main lithospheric thinning trend (Figure 3). At BU (A; Figure 2), the unconformity signals that magmatic production is essentially switched off (Figures 3b and 3c) and ΔT p is interpreted to drop to ∼50°C, or less.…”