ALBERT

Description: During the Meteor 79/3 expedition on the Cabo Verde Archipelago/NE Atlantic region, rock samples were obtained using a dredge haul. 40Ar-39Ar laser total fusion geochronological analyses have been performed on K-feldspar and biotite single-/multi-crystal separates from one sample (950-2). The 40Ar-39Ar analyses were determined using a MAP216 mass spectrometer at GEOMAR, a MAP argon extraction line and a 25 W Coherent Sabre 25TSM argon ion laser, from 30th August 2017 until 2nd September 2017. The Taylor Creek sanidine (TCS2) flux standard was used with an age of 27.344 +/- 0.011 Ma (1 sigma error; Fleck et al. 2019).

Description: Sediment gravity cores recovered during the RV METEOR cruise M80/3 in 2010 around the northwestern end of the Cape Verde Archipelago contain three widespread hyaloclastic tephra layers. One of these layers occurs in two sediment cores 40 km apart. The blocky shapes of the vesicle-poor/-free glass-shards clearly indicate their origin from a subaqueous eruption. There are three potential sources in the northwestern Cape Verdean Seamount Province: (1) the Nola Seamount, (2) the Sodade Seamount and (3) the Charles Darwin Volcanic Field. Using geochemical fingerprinting the hyaloclastic glass-shards could be unambiguously correlated to the Charles Darwin Volcanic Field. This is a deep-sea volcanic field consisting of at least 14 eruption centers all at 〉2,850 m below sea level, located about 100 km east of the core locations. Previous studies have documented widespread tephra distributions from relatively shallow (〈 500 mbsl) submarine explosive eruptions, but here we record such a widespread tephra from a deep-sea (probably 〉3000 mbsl) eruption. We discuss the mechanisms of formation and far transport of the hyaloclastic particles.

Description: Maio Island is situated in the eastern part of Cape Verde archipelago, and comprises early Mesozoic MORB-type pillow lavas and deep-sea sediments, and Miocene-Pliocene igneous rocks. The island is deeply eroded, indicating several phases of intensive erosion and collapse both during and after igneous growth. Maio exposes a central intrusive complex of Miocene age, which is surrounded by Lower Cretaceous pelagic limestones that were uplifted from the surrounding seafloor by igneous and tectonic activity1. The limestones were intruded by dykes and sills during the Miocene (12-8 Ma), with a peak in activity around 11 Ma2. These successions are overlain by lava flows and conglomerates of the Casas Velhas Formation (Fm.) and the Pedro Vaz Fm., which formed between 12-11 Ma, and by plateau lavas of the Malhada Pedra Fm. (9-7 Ma)2. The youngest volcanic units exposed on the island are lava flows of the Monte Penoso Fm., which formed a stratovolcano and have been K-Ar dated at 6.9 ± 0.4 Ma and 6.7 ± 0.4 Ma (2σ )2. At the base of the Monte Penoso Fm., polymict conglomerates up to 100 m thick, which we interpret as landslide deposits, occur at several locations. They contain well rounded clasts of various lithologies. Fresh biotite grains from a xenolith-rich basanite clast within the conglomerate were analysed. Single crystal laser total fusion biotite 40Ar-39Ar ages range from 8.50 ± 0.03 Ma to 11.55 ± 0.34 Ma (2σ, using the decay constants and atmospheric ratio of 3, and age standard TCS2 (27.87 ± 0.04 Ma; 1σ)). The older biotite ages are probably xenocrysts, but the youngest biotites yield a well constrained weighted mean 40Ar-39Ar age of 8.50 ± 0.02 Ma (2σ; n=22). Combining the K-Ar ages of the overlying Monte Penoso Fm.2 and our 40Ar-39Ar single biotite ages for the conglomerate clast, the Miocene period of large scale collapse and erosion on Maio can be confined to a period between 8.50-6.80 Ma, before the final phase of volcanism on the island began with the formation of the Monte Penoso stratovolcano. References: 1 Stillman, C.J. et al. (1982) J. Geol. Soc. 139 (3), 347–361. 2 Mitchell, J.G. et al. (1983) EPSL 64, 61–76. 3 Steiger R.H. & Jäger E. (1977) EPSL 36, 359–362.

Description: Highlights • Extensive set of 170 40Ar39Ar single-crystal ages for Cadamosto Seamount. • Volcanic eruption ages at Cadamosto Seamount are all young (〈 100 ka). • Three samples dominated by sanidine phenocrysts preserve a 21.04 ± 0.62 ka age. • Sanidine antecrysts in two samples show complex chemical zonation patterns. • Antecryst ages suggest long-lived magmatic activity in the seamount, up to 1.52 Ma. Abstract Cadamosto Seamount is located in the SW of the Cape Verde Archipelago in the central Atlantic Ocean off the west coast of Africa. Many radiometric dates exist for the islands in the archipelago; however, no geochronological information has been obtained from the numerous seamounts. The timescales for igneous processes in the submarine realm are thus poorly understood. In this study, we investigated five lavas that were sampled by dredging and ROV (remotely operated vehicle) from the flanks and summit areas of the largely phonolitic Cadamosto Seamount during two different research cruises. Chemical zonation patterns of minerals were determined by electron microprobe, and radiometric ages were obtained from single-crystal total-fusion and single−/multi-grain step-heating 40Ar39Ar analyses of sanidine, nepheline and sodalite-group minerals. Our 40Ar39Ar results reveal young sanidine eruption ages (all 〈100 ka) at Cadamosto Seamount: (1) Three western flank/summit lavas have a relatively simple petrology dominated by phenocrysts, and overlap with mean sanidine ages of 20.98 ± 0.87 ka, 21.44 ± 0.80 ka and 22.3 ± 2.0 ka, with a combined mean age of 21.04 ± 0.62 ka from the three samples (all uncertainties are quoted at 2σ). The remaining two samples from the summit/NE flank are dominated by complex zoned sanidines with resorbed antecrystic cores and phenocrystic rims. These samples yield older sanidine ages of 51.8 ± 2.4 ka and 97 ± 14 ka, which are interpreted to be maximum eruption ages. This is due to the dominance of antecrysts in these two samples and the possibility that the analyzed sanidine grains may be a mixture of older antecrystic cores and younger phenocrystic rims. The older 40Ar39Ar ages of many sanidine and nepheline antecrysts also give us clues regarding older magmatic events at Cadamosto Seamount, despite these grains having undergone resorption and phenocrystic rim overgrowths, resulting in some radiogenic 40Ar loss during entrainment in the subsequent magmas. The antecrysts minimum ages extend back to 1.5215 ± 0.0083 Ma, which supports the age progression of magmatism observed in the southern islands chain of the Cape Verde Archipelago. The youngest volcanic eruption period (21.14 ± 0.62 ka) occurred during the Last Glacial Maximum, a period of global sea level lowstands. We suggest that the comparatively rapid unloading leading up to the lowstand may have reduced pressure conditions within the Cadamosto Seamount magma plumbing system, and thus led to enhanced submarine eruption activity.