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We present a high‐resolution 2‐D P‐wave velocity model from a 225‐km‐long active seismic profile, collected over ~60–75 Ma central Atlantic crust. The profile crosses five ridge segments separated by a transform and three nontransform offsets. All ridge discontinuities share similar primary characteristics, independent of the offset. We identify two types of crustal segment. The first displays a classic two‐layer velocity structure with a high gradient Layer 2 (~0.9 s−1) above a lower gradient Layer 3 (0.2 s−1). Here, PmP coincides with the 7.5 km s−1 contour, and velocity increases to >7.8 km s−1 within 1 km below. We interpret these segments as magmatically robust, with PmP representing a petrological boundary between crust and mantle. The second has a reduced contrast in velocity gradient between the upper and lower crust and PmP shallower than the 7.5 km s−1 contour. We interpret these segments as tectonically dominated, with PmP representing a serpentinized (alteration) front. While velocity‐depth profiles fit within previous envelopes for slow‐spreading crust, our results suggest that such generalizations give a misleading impression of uniformity. We estimate that the two crustal styles are present in equal proportions on the floor of the Atlantic. Within two tectonically dominated segments, we make the first wide‐angle seismic identifications of buried oceanic core complexes in mature (>20 Ma) Atlantic Ocean crust. They have a ~20‐km‐wide "domal" morphology with shallow basement and increased upper crustal velocities. We interpret their midcrustal seismic velocity inversions as alteration and rock‐type assemblage contrasts across crustal‐scale detachment faults. Key Points: We identify that magmatic and tectonic modes of slow Atlantic crustal accretion are distinct and equal, based on seismic characteristicsThe structure of crustal discontinuities is controlled by the accretion mode of neighboring segments, independent of ridge offset and orderTwo buried oceanic core complexes in mature Atlantic Ocean crust are identified, based on their dimensions and seismic structure [ABSTRACT FROM AUTHOR]

Assessing the impact of crustal assimilation on the composition of oceanic arc lavas is important if source composition is to be correctly interpreted. This is particularly the case in the Lesser Antilles where lavas encompass a very large range in radiogenic isotope compositions. Here we present new 176Hf/177Hf and trace element data for a suite of samples from St Lucia in the southern Lesser Antilles arc where assimilation of sediments located within the arc crust has been shown to influence significantly Sr-Nd-Pb isotope compositions. We show that a high rate of assimilation (r=0·8) of sediment is responsible for the co-variation of Th/Th*, La/Sm, 87Sr/86Sr, 206/207/208Pb/204Pb, 143Nd/144Nd and 176Hf/177Hf towards extreme 'continental' compositions. Lavas that escaped sediment assimilation have a typical oceanic arc signature and provide the best indication of mantle source characteristics beneath St Lucia. They display similar Ba/Th, La/Sm and Nd isotopic compositions to lavas further north in the arc, but with slightly more radiogenic Sr and Pb. Addition of less than 2% of local bulk subducting sediment, or less than 3·5% of sediment partial melt or fluid, to the mantle wedge can explain these compositions; these estimates are similar to those previously proposed for the northern arc. After correction for the effects of sediment assimilation, the St Lucia lavas have only slightly more radiogenic Pb and Sr isotope signatures compared with the northern islands; this can be attributed to differences in the isotopic composition of the local subducting sediment rather than to greater sediment input, as has been previously proposed. Comparison of St Lucia with the other southern Lesser Antilles islands suggests that similar mantle source compositions exist beneath Martinique, St Vincent and perhaps Bequia, whereas a more 'continental' source might characterize Ile de Caille, Kick 'em Jenny and Grenada. [ABSTRACT FROM AUTHOR]

The geological carbon cycle has played a key role in controlling climate throughout Earth's history. For the last ∼3 billion years plate tectonics has driven subduction. Subducted slabs have transported CO 2 from the lithosphere, hydrosphere, and atmosphere into the Earth, from where it may be released back to the surface through processes such as arc volcanism or can be stored in the deep interior over geological time. Carbonate-bearing sediments and basalts of altered oceanic crust are the primary media by which carbon is subducted. Therefore, quantifying the depth and amount of CO 2 released from different carbonate-bearing lithologies during subduction is fundamental to understanding whether CO 2 is recycled through arc volcanism or buried in the mantle. The magnitude of CO 2 released from subducting slabs at fore- and sub-arc depths is controlled by processes including ocean crust alteration (i.e., carbonation), metamorphic decarbonation, carbonate dissolution and slab-melting. However, the relative contribution of these processes to overall slab decarbonation is still debated, and will be complex given the variety of sedimentary lithologies and subduction geodynamics. Here, we present a global arc-by-arc lithology-specific analysis of the magnitude of slab CO 2 released purely by metamorphic decarbonation of carbonate-bearing sediment and basalt during subduction of altered oceanic crust, using a thermodynamically rigorous model. We find that metamorphic decarbonation is highly efficient in low carbonate sediments, such as carbonated clay, and in carbonated basalts of altered oceanic crust, causing all of their CO 2 to be removed. Sediments with medium and higher carbonate contents, such as chalk and limestone, are only partially decarbonated, but the combination of metamorphic decarbonation and carbonate dissolution promotes efficient carbon loss. Together they can explain observed magmatic CO 2 emissions in carbonate-rich arcs. Warm slabs, such as Mexico and Cascadia, produce complete metamorphic decarbonation of carbonate minerals beneath fore-arcs. Under more common cold and intermediate thermal regimes metamorphic decarbonation of carbonate minerals occurs at depths between ∼80 and 170 km (∼2.3 to 5.5 GPa) promoting CO 2 input into the mantle sources of volcanic arcs. Overall, our results demonstrate that sub-arc decarbonation is typically considered an important potential source of slab-derived CO 2 , which needs to be considered together with carbonate dissolution to explain observed volcanic CO 2 emissions. In many arcs the modelled CO 2 flux from sediment and basalts of altered oceanic crust into the wedge exceeds the observed CO 2 output suggesting that the mantle wedge and arc lithosphere may sequester some CO 2. • Decarbonation is an important source of slab-derived CO 2 for arc volcanoes. • Cold and intermediate slabs produce decarbonation beneath sub-arcs. • Warm slabs produce decarbonation beneath fore-arcs. • The mantle wedge or arc lithosphere may accumulate and store slab-derived CO 2. [ABSTRACT FROM AUTHOR]

Abstract: Arc magmas are commonly assumed to form by melting of sub-arc mantle that has been variably enriched by a component from the subducted slab. Although most magmas that reach the surface are not primitive, the impact of assimilation of the arc crust is often ignored with the consequence that trace element and isotopic compositions are commonly attributed only to varying contributions from different components present in the mantle. This jeopardises the integrity of mass balance recycling calculations. Here we use Sr and O isotope data in minerals from a suite of volcanic rocks from St Lucia, Lesser Antilles arc, to show that assimilation of oceanic arc basement can be significant. Analysis of 87Sr/86Sr in single plagioclase phenocrysts from four Soufrière Volcanic Complex (SVC; St Lucia) hand samples with similar composition (87Sr/86Sr=0.7089–0.7091) reveals crystal isotopic heterogeneity among hand samples ranging from 0.7083 to 0.7094 with up to 0.0008 difference within a single hand sample. measurements in the SVC crystals show extreme variation beyond the mantle range with +7.5 to for plagioclase ( ), +10.6 to for quartz ( ), +9.4 to for amphibole ( ) and +9 to for pyroxene ( ) while older lavas (Pre-Soufriere Volcanic Complex), with less radiogenic whole rock Sr composition (87Sr/86Sr=0.7041–0.7062) display values closer to mantle range: +6.4 to for plagioclase ( ) and +6 to for pyroxene ( ). We argue that the 87Sr/86Sr isotope disequilibrium and extreme values provide compelling evidence for assimilation of material located within the arc crust. Positive correlations between mineral and whole rock 87Sr/86Sr, 143Nd/144Nd and 206,207,208Pb/204Pb shows that assimilation seems to be responsible not only for the isotopic heterogeneity observed in St Lucia but also in the whole Lesser Antilles since St Lucia encompasses almost the whole-arc range of isotopic compositions. This highlights the need for detailed mineral-scale investigation of oceanic arc suites to quantify assimilation that could otherwise lead to misinterpretation of source composition and subduction processes. [Copyright &y& Elsevier]