Yannick Bussweiler

Diamonds from the Guiratinga region (southeastern Mato Grosso, central Brazil) provide constraints on the structure, metasomatism, redox state, and thermal evolution of the subcontinental lithospheric mantle beneath the northern Paraná Basin, along the southeastern margin of the Amazonian Craton. A multi-analytical approach integrating synchrotron μ-XRD, Raman spectroscopy, μ-FTIR, EPMA, and LA-ICP-MS was applied to characterize mineral inclusions, mineral chemistry, and nitrogen systematics. The inclusion assemblage is dominated by a peridotitic suite comprising forsteritic olivine (Mg# = 92.1–93.3), enstatite (Mg# = 94.3–94.8), clinopyroxene, and garnet, indicating derivation from a strongly depleted lithospheric mantle keel. Temperatures derived from the Al-in-olivine thermometer projected onto a 40 mW/m2 geotherm yield P–T estimates of 4.5–5.0 GPa and 1059–1156 °C. The occurrence of phlogopite indicates infiltration of volatile-rich metasomatic agents. A subordinate eclogitic signature is recorded by high-pressure phases (coesite and kyanite) and mineral associations compatible with transformed crustal protoliths. Hydrous to retrograde phases (amphibole, sodic plagioclase, epidote, and quartz) are interpreted as products of post-entrapment re-equilibration during hydration–oxidation processes. Hematite occurs as an isolated inclusion lacking microfracture connectivity, suggesting deep redox re-equilibration. FTIR spectra reveal total nitrogen contents of 0–970 at.ppm and aggregation states dominated by Type IaAB diamonds, with subordinate IaB, IaA, Ia, and IIa populations, indicating heterogeneous time–temperature histories. Nitrogen aggregation thermochronometry suggests prolonged mantle residence at ~1145–1261 °C for ~1.0 Ga.

Subduction of oceanic crust buries an average thickness of 300–500 m of sediment that eventually dehydrates or partially melts. Progressive release of fluid/melt metasomatizes the fore-arc mantle, forming serpentinite at low temperatures and phlogopite-bearing pyroxenite where slab surface reaches 700–900 °C. This is sufficiently high to partially melt subducted sediments before they approach the depths where arc magmas are formed. Here, we present experiments on reactions between melts of subducted sediments and peridotite at 2–6 GPa/750–1100 °C, which correspond to the surface of a subducting slab. The reaction of volatile-bearing partial melts derived from sediments with depleted peridotite leads to separation of elements and a layered arrangement of metasomatic phases, with layers consisting of orthopyroxene, mica-pyroxenite, and clinopyroxenite. The selective incorporation of elements in these metasomatic layers closely resembles chemical patterns found in K-rich magmas. Trace elements were imaged using LA-ICP-TOFMS, which is applied here to investigate the distribution of trace elements within the metasomatic layers. Experiments of different duration enabled estimates of the growth of the metasomatic front, which ranges from 1–5 m/ky. These experiments explain the low contents of high-field strength elements in arc magmas as being due to their loss during melting of sedimentary materials in the fore-arc.

The magmatic architecture and physicochemical processes inside volcanoes influence the style and timescale of eruptions. A long-standing challenge in volcanology is to establish the rates and depths of magma storage and the events that trigger eruption. Magma feeder systems are remarkably crystal-rich, and the growth stratigraphy of minerals sampled by erupted magmas can reveal a wealth of information on pre-eruptive processes. Here we combine detailed textural and chemical data acquired on large (>5 mm), euhedral augite megacrysts from Roman era activity (Pizzo scoria cone, 2.4–1.8 ka) at Stromboli (Italy) to investigate the plumbing system prior to the onset of current steady-state activity. Our dataset includes novel laser ablation time-of-flight mass spectrometry (LA-ICP-TOFMS) maps, which rapidly visualise multi-element zoning patterns across entire megacryst sections. The clinopyroxene data are complemented with geochemical constraints on mineral and melt inclusions, and adhering glassy tephra. Megacrysts are sector and oscillatory zoned in trace elements, yet their major element compositions are relatively uniform and in equilibrium with shoshonite-buffered melts. Mild sector zoning documents dynamic crystallisation under conditions of low undercooling during magma residence and growth. Clinopyroxene-melt thermobarometric and hygrometric calibrations, integrated with thermodynamically derived equilibrium equations, accurately track the P-T-H₂O path of magmas. The refined models return restricted crystallisation depths that are deeper than those reported previously for historical and current eruptions, but consistent with deep clinopyroxene-dominated crystallisation (≥10 km), resembling other water-rich alkaline mafic systems. Megacryst cores are overgrown by oscillatory zoned mantles recording continuous input of magma that failed to trigger eruption. Crystal rims are characterised by a mild increase in compatible transition metals Cr and Ni, and depletion in incompatible elements, indicative of pre-eruptive mafic replenishment and magma mixing. The volcanic system appears to have been dominated by protracted periods of replenishment, convection, and crystal residence, punctuated by rapid megacryst evacuation and eruption upon arrival of more mafic magma (days-weeks). Since the inception of current steady-state activity, eruption-triggering melts have become appreciably more mafic, suggesting that intrusion of primitive magma may be a key driver of the steady-state regime.