Constraining eruption age and quartz formation in a basaltic lava flow (Martinique) using trapped-charge and geochemical methods

In south-western Martinique (Lesser Antilles), the basaltic lava flow and associated strombolian cone of Pointe Burgos transect the porphyritic dacitic lava dome of Morne Champagne, which has been dated to 617 ± 52 ka (Germa et al., 2011). A striking characteristic of the basaltic lava is an unusually high abundance (~4%) of large quartz crystals reaching up to 2 cm. These have previously been interpreted as xenocrysts incorporated into the basaltic magma through mechanical mixing with a shallow, cooled dacitic reservoir at an approximate 9:1 basalt–dacite ratio. Support for this interpretation includes resorbed plagioclase phenocrysts with reaction rims, commonly regarded as indicators of crystal remobilisation. However, the eruption products lack other textural features typically associated with magma mixing. Moreover, the quartz crystals display atypical morphologies, extensive internal fracturing, and occur as apparent void-fillings within the basalt, prompting a reassessment of their origin.

To better constrain the timing and mechanism of quartz incorporation, we investigated both the eruption age of the basaltic lava and the formation history of the quartz crystals. K–Ar dating of the basaltic groundmass yields an age of 379 ± 25 ka, indicating that the basalt erupted ~240 ka after the dacitic dome it crosscuts. This substantial time gap implies that the shallow dacitic reservoir would have been fully solidified during basalt ascent, a scenario in which entrainment of dacitic enclaves might be expected but is not observed.

Thermoluminescence (TL) dating provides a means to estimate the time elapsed since mineral crystallisation or cooling to ambient temperature, rendering it well suited to evaluate whether the quartz formed contemporaneously with the basaltic eruption or represents a later generation of minerals (substitution minerals or hydrothermal void fillings). Moreover, TL can inform on thermal conditions during signal acquisition through the thermal stability of selected TL signals. We applied red TL measurements using multiple dose determination protocols to calculate an apparent age, which yielded internally consistent results. Dose-rate calculations account for the grain-size distribution of the quartz xenocrysts, radioelement concentrations and the erosional evolution of the site.

Apparent TL ages range from ~104 ka assuming no erosion, to ~122 ka for ~100 m of surface erosion, each with an ~17% uncertainty. New LA-ICP-MS geochemical data obtained from three quartz xenocrysts provide further evidence for a magmatic formation environment, lending support to the magma mixing hypothesis. The younger TL ages relative to the K–Ar eruption age may thus reflect partial thermal resetting of the TL signal due to prolonged hydrothermal activity. Kinetic parameters derived from the TL data enable forward modelling of thermal scenarios compatible with the observed ages. Together, the geochronological, kinetic, and geochemical results allow us to reassess the origin of quartz in the Pointe Burgos lava and to explore the post-eruptive hydrothermal evolution of the system.

References

Germa, A., Quidelleur, X., Lahitte, P., Labanieh, S., Chauvel, C., 2011. The K–Ar Cassignol–Gillot technique applied to western Martinique lavas: a record of Lesser Antilles arc activity from 2 Ma to Mount Pelée volcanism. Quaternary Geochronology 6, 341-355.