Contrasting P-T paths of shield and rejuvenated volcanism at Robinson Crusoe Island, Juan Fernández Ridge, SE Pacific

- Pressure-temperature pre-eruptive conditions for shield and rejuvenated volcanism in Robinson Crusoe Island are estimated
- From P-T paths, whole rock chemistry and textural analysis contrasting patterns of ascent and storage are recognized
- Low-pressure differentiation occurs in shallow magma reservoirs during shield stage
- Direct ascent and polybaric crystallization characterize the rejuvenated stage
- Shallow reservoirs for shield stage and deeper P conditions for rejuvenated volcanism seems to be a global pattern

A remarkable expression of intraplate volcanism is the occurrence of evolutionary stages with important variations of magmatic processes and products. Plumbing systems and storage conditions seem to be different for shield and rejuvenated volcanism, two classical stages notably preserved in Robinson Crusoe Island, Juan Fernández Ridge in the SE Pacific Ocean. We here present first order geochemical features for rocks from both shield and rejuvenated stages and through geothermobarometry and textural analysis we unravel their contrasting ascent and storage history. The shield stage (~ 3.8 Ma) is represented by a ~ 900 m thick sequence of basalt, picrobasalt and picrite lava flows forming subsets according their chemistry and mineralogy: 'differentiated', 'near-primitive' and 'olivine-rich' lavas. Pressure estimates for in equilibrium assemblages are < 3.2 kbar, and temperature ranges around 1321 °C for the 'near-primitive' and 1156-1181 °C for the 'differentiated' groups. Volcanic rocks from the rejuvenated stage (~ 0.9 Ma) fill the eroded morphology of the shield pile with basanite and picrite lava flows with two compositional varieties: the primitive 'high-Mg' group that crystallized clinopyroxene at pressures < 3.7 kbar and olivine at temperatures in the range 1316-1354 °C; and the 'low-Mg' group that carries notably zoned crystals formed at a wide range of pressures (0-10.8 kbar) and temperatures (1256-1295 °C). This allows us to infer contrasting patterns of ascent and storage during these archetypical stages in Robinson Crusoe Island, which also controlled volcanic processes on surface and finally shaped the island. We propose the existence of shallow magmatic reservoirs in the shield stage, where the ascending magmas would have been stored and differentiated. On the other hand, rejuvenated magmas experimented rapid ascent with polybaric crystallization and sometimes short-time storage in low-volume reservoirs. Similar conditions have been proposed in other oceanic islands suggesting that shallow reservoirs in the shield stage and deeper crystallization of more alkaline magmas in the rejuvenated stage seems to describe a global pattern.

Schematic model of the magmatic ascent and storage in Robinson Crusoe Island showing the contrasting pattern for shield and rejuvenated volcanism: storage in shallow reservoirs for shield and rapid ascent with polybaric crystallization for the rejuvenated stage (details in Section 5.2). Qualitative differences in depth (from pressure estimations of clinopyroxene geobarometer from Nimis and Ulmer, 1998) and temperature conditions (according olivine-liquid geothermometer from Herzberg and O'Hara, 2002) of each compositional group are also shown. Pressure conditions estimated are showed in histograms for both units.279