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Isótopos U-Th en magmas de arco: implicaciones para la transferencia de elementos desde la corteza subducida
Hawkesworth1, Turner, McDermott
1C. J. Hawkesworth, S. P. Turner, D. W. Peate, and P. van Calsteren are with the Department of Earth Sciences, The Open University, Milton Keynes, MK7 6AA, UK. F. McDermott is with the Department of Geology, University College, Dublin, Eire.
Science (New York, N.Y.)
|April 25, 1997
Resumen
Los isótopos de uranio-torio revelan las tasas de transferencia de fluidos y sedimentos de las placas tectónicas de subducción. Las rocas volcánicas del arco de la isla indican que los fluidos movilizan el uranio, mientras que los sedimentos contribuyen significativamente al torio en las rocas de arco.
Área de la Ciencia:
- La geoquímica es la geoquímica.
- La geoquímica de los isótopos es la geoquímica de los isótopos.
- La tectónica es la tectónica.
Sus antecedentes:
- Las zonas de subducción son críticas para el reciclaje de la corteza terrestre.
- La comprensión de la transferencia de material de losas de subducción a la cuña del manto es clave para descifrar el magmatismo de arco.
- Los isótopos de uranio (U) y torio (Th) ofrecen potentes marcadores para los procesos geológicos.
Objetivo del estudio:
- Para investigar las tasas de transferencia de fluidos y sedimentos de una losa descendente a la cuña del manto utilizando rocas volcánicas de arco de isla.
- Para cuantificar la contribución de los sedimentos subducidos a la composición de la roca de arco.
- Para determinar las escalas de tiempo del transporte de fluidos y sedimentos por la fusión.
Principales métodos:
- Análisis de la sistemática de isótopos de uranio-Th-Nd en las rocas volcánicas de arco insular.
- Utilizando la movilidad diferencial del uranio y el torio durante la alteración del fluido.
- Interpretación de las covariaciones isotópicas (por ejemplo, Th/Ce frente a 143Nd/144Nd) para distinguir fuentes.
Principales resultados:
- El uranio se moviliza fácilmente en fluidos, mientras que el torio es menos móvil.
- Las correlaciones negativas entre Th/Ce y 143Nd/144Nd indican un importante aporte de sedimentos de la losa descendente.
- Los tiempos de transferencia estimados para los fluidos son de 30.000 a 120.000 años, y para los sedimentos fundidos, varios millones de años.
- Solo alrededor del 30% del sedimento subducido, el torio, vuelve a la corteza.
Conclusiones:
- El volcanismo de arco insular registra la compleja interacción del fluido y el reciclaje de sedimentos durante la subducción.
- Las distintas firmas isotópicas destacan los diferentes mecanismos de transporte de los fluidos frente a los sedimentos fundidos.
- La composición de la corteza arquea refleja distintos procesos de generación de la corteza, que influyen en las relaciones promedio U/Th.
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