An investigation of the influence of the melting process on the systematics of uranium-series elements in arc lavas: A study of uranium-thorium-radium-protactinium isotopes in Costa Rica and Nicaragua.

摘要

In order to constrain timing and mass transfer during melting in subduction zones, U-series isotopes (238U-230Th- 226Ra and 235U-231Pa) were measured in young volcanic rocks from Costa Rica and Nicaragua. Until recently, 231Pa and 226Ra were not routinely measured by mass spectrometry methods. As part of this project, chemical separation and thermal ionization mass spectrometry (TIMS) methods developed to analyze U, Th and Pa in carbonate samples were modified to be applied to volcanic samples. Because Ra isotopes have not been measured at the University of Minnesota, it was necessary to develop new separation chemistry and TIMS methods. Inductively coupled plasma mass spectrometry methods were also developed and tested for the measurement of U, Th, and Ra isotopes. The U-series data for these lavas fall into two groups. Costa Rican lavas are characterized by low ( 230Th/232Th) (1.1--1.2), (230Th/ 231U) of 1.07--1.2, high (231Pa/235U) (1.4 to 1.8), low (226Ra/230Th) (1.05--1.2), and low Ba/Th (100--200). The Nicaraguan lavas are characterized by high (230Th/232Th) (2.2--2.7), low ( 230Th/238U) (0.85--0.97), high (231Pa/ 235U) (1.27--1.77), high Ba/Th (500--1100), and ( 226Ra/230Th) that increase from 1.2--2.4 as Ba/Th and (238U/230Th) increases. Lavas from the geochemical transition region (northern Costa Rica and southern Nicaragua) tend to have U-series characteristics of both these groupings. In general, melting processes will increase (230Th/238U), (231Pa/ 235U) and (226Ra/230Th). The addition of a subduction fluid will decrease (230Th/238U), (231Pa/235U) and increase (226Ra/ 230Th). The data implies that the 230Th-238 U systematics are controlled by melting in Costa Rica and fluid addition in Nicaragua. However, all but one sample have (231Pa/ 235U) > 1, even when (230Th/238U) 1. Therefore, 231Pa-235U systematics are dominantly controlled by the melting process, and are not significantly affected by fluid addition. Finally the 230Th-226Ra systematics are strongly affected by fluid addition, as indicated by the correlation between (226Ra/230Th) and both Ba/Th and (238 U/230Th). A flux-ingrowth numerical model was developed to explain the observed data. It is an incremental melting model in which increments of a fluid from the subducting slab instigate melting in the mantle wedge and time between fluid addition and subsequent melting events accounts for isotopic ingrowth and decay, thus reproducing the observed isotopic ratios.