Mantle-derived noble gases in carbonatites

Isotopic compositions of the noble gases (He, Ne, Ar, Kr, Xe) were obtained by stepwise heating of separated mineral phases from carbonatites from Brazil (Jacupiranga, Tapira) and Canada (Borden, Oka, and Prairie Lake) as well as one syenite (Poohbah Lake, Canada). Mineral phases used were apatite, forsterite, diopside, calcite, monticellite, and perovskite. Large amount of in situ produced U-238-fissiogenic Xe131-136, along with lesser amounts of U-238-fissiogenic Kr83-86 and in situ He-4 were found in most samples. An apatite sample from the Prairie Lake carbonatite showed a Xe-136/Xe-130 ratio of about 1400. Some samples, mostly apatites, showed excess Xe-129 relative to air in the high temperature (1800 degrees C) gas fractions. The highest Xe-129/Xe-130 ratios, between 7.0 and 8.6, are similar to those found in other mantle-derived materials such as MORBs and diamonds. The excess Xe-129 is considered to be primordial and attributed to now extinct nuclide I-129 once present in the early history of the Earth. Neon isotopic compositions were also anomalous showing very low Ne-20/Ne-22 ratios (down to 0.01) and high Ne-21/Ne-22 ratios (up to 1.25). They are attributed to Wetherill reactions, such as O-18(alpha,n)Ne-21, F-19(alpha,n)Na-22(beta(+))Ne-22, and so on. The measured Ar-40/Ar-36 ratios are extremely variable and range from values that are close to atmospheric to values as high as 42400. Stepwise heating studies of an apatite from the Jacupiranga carbonatite indicate that the high temperature fractions retain the original noble gas signature of the carbonatite source. The Ar-40/Ar-36 ratios for the high temperature gas fractions are about 6400 or smaller. A plot of Ar-40/Ar-36 vs. Xe-129/Xe-130 shows that the source of carbonatites is different and less degassed than that of MORBs. The presence of excess Xe-129 in carbonatites suggests that carbon in carbonatites is unlikely to be recycled C related to subduction processes. Copyright (C) 1997 Elsevier Science Ltd.