The Statistics and Distribution of Helium in the Mantle

Abstract

An unbiased estimate of the ^3He/^4He ratio (R) along the Earth's spreading ridge system is 9.14 ± 3.59 Ra (n = 503) where Ra is the atmospheric ratio (1.38 × 10^6). By arbitrarily excluding all values with R > 11 Ra and from ocean depths less than 2500 meters, I obtain 7.91 ± 1.50 Ra (n = 212), which is close to previous “filtered” estimates based on the hypothesis that the excluded values have been influenced by plumes. These are biased estimates. Based on unbiased statistics, many of the so-called high^(-3)He hotspot regions have isotopic ratios well within the MORB range, and all have absolute ^3He concentrations much less than MORB. The high variance of some oceanic-island data compared to MORB reflects, in part, the difference between a small sample and a large sample, and magma-chamber processes. Values of 11 to 15 Ra are commonly attributed to deep mantle plumes and “indicative of lower mantle involvement,” but these values are within 2σ of the mean and are not exceptional. Much higher values combined with low absolute helium concentrations are commonly associated with the onset of rifting, or volcanism, and may reflect a shallow, or lithospheric, low^(-238)U/^3He (LONU) source. The temporal progression to average MORB-like values suggests that the bulk of the magmas at spreading ridges and large oceanic constructs comes from below the LONU level. The termination of spreading is associated with low ratios, 6 to 7 Ra, similar to values associated with the high^(-238)U/^(204)Pb (HIMU) mantle component and some oceanic islands.