THE SMOOTHNESS AND SHAPES OF CHONDRITE-NORMALIZED RARE EARTH ELEMENT PATTERNS IN BASALTS

The atomic structure of the REE disposes them to form 3+ cations in natural systems, with a few exceptions (mainly Eu, which occurs also as 2+). Their usefulness in igneous petrology stems from REE³⁺ mineral/melt partition coefficients usually spanning a wide range for a given mineral (for garnets, three orders of magnitude), but within that range varying smoothly with atomic number. ICP-MS methods now enable all 14 REEs to be determined routinely to high precision. A set of REE abundances is usually displayed by normalizing to chondritic (CI) abundances, and then plotting the normalized abundances against atomic number or, equivalently, ionic radius, r_REE. In basalts, the resulting patterns are usually smooth (except Eu), such that they can be fitted to polynomials in r_REE, often with standard deviations of only ~ 3% for the 4-term polynomial.

The polynomials can be re-arranged into the orthogonal form:

ln([REE]/[REE]_CI) = λ₀f₀ + λ₁f₁ + λ₂f₂+ ….

where f₀, f₁, f₂ etc. are special polynomials of r_REE, of order 0, 1, 2, etc., chosen to make the coefficients λ₀, λ₁, λ₂ etc. independent of each other, and of the number of terms used in the fitting. The λ coefficients have simple, intuitive meaning: λ₀ is the average abundance, λ₁ the linear slope, λ₂ the quadratic curvature. The higher order terms are of minor significance in common basalts. The λ coefficients can be used to compare REE patterns quantitatively, highlighting similarities and differences between types of basalts. Especially instructive are λ₂ vs. λ₁ (curvature vs. slope) diagrams, which discriminate well between basalts produced in different environments. Petrogenetic processes such as partial melting or fractional crystallization can be treated analogously, by fitting to the same orthogonal polynomials. A process may then be plotted as a “vector” in the λ₂ vs. λ₁ diagram, to link the REE pattern of a basalt to that of its source.

While the resulting insights confirm much geochemical folk wisdom, e.g., Ocean Island Basalts (OIBs) do carry a “garnet signature”, other commonplaces are questioned, e.g., most if not all adakites do not. Hawaiian shield basalts have REE patterns unlike other OIBs, partly due to higher degree of melting, although still much lower than that for Ocean Floor Basalts. The REE patterns of subduction-related basalts are far more varied than other types.