John D. Greenough and Kevin MacKenzie
Abstract
Incompatible elements and isotopic ratios identify three endmember
mantle components in oceanic island basalt (OIB); EM1, EM2, and
HIMU. We estimate compatible to mildly incompatible transition metal
abundance trends (Ni, Co, Fe, Cu, Cr, V, Mn, Sc, and Zn) in
‘primitive’ basalt suites (Mg# = Mg/(Mg + 0.9*Fe) atomic = 0.72)
from 12 end-member oceanic islands by regressing metals against
Fe/Mg ratios in sample suites, and solving for concentrations at
Mg/Fe = 1 (Mg# = 0.72). Using the transition metal estimates,
exploratory statistics reveal that islands ‘group’ based on mantle
component type even when La/Yb ratios are used to compensate metal
concentrations for percentage melting. Higher chalcophile Zn (and
Pb, earlier work) in EM1 and EM2 compared to HIMU, and higher Cr
(3+) and Sc in HIMU relative to EM1, support views that HIMU
represents subductionprocessed ocean floor basalt. Incompatible
elements, ratios and isotopes indicate that EM1 is Archean, EM2 is
Proterozoic or younger, and both are related to sediment subduction.
As found with incompatible elements, EM1 and EM2 show similar
‘compatible’ element concentrations, but lower (multivalence) Cr, Fe
and Mn in EM1 could indirectly reflect increasing oxidation of
subducted sediment between the Archean and Proterozoic.
Alternatively, changes in subduction processes that yielded peak
continental formation in the Neoarchean, and craton-suturing in the
Paleoproterozoic may account for EM1–EM2 differences. EM1 shows
similar or lower Cr, Ni and Co compared to HIMU and EM2 suggesting
that economic viability of layered intrusions, which have extreme
EM1-like signatures, is unrelated to high metals in EM1 mantle
sources, but that high % melting appears important. Because
core-concentrated transition metals correlate with mantle component
type, lithospheric recycling apparently controls their
concentrations in OIB and core-mantle interaction may be
unimportant.