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Abstract:
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A fundamental question in the genesis of komatiites is whether 30 these rocks originate
from partial melting of dry and hot mantle, 400−500°C hotter than typical sources of MORB and
OIB magmas, or if they were produced by hydrous melting of the source at much lower
temperatures, similar or only moderately higher than those known today. Gorgona Island,
Colombia, is a unique place where Phanerozoic komatiites occur and whose origin is directly
connected to the formation of the Caribbean Large Igneous Province. The genesis of Gorgona
komatiites remains controversial, mostly because of the uncertain origin of volatile components
which they appear to contain. These volatiles could equally result from shallow level magma
contamination, melting of a “damp” mantle or fluid-induced partial melting of the source due to
devolatilization of the ancient subducting plate. We have analyzed boron isotopes of olivine40
hosted melt inclusions from the Gorgona komatiites. These inclusions are characterized by
relatively high contents of volatile components and boron (0.2−1.0 wt.% H2O, 0.05−0.08 wt.%
S, 0.02−0.03 wt.% Cl, 0.6−2.0 μg/g B), displaying positive anomalies in the overall depleted,
primitive mantle (PM) normalized trace element and REE spectra ([La/Sm]n = 0.16−0.35;
[H2O/Nb]n = 8−44; [Cl/Nb]n = 27−68; [B/Nb]n = 9-30, assuming 300 μg/g H2O, 8 μg/g Cl and
0.1 μg/g B in PM; Kamenetsky et al., 2010. Composition and temperature of komatiite melts
from Gorgona Island constrained from olivine-hosted melt inclusions. Geology 38, 1003–1006).
The inclusions range in δ11B values from −11.5 to +15.6 ± 2.2‰ (1 SE), forming two distinct
trends in a δ11B vs. B-concentration diagram. Direct assimilation of seawater, seawater-derived
components, altered oceanic crust or marine sediments by ascending komatiite magma cannot
readily account for the volatile contents and B isotope variations. Alternatively, injection of <3%
of a 11B enriched fluid to the mantle source could be a plausible explanation for the δ11B range
that also may explain the H2O, Cl and B excess. |