查看更多>>摘要：Komatiites are crystallized samples of high-temperature, high-MgO lavas that were common during the Archean, but became increasingly rarer in the Proterozoic and Phanerozoic. Although the origin of komatiites remains a subject of debate, all komatiites included in this review, ranging in age from 3.6 to 2.0 Ga, are interpreted to have most likely been derived from anhydrous melting in mantle plumes. These plumes are estimated to have been initiated at different depths in the mantle, thus, providing important information about the chemical evolution of the early Earth. The Nd-142,Nd-143, Hf-176, W-182, Os-187, and Os-186 systematics and trace- and highly siderophile element (HSE) abundances of these komatiites provide strong evidence for the presence of isotopic and chemical heterogeneities in the mantle during the first half of Earth history. These heterogeneities likely reflect the combined effects of (1) the co-existence of diverse post-magma ocean silicate domains that were characterized by variably-fractionated lithophile and sidemphile element abundances; (2) the presence of distinct reservoirs that included mantles and cores of late accreted, differentiated planetesimals; and (3) isotopic exchange across the core-mantle boundary. These data highlight the complexity of komatiite mantle sources, none of which were similar in composition to estimates for the modern bulk silicate Earth (BSE). Moreover, no single petrogenetic model can account for the remarkably diverse chemical and isotopic compositions of komatiites. The disappearance of resolvable positive and negative Nd-142 anomalies, as well as decoupled Nd-143-Hf-176 isotopic signatures, in the mafic-ultramafic rock record by similar to 2.5 Ga indicate that, by the end of the Archean, the earliest silicate reservoirs, formed through primordial magma ocean crystallization, had been largely destroyed as a result of vigorous convective mantle mixing. This implies that, during the Hadean and Archean, it took the mantle similar to 1.5 Ga to mix away the early formed Nd-142 heterogeneities via wholesale mantle convection. Similar to Nd-142 systematics, there appears to have been a shift from mostly positive W-182 anomalies in pre-2.5 Ga komatiite mantle sources to no W-182 offsets in post-2.5 Ga komatiite mantle sources. Coupled with the dis-apperance of projected HSE depletions in komatiite mantle sources at similar to 2.5 Ga, relative to the modern BSE, this shift may indicate that, by the end of the Archean, late accreted planetesimals had become largely homogenized within the mantle, and core-mantle interaction took over as the main driving force of creating W-182 isotope anomalies, possibly coincident with the timing of the onset of modern-style plate tectonics on Earth.
NSTL
Elsevier