Mantle extraction age

These model mantle extraction ages can be interpreted either as a minimum age at which the material separated from the depleted mantle or chondrite uniform... 

Like Hf isotope ratios, Os/ Os ratios can be used to derive model ages (Figure 9.7), in this case for residues of mantle melting, such as some sulfides or alloys are believed to be. The mantle extraction age (Tma) is defined as... 

Figure 9.7 Schematic illustration of the concept of mantle extraction ages (Tma) and rhenium depletion ages (Trd) for a residue of partial melting, of which the present day Os/ Os ratio is indicated by the circle on the y-axis. The mantle extraction age uses the measured Re/ Os ratio to calculate the age at which the Os/ Os ratio of the sampl was the same as that of the chondritic mantle. The rhenium depletion age assumes that all rhenium was removed during the partial melting event, and that any rhenium present in the sample now was added recently.

In this case, it is assumed that the residue lost all of its rhenium during the partial melting event, and that any rhenium present was added recently, and therefore did not contribute any radiogenic Os. The rhenium depletion age is always younger than the mantle extraction age. 

Figure 13 An evolutionary model of time versus the ei82- v composition of the silicate Earth for the first 50 of Earth s history. The higher composition of the Earth relative to chondrites can only be balanced by a complementary lower than chondrites reservoir in the core. Extraction age models for the core are a function of the decay constant, the difference between the silicate Earth and chondrites, the proportion of W and Hf in the mantle and core and the rate of mass extraction to the core. Details of these models are given in the above citations, with the upper limit of the age curves shown here (sources Yin et al, 2002 Kleine et al, 2002 ...

Osmium isotopes currently provide the strongest case for mineral-to-mineral disequilibrium, and for mineral-melt disequilibrium available from observations on natural rocks. Thus, both osmium alloys and sulfides from ophiolites and mantle xenoliths have yielded strongly heterogeneous osmium isotope ratios (Alard et al., 2002 Meibom et al., 2002). The most remarkable aspect of these results is that these ophiolites were emplaced in Phanerozoic times, yet they contain osmiumbearing phases that have retained model ages in excess of 2 Ga in some cases. The melts that were extracted from these ophiolitic peridotites contained almost certainly much more radiogenic osmium and could, in any case, not have been in osmium-isotopic equilibrium with all of these isotopically diverse residual phases. 

The robustness of the Lu-Hf isotope system in some mantle environments is demonstrated by the precise Lu-Hf isochron of 1,413 67 Myr dehned by clinopyroxene separates from the Beni Bousera peridotite massif (Pearson and Nowell, 2003). This age probably dates the time of melt extraction from these rocks and is considerably more precise than the Sm-Nd isochron or the scattered Re-Os isotope systematics of these rocks. This indicates the potential power of this system in dating mantle rocks. The initial results from the Lu-Hf isotope system indicate that of the incompatible element isotope systems, it is the more robust to metasomatic effects, with signatures frequently recording the time-integrated response to melt depletion. 

Direct evidence for the compositional effects of partial melt extraction is preserved in samples of upper-mantle lithosphere with a range of ages, including Archean cratonic mantle, Proterozoic subcontinental mantle, and modern oceanic mantle. Samples of upper mantle are collected as xenoliths, peridotites dredged from oceanic fracture zones, and slices of upper mantle tectonically exposed at the surface, and extensive samples exist from both oceanic and continental settings (see Chapters 2.04 and 2.05). Here, data sets are assembled for oceanic and subcontinental mantle lithosphere, and compositional trends are compared to those predicted for partial melt extraction from fertile peridotite in order to deduce the role that melt extraction has played in producing compositional variability in upper-mantle lithosphere, and to place constraints on the thermal evolution of the mantle. 

Mineralogic and major-element variations in samples from the upper mantle provide strong evidence that melt extraction has played a predominant role in the origin of mantle lithosphere of all ages and provenance. There are compositional distinctions among samples of... 

As an initial premise, it is assumed in the following that in general water played a relatively minor role in the melt extraction events that led to the primary depletion characteristics of upper-mantle lithosphere. In this case, compositional distinctions among oceanic and continental lithosphere of variable age can be related directly to average depth and temperature of melt... 

The time at which melt was extracted from a mantle peridotite. More precisely it is a model age calculated relative to the187Os/188 Os ratio of the BSE, assuming that the Re/Os ratio of the sample is zero. Note, however, that BSE reference... 

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