11:00 AM - 11:15 AM
[SGC38-02] Osmium isotope heterogeneity in abyssal peridotites from Kane Fracture Zone, Mid Atlantic ridge
The 187Os/188Os ratios of mantle peridotites are frequently used to constrain the timing of melt depletion, assuming that the 187Os/188Os evolution of Earth's mantle are broadly chondritic. To improve the accuracy of model age estimates based on the Re-Os isotope system, it is therefore important to better understand the 187Os/188Os evolution of the Earth's mantle. Since abyssal peridotites are generally considered to be the residue of partial melting of the convective upper mantle, 187Os/188Os data from abyssal peridotites have been used to constrain the 187Os/188Os ratio in the present-day mantle. However, previous studies have shown that there are significant variations in the 187Os/188Os ratios of abyssal peridotites, suggesting that the present-day mantle preserves 187Os/188Os heterogeneity generated in ancient melting events.
In order to investigate the origin of 187Os/188Os heterogeneity in abyssal peridotites, we performed whole-rock Re-Os isotope analyses of 38 abyssal peridotites collected by Ocean Drilling Program Leg 153 in Hole 920, Kane fracture zone, the Mid-Atlantic Ridge. Our new data demonstrate that the mean value of 187Os/188Os ratio is 0.1260±0.0017 (2S.D.), in agreement with that reported in previous studies for global abyssal peridotites. One sample has a distinctively lower 187Os/188Os ratio about 0.1200, which is the lowest value reported for the Kane fracture zone. However, there are no obvious correlations between the 187Os/188Os ratio and the concentrations of other elements, suggesting that elemental concentrations were largely controlled by the recent melting process beneath the mid-ocean ridge. When compared to the data from the nearby 15°20' fracture zone, peridotites from the Kane fracture zone have more fertile in melt components in terms of major elements (e.g. MgO, Al2O3 and CaO), incompatible trace elements (e.g. rare earth elements), and highly siderophile element concentrations. However, the data from these two sites do not follow an identical partial melting trend, suggesting that the peridotite from the 15°20' fracture zone may have undergone ancient partial melting in a different geological setting, as inferred from its unradiogenic 187Os/188Os ratios. This suggests that some parts of the abyssal peridotites in the Mid-Atlantic Ridge are not derived from the modern convective mantle, and such data are not suitable for estimating the 187Os/188Os ratio in the present-day mantle.
In order to investigate the origin of 187Os/188Os heterogeneity in abyssal peridotites, we performed whole-rock Re-Os isotope analyses of 38 abyssal peridotites collected by Ocean Drilling Program Leg 153 in Hole 920, Kane fracture zone, the Mid-Atlantic Ridge. Our new data demonstrate that the mean value of 187Os/188Os ratio is 0.1260±0.0017 (2S.D.), in agreement with that reported in previous studies for global abyssal peridotites. One sample has a distinctively lower 187Os/188Os ratio about 0.1200, which is the lowest value reported for the Kane fracture zone. However, there are no obvious correlations between the 187Os/188Os ratio and the concentrations of other elements, suggesting that elemental concentrations were largely controlled by the recent melting process beneath the mid-ocean ridge. When compared to the data from the nearby 15°20' fracture zone, peridotites from the Kane fracture zone have more fertile in melt components in terms of major elements (e.g. MgO, Al2O3 and CaO), incompatible trace elements (e.g. rare earth elements), and highly siderophile element concentrations. However, the data from these two sites do not follow an identical partial melting trend, suggesting that the peridotite from the 15°20' fracture zone may have undergone ancient partial melting in a different geological setting, as inferred from its unradiogenic 187Os/188Os ratios. This suggests that some parts of the abyssal peridotites in the Mid-Atlantic Ridge are not derived from the modern convective mantle, and such data are not suitable for estimating the 187Os/188Os ratio in the present-day mantle.