Re-partitioning of Cu and Zn isotopes by modified protein expression

Cu and Zn have naturally occurring non radioactive isotopes, and their isotopic systematics in a biological context are poorly understood. In this study we used double focussing mass spectroscopy to determine the ratios for these isotopes for the first time in mouse brain. The Cu and Zn isotope ratios for four strains of wild-type mice showed no significant difference (δ65Cu -0.12 to -0.78 permil; δ66Zn -0.23 to -0.48 permil). We also looked at how altering the expression of a single copper binding protein, the prion protein (PrP), alters the isotope ratios. Both knockout and overexpression of PrP had no significant effect on the ratio of Cu isotopes. Mice brains expressing mutant PrP lacking the known metal binding domain have δ65Cu isotope values of on average 0.57 permil higher than wild-type mouse brains. This implies that loss of the copper binding domain of PrP increases the level of 65Cu in the brain. δ66Zn isotope values of the transgenic mouse brains are enriched for 66Zn to the wild-type mouse brains. Here we show for the first time that the expression of a single protein can alter the partitioning of metal isotopes in mouse brains. The results imply that the expression of the prion protein can alter cellular Cu isotope content

Formation of podiform chromitite deposits : implications from PGE abundances and Os isotopic compositions of chromites from the Troodos complex, Cyprus

Podiform chromitite deposits occur in the mantle sequences of many ophiolites that were formed in supra-subduction zone settings (SSZ). We have measured PGE abundances and Os isotopic compositions of three major chromitite deposits (Kannoures, Hadji Pavlou, Kokkinorostos) and associated mantle peridotites from the Troodos Ophiolite Complex in order to investigate the petrogenesis of these rocks, and their genetic relationships and to examine the geochemical behaviour of the PGE. Spinels from the chromitite deposits have flat chondrite-normalized PGE patterns, but have distinct negative Pt anomalies. Thus, Pd, Os, Ru and Ir concentrations are very high compared to the Pt concentrations (Os: 13.7-104 ng/g, Ir 11.3-19.0 ng/g, Ru 34.3-83.6 ng/g, Pt 0.41-9.07 ng/g, Pd 11.1-76.8 ng/g). With the exception of Pd, this appears to be a general feature of chromitites from ophiolites worldwide. However, Pd concentrations determined in this study are high compared to other studies where whole rock samples were analysed. There is no simple explanation for this difference because mass balance constraints would not allow that this is solely due to Pd-depletion in the interstitial component. Rather, it implies that chromitites display large variations of relative PGE abundances, even on a local scale. Podiform chromitite deposits form as a result of the interaction of fluid-rich, percolating melts with surrounding mantle peridotites. Osmium, Ir, Ru and Cr concentrations decrease systematically from harzburgite to dunite surrounding the deposits. In addition, dunites and chromites have complementary PGE distribution patterns. Thus, the mantle peridotite is the source of these metals in chromitites. This also indicates that these elements behave incompatibly and are mobilized during continuous melt percolation. However, the low Pt concentrations in the chromitites suggest that Pt is not as effectively mobilized during melt percolation. Uniformly high Pt concentrations in harzburgite and dunite (ca. 11 ppb) also imply that most Pt remains in the mantle peridotite. This can be explained if residual Pt-rich phases, such as PtFe alloys, limit the mobility of Pt. PGE and Cr become concentrated when chromite and sulfide liquids precipitate as a result of the mixing of percolating melts in magma pools near the crust-mantle boundary. The Os-187/Os-188 ratios of the chromite separates (0.1265-0.1301) are less variable than those of the associated peridotites (0.1235-0.1546). The average isotopic composition of the chromites (Os-187/O-188: 0.1284 +/- 0.0021) is superchondritic compared with the carbonaceous chondrite value (Os-187/Os-188: 0.1260 +/- 0.0013 after Geochim. Cosmochim. Acta 66 (2002) 329; Geochim. Cosmochim. Acta 66 (2002) 4187) and similar to the average value measured for podiform chromitites worldwide (0.12809 +/- 0.00085 after Geochim. Cosmochim. Acta 66 (2002) 329; Geochim. Cosmochim. Acta 66 (2002) 4187). Radiogenic melts/fluids derived from the subducting slab trigger partial melting in the overlying mantle wedge and add significant amounts of radiogenic Os to the peridotites. Mass balance calculations suggest that a melt/rock ratio of approximately 17:1 (melt:Os-187/Os-188: 0.163. Os: 0.01 ng/g, mantle peridotite: Os-187/Os-188: 0.127, Os 4.2 ng/g) is necessary in order to increase the Os isotopic composition of the chromitite deposits to its observed average value. This value implies a surprisingly low average melt/rock ratio during the formation of chromitite deposits. The percolating melts likely have variable isotopic composition and PGE concentration. However, in the chromitite pods the Os from many melts is pooled and homogenized, which is the reason why the chromitite deposits show such a small variation in their Os isotopic composition. The results of this study suggest that the Os-187/Os-188 ratio of chromitites is not representative for the DMM, but only reflects an upper limit

Os mobilization during melt percolation : the evolution of Os isotope heterogeneities in the mantle sequence of the Troodos ophiolite, Cyprus

This study focuses on the origin of the Os isotope heterogeneities and the behaviour of Os and Re during melt percolation and partial melting processes in the mantle sequence of the Troodos Ophiolite Complex. The sequence has been divided into an eastern (Unit 1) and a western part (Unit 2) (Batanova and Sobolev, 2000). Unit 1 consists mainly of spinel-Iherzolites and a minor amount of dunites, which are surrounded by cpx-bearing harzburgites. Unit 2 consists of harzburgites, dunites, and contains chromitite deposits. Unit 1 (Os-187/(OS)-O-188: 0.1169 to 0.1366) and Unit 2 (Os-187/Os-188 0.1235 to 0.1546) peridotites both show large ranges in their Os isotopic composition. Most of the Os-187/Os-188 ratios of Unit 1 Iherzolites and harzburgites are chondritic to subchondritic, and this can be explained by Re depletion during ancient partial melting and melt percolation events. The old Os isotope model ages (>800 Ma) of some peridotites in a young ophiolitic mantle show that ancient Os isotopic heterogeneities can survive in the Earth upper mantle. Most harzburgites and dunites of Unit 2 have suprachondritic Os-187/Os-188 ratios. This is the result of the addition of radiogenic Os during a younger major melt percolation event, which probably occurred during the formation of the Troodos crust 90 Ma ago. This study shows that Unit 1 and Unit 2 of the Troodos mantle section have a complex and different evolution. However, the Os isotopic characteristics are consistent with a model where the harzburgites and dunites of both units belong to the same melting regime producing the Troodos oceanic crust

Ancient, highly heterogeneous mantle beneath Gakkel ridge, Arctic Ocean

The Earth's mantle beneath ocean ridges is widely thought to be depleted by previous melt extraction, but well homogenized by convective stirring. This inference of homogeneity has been complicated by the occurrence of portions enriched in incompatible elements. Here we show that some refractory abyssal peridotites from the ultraslow-spreading Gakkel ridge (Arctic Ocean) have very depleted 187Os/188Os ratios with model ages up to 2 billion years, implying the long-term preservation of refractory domains in the asthenospheric mantle rather than their erasure by mantle convection. The refractory domains would not be sampled by mid-ocean-ridge basalts because they contribute little to the genesis of magmas. We thus suggest that the upwelling mantle beneath mid-ocean ridges is highly heterogeneous, which makes it difficult to constrain its composition by mid-ocean-ridge basalts alone. Furthermore, the existence of ancient domains in oceanic mantle suggests that using osmium model ages to constrain the evolution of continental lithosphere should be approached with caution

Ancient, highly heterogeneous mantle beneath Gakkel ridge, Arctic Ocean

The Earth\u27s mantle beneath ocean ridges is widely thought to be depleted by previous melt extraction, but well homogenized by convective stirring. This inference of homogeneity has been complicated by the occurrence of portions enriched in incompatible elements. Here we show that some refractory abyssal peridotites from the ultraslow-spreading Gakkel ridge (Arctic Ocean) have very depleted 187Os/188Os ratios with model ages up to 2 billion years, implying the long-term preservation of refractory domains in the asthenospheric mantle rather than their erasure by mantle convection. The refractory domains would not be sampled by mid-ocean-ridge basalts because they contribute little to the genesis of magmas. We thus suggest that the upwelling mantle beneath mid-ocean ridges is highly heterogeneous, which makes it difficult to constrain its composition by mid-ocean-ridge basalts alone. Furthermore, the existence of ancient domains in oceanic mantle suggests that using osmium model ages to constrain the evolution of continental lithosphere should be approached with caution. ©2008 Nature Publishing Group