Diamond preservation in the lithospheric mantle recorded by olivine in kimberlites.

The diamond potential of kimberlites is difficult to assess due to several mantle and magmatic processes affecting diamond content. Traditionally, initial evaluations are based on the compositions of mantle-derived minerals (garnet, chromite, clinopyroxene), which allow an assessment of pressure-temperature conditions and lithologies suitable for diamond formation. Here we explore a complementary approach that considers the conditions of diamonds destruction by interaction with melts/fluids (metasomatism). We test the hypothesis that carbonate-rich metasomatism related to kimberlite melt infiltration into the deep lithosphere is detrimental to diamond preservation. Our results show that high diamond grades in kimberlites worldwide are exclusively associated with high-Mg/Fe olivine, which corresponds to mantle lithosphere minimally affected by kimberlite-related metasomatism. Diamond dissolution in strongly metasomatised lithosphere containing low-Mg/Fe olivine provides a causal link to the empirical associations between low diamond grades, abundant Ti-Zr-rich garnets and kimberlites with high Ti and low Mg contents. This finding show-cases olivine geochemistry as a viable tool in diamond exploration.

Genetic dissection of attractant-induced conductances in Paramecium.

Paramecium tetraurelia is attracted to acetate and biotin by swimming smoothly and fast up gradients of these attractants, and turning immediately and slowing down when leaving these stimuli. We use a group of mutants, each with a different defect in an identified ion conductance, to show that these two stimuli open different ion channels, and the behaviors that occur upon application of stimulus (on-response) and removal of stimulus (off-response) have different roles in attraction to these two stimuli. The most important parameters for successful attraction to acetate are the on-response behaviors of fast swimming with few turns, and the mutants' behavior suggests that I(K(Ca,h)) is the conductance involved that initiates this behavior. I(K(Ca,h or d)) appears to be important to the on-response in biotin; the results with mutants suggest that the biotin off-response depolarization is initiated by an I(Ca), which can be large enough or close enough to channels to open I(K(Ca,d)), I(Na(Ca)) and I(Mg(Ca)).

An exchanger-like protein underlies the large Mg2+ current in Paramecium.

There are very few molecules known to transport Mg(2+) in eukaryotes. The membrane of Paramecium tetraurelia passes a large Mg(2+)-selective current and exhibits a corresponding backward swimming behavior. Both are missing in a group of mutants called eccentric. By sorting an indexed WT genomic library through microinjection into the macronucleus, we have isolated a DNA fragment that complements the eccentric mutations. The Mg(2+) currents and behavior are restored fully in the transformed cells. Surprisingly, the conceptually translated protein is not homologous to any known ion channel but instead has some similarity to K(+)-dependent Na(+)Ca(2+) exchangers. Exchangers are either electrically silent or only pass very small and slow currents compared with ion-channel currents. In light of recent ion-channel crystal structures and considering the need to have narrow ion-selective filters, we speculate on how an exchanger might evolve to show channel-like activities in special circumstances. The significance of finding the molecular basis of a Mg(2+)-specific pathway is also discussed.