Karim Benzerara

Experimental investigation of the stability of Fe-rich carbonates in the lower mantle

The fate of carbonates in the Earths mantle plays a key role in the geodynamical carbon cycle. Although iron is a major component of the Earths lower mantle, the stability of Fe-bearing carbonates has rarely been studied. Here we present experimental results on the stability of Fe-rich carbonates at pressures ranging from 40 to 105 GPa and temperatures of 1450-3600 K, corresponding to depths within the Earths lower mantle of about 1000-2400 km. Samples of iron oxides and iron-magnesium oxides were loaded into CO2 gas and laser heated in a diamond-anvil cell. The nature of crystalline run products was determined in situ by X-ray diffraction, and the recovered samples were studied by analytical transmission electron microscopy and scanning transmission X-ray microscopy. We show that Fe-(II) is systematically involved in redox reactions with CO2 yielding to Fe-(III)-bearing phases and diamonds. We also report a new Fe-(III)-bearing high-pressure phase resulting from the transformation of FeCO3 at pressures exceeding 40 GPa. The presence of both diamonds and an oxidized C-bearing phase suggests that oxidized and reduced forms of carbon might coexist in the deep mantle. Finally, the observed reactions potentially provide a new mechanism for diamond formation at great depth.

Bacteria in the Tatahouine meteorite: nanometric-scale life in rocks.

We present a study of the textural signature of terrestrial weathering and related biological activity in the Tatahouine meteorite. Scanning and transmission electron microscopy images obtained on the weathered samples of the Tatahouine meteorite and surrounding soil show two types of bacteria-like forms lying on mineral surfaces: (1) rod-shaped forms (RSF) about 70-80 nm wide and ranging from 100 nm to 600 nm in length; (2) ovoid forms (OVF) with diameters between 70 and 300 nm. They look like single cells surrounded by a cell wall. Only Na, K, C, O and N with traces of P and S are observed in the bulk of these objects. The chemical analyses and electron diffraction patterns confirm that the RSF and OVF cannot be magnetite or other iron oxides, iron hydroxides, silicates or carbonates. The sizes of the RSF and OVF are below those commonly observed for bacteria but are very similar to some bacteria-like forms described in the Martian meteorite ALH84001. All the previous observations strongly suggest that they are bacteria or their remnants. This conclusion is further supported by microbiological experiments in which pleomorphic bacteria with morphology similar to the OVF and RSF objects are obtained from biological culture of the soil surrounding the meteorite pieces. The present results show that bacteriomorphs of diameter less than 100 nm may in fact represent real bacteria or their remnants.

Garnet-filled trails associated with carbonaceous matter mimicking microbial filaments in Archean basalt

The study of the earliest traces of life on Earth can be complicated by abiotically formed biomorphs. We report here the finding of clustered micrometer-sized filaments of iron- and calcium-rich garnets associated with carbonaceous matter in an agate amygdale from a 2.7-billion-year-old basalt of the Maddina Formation, Western Australia. The distribution of carbonaceous matter and the mineral phases composing the filaments were analyzed using a combination of confocal laser scanning microscopy, laser-Raman micro-spectroscopy, focused ion beam sectioning and transmission electron microscopy. The results allow consideration of possible biogenic and abiotic processes that produced the filamentous structures. The filaments have a range of sizes, morphologies and distributions similar to those of certain modern iron-mineralized filamentous bacteria and some ancient filamentous structures interpreted as microfossils. They also share a high morphological similarity with tubular structures produced by microbial boring activity. However, the microstructures and the distribution of carbonaceous matter are more suggestive of an abiotic origin for the filaments. They are characteristic features of trails produced by the displacement of inclusions associated with local dissolution of their silica matrix. Organic compounds found in kerogen or bitumen inclusions may have contributed significantly to the dissolution of the quartz (or silica gel) matrix driving filamentous growth. Discriminating the products of such abiotic organic-mediated processes from filamentous microfossils or microbial borings is important to the interpretation of the scarce Precambrian fossil record and requires investigation down to the nanoscale.

Experimental Colonization and Alteration of Orthopyroxene by the Pleomorphic Bacteria Ramlibacter tataouinensis

The colonization of orthopyroxene crystals by a pleomorphic bacterium and the mineralogical products resulting from a prolonged interaction have been studied. We used Ramlibacter tataouinensis (strain TTB310), which is an aerobic β-Proteobacterium moving over surfaces by gliding motility and whose life cycle includes rods and spherical cysts. Analysis of cultures grown on solid media with micrometer-sized pyroxene and quartz crystals scattered over the surface revealed a taxis of the bacteria toward the crystals. Given the mineralogical non-specificity of the interaction, a mechanism of elasticotaxis is inferred. After the rods had adhered to the pyroxene surface, they differentiated into cysts leading to the formation of microcolonies that were centered on a crystal grain. This suggests an original coupling between the life cycle of R. tataouinensis and the interaction with the crystals. The alteration of orthopyroxene was studied by high-resolution transmission electron microscopy at the interface between cysts and pyroxene crystals. The pyroxene surface showed an amorphous layer that was more developed than that of abiotic control samples processed under the same conditions. Moreover, chemical analyses showed that the dissolution of pyroxene was reduced in the presence of R. tataouinensis. The origin and the significance of the amorphous layers is discussed.

Cristobalite inclusions in the Tatahouine achondrite: Implications for shock conditions

Abstract The mineralogy of the Tatahouine diogenite was investigated by optical microscopy, Raman micro-spectrometry, and scanning and transmission electron microscopies. Inclusions of α-cristobalite in orthopyroxenes, locally in symplectic association with chromites, or associated with metal, have been characterized for the first time in a diogenite. Mosaicism of the orthopyroxenes indicates shock effects in the meteorite. The shock history of the meteorite must be consistent with the presence of vein-like structures containing inclusions of well-crystallized cristobalite, a low-pressure, high-temperature phase. Several possible mechanisms to account for these observations are discussed. The simplest one, consistent with all observations, is that a shock event would have occurred in a hot orthopyroxenite, either before extensive cooling of the asteroid, or in materials heated by previous impacts and maintained hot under an ejecta blanket.

Acceptance of the Mineralogical Society of America Award for 2012

Thank you very much, Gordon, for these very kind words. I receive the MSA Award with a lot of pleasure and much pride. Checking the impressive list of the former recipients, I can only feel very humble and surprised but happy to be here. It means a lot to me. Considering the worldwide reach and breadth of MSA, this is a fantastic recognition and looking back to my childhood, the distance to here was clearly huge. I first came to the U.S.A. in 2003 for a post-doc in Stanford, California. Funnily enough, the first workshop I attended was the MSA short course on biomineralization in the Nappa Valley and I remember exciting and stimulating talks. At that time, I could not imagine that one day I would receive the MSA award. I also take it as a huge responsibility: an incentive to work harder so that some day I may contribute significantly, like the former recipients of the award, to our field of mineralogy.nnTo tell a few words about my background, I received a general background in science including Life and Earth sciences. When I was admitted at l’Ecole Normale Superieure in Paris, I faced the dilemma of choosing between biology or geology. Because of the charisma of some teachers, in particular Laurent Jolivet, I chose geology. I had a lot of fun learning many things about geodynamics but …