L. Daly

Defining the mechanism for compaction of the CV chondrite parent body

The Allende meteorite, a relatively unaltered member of the CV carbonaceous chondrite group, contains primitive crystallographic textures that can inform our understanding of early Solar System planetary compaction. To test between models of porosity reduction on the CV parent body, complex microstructures within ~0.5-mm-diameter chondrules and ~10-μm-long matrix olivine grains were analyzed by electron backscatter diffraction (EBSD) techniques. The large area map presented is one of the most extensive EBSD maps to have been collected in application to extraterrestrial materials. Chondrule margins preferentially exhibit limited intragrain crystallographic misorientation due to localized crystal-plastic deformation. Crystallographic preferred orientations (CPOs) preserved by matrix olivine grains are strongly coupled to grain shape, most pronounced in shortest dimension , yet are locally variable in orientation and strength. Lithostatic pressure within plausible chondritic model asteroids is not sufficient to drive compaction or create the observed microstructures if the aggregate was cold. Significant local variability in the orientation and intensity of compaction is also inconsistent with a global process. Detailed microstructures indicative of crystal-plastic deformation are consistent with brief heating events that were small in magnitude. When combined with a lack of sintered grains and the spatially heterogeneous CPO, ubiquitous hot isostatic pressing is unlikely to be responsible. Furthermore, Allende is the most metamorphosed CV chondrite, so if sintering occurred at all on the CV parent body it would be evident here. We conclude that the crystallographic textures observed reflect impact compaction and indicate shock-wave directionality. We therefore present some of the first significant evidence for shock compaction of the CV parent body.

Aqueous alteration of the Martian meteorite Northwest Africa 817: Probing fluid–rock interaction at the nakhlite launch site

The nakhlite meteorites characteristically contain iddingsite, a hydrous iron–magnesium silicate that formed by aqueous alteration on Mars. Iddingsite is most abundant in Northwest Africa (NWA) 817, and alteration products in this meteorite also have the lowest deuterium/hydrogen ratio of any nakhlite. Taken together, these distinctive properties could be interpreted to show that NWA 817 was altered under different physico‐chemical conditions than the other nakhlites and by liquid water from a separate reservoir. Here this interpretation is tested through a petrographic, mineralogical, chemical, and isotopic study of NWA 817. We find that its iddingsite occurs as olivine‐hosted veins of nanocrystalline smectite and Fe‐oxyhydroxide. Strong similarities in the mineralogy of iddingsite between NWA 817 and other nakhlites suggest that these meteorites were altered under comparable physico‐chemical conditions, with the Fe‐rich composition of NWA 817 olivine grains rendering them especially susceptible to aqueous alteration. Analyses of NWA 817 bulk samples by stepwise pyrolysis confirm that its iddingsite has unusually low deuterium/hydrogen ratios, but owing to terrestrial weathering of this meteorite, the hydrogen isotopic data cannot be used with confidence to infer the origin of Martian aqueous solutions. NWA 817 was most probably altered along with the other nakhlites over a short time period and in a common aqueous system. One interpretation of a correlation between the eruption ages of three of the nakhlites and the chemical composition of their iddingsite is that water originated from close to the surface of Mars and flowed through the nakhlite lava pile under the influence of gravity.

Nebula sulfidation and evidence for migration of “free-floating” refractory metal nuggets revealed by atom probe microscopy

Disk models have been proposed that imply particles migrate rapidly in a protoplanetary disk. However, the only physical constraints on these processes from meteorites are observations of refractory inclusions in cometary material from the NASA Stardust mission. Atom probe microscopy (APM) of sub-micrometer refractory metal nuggets (RMNs) contained within a Sc-Zr–rich ultrarefractory inclusion (URI) from the ALH 77307 carbonaceous Ornans (CO) 3.0 meteorite revealed the presence of sulfur at 0.06–1.00 atomic percent (at%) abundances within RMNs. The mineralogical assemblage, petrographic texture, and flat chondrite-normalized highly siderophile element ratios indicate S exposure was unlikely to have occurred after the RMNs were incorporated into the URI. APM analyses suggest these RMNs were likely “free floating” when they were exposed to a S-condensing gas. This requires early, rapid migration of RMNs to cooler regions of the disk to incorporate S and then cycling back to the Ca-Al–rich inclusion (CAI)–forming region for incorporation in the URI, or conditions in the CAI-forming region that promote the incorporation of S into RMNs.

Coastal acidification impacts on shell mineral structure of bivalve mollusks

Abstract Ocean acidification is occurring globally through increasing CO 2 absorption into the oceans creating particular concern for calcifying species. In addition to ocean acidification, near shore marine habitats are exposed to the deleterious effects of runoff from acid sulfate soils which also decreases environmental pH. This coastal acidification is being exacerbated by climate change‐driven sea‐level rise and catchment‐driven flooding. In response to reduction in habitat pH by ocean and coastal acidification, mollusks are predicted to produce thinner shells of lower structural integrity and reduced mechanical properties threatening mollusk aquaculture. Here, we present the first study to examine oyster biomineralization under acid sulfate soil acidification in a region where growth of commercial bivalve species has declined in recent decades. Examination of the crystallography of the shells of the Sydney rock oyster, Saccostrea glomerata, by electron back scatter diffraction analyses revealed that the signal of environmental acidification is evident in the structure of the biomineral. Saccostrea glomerata, shows phenotypic plasticity, as evident in the disruption of crystallographic control over biomineralization in populations living in coastal acidification sites. Our results indicate that reduced sizes of these oysters for commercial sale may be due to the limited capacity of oysters to biomineralize under acidification conditions. As the impact of this catchment source acidification will continue to be exacerbated by climate change with likely effects on coastal aquaculture in many places across the globe, management strategies will be required to maintain the sustainable culture of these key resources.