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Earth’s largest aquifer ecosystem resides in igneous oceanic crust, where chemosynthesis and water-rock reactions provide the carbon and energy that support an active deep biosphere. The Calvin Cycle is the predominant carbon fixation pathway in cool, oxic, crust; however, the energy and carbon metabolisms in the deep thermal basaltic aquifer are poorly understood. Anaerobic carbon fixation pathways such as the Wood-Ljungdahl pathway, which uses hydrogen (H(2)) and CO(2), may be common in thermal aquifers since water-rock reactions can produce H(2) in hydrothermal environments and bicarbonate is abundant in seawater. To test this, we reconstructed the metabolisms of eleven bacterial and archaeal metagenome-assembled genomes from an olivine biofilm obtained from a Juan de Fuca Ridge basaltic aquifer. We found that the dominant carbon fixation pathway was the Wood-Ljungdahl pathway, which was present in seven of the eight bacterial genomes. Anaerobic respiration appears to be driven by sulfate reduction, and one bacterial genome contained a complete nitrogen fixation pathway. This study reveals the potential pathways for carbon and energy flux in the deep anoxic thermal aquifer ecosystem, and suggests that ancient H(2)-based chemolithoautotrophy, which once dominated Earth’s early biosphere, may thus remain one of the dominant metabolisms in the suboceanic aquifer today.

Microtubes within 145 Ma volcanic glass from the Argo Abyssal Plain possess intriguing internal microtextures that under light microscopy resemble biological septa and ovoid microbial cells. These microtextures have previously been used as part of a suite of evidence to support the biogenicity of such microtubes, and similar textures are beginning to be used in attempts to taxonomically classify microtubes from both the modern and ancient oceanic crust within an ichnofossil (trace fossil) hierarchy. Here we use high spatial resolution correlative microscopy to characterize the morphology and chemistry of the Argo microtubes in order to critically assess the origin of these microtextures and increase our understanding of the potential formation mechanisms of microtubes in volcanic glass. Electron microscopy shows that the microtubes contain abundant elongated void spaces and when these are reconstructed in three dimensions they closely replicate the morphology and distribution of the previously described ‘septa’. No organic material is associated with the void spaces and so we reinterpret the ‘septa’ as cracks within the clay mineral phase that infills the microtubes, probably formed during sample collection and/or preparation. One ovoid body also appears to correlate with void space but further data are required to substantiate such an origin. We caution that the study of micro-textures within volcanic glass-hosted microtubes by optical microscopy alone may be misleading, hence each individual occurrence should be subject to detailed micro- to nano-scale in situ morphological and chemical investigation before being used as a potential biosignature. Several microtubes do contain elevated levels of carbon, typically found within amorphous carbonate minerals that, along with nontronite clay, have precipitated within the microtubes. One microtube contains organic carbon; this is heterogeneously distributed, occurs away from void spaces and is spatially associated with elevated levels of titanium. This organic carbon could originate from in situ biological activity but it could also have been introduced by circulating seawater. Titanium adsorbed onto this organic material may provide a titanium source for the commonly observed titanite mineralization found in ancient volcanic glass-hosted microtubes within greenstone belts and ophiolites that have experienced low grade metamorphism. Elemental enrichments and depletions in three chemically distinct regions (glass, alteration rim, and tube interior) provide further insights into microtube formation mechanisms. Alteration rims have sharply defined edges, are about 0.1 μm wide independent of microtube diameter, and are primarily composed of Si, Al, O ± Ti. The tube interiors are depleted in Si and Al, and most other elements (Ca, Mg, Na, Mn) relative to fresh glass but K and Fe may be enriched. There is no evidence for depletion of elements in the glass immediately exterior to the alteration rim. This favours a mechanism whereby microtubes grow by increasing in length, rather than increasing in diameter. In this model protons are the major agent of glass alteration and the supply of protons and the kinetics of the formation of the Si-Al alteration rim control the diameter of the microtubes.

Diagenetic silica enrichment in fracture-associated halos that crosscut lacustrine and unconformably overlying aeolian sedimentary bedrock is observed on the lower north slope of Aeolis Mons in Gale crater, Mars. The diagenetic silica enrichment is colocated with detrital silica enrichment observed in the lacustrine bedrock yet extends into a considerably younger, unconformably draping aeolian sandstone, implying that diagenetic silica enrichment postdates the detrital silica enrichment. A causal connection between the detrital and diagenetic silica enrichment implies that water was present in the subsurface of Gale crater long after deposition of the lacustrine sediments and that it mobilized detrital amorphous silica and precipitated it along fractures in the overlying bedrock. Although absolute timing is uncertain, the observed diagenesis likely represents some of the most recent groundwater activity in Gale crater and suggests that the timescale of potential habitability extended considerably beyond the time that the lacustrine sediments of Aeolis Mons were deposited.

Volcanic ocean crust contains a global chemosynthetic microbial ecosystem that impacts ocean productivity, seawater chemistry and geochemical cycling. We examined the mineralogical effect on community structure in the aquifer ecosystem by using a four-year in situ colonization experiment with igneous minerals and glasses in Integrated Ocean Drilling Program Hole 1301A on the Juan de Fuca Ridge. Microbial community analysis and scanning electron microscopy revealed that olivine phases and iron-bearing minerals bore communities that were distinct from iron-poor phases. Communities were dominated by Archaeoglobaceae, Clostridia, Thermosipho, Desulforudis and OP1 lineages. Our results suggest that mineralogy determines microbial composition in the subseafloor aquifer ecosystem.

The Curiosity rover observed high Mn abundances (>25 wt % MnO) in fracture-filling materials that crosscut sandstones in the Kimberley region of Gale crater, Mars. The correlation between Mn and trace metal abundances plus the lack of correlation between Mn and elements such as S, Cl, and C, reveals that these deposits are Mn oxides rather than evaporites or other salts. On Earth, environments that concentrate Mn and deposit Mn minerals require water and highly oxidizing conditions; hence, these findings suggest that similar processes occurred on Mars. Based on the strong association between Mn-oxide deposition and evolving atmospheric dioxygen levels on Earth, the presence of these Mn phases on Mars suggests that there was more abundant molecular oxygen within the atmosphere and some groundwaters of ancient Mars than in the present day.

Key Points: • Mean K2O abundance in sedimentary rocks >5 times higher than that of the average Martian crust • Presence of alkali feldspars and K-phyllosilicates in basaltic sedimentary rocks along the traverse • The K-bearing minerals likely have a detrital origin

We propose a model whereby microscopic tunnels form in basalt glass in response to a natural proton flux from seawater into the glass. This flux is generated by the alteration of the glass as protons from water replace cations in the glass. In our proton gradient model, cells are gateways through which protons enter and alter the glass and through which cations leave the glass. In the process, tunnels are formed, and cells derive energy from the proton and ion fluxes. Proton flux from seawater into basalt glass would have occurred on Earth as soon as water accumulated on the surface and would have preceded biological redox catalysis. Tunnels in modern basalts are similar to tunnels in Archean basalts, which may be our earliest physical evidence of life. Proton gradients like those described in this paper certainly exist on other planetary bodies where silicate rocks are exposed to acidic to slightly alkaline water.

International audience; The Yellowknife Bay formation represents a similar to 5m thick stratigraphic section of lithified fluvial and lacustrine sediments analyzed by the Curiosity rover in Gale crater, Mars. Previous works have mainly focused on the mudstones that were drilled by the rover at two locations. The present study focuses on the sedimentary rocks stratigraphically above the mudstones by studying their chemical variations in parallel with rock textures. Results show that differences in composition correlate with textures and both manifest subtle but significant variations through the stratigraphic column. Though the chemistry of the sediments does not vary much in the lower part of the stratigraphy, the variations in alkali elements indicate variations in the source material and/or physical sorting, as shown by the identification of alkali feldspars. The sandstones contain similar relative proportions of hydrogen to the mudstones below, suggesting the presence of hydrous minerals that may have contributed to their cementation. Slight variations in magnesium correlate with changes in textures suggesting that diagenesis through cementation and dissolution modified the initial rock composition and texture simultaneously. The upper part of the stratigraphy (similar to 1m thick) displays rocks with different compositions suggesting a strong change in the depositional system. The presence of float rocks with similar compositions found along the rover traverse suggests that some of these outcrops extend further away in the nearby hummocky plains.

A suite of eight rocks analyzed by the Curiosity Rover while it was stopped at the Rocknest sand ripple shows the greatest chemical divergence of any potentially sedimentary rocks analyzed in the early part of the mission. Relative to average Martian soil and to the stratigraphically lower units encountered as part of the Yellowknife Bay formation, these rocks are significantly depleted in MgO, with a mean of 1.3 wt %, and high in Fe, averaging over 20 wt % FeO_T, with values between 15 and 26 wt % FeO_T. The variable iron and low magnesium and rock texture make it unlikely that these are igneous rocks. Rock surface textures range from rough to smooth, can be pitted or grooved, and show various degrees of wind erosion. Some rocks display poorly defined layering while others seem to show possible fractures. Narrow vertical voids are present in Rocknest 3, one of the rocks showing the strongest layering. Rocks in the vicinity of Rocknest may have undergone some diagenesis similar to other rocks in the Yellowknife Bay Formation as indicated by the presence of soluble calcium phases. The most reasonable scenario is that fine-grained sediments, potentially a mixture of feldspar-rich rocks from Bradbury Rise and normal Martian soil, were lithified together by an iron-rich cement.

Pits in ocean basalt glass are often attributed to the activity of microorganisms, however, neither the mechanism of formation of pits in glass nor the involvement of microorganisms have been confirmed by experimentation. Experimental abiotic corrosion of basalt glass with 1% hydrofluoric acid (HF), a proxy for more slowly acting organic acids, produces pits that are similar in size, shape, and distribution to pits found in basalt glass collected from the ocean floor and basalt glass incubated with microorganisms. This pit formation by HF etching was demonstrated by comparing secondary electron images taken before and after the acid treatment. The formation of pits by abiotic corrosion of basalt glass is an alternate hypothesis for the origin of these features, and the interpretation of these features as biomarkers may require a resolution of these alternative hypotheses.

We provide a comprehensive photographic atlas of the intricate alteration features found in glass in igneous rocks from the ocean basins. The samples come from surface and subsurface rocks from oceanic rises and seamounts of the ocean basins and some marginal seas. These textures have previously been termed “bioalteration textures” by those who consider them as potentially biogenic in origin, or as “etch pits” by those who prefer a non-biogenic interpretation. Here, transmitted-light color photomicrographs are provided to illustrate the range of granular and tubular textures as well as their relation to fractures, minerals, vesicles, and multiple episodes of alteration in the same sample. The tubular forms are described using seven morphological characteristics: (1) length and width; (2) density; (3) curvature; (4) roughness; (5) variations in width; (6) branching; and (7) tunnel contents. The photomicrographs are a starting point for understanding the factors that control the formation of the alteration textures, for evaluating the biogenicity of the various forms, for inferring subsurface conditions during alteration, and for making comparisons to similar textures in ancient ophiolites, some of which have been attributed to the earliest life on Earth

International audience; The Curiosity rover has analyzed various detrital sedimentary rocks at Gale Crater, among which fluvial and lacustrine rocks are predominant. Conglomerates correspond both to the coarsest sediments analyzed and the least modified by chemical alteration, enabling us to link their chemistry to that of source rocks on the Gale Crater rims. In this study, we report the results of six conglomerate targets analyzed by Alpha-Particle X-ray Spectrometer and 40 analyzed by ChemCam. The bulk chemistry derived by both instruments suggests two distinct end-members for the conglomerate compositions. The first group (Darwin type) is typical of conglomerates analyzed before sol 540; it has a felsic alkali-rich composition, with a Na2O/K2O\textgreater5. The second group (Kimberley type) is typical of conglomerates analyzed between sols 540 and 670 in the vicinity of the Kimberley waypoint; it has an alkali-rich potassic composition with Na2O/K2O\textless2. The variety of chemistry and igneous textures (when identifiable) of individual clasts suggest that each conglomerate type is a mixture of multiple source rocks. Conglomerate compositions are in agreement with most of the felsic alkali-rich float rock compositions analyzed in the hummocky plains. The average composition of conglomerates can be taken as a proxy of the average igneous crust composition at Gale Crater. Differences between the composition of conglomerates and that of finer-grained detrital sediments analyzed by the rover suggest modifications by diagenetic processes (especially for Mg enrichments in fine-grained rocks), physical sorting, and mixing with finer-grained material of different composition.

The boundary between ice and basalt on Earth is an analogue for some near-surface environments of Mars. We investigated neutrophilic iron-oxidizing microorganisms from the basalt-ice interface in a lava tube from the Oregon Cascades with perennial ice. One of the isolates (Pseudomonas sp. HerB) can use ferrous iron Fe(II) from the igneous mineral olivine as an electron donor and O2 as an electron acceptor. The optimum growth temperature is ∼12–14°C, but growth also occurs at 5°C. Bicarbonate is a facultative source of carbon. Growth of Pseudomonas sp. HerB as a chemolithotrophic iron oxidizer with olivine as the source of energy is favored in low O2 conditions (e.g., 1.6% O2). Most likely, microbial oxidation of olivine near pH 7 requires low O2 to offset the abiotic oxidation of iron. The metabolic capabilities of this bacterium would allow it to live in near-surface, icy, volcanic environments of Mars in the present or recent geological past and make this type of physiology a prime candidate in the search for life on Mars. Key Words: Extremophiles—Mars—Olivine—Iron-oxidizing bacteria—Redox. Astrobiology 12, 9–18.

The European Space Agency will launch the ExoMars mission in 2016 with a primary goal of surveying the martian subsurface for evidence of organic material. We have recently investigated the utility of including either a 365 nm light-emitting diode or a 375 nm laser light source in the ExoMars rover panoramic camera (PanCam). Such a modification would make it feasible to monitor rover drill cuttings optically for the fluorescence signatures of aromatic organic molecules and map the distribution of polycyclic aromatic hydrocarbons (PAHs) as a function of depth to the 2 m limit of the ExoMars drill. The technique described requires no sample preparation, does not consume irreplaceable resources, and would allow mission control to prioritize deployment of organic detection experiments that require sample destruction, expenditure of non-replaceable consumables, or both. We report here for the first time laser-induced fluorescence emission (L.I.F.E.) imaging detection limits for anthracene, pyrene, and perylene targets doped onto a Mars analog granular peridotite with a 375 nm Nichia laser diode in optically uncorrected wide-angle mode. Data were collected via the Beagle 2 PanCam backup filter wheel fitted with original blue (440 nm), green (530 nm), and red (670 nm) filters. All three PAH species can be detected with the PanCam green (530 nm) filter. Detection limits in the green band for signal-to-noise ratios (S/N)10 are 49 parts per million (ppm) for anthracene, 145 ppm for pyrene, and 20 ppm for perylene. The anthracene detection limit improves to 7 ppm with use of the PanCam blue filter. We discuss soil-dependent detection limit constraints; use of UV excitation with other rover cameras, which provides higher spatial resolution; and the advantages of focused and wide-angle laser modes. Finally, we discuss application of L.I.F.E. techniques at multiple wavelengths for exploration of Mars analog extreme environments on Earth, including Icelandic hydrothermally altered basalts and the ice-covered lakes and glaciers of Dronning Maud Land, Antarctica.

We used molecular techniques to analyze basalts of varying ages that were collected from the East Pacific Rise, 9 degrees N, from the rift axis of the Juan de Fuca Ridge and from neighboring seamounts. Cluster analysis of 16S rDNA terminal restriction fragment polymorphism data revealed that basalt endoliths are distinct from seawater and that communities clustered, to some degree, based on the age of the host rock. This age-based clustering suggests that alteration processes may affect community structure. Cloning and sequencing of bacterial and archaeal 16S rRNA genes revealed 12 different phyla and subphyla associated with basalts. These include the Gemmatimonadetes, Nitrospirae, the candidate phylum SBR1093 in the bacteria, and in the Archaea Marine Benthic Group B, none of which have been previously reported in basalts. We delineated novel ocean crust clades in the gamma-Proteobacteria, Planctomycetes and Actinobacteria that are composed entirely of basalt-associated microflora, and may represent basalt ecotypes. Finally, microarray analysis of functional genes in basalt revealed that genes coding for previously unreported processes such as carbon fixation, methane oxidation, methanogenesis and nitrogen fixation are present, suggesting that basalts harbor previously unrecognized metabolic diversity. These novel processes could exert a profound influence on ocean chemistry.

International audience; Understanding of the geologic evolution of Mars has been greatly improved by recent orbital(1-3), in situ(4,5) and meteorite(6-8) data, but insights into the earliest period of Martian magmatism (4.1 to 3.7 billion years ago) remain scarce(9). The landing site of NASA's Curiosity rover, Gale crater, which formed 3.61 billion years ago(10) within older terrain(11), provides a window into this earliest igneous history. Along its traverse, Curiosity has discovered light-toned rocks that contrast with basaltic samples found in younger regions(12). Here we present geochemical data and images of 22 specimens analysed by Curiosity that demonstrate that these light-toned materials are feldspar-rich magmatic rocks. The rocks belong to two distinct geochemical types: alkaline compositions containing up to 67 wt% SiO2 and 14 wt% total alkalis (Na2O + K2O) with fine-grained to porphyritic textures on the one hand, and coarser-grained textures consistent with quartz diorite and granodiorite on the other hand. Our analysis reveals unexpected magmatic diversity and the widespread presence of silica- and feldspar-rich materials in the vicinity of the landing site at Gale crater. Combined with the identification of feldspar-rich rocks elsewhere(9,13,14) and the low average density of the crust in the Martian southern hemisphere(15), we conclude that silica-rich magmatic rocks may constitute a significant fraction of ancient Martian crust and may be analogous to the earliest continental crust on Earth.

International audience; The ChemCam campaign at the fluvial sedimentary outcrop ‘‘Shaler’’ resulted in observations of 28non-soil targets, 26 of which included active laser induced breakdown spectroscopy (LIBS), and all ofwhich included Remote Micro-Imager (RMI) images. The Shaler outcrop can be divided into seven faciesbased on grain size, texture, color, resistance to erosion, and sedimentary structures. The ChemCamobservations cover Facies 3 through 7. For all targets, the majority of the grains were below the limitof the RMI resolution, but many targets had a portion of resolvable grains coarser than 0.5 mm. The Shalerfacies show significant scatter in LIBS spectra and compositions from point to point, but several keycompositional trends are apparent, most notably in the average K2O content of the observed facies. Facies3 is lower in K2O than the other facies and is similar in composition to the ‘‘snake,’’ a clastic dike thatoccurs lower in the Yellowknife Bay stratigraphic section. Facies 7 is enriched in K2O relative to the otherfacies and shows some compositional and textural similarities to float rocks near Yellowknife Bay. Theremaining facies (4, 5, and 6) are similar in composition to the Sheepbed and Gillespie Lake members,although the Shaler facies have slightly elevated K2O and FeOT. Several analysis points within Shaler suggestthe presence of feldspars, though these points have excess FeOT which suggests the presence of Feoxide cement or inclusions. The majority of LIBS analyses have compositions which indicate that theyhttp://dx.doi.org/10.1016/j.icarus.2014.07.0250019-1035/Published by Elsevier Inc.⇑ Corresponding author.E-mail addresses: rbanderson@usgs.gov (R. Anderson), jcb36@leicester.ac.uk (J.C. Bridges), amywill@ucdavis.edu (A. Williams), ledgar1@asu.edu (L. Edgar), aollila@unm.edu (A. Ollila), josh505@unm.edu (J. Williams), marion.nach@gmail.com (M. Nachon), nicolas.mangold@univ-nantes.fr (N. Mangold), Martin.Fisk@oregonstate.edu (M. Fisk),jschiebe@indiana.edu (J. Schieber), s.gupta@imperial.ac.uk (S. Gupta), gilles.dromart@ens-lyon.fr (G. Dromart), rwiens@lanl.gov (R. Wiens), stephane.lemouelic@univ-nantes.fr (S. Le Mouélic), olivier.forni@irap.omp.eu (O. Forni), nlanza@lanl.gov (N. Lanza), amezzacappa09@students.desu.edu (A. Mezzacappa), vsautter@mnhn.fr (V. Sautter), diana.l.blaney@jpl.nasa.gov (D. Blaney), bclark@spacescience.org (B. Clark), sclegg@lanl.gov (S. Clegg), olivier.gasnault@irap.omp.eu (O. Gasnault), jeremie.lasue@irap.omp.eu (J.Lasue), rich.leveille@gmail.com (R. Léveillé), Eric.LEWIN@obs.ujf-grenoble.fr (E. Lewin), sylvestre.maurice@irap.omp.eu (S. Maurice), newsom@unm.edu (H. Newsom),dvaniman@psi.edu (D. Vaniman).Icarus 249 (2015) 2–21Contents lists available at ScienceDirectIcarusjournal homepage: www.elsevier.com/ locate/ icarusare mixtures of pyroxene and feldspar. The Shaler feldspathic compositions are more alkaline than typicalfeldspars from shergottites, suggesting an alkaline basaltic source region, particularly for the K2OenrichedFacies 7. Apart from possible iron-oxide cement, there is little evidence for chemical alterationat Shaler, although calcium-sulfate veins comparable to those observed lower in the stratigraphic sectionare present. The differing compositions, and inferred provenances at Shaler, suggest compositionally heterogeneousterrain in the Gale crater rim and surroundings, and intermittent periods of deposition

There is widespread interest in developing methods to investigate in situ microbial activity in subsurface environments. Novel experiments based on single borehole push–pull methods were conducted to measure in situ microbial activity at the Aspo Hard Rock Laboratory (HRL). Microbial nitrate reduction and lactate consumption were measured at in situ conditions at a depth of 450 m in the HRL tunnel. A circulation system was used to circulate ground water from the aquifer through pressure-maintaining flow cells containing coupons for biofilm growth. The system allows microbial investigations at in situ pressure, temperature and chemistry. Four experiments were conducted in which a combination of a conservative tracer, nitrate and lactate was injected into the circulation system. Rate of nitrate utilization was 5 µm h-1 without lactate and 13 µm h-1 with lactate. Lactate consumption increased from 30 to 50 µm h-1 with the addition of an exogenous electron acceptor (nitrate). Attached and unattached cells were enumerated using epifluorescence microscopy to calculate cell-specific rates of activity. The biofilm had an average cell density of 1 × 106 cells cm-2 and there was an average of 6 × 105 unattached cells mL-1 in circulation. Cell-specific rates of lactate consumption were higher than previously reported using radiotracer methods in similar environments. The differences highlight the importance of conducting microbial investigations at in situ conditions. The results demonstrate that an indigenous community of microbes survives at a depth of 450 m in the Fennoscandian shield aquifer with the potential to oxidize simple organic molecules such as lactate.

We examined the common, iron-magnesium silicate minerals olivine and pyroxene in basalt and in mantle rocks to determine if they exhibit textures similar to bioweathering textures found in glass. Our results show that weathering in olivine may occur as long, narrow tunnels (1-3 microm in diameter and up to 100 microm long) and as larger irregular galleries, both of which have distinctive characteristics consistent with biological activity. These weathering textures are associated with clay mineral by-products and nucleic acids. We also examined olivine and pyroxene in martian meteorites, some of which experienced preterrestrial aqueous alteration. Some olivines and pyroxenes in the martian meteorite Nakhla were found to contain tunnels that are similar in size and shape to tunnels in terrestrial iron-magnesium silicates that contain nucleic acids. Though the tunnels found in Nakhla are similar to the biosignatures found in terrestrial minerals, their presence cannot be used to prove that the martian alteration features had a biogenic origin. The abundance and wide distribution of olivine and pyroxene on Earth and in the Solar System make bioweathering features in these minerals potentially important new biosignatures that may play a significant role in evaluating whether life ever existed on Mars.

Major element chemistry of basalt from the southern East Pacific Rise (EPR) is different from that of the EPR at the time of the formation of the Pacific Plate at 170 Ma.Glass recovered from Jurassic age (170 Ma) Pacific ocean crust [Bartolini and Larson, Geology 29 (2001) 735^738] at Ocean Drilling Program Hole 801C records higher Fe8 (10.77 wt%) and marginally lower Na8 (2.21 wt%) compared to the modern EPR, suggesting deeper melting and a temperature of initial melting that was 60‡C hotter than today.Trace element ratios such as La/Sm and Zr/Y, on the other hand, show remarkable similarities to the modern southern EPR, indicating that Site 801 was not generated on a hotspot-influenced ridge and that mantle composition has changed little in the Pacific over the past 170 Ma.Our results are consistent with the observation that mid-ocean ridge basalts (MORBs) older than 80 Ma were derived by higher temperature melting than are modern MORBs [Humler et al., Earth Planet. Sci. Lett. 173 (1999) 7^23], which may have been a consequence of the Cretaceous superplume event in the Pacific.Site 801 predates the formation of Pacific oceanic plateaus and 801C basalt chemistry indicates that higher temperatures of mantle melting beneath Pacific ridges preceded the initiation of the superplume.F 2002 Published by Elsevier Science B.

The ChemCam instrument on the Mars Science Laboratory (MSL) rover, Curiosity, observed numerous igneous float rocks and conglomerate clasts, reported previously. A new statistical analysis of single-laser-shot spectra of igneous targets observed by ChemCam shows a strong peak at ~55 wt% SiO2 and 6 wt% total alkalis, with a minor secondary maximum at 47–51 wt% SiO2 and lower alkali content. The centers of these distributions, together with the rock textures, indicate that many of the ChemCam igneous targets are trachybasalts, Mg#=27 but with a secondary concentration of basaltic material,with a focus of compositions around Mg#=54. We suggest that all of these igneous rocks resulted from low-pressure, olivine-dominated fractionation of Adirondack (MER) class-type basalt compositions. This magmatism has subalkaline, tholeiitic affinities. The similarity of the basalt endmember to much of the Gale sediment compositions in the first 1000 sols of the MSL mission suggests that this type of Fe-rich, relatively low-Mg#, olivine tholeiite is the dominant constituent of the Gale catchment that is the source material for the fine-grained sediments in Gale. The similarity to many Gusev igneous compositions suggests that it is a major constituent of ancient Martian magmas, and distinct from the shergottite parental melts thought to be associated with Tharsis and the Northern Lowlands. The Gale Crater catchment sampled a mixture of this tholeiitic basalt along with alkaline igneous material, together giving some analogies to terrestrial intraplate magmatic provinces.

H 2 O, CO 2 , SO 2 , O 2 , H 2 , H 2 S, HCl, chlorinated hydrocarbons, NO, and other trace gases were evolved during pyrolysis of two mudstone samples acquired by the Curiosity rover at Yellowknife Bay within Gale crater, Mars. H 2 O/OH-bearing phases included 2:1 phyllosilicate(s), bassanite, akaganeite, and amorphous materials. Thermal decomposition of carbonates and combustion of organic materials are candidate sources for the CO 2 . Concurrent evolution of O 2 and chlorinated hydrocarbons suggests the presence of oxychlorine phase(s). Sulfides are likely sources for sulfur-bearing species. Higher abundances of chlorinated hydrocarbons in the mudstone compared with Rocknest windblown materials previously analyzed by Curiosity suggest that indigenous martian or meteoritic organic carbon sources may be preserved in the mudstone; however, the carbon source for the chlorinated hydrocarbons is not definitively of martian origin.

Hydrothermal vents appear to be the tip of the subsurface biosphere in the ocean crust. The primary prducers in this biosphere are prokaryotes that tolerate a wide variety of physical and chemical conditions and are versatile in their use of inorganic compounds to drive metabolism. A synthesis of chemical and mineralogical data from Martian meteorites and measurements of the Martian surface suggest that conditions similar to those that make life possible in Earth's oceanic crust, namely, water, carbon, nutrients, appropriate temperatures, and gradients in redox conditions, also occur within Mars. Chemolithoautotrophic microorganisms capable of living below the seafloor on Earth would probably survive in some regions of the Martian subsurface.

The use of ocean floor basalt chemistry as a tool to investigate mantle composition and processes requires that we work with basalts that have been modified little since leaving the mantle. One source of such basalts is melt inclusions trapped in primitive crystals. However, obtaining information from these melt inclusions is complicated by the fact that melt inclusions in natural basalts are essentially always altered by post-entrapment crystallization. This requires that we develop techniques for reconstructing the original trapped liquid compositions. We conducted a series of experiments to reverse the effects of post-entrapment crystallization by re-heating the host crystals to their crystallization temperature. For these experiments we used plagioclase crystals separated from a single Gorda Ridge lava. The crystallization temperature for these crystals was determined by a set of incremental re-heating experiments to be ∼1240–1260° C. The inclusions are primitive, high Ca-Al basaltic melts, saturated with plagioclase, olivine and Al-rich chromite at low pressure. The inclusion analyses can be linked to the host lava composition by low pressure fractionation. The major element composition of the re-homogenized melt inclusions within each crystal is relatively constant. However, the incompatible element analyses have extremely wide ranges. The range of La/Sm and Ti/Zr from inclusions analyzed from a single sample from the Gorda Ridge exceeds the range reported for lavas sampled from the entire ridge. The pyroxene compositions predicted to be in equilibrium with the melt inclusion trace element signature cover much of the range represented by pyroxenes from abyssal peridotites. The volumetric proportions of the magmas entering the base of the crust can be evaluated using frequency distribution of melt inclusion compositions. This distribution suggests that the array of magmas was skewed towards the more depleted compositions, with little evidence for an enriched component in this system. This pattern is more consistent with a dynamic flow model of the mantle or of a passive flow model where the melts produced in the peripheral areas of the melting regime were not focused to the ridge.

The island of Tahiti, the largest in French Polynesia, comprises two major volcanoes aligned NW-SE, parallel with the general trend of the Society Islands hotspot track. Rocks from this volcanic system are basalts transitional to tholeiites, alkali basalts, basanites, picrites, and evolved lavas. Through K-Ar radiometric dating we have established the age of volcanic activity. The oldest lavas (--1.7 Ma) crop out in deeply eroded valleys in the center of the NW volcano (Tahiti Nui), while the main exposed shield phase erupted between 1.3 and 0.6 Ma, and a late- stage, valley-filling phase occurred between 0.7 and 0.3 Ma. The SW volcano (Tahiti Iti) was active between 0.9 and 0.3 Ma. There is a clear change in the composition of lavas through time. The earliest lavas are moderately high SiO2, evolved basalts (Mg number (Mg# = Mg/Mg+Fe2+) 42-49), probably derived from parental liquids of composition transitional between those of tholeiites and alkali basalts. The main shield lavas are predominantly more primitive olivine and clinopyroxene-phyric alkali basalts (Mg# 60-64), while the later valley-filling lavas are basanitic (Mg# 64-68) and commonly contain peridotitic xenoliths (olivine+orthopyroxene+ clinopyroxene+ spinel). Isotopic compositions also change systematically with time to more depleted signatures. Rare earth element patterns and incompatible element ratios, however, show no systematic variation with time. We focused on a particularly well exposed sequence of shield- building lavas in the Punaruu Valley, on the western side of Tahiti Nui. Combined K-Ar ages and magnetostratigraphic boundaries allow high-resolution age assignments to this -0.7-km-thick flow section. We identified an early period of intense volcanic activity, from 1.3 to 0.9 Ma, followed by a period of more intermittent activity, from 0.9 to 0.6 Ma. Flow accumulation rates dropped by a factor of 4 at about 0.9 Ma. This change in rate of magma supply corresponds to a shift in activity to Tahiti Iti. We calculated the composition of the parent magma for the shield- building stage of volcanism, assuming that it was in equilibrium with Fo89 olivine and that the most primitive aphyric lavas were derived from this parent by the crystallization of olivine alone. The majority of the shield lavas represent 25 to 50% crystallization of this parent magma, but the most evolved lavas represent about 70% crystallization. From over 50 analyzed flow units we recognize a quasi-periodic evolution of lava compositions within the early, robust period of volcanic activity, which we interpret as regular recharge of the magma chamber (approximately every 25 + 10 kyr). Volcanic evolution on Tahiti is similar to the classic Hawaiian pattern. As the shield-building stage waned, the lavas became more silica undersaturated and isotopic ratios of the lavas became more MORB-like. We propose that the Society plume is radially zoned due to entrainment of a sheath of viscously coupled, depleted mantle surrounding a central core of deeper mantle material. All parts of the rising plume melt, but the thermal and compositional radial gradient ensures that greater proportions of melting occur over the plume center than its margins. The changing composition of Tahitian magmas results from lithospheric motion over this zoned plume. Magmas erupted during the main shield-building stage are derived mainly from the hot, incompatible element-enriched central zone of the plume; late-stage magmas are derived from the cooler, incompatible element-depleted, viscously coupled sheath. A correlation between Pb/Ce and isotope ratios suggests that the Society plume contains deeply recycled continental material.

The effects of composition and temperature on pyroxene-melt partitioning of the high-field-strength elements (HFSE) — Ti, Zr, Nb and Ta — were evaluated from doped experiments on natural mafic to intermediate composition lavas at pressures from 0.1 MPa to 0.9 GPa (0.001 to 9 kbar). The HFSE partition coefficients (D) maintain similar relative relationships: DTi >DZr >DTa >DNb, but vary absolutely as a function of composition and temperature, often exhibiting a range of over a factor of 5 at a single temperature. For example, DZr ranges from 0.1 in a tholeiitic melt to 0.6 for a dacitic melt at 1100°C, 0.1 MPa. DZr, DTa and DNb for high- and low-Ca pyroxene can be described as linear functions of DTi. For high-Ca pyroxenes, the functions are DZr = 0.64DTi − 0.13, DTa = 0.14DTi − 0.02 and DNb = 0.04DTi − 0.01. The low-Ca pyroxene expressions are DZr = 0.60DTi − 0.06, DTa = 0.27DTi − 0.005 and DNb = 0.08DTi − 0.005. This linear relationship suggests similar substitution mechanisms for Ti and the other HFSE in both pyroxene and silicate melts. An expression was derived to calculate the Ti content of pyroxene based on the melt composition, Ca content of the pyroxene, temperature and pressure. This expression uses an approximation of the equilibrium constant for an exchange reaction of a Ti/Al-bearing component with a Ca-bearing component in the pyroxene. Over the experimental temperature range (1170-1070°C), the clinopyroxene Ti contents can be reproduced with a precision of ±20% (1σ).

The effects of composition and temperature on the partitioning behavior of Sc, Ni, V, U and Th, and the high field strength elements (HFSE) Zr, Nb, Ta and Hf between magnetite and natural silicate melts were evaluated from doped experiments on natural mafic- to intermediate-composition lavas at 1-atm pressure. Composition was found to be the strongest controlling factor on partitioning behavior. The partition coefficients (D) for Zr, Nb, Hf and Ta correlate with DTi and are similar to one another for any given magnetite-melt pair, but vary from 2 in titanomagnetite. DSc is higher than DZr, DNb, DHf and DTa, and also correlated to DTi. In contrast, Ni is more compatible in Al-Cr-rich magnetites than in titanomagnetites, but is compatible in all magnetite-melt pairs in our experiments. V is generally more compatible than Zr, Nb, Ta and Hf, but its behavior is complicated by its multiple valence states. U and Th are incompatible (D < 0.035) in all magnetites. Expressions were derived to describe the relationship between D and the most strongly correlated parameters, oxide Fe/Mg ratio and Al, and DTi. These patterns of behavior are consistent with the observed miscibility gap between the spinel-group end-members. Equilibrium constants for spinel end-member-melt reactions were parameterized in terms of temperature, pressure and composition. These expressions can be used to predict the temperature and composition of equilibrium spinels. These major-element constraints can also be used to predict spinel-melt partition coefficients using the expressions describing DHFSE as a function of DTi and composition.

The Integrated Ocean Drilling Program (IODP) Hole 1301A on the eastern flank of Juan de Fuca Ridge was used in the first long-term deployment of microbial enrichment flow cells using osmotically driven pumps in a subseafloor borehole. Three novel osmotically driven colonization systems with unidirectional flow were deployed in the borehole and incubated for 4 years to determine the microbial colonization preferences for 12 minerals and glasses present in igneous rocks. Following recovery of the colonization systems, we measured cell density on the minerals and glasses by fluorescent staining and direct counting and found some significant differences between mineral samples. We also determined the abundance of mesophilic and thermophilic culturable organotrophs grown on marine R2A medium and identified isolates by partial 16S or 18S rDNA sequencing. We found that nine distinct phylotypes of culturable mesophilic oligotrophs were present on the minerals and glasses and that eight of the nine can reduce nitrate and oxidize iron. Fe(II)-rich olivine minerals had the highest density of total countable cells and culturable organotrophic mesophiles, as well as the only culturable organotrophic thermophiles. These results suggest that olivine (a common igneous mineral) in seawater-recharged ocean crust is capable of supporting microbial communities, that iron oxidation and nitrate reduction may be important physiological characteristics of ocean crust microbes, and that heterogeneously distributed minerals in marine igneous rocks likely influence the distribution of microbial communities in the ocean crust.

The Mendocino Fracture Zone, a 3,000-km-long transform fault, extends from the San Andreas Fault at Cape Mendocino, California due west into the central Pacific basin. The shallow crest of this fracture zone, known as the Mendocino Ridge, rises to within 1,100 m of the sea surface at 270 km west of the California Coast. Rounded basalt pebbles and cobbles, indicative of a beach environment, are the dominant lithology at two locations on the crest of Mendocino Ridge and a40Ar/39 Ar incremental heating age of 11.0 ± 1.0 million years was determined for one of the these cobbles. This basalt must have been erupted on the Gorda Ridge because the crust immediately to the south of the fracture zone is older than 27 Ma. This age also implies that the crest of Mendocino Ridge was at sea level and would have blocked Pacific Ocean eastern boundary currents and affected the climate of the North American continent at some time since the late Miocene. Basalts from the Mendocino Fracture Zone (MFZ) are FeTi basalts similar to those commonly found at intersections of mid-ocean ridges and fracture zones. These basalts are chemically distinct from the nearby Gorda Ridge but they could have been derived from the same mantle source as the Gorda Ridge basalts. The location of the 11 Ma basalt suggests that Mendocino Ridge was transferred from the Gorda Plate to the Pacific Plate and the southern end of Gorda Ridge was truncated by a northward jump in the transform fault of MFZ.

Partially crystalline melt inclusions in anorthite phenocrysts from a Galapagos seamount have been rehomogenized in a series of heating experiments in order to accurately determine their initial compositions. They are primitive tholeiites, high in CaO and Al2O3, and low in TiO2 and alkalies. We infer that they were entrapped at a temperature close to 1270°C. At this temperature, the trapped melt composition is multiply saturated with olivine, plagioclase and spinel at 1 atmosphere pressure, but it is also near-primary requiring addition of less than 5% olivine plus or minus plagioclase and spinel in order to be in equilibrium with mantle olivine. The inclusions occur in association with, and are potentially parental to, a suite of MORB-like pillow lavas which range in MgO content from 10 to 8 wt% and which could have been derived by up to 40% fractionation of olivine plus plagioclase from the inclusion composition. Although the host anorthites are remarkably homogeneous and unzoned and the inclusions themselves are uniform in major element composition, minor element contents vary by a factor of two or more. Unlike many other plagioclase megacrysts, these anorthites must have crystallized under conditions in which the major element compositions were buffered but minor (and presumably trace) elements were free to vary. Such conditions could be achieved by liquid-solid interactions either in the melting regime or, perhaps more likely, in a liquid-crystal mush within the oceanic crust. These melt inclusions are not unique. They appear to belong to a class of primitive, high CaO and Al2O3 MORB that occur in association with high-An plagioclase in a variety of oceanic settings, usually where magma supply is low. We propose that such magmas are derived from the shallowest, most depleted part of a mantle melt column, and that they constitute one end member of a spectrum of primary magmas which gives rise to the global array of MORB compositions.

colonization. SSA ranged from 0.3 to 52 m 2 /g of basalt with the lowest value coming from pillow basalt and the highest value from breccia. The average for massive and pillow basalts combined was 2.3 m 2 /g. The total specific surface area of the extrusive volcanic rocks of the ocean crust is estimated to be 10 24 m 2 . This surface area could provide attachment for up to 10 34 cells if cell density is the same as that of experimentally colonized basalt surfaces. Independent measures and calculations of biomass in basalts suggest that cell densities on surfaces are only 10 −4 times those in laboratory experiments and, therefore, the surface area of basalt does not limit microbial biomass in the igneous ocean crust. Citation: Nielsen, M. E., and M. R. Fisk (2010), Surface area measurements of marine basalts: Implications for the subseafloor microbial biomass, Geophys. Res. Lett., 37, L15604, doi:10.1029/2010GL044074.

Bransfield Strait is a narrow marginal basin separating the South Shetland Islands from the northern end of the Antarctic Peninsula. Quaternary volcanism occurs in the strait as subaerial and submarine volcanoes aligned on the inferred axis of rifting, and as two subaerial, off-axis volcanoes on the northern margin of the strait. The Bransfield Strait lavas are similar to published analyses from other marginal basins, ranging from basalts and basaltic andesites to trachytes. They exhibit moderate enrichments in alkali and alkaline earth elements relative to high-field-strength elements that are typical of many back-arc basin basalts. The seamount basalts have trace element chemistry similar to enriched mid-ocean ridge basalts (E-MORB), but with variously higher alkali and alkaline earth element concentrations and, frequently, lower Nb. Low-pressure fractional crystallization accounts for most of the compositional variation within individual volcanoes, but it does not explain intervolcano differences even though the volcanoes are closely spaced and presently or recently active. Melting of depleted mantle mixed with 0.5–2% crust or sediment explains the isotopic and trace element variations not accounted for by magma chamber processes. The off-axis volcanoes are the products of less partial melting than the on-axis volcanoes. One of the off-axis volcanoes also contains more of an enriched component in its source. Some of the lavas have Rb/Sr too low to account for their 87Sr/86Sr, perhaps due the loss of Rb during mantle metasomatism or interaction between slab-derived fluids and the mantle source of the basalts. Strontium, neodymium, and 207Pb/204Pb isotopic signatures remained constant during the transition from Tertiary island arc volcanism to Quaternary marginal basin volcanism, but 206Pb/204Pb increased. High CeN/SmN of basalts from some volcanoes requires residual garnet in the source. Thus the young (15–25 Ma at the trench) subducted slab is either deeper than 60 km beneath the volcanoes, or it has fractured and is no longer a coherent barrier to melts generated below it.

Rapid discovery of the distribution and relative abundance of organic material without sample destruction or the expenditure of irreplaceable resources is one of the primary requirements for exploration of novel extreme environments both in remote locations on Earth and on the Mars regolith. A wide variety of organic and biogenic molecular targets including polycyclic aromatic hydrocarbons, aromatic amino acids, nucleic acids, photosynthetic pigments, and critical metabolic components such as flavin adenine dinucleotide and nicotinamide adenine dinucleotide exhibit strong, distinctive fluorescent signatures following excitation by ultraviolet (UV) light sources. These fluorescence signatures are easily imaged with camera systems currently employed on Mars rovers and imaging equipment available during human or robotic exploration of remote extreme environments on Earth. In this paper we discuss recent results with epifluorescent imaging of organic and biological targets using filter bands comparable to those available to ExoMars and review plans for epifluorescence surveys of the Dry Valleys of Eastern Antarctica including the Schirmacher Oasis and the perennially ice-covered, merimictic, oligotrophic Lake Untersee.

We investigate the utility of adding a 365 nm ultraviolet (UV) light source to the ExoMars panoramic camera (PanCam) scheduled for launch in 2013. The modification makes it feasible to monitor rover drill cuttings for aromatic organic molecules and provide constraints on polycyclic aromatic hydrocarbons (PAH) as a function of depth to the 2-meter limit of the ExoMars drill. This non-destructive triage allows prioritized deployment of organic detection experiments requiring sample destruction and/or expenditure of non-replaceable resources. Utilizing the Beagle 2 PanCam backup filter wheel fitted with original blue (440 nm), green ( 530 nm), and red ( 670 nm) filters we captured fluorescent images following 365 nm excitation of 3-, 4- and 5-ring PAH species doped on Mars analog peridotite grains. We demonstrate a detection limit for pyrene of 1.5 mu g in granular peridotite doped at pyrene levels of 50 +/- 5 ppm for camera-to-target distance of 1 meter.

Back-arc and marginal basins make up a significant portion of the earth's crust and they can represent the transition from continental to oceanic crust. The Bransfield Strait is a young marginal basin of the arc-trench system that lies off the northwestern edge of the Antarctic Peninsula. The strait is about 65 km wide and has a maximum water depth of 2000 m. “Active” volcanoes in the Bransfield Strait include two seamounts, which are south of the eastern end of King George Island, and three island volcanoes — Penguin, Deception, and Bridgeman Islands. Alkaline and calc-alkaline suites occur on these islands, and the seamounts are composed of tholeiites and basaltic andesites. This diversity is similar to that found in some back-arc basins, but the Bransfield Strait basalts as a group cannot be classified as back-arc basin or island-arc basalts. The diverse rock types and the chemical similarity of some of the Bransfield Strait basalts to ophiolite basalts suggests that some ophiolites were generated in back-arc basins.

We examined the phylogenetic diversity of microbial communities associated with marine basalts, using over 300 publicly available 16S rDNA sequences and new sequence data from basalt enrichment cultures. Phylogenetic analysis provided support for 11 monophyletic clades originating from ocean crust (sediment, basalt and gabbro). Seven of the ocean crust clades (OCC) are bacterial, while the remaining four OCC are in the Marine Group I (MGI) Crenarchaeota. Most of the OCC were found at diverse geographic sites, suggesting that these microorganisms have cosmopolitan distributions. One OCC in the Crenarchaeota consisted of sequences derived entirely from basalts. The remaining OCC were found in both basalts and sediments. The MGI Crenarchaeota were observed in all studies where archaeal diversity was evaluated. These results demonstrate that basalts are occupied by cosmopolitan clades of microorganisms that are also found in marine sediments but are distinct from microorganisms found in other marine habitats, and that one OCC in the ubiquitous MGI Crenarchaeota clade may be an ecotype specifically adapted to basalt.

The subsurface biosphere may constitute as much as 50 percent of Earth's biomass. Direct and indirect evidence suggests that an extensive biosphere exists in the rocks below the sea floor. This survey of basalts of the Atlantic, Pacific, and Indian Oceans supports the hypothesis that bacteria have colonized much of the upper oceanic crust, which has a volume estimated at 10 18 cubic meters. Although this is the largest habitat on Earth, its low abundance of bacteria constitutes much less than 1 percent of Earth's biomass.

Three quarters of Earth's near surface rock is volcanic and virtually all of it has persistent or intermittent exposure to water. Hydration of the primary igneous silicate minerals (feldspar, pyroxene, olivine, and amorphous glass) and chemical exchange between these minerals and water produces secondary phyllosilicate minerals. The mineral-water interface is energy rich and supports diverse microbial communities that take up residence along cooling cracks and fractures. Microbes bore into minerals and leave trace fossils and organic evidence of their presence. The Martian surface is also dominated by volcanic rocks and some of these have been exposed to water long enough for phyllosilicates to form. These phyllosilicates are found in some Martian meteorites and a widespread distribution of phyllosilicates is indicated by reflected infrared light from some areas of Mars. As microbial trace fossils have been preserved for billions of years on Earth, if life ever existed at water-rock interfaces on Mars, then evidence of this life will have been preserved in the rocks. Areas of Mars that are likely to contain rocks that once were in contact with water can be located with orbital imagery. A rover on the Martian surface can locate outcrops likely to contain evidence of water-rock interaction based on the geological context and outcrop morphology. Examination of prepared surfaces of outcrops with a microscopic imager could reveal microbial trace fossils. Chemical analysis of the same surfaces prepared for microscopic imaging could reveal complex organic compounds. Here we report on a strategy for evaluating landing sites on Mars for their potential for containing evidence of microbial activity in volcanic rocks.

Analysis of spectral and imaging data from meteoritic samples and sample return missions would benefit significantly from a systematic, quantitative statistical classification methodology and a common set of standards for data collection [McDonald and Storrie-Lombardi, 2006]. Stochastic artificial neural networks can be trained using elemental abundance distributions for the detection of macroscopic fossils [Storrie-Lombardi and Hoover, 2004] and extant microbial life [Storrie-Lombardi and Hoover, 2005]. These non-linear algorithms are particularly attractive since they can produce a Bayesian estimate of the classification accuracy of either human experts or automated, unsupervised classification algorithms. In sub-ocean and surface basalts on earth the networks can distinguish regions of biotic and abiotic alteration of basalt glass from unaltered samples using only elemental abundances as inputs [Storrie-Lombardi and Fisk, 2004b]. Recently, evidence has been presented documenting the presence of morphologic signatures in the Mars meteorite Nakhla [Fisk et al., 2004; Fisk et al., 2006] previously noted in regions of biotic alteration in sub-ocean and surface terrestrial basalts [Fisk et al., 2003; Furnes et al., 2004]. The tunneling alterations are not conclusive evidence of biotic alteration of Nakhla on Mars. However, the meteorite is well known to have experienced aqueous alteration prior to arrival on earth and is rich in carbon [Gibson et al., 2006; McKay et al., 2006]. We here present an initial application of our probabilistic classification strategy to assess elemental abundance distributions from multiple target regions in Nakhla and lunar dust samples collected by Apollo 17 astronauts. We present scanning electron microscope images and elemental abundance point distributions (C, N, O, Na2O, MgO, Al2O3, SiO2, P2O5, S, Cl, K2O, CaO, and FeO) for a series of target regions. We discuss our observations in the context of data previously presented in these meetings for extant cyanobacteria, fossil trilobites, Orgueil meteorite, and terrestrial basalt targets. These data are being added to a database that will made available to the biogeology and astrobiology communities as part of an ongoing effort to provide a quantitative probabilistic methodology for analysis of putative elemental abundance geobiological signatures.

Microbial activity in subsurface basalts produces characteristic alteration in the volcanic glass readily observed in petrographic microphotographs 1 . Basalts containing volcanic glass cored from three different sites beneath the ocean floor along a mid-ocean ridge exhibiting such alteration was determined to also exhibit co-registered regions of elemental abundance alteration. 2 Further investigation one of these sites presenting examples of all three states (unaltered glass and glass altered to produce clays by abiotic and biotic processes) now reveals that lossless compression analysis 3 of the RGB photomicrographs detects significant differences between altered and unaltered basalt glass. The technique can distinguish images of all three groups without knowledge of the opinion of the expert human observer. Electron microprobe elemental abundance measurements were obtained from sites in all three regions. Principal Component Analysis using eleven elements as input (Na 11 , Mg 12 , Al 13 , Si 14 , P 15 , Cl 17 , K 19 , Ca 20 , Ti 22 , Mn 25 , and Fe 26 ) extracted two factors accounting for more than 75% of the data variance. When these same elements were used as inputs to a Hierarchical Cluster Analysis algorithm to provide unsupervised sample classification all samples clustered as predicted both by human visual observation and complexity analysis prediction. Finally, the PCA components extracted were used as inputs to a stochastic, non-linear and Artificial Neural Network to produce a Bayesian probability of correct classification. The technique provides a quantitative methodology for discrimination of biotic and abiotic alteration of sub-ocean basalt glass using both structural and chemical information.

[1] The Hawaii Scientific Drilling Program (HSDP) cored and recovered igneous rock from the surface to a depth of 3109 m near Hilo, Hawaii. Much of the deeper parts of the hole is composed of hyaloclastite (fractured basalt glass that has been cemented in situ with secondary minerals). Some hyaloclastite units have been altered in a manner attributed to microorganisms in volcanic rocks. Samples from one such unit (1336 m to 1404 m below sea level) were examined to test the hypothesis that the alteration was associated with microorganisms. Deep ultraviolet native fluorescence and resonance Raman spectroscopy indicate that nucleic acids and aromatic amino acids are present in clay inside spherical cavities (vesicles) within basalt glass. Chemical mapping shows that phosphorus and carbon were enriched at the boundary between the clay and volcanic glass of the vesicles. Environmental scanning electron microscopy (ESEM) reveals two to three micrometer coccoid structures in these same boundaries. ESEM-linked energy dispersive spectroscopy demonstrated carbon, phosphorous, chloride, and magnesium in these bodies significantly differing from unoccupied neighboring regions of basalt. These observations taken together indicate the presence of microorganisms at the boundary between primary volcanic glass and secondary clays. Amino acids and nucleic acids were extracted from bulk samples of the hyaloclastite unit. Amino acid abundance was low, and if the amino acids are derived from microorganisms in the rock, then there are less than 100,000 cells per gram of rock. Most nucleic acid sequences extracted from the unit were closely related to sequences of Crenarchaeota collected from the subsurface of the ocean floor.

[1] Bransfield Strait is a Quaternary, ensialic back arc basin at the transition from rifting to spreading. Fresh volcanic rocks occur on numerous submarine features distributed along the rift axis, including a discontinuous neovolcanic ridge similar to the nascent spreading centers seen in some other back arc basins. Smaller edifices near the northeast end of the rift yielded basalts with the most arc-like compositions (e.g., high large-ion lithophile element/high field strength element and 87 Sr/ 86 Sr). The most mid-ocean ridge basalt (MORB)-like basalts are from a large, caldera-topped seamount and a 30-km-long axial neovolcanic ridge toward the southwest end of the rift, but these two features also yielded andesite and rhyolite, respectively. The volcanic and geochemical variations are not systematic along axis and do not reflect the unidirectional propagation of rifting suggested by geophysical data. The most depleted basalts have major and trace element characteristics indistinguishable from MORB except for slightly higher Cs and Pb concentrations. Pb isotopic ratios show little variation compared to Sr and Nd isotopic ratios and do not extend to the depleted Pb isotopic ratios found in other back arc basins. Either the depleted mantle beneath Bransfield Strait has higher than normal Pb isotopic ratios or the subducted component beneath Bransfield Strait has such high Pb concentrations that it dominates the Pb isotopic composition of the Bransfield Strait mantle without significantly affecting the Sr and Nd isotopic compositions. Metalliferous sediments and fluids extracted from a subducting slab may have the necessary high concentrations of Pb. INDEX TERMS: 1040 Geochemistry: Isotopic composition/chemistry; 3640 Mineralogy and Petrology: Igneous petrology; 3655 Mineralogy and Petrology: Major element composition; 3670 Mineralogy and Petrology: Minor and trace element composition; KEYWORDS: back arc basin, Bransfield Strait, Antarctica, geochemistry, volcanism, recycling

When a mantle plume interacts with a mid-ocean ridge, both are noticeably affected. The mid-ocean ridge can display anomalously shallow bathymetry, excess volcanism, thickened crust, asymmetric sea-floor spreading and a plume component in the composition of the ridge basalts. The hotspot-related volcanism can be drawn closer to the ridge, and its geochemical composition can also be affected. Here we present Sr-Nd-Pb isotopic analyses of samples from the next-to-oldest seamount in the Hawaiian hotspot track, the Detroit seamount at 51 degrees N, which show that, 81 Myr ago, the Hawaiian hotspot produced volcanism with an isotopic signature indistinguishable from mid-ocean ridge basalt. This composition is unprecedented in the known volcanism from the Hawaiian hotspot, but is consistent with the interpretation from plate reconstructions that the hotspot was located close to a mid-ocean ridge about 80 Myr ago. As the rising mantle plume encountered the hot, low-viscosity asthenosphere and hot, thin lithosphere near the spreading centre, it appears to have entrained enough of the isotopically depleted upper mantle to overwhelm the chemical characteristics of the plume itself. The Hawaiian hotspot thus joins the growing list of hotspots that have interacted with a rift early in their history.

as a plat-form for deep-biosphere research requires that the recovered cores aresuitable for microbiological study. The major concern is whether mi-crobes from the drilling fluid are introduced into the recovered corematerial during coring. Therefore, it is critical to verify whether recov-ered cores are contaminated. Here we present details of two tracermethods used to quantify the amount of contamination. These meth-ods were modified from land-based drilling operations for use on the

Bransfield Strait is a marginal basin landward of the South Shetland Trench. It lies between the South Shetland Islands and the tip of the Antarctic Peninsula and is an example of an extensional basin formed by rifting within a continental volcanic arc. The Antarctic Peninsula is the product of at least 200 m.y. of subduction with the majority of the exposed rocks related to continental arc volcanism older than 20 Ma. Volcanism in Bransfield Strait started by 0.3 Ma and continues today. This new volcanism maintains some of the chemical signatures of the old arc volcanism but also has signatures transitional between arc rocks and backarc basin rocks. On the basis of high heat flow, active volcanism, extensional faulting, and earthquake fault plane mechanisms, Bransfield Strait is an active extensional basin forming within the Antarctic Peninsula. Seismic refraction work in Bransfield Strait indicates some thinning of continental crust, but the basin itself is underlain by as much as 30 km of anomalous crustal material. The observed extension seems to be confined to Bransfield Strait, which is bounded by the landward projections of the Hero and Shackleton fracture zones. The present extension in Bransfield Rift started less than 4 m.y. ago, and possibly less than 1.5 m.y. ago, following the demise of the Antarctic-Phoenix spreading center (ANT-PHO ridge), which ceased spreading about 4 Ma. Apparent, continued subduction at the South Shetland Trench after the ANT-PHO ridge stopped spreading may occur as trench rollback. The amount of trench rollback should be comparable to the amount of extension in Bransfield Strait.