Martin R. Fisk

Curiosity at Gale Crater, Mars: Characterization and Analysis of the Rocknest Sand Shadow

The Rocknest aeolian deposit is similar to aeolian features analyzed by the Mars Exploration Rovers (MERs) Spirit and Opportunity. The fraction of sand <150 micrometers in size contains ~55% crystalline material consistent with a basaltic heritage and ~45% x-ray amorphous material. The amorphous component of Rocknest is iron-rich and silicon-poor and is the host of the volatiles (water, oxygen, sulfur dioxide, carbon dioxide, and chlorine) detected by the Sample Analysis at Mars instrument and of the fine-grained nanophase oxide component first described from basaltic soils analyzed by MERs. The similarity between soils and aeolian materials analyzed at Gusev Crater, Meridiani Planum, and Gale Crater implies locally sourced, globally similar basaltic materials or globally and regionally sourced basaltic components deposited locally at all three locations.

Soil Diversity and Hydration as Observed by ChemCam at Gale Crater, Mars

The ChemCam instrument, which provides insight into martian soil chemistry at the submillimeter scale, identified two principal soil types along the Curiosity rover traverse: a fine-grained mafic type and a locally derived, coarse-grained felsic type. The mafic soil component is representative of widespread martian soils and is similar in composition to the martian dust. It possesses a ubiquitous hydrogen signature in ChemCam spectra, corresponding to the hydration of the amorphous phases found in the soil by the CheMin instrument. This hydration likely accounts for an important fraction of the global hydration of the surface seen by previous orbital measurements. ChemCam analyses did not reveal any significant exchange of water vapor between the regolith and the atmosphere. These observations provide constraints on the nature of the amorphous phases and their hydration.

The Petrochemistry of Jake_M: A Martian Mugearite

“Jake_M,” the first rock analyzed by the Alpha Particle X-ray Spectrometer instrument on the Curiosity rover, differs substantially in chemical composition from other known martian igneous rocks: It is alkaline (>15% normative nepheline) and relatively fractionated. Jake_M is compositionally similar to terrestrial mugearites, a rock type typically found at ocean islands and continental rifts. By analogy with these comparable terrestrial rocks, Jake_M could have been produced by extensive fractional crystallization of a primary alkaline or transitional magma at elevated pressure, with or without elevated water contents. The discovery of Jake_M suggests that alkaline magmas may be more abundant on Mars than on Earth and that Curiosity could encounter even more fractionated alkaline rocks (for example, phonolites and trachytes).

Major- and trace-element magnetite-melt equilibria

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.

Sources of nutrients and energy for a deep biosphere on Mars

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 Earths 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.

First Investigation of the Microbiology of the Deepest Layer of Ocean Crust

The gabbroic layer comprises the majority of ocean crust. Opportunities to sample this expansive crustal environment are rare because of the technological demands of deep ocean drilling; thus, gabbroic microbial communities have not yet been studied. During the Integrated Ocean Drilling Program Expeditions 304 and 305, igneous rock samples were collected from 0.45-1391.01 meters below seafloor at Hole 1309D, located on the Atlantis Massif (30 °N, 42 °W). Microbial diversity in the rocks was analyzed by denaturing gradient gel electrophoresis and sequencing (Expedition 304), and terminal restriction fragment length polymorphism, cloning and sequencing, and functional gene microarray analysis (Expedition 305). The gabbroic microbial community was relatively depauperate, consisting of a low diversity of proteobacterial lineages closely related to Bacteria from hydrocarbon-dominated environments and to known hydrocarbon degraders, and there was little evidence of Archaea. Functional gene diversity in the gabbroic samples was analyzed with a microarray for metabolic genes (“GeoChip”), producing further evidence of genomic potential for hydrocarbon degradation - genes for aerobic methane and toluene oxidation. Genes coding for anaerobic respirations, such as nitrate reduction, sulfate reduction, and metal reduction, as well as genes for carbon fixation, nitrogen fixation, and ammonium-oxidation, were also present. Our results suggest that the gabbroic layer hosts a microbial community that can degrade hydrocarbons and fix carbon and nitrogen, and has the potential to employ a diversity of non-oxygen electron acceptors. This rare glimpse of the gabbroic ecosystem provides further support for the recent finding of hydrocarbons in deep ocean gabbro from Hole 1309D. It has been hypothesized that these hydrocarbons might originate abiotically from serpentinization reactions that are occurring deep in the Earths crust, raising the possibility that the lithic microbial community reported here might utilize carbon sources produced independently of the surface biosphere.

High-field-strength element partitioning between pyroxene and basaltic to dacitic magmas

Abstract 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).

Near-primary melt inclusions in anorthite phenocrysts from the Galapagos Platfrom

Abstract 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 Al 2 O 3 , and low in TiO 2 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 Al 2 O 3 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.

Bransfield Strait, Antarctic Peninsula Active Extension behind a Dead Arc

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.

Isotopic and trace element constraints on mixing and melting models of marginal basin volcanism, Bransfield Strait, Antarctica

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.