Eugene G. Grosch

XMapTools: A MATLAB©-based program for electron microprobe X-ray image processing and geothermobarometry

XMapTools is a MATLAB^(C)-based graphical user interface program for electron microprobe X-ray image processing, which can be used to estimate the pressure-temperature conditions of crystallization of minerals in metamorphic rocks. This program (available online at http://www.xmaptools.com) provides a method to standardize raw electron microprobe data and includes functions to calculate the oxide weight percent compositions for various minerals. A set of external functions is provided to calculate structural formulae from the standardized analyses as well as to estimate pressure-temperature conditions of crystallization, using empirical and semi-empirical thermobarometers from the literature. Two graphical user interface modules, Chem2D and Triplot3D, are used to plot mineral compositions into binary and ternary diagrams. As an example, the software is used to study a high-pressure Himalayan eclogite sample from the Stak massif in Pakistan. The high-pressure paragenesis consisting of omphacite and garnet has been retrogressed to a symplectitic assemblage of amphibole, plagioclase and clinopyroxene. Mineral compositions corresponding to ~165,000 analyses yield estimates for the eclogitic pressure-temperature retrograde path from 25kbar to 9kbar. Corresponding pressure-temperature maps were plotted and used to interpret the link between the equilibrium conditions of crystallization and the symplectitic microstructures. This example illustrates the usefulness of XMapTools for studying variations of the chemical composition of minerals and for retrieving information on metamorphic conditions on a microscale, towards computation of continuous pressure-temperature-and relative time path in zoned metamorphic minerals not affected by post-crystallization diffusion.

Geochemistry and tectonic setting of mafic rocks in western Dronning Maud Land, East Antarctica: implications for the geodynamic evolution of the Proterozoic Maud Belt

On the basis of new bulk major and trace element (including REE) as well as Sm–Nd and Rb–Sr isotope data, used in conjunction with available geochronological data, a post-tectonic mafic igneous province and four groups of pre- to syntectonic amphibolite are distinguished in the polymetamorphic Maud Belt of western Dronning Maud Land, East Antarctica. Protoliths of the Group 1 amphibolites are interpreted as volcanic arc mafic intrusions with Archaean to Palaeoproterozoic Nd model ages and depletion in Nb and Ta. Isotopic and lithogeochemical characteristics of this earliest group of amphibolite indicate that the Maud Belt was once an active continental volcanic arc. The most likely position of this arc, for which a late Mesoproterozoic age (c. 1140 Ma) is indicated by available U–Pb single-zircon age data, was on the southeastern margin of the Kaapvaal–Grunehogna Craton. The protoliths of Group 2 amphibolites are attributed to the 1110 Ma Borgmassivet–Umkondo thermal event on the basis of comparable Nd model ages and trace element distributions. Group 3 amphibolite protoliths are characterized by mid-ocean ridge basalt-type REE patterns and low Th/Yb ratios, and they are related to Neoproterozoic extension. Group 4 amphibolite protoliths are distinguished by high Dy/Yb ratios and are attributed to a phase of syntectonic Pan-African magmatism as indicated by Rb–Sr isotope data.

Sulfur isotope evidence for a Paleoarchean subseafloor biosphere, Barberton, South Africa

The Archean sub-seafloor has been proposed as an environment for the emergence of life, with septate clusters of titanite microtextures in pillow lava rims argued to be the earliest traces of microbial microboring. Here we use nanoscale secondary ion mass spectrometry (NanoSIMS) to test possible geochemical traces of life in ca. 3.45 Ga pillow lavas of the Barberton Greenstone Belt, South Africa. Sulfide inclusions in the titanite microtextures record strongly negative sulfur isotope fractionations of δ 34 S VCDT –39.8‰ to –3.2‰ (VCDT—Vienna Canyon Diablo Troilite). These represent the largest range and most negative δ 34 S values so far reported from the Archean, and are consistent with an early biogenic origin for the sulfides. Extensive in situ elemental mapping did not find any organic linings associated with the microtextures, despite the high spatial resolution and sensitivity of the NanoSIMS. The absence of organic linings thus excludes a key line of evidence previously used to support the biogenicity of the microtextures. In contrast, in situ sulfur isotope analysis of basalt-hosted sulfides provides an alternative approach to investigating the existence and nature of an Archean subseafloor biosphere.

Reassessing the biogenicity of Earth’s oldest trace fossil with implications for biosignatures in the search for early life

Significance It has been argued that Archean subseafloor pillow lava sequences provide an environment in which to seek evidence for the earliest traces of life. Candidate titanite biosignatures of microbial activity have been reported in ∼3.45-Ga metavolcanic glass from the Barberton greenstone belt of South Africa. In this paper we present new in situ U–Pb age data, metamorphic constraints, and morphological observations on these titanite microtextures. Our data challenges a biological origin for these oldest purported trace fossils, with implications for the ecological niches where life may have first emerged. We therefore suggest alternative biosignatures and approaches should be considered in the search for subsurface life on early Earth and in extraterrestrial mafic–ultramafic rocks, for example, in martian basalts. Microtextures in metavolcanic pillow lavas from the Barberton greenstone belt of South Africa have been argued to represent Earth’s oldest trace fossil, preserving evidence for microbial life in the Paleoarchean subseafloor. In this study we present new in situ U–Pb age, metamorphic, and morphological data on these titanite microtextures from fresh drill cores intercepting the type locality. A filamentous microtexture representing a candidate biosignature yields a U–Pb titanite age of 2.819 ± 0.2 Ga. In the same drill core hornfelsic-textured titanite discovered adjacent to a local mafic sill records an indistinguishable U–Pb age of 2.913 ± 0.31 Ga, overlapping with the estimated age of intrusion. Quantitative microscale compositional mapping, combined with chlorite thermodynamic modeling, reveals that the titanite filaments are best developed in relatively low-temperature microdomains of the chlorite matrix. We find that the microtextures exhibit a morphological continuum that bears no similarity to candidate biotextures found in the modern oceanic crust. These new findings indicate that the titanite formed during late Archean ca. 2.9 Ga thermal contact metamorphism and not in an early ca. 3.45 Ga subseafloor environment. We therefore question the syngenicity and biogenicity of these purported trace fossils. It is argued herein that the titanite microtextures are more likely abiotic porphyroblasts of thermal contact metamorphic origin that record late-stage retrograde cooling in the pillow lava country rock. A full characterization of low-temperature metamorphic events and alternative biosignatures in greenstone belt pillow lavas is thus required before candidate traces of life can be confirmed in Archean subseafloor environments.

Questioning the biogenicity of titanite mineral trace fossils in Archean pillow lavas

Staudigel et al. (1) compare early Archean titanite microtextures to recent microtubules in Cenozoic volcanic seafloor glass to support a biogenic origin. However, given the 3.5 billion years of Earth history since eruption of the Archean lavas, many geological processes have affected these rocks, complicating the simple case for trace fossils. Using hollow and partially mineralized microtextures in modern seafloor basalt as an analog for argued microbial alteration of Archean glass is, in our opinion, a weak line of argument and an overextrapolated interpretation in support of biogenicity. The many assumptions required in their proposed bioalteration model are not supported by microbiological experiments or geological observations. For example, Staudigel et al. (1) require that hollow microbial tunnels are filled in by some process forming titanite, but when and how this occurs is not substantiated. The authors also contradict earlier work by abandoning organic carbon linings to the microtextures as evidence in support of biogenicity. Staudigel et al. provide no new data to support a biogenic origin, and we highlight that they have further complicated their lines of argument.

Metamorphic and age constraints on crustal reworking in the western H.U. Sverdrupfjella: implications for the evolution of western Dronning Maud Land, Antarctica

A petrological and metamorphic comparison of Mesoproterozoic metabasic rocks was conducted on the eastern margin of the Archaean Kaapvaal–Grunehogna Craton and the adjacent westernmost Maud Belt (western H.U. Sverdrupfjella), across a major structural discontinuity known as the Pencksökket–Jutulstraumen Discontinuity. Thermodynamic phase diagram modelling of the low- and high-grade metabasaltic assemblages on either side of the discontinuity revealed a difference in peak metamorphic conditions from T = 340 ± 25 to 700 ± 30°C and P = 0.29 ± 0.08 to 0.90 ± 0.10 GPa over an extrapolated orthogonal distance of 32 km across the strike of the discontinuity. Laser ablation inductively coupled plasma mass spectrometry U–Pb dating of titanite in a hornblende–plagioclase–quartz corona around garnet yielded a late Pan-African age of 491 ± 27 Ma for high-grade metamorphism in the western H.U. Sverdrupfjella of the Maud Belt. The new petrological and geochronological constraints indicate that peak upper amphibolite-facies conditions were achieved in the western H.U. Sverdrupfjella at c. 500 Ma; namely, c. 70–80 Ma after peak eclogite–high-pressure granulite-facies metamorphism in the eastern H.U. Sverdrupjella. The U–Pb age data argue against previous models that invoke only late Mesoproterozoic (c. 1060–1030 Ma) granulite-facies metamorphism in the western H.U. Sverdrupfjella and Kaapvaal–Grunehogna Craton margin, and support the concept of a major diachronous Pan-African orogenic episode. A new geodynamic model is presented for western Dronning Maud Land, involving earliest Pan-African eclogite–high-pressure granulite- and upper amphibolite-facies metamorphism in a separate eastern H.U. Sverdrupjella terrane between c. 565 and 540 Ma, followed by later diachronous tectonic accretion of the eastern H.U. Sverdrupjella arc segment onto the western H.U. Sverdrupfjella crustal segment (including the easternmost Kaapvaal–Grunehogna Craton margin) that involved only upper amphibolite-facies metamorphism at c. 500 Ma. The new petrological data indicate that the inferred sub-glacial boundary (Pencksökket–Jutulstraumen Discontinuity) between the Kaapvaal–Grunehogna Craton and the western H.U. Sverdrupfjella region represents a major cratonic Pan-African thrust, which was subsequently reactivated by normal faulting during Gondwana break-up. Supplementary materials: Analytical details, representative mineral compositions, and geothermometry and geobarometry results are available at http://www.geolsoc.org.uk/SUP18832.

A Tribute to Martin D. Brasier: Palaeobiologist and Astrobiologist (April 12, 1947-December 16, 2014).

The late Martin Brasier, emeritus professor of palaeobiology at the University of Oxford, was perhaps best known among the astrobiology community for his research on the Archean biosphere and for testing the oldest microfossil evidence in the rock record. But this would overlook the broad-ranging and multifaceted scientist that Martin was, with research interests spanning the entire history of life on Earth, which we have attempted to capture in this tribute. Martin was at the center of many important paleobiological debates over the last 40 years and contributed to our understanding of Earth’s biosphere at key transitions in Earth’s history. He was a keen advocate of the field of astrobiology, bringing his extensive geological and paleontological experience to bear. In this tribute we have compiled the reflections of several former students, international collaborators, and academic colleagues with the aim of describing Martin’s broad-ranging and far-reaching contributions. First we present a brief overview of Martin’s academic research, which is by no means intended to be exhaustive, before reporting the personal accounts of several scientists who had the privilege of working and/or interacting with Martin during his extensive career. Martin undertook a PhD at University College London on the ecology and microhabitats of modern benthic foraminifera, algae, and sea-grass communities of the Caribbean island of Barbuda. Martin was engaged as ship’s scientist aboard the Royal Navy ship HMS Fox and Fawn, an opportunity which he likened to Darwin’s position on the HMS Beagle, and during that time he undertook much fundamental research. Martin was both a conventional and unconventional micropaleontologist, as described in the contribution by Owen Green below. In the 1970s Martin undertook mathematical studies of foraminiferid morphospace to investigate the evolution of foraminifera photosymbioses through time. The concept of morphospace analysis was an approach that Martin would later return to in other areas of his research, for example, to test the biogenicity of carbonaceous microfossils, also to investigate the evolution and growth of the Ediacaran biota. In the 1980s while based at the University of Hull, Martin worked on Lower Cambrian reef systems, particularly on several expeditions to Mongolia, one important outcome of which was to reveal the sponge-like biology of archaeocyathids. At this time Martin was also undertaking an ecological and taphonomic assessment of the Cambrian diversification of skeletal fossils. Throughout his career Martin was an advocate of the high-resolution analysis of fossils in their sedimentological and geological context (see, e.g., the contribution by Duncan McIlroy below, reflecting on work to characterize the Ediacaran and Cambrian evolutionary radiations). In 1988 Martin moved to the Department of Earth Sciences at the University of Oxford and took a leading role in the International Geological Correlation Programme (IGCP), particularly in their work to formally define the Cambrian time period, and in the selection of the global stratotype section in Newfoundland. Martin worked extensively on Proterozoic geobiological evolution; for instance, he coined the phrase ‘‘the Boring Billion’’ to refer to the apparent evolutionary quiescence of the Mesoproterozoic. In Australia he worked with John Lindsay,

Biosignatures across Space and Time-Special Collection.

The Centre for Geobiology at the University of Bergen together with the Nordic Network of Astrobiology hosted an international conference ‘‘Biosignatures across Space and Time’’ between May 20 and 22, 2014 (Fig. 1). Delegates from more than 19 countries attended the 3-day meeting, with presentations on different aspects of biosignatures in the early Archean rock record, at extreme terrestrial sites, and in potential extraterrestrial environments. Session themes and key topics at the meeting included the question of life beyond Earth, how many Earth-sized planets might exist in the Universe, and whether life had its origin at hydrothermal vents. The meeting also addressed the evidence for biosignatures and habitable environments on early Earth and what this could potentially say about habitability in the wet subsurface of Mars and other rocky planets. The Biosignatures across Space and Time meeting was aimed at bringing a wide cross section of astrobiologists together to tackle these big questions by doing big science.