Impact-generated Hydrothermal Systems Research Articles

Evolution of impact-generated hydrothermal systems in basaltic targets on Earth and implications for habitats on Mars

Impact-generated hydrothermal systems are a potential habitat for life on Earth and other planetary bodies. The ubiquity of impact craters on Mars makes them of particular interest for the search for life on this planet although their viability as a habitat is not well understood. To better understand such systems, two analogue impact structures on Earth were investigated, as their dominantly basaltic target rock makes them similar to impact structures in the Martian crust: the Vista Alegre and Vargeão Dome impact structures in Brazil.The goals of this paper are to 1) better understand the Vista Alegre and Vargeão Dome impact structures, and 2) understand how impact-generated hydrothermal systems on Earth can be compared with similar systems on Mars. To enable comparison, the software HYDROTHERM is used to reconstruct the hydrothermal evolution of both impact systems on Earth. Permeability, porosity, thermal conductivity, and initial temperature distributions are varied to simulate slow--, medium-, and fast-cooling models. The medium cooling rate models are then adapted to Martian conditions and scaled to 1×, 2×, 4×, and 8× the size on Earth, also adapting the impact energy and initial heat distribution, to check the comparability using similar target rocks. Finally, target rocks are changed to a basaltic composition only, to check how a lower-permeability Martian surface composition would affect the hydrothermal system.The models indicate between 150 and 650 thousand years (ka) of hydrothermal activity for both the Vista Alegre and Vargeão Dome impact structures. Due to the limited availability of water in the initial, high-temperature situation in the crater centre, water flux first increases and then decreases, although after 650 kyr the water flux towards the crater centre remains active in Vargeão Dome only. This sustained water flux is likely related to the low relative pressure in the crater centre due to the lack of overlying rock in comparison to outside the crater, as well as the direct connection of an underlying sandstone aquifer with high-permeability, fractured basalt in the centre of the structure.The Martian models with the same lithologies as on Earth show overall lower water flux and contain permafrost, which prevents the water flux within the aquifer from outside the crater towards the crater centre as seen in Vargeão Dome. The cooling times increase with scaling from 1× to 8× the size to ∼0.2 and 5 Myr, respectively. With only basalts in the target, cooling times reach ∼0.2 to 9 Myr. However, in that case, the water flow only remains active for up to the first Myr, and a minimum crater diameter of approximately 40 km is needed to achieve surface water fluxes close to those in Vargeão Dome and Vista Alegre.Below the Martian surface, hydrothermal system activity can persist for millions of years, providing potential for supporting life. Unfortunately, these parts of the system are currently unreachable for both sampling and spectroscopic techniques, and the short lifetime and low water fluxes at the surface related to the permafrost layers do not favour impact-generated hydrothermal systems on present-day Mars as targets for the search for life. However, these systems are highly dependent on the rock types and related permeabilities and could have provided habitats before 3.5 Ga, when the planet was warmer and liquid water was present at the surface.

Hydrothermal alteration at the basalt‐hosted Vista Alegre impact structure, Brazil

AbstractHydrothermal systems provide a possible habitat for early life and are key targets in the quest for life outside Earth. In impact craters on Mars, hydrous minerals can represent products of impact‐generated hydrothermal systems (IGHS) or minerals already present in the crust and exposed during impact‐caused excavation. Because of its basaltic target rock, similar in composition to Martian crust, the Vista Alegre impact structure in Brazil is one of the very few analog structures that may reveal the origin of these minerals, if evidence of hydrothermal alteration is established. This work presents the results of a systematic search for evidence of hydrothermal alteration at the Vista Alegre impact structure. Four types of alteration were identified, all within a 2.5–3.0 km radius from the crater center: a zircon‐bearing melt veinlet, two sets of hydrothermal veins consisting predominantly of calcite and chabazite, and local alteration comprising saponite. Thermodynamic modeling suggests subsequent heating and cooling for each of the hydrothermal vein sets. Combined thermodynamic and spectrometric evidence indicates that development of a vigorous IGHS is unlikely. If similar processes occur on Mars, hydrous minerals are more likely preimpact phases exposed by excavation, rather than being formed through an IGHS.

Hydrogen Production from Alteration of Chicxulub Crater Impact Breccias: Potential Energy Source for a Subsurface Microbial Ecosystem.

A sulfate-reducing population of thermophiles grew in porous, permeable niches within glass-bearing impact breccias of the Chicxulub impact crater. The microbial community grew in an impact-generated hydrothermal system that vented on the seafloor several hundred meters beneath the sea surface. Potential electron donors for that metabolism are hydrocarbons, although a strong C-isotope signature of that source does not exist. Model calculations explored here suggest that alteration of glass within the impact breccias may have produced H2 in sufficient quantities for population growth as the hydrothermal system cooled through thermophilic temperatures, although it is sensitive to the oxidation state of iron in the melt rock prior to hydrothermal alteration and the secondary mineral assemblage. At high water-to-rock ratios and temperatures below 45°C, H2 yields are insufficient to maintain a population of hydrogenotrophic sulfate-reducing bacteria, but yields double with a higher proportion of ferrous iron between 45 and 65°C. The most reduced rocks (i.e., highest proportion of ferrous iron) that are allowed to form andradite, which is observed in core samples, produce copious amounts of H2 in the temperature window for thermophiles and hyperthermophiles. Mixtures of melt rock and carbonate, which is observed in breccia matrices, produce somewhat less H2, and the onset of massive H2 production is shifted to higher temperatures (i.e., lower W/R).

Stability of α-ketoglutaric acid simulating an impact-generated hydrothermal system: implications for prebiotic chemistry studies

AbstractLife originated on Earth possibly as a physicochemical process; thus, geological environments and their hypothetical characteristics on early Earth are essential for chemical evolution studies. Also, it is necessary to consider the energy sources that were available in the past and the components that could have contributed to promote chemical reactions. It has been proposed that the components could have been mineral surfaces. The aim of this work is to determine the possible role of mineral surfaces on chemical evolution, and to study of the stability of relevant molecules for metabolism, such as α-ketoglutaric acid (α-keto acid, Krebs cycle participant), using ionizing radiation and thermal energy as energy sources and mineral surfaces to promote chemical reactions. Preliminary results show α-ketoglutaric acid can be relatively stable at the simulated conditions of an impact-generated hydrothermal system; thus, those systems might have been plausible environments for chemical evolution on Earth.

Re-Evaluating the Age of Deep Biosphere Fossils in the Lockne Impact Structure

Impact-generated hydrothermal systems have been suggested as favourable environments for deep microbial ecosystems on Earth, and possibly beyond. Fossil evidence from a handful of impact craters worldwide have been used to support this notion. However, as always with mineralized remains of microorganisms in crystalline rock, certain time constraints with respect to the ecosystems and their subsequent fossilization are difficult to obtain. Here we re-evaluate previously described fungal fossils from the Lockne crater (458 Ma), Sweden. Based on in-situ Rb/Sr dating of secondary calcite-albite-feldspar (356.6 ± 6.7 Ma) we conclude that the fungal colonization took place at least 100 Myr after the impact event, thus long after the impact-induced hydrothermal activity ceased. We also present microscale stable isotope data of 13C-enriched calcite suggesting the presence of methanogens contemporary with the fungi. Thus, the Lockne fungi fossils are not, as previously thought, related to the impact event, but nevertheless have colonized fractures that may have been formed or were reactivated by the impact. Instead, the Lockne fossils show similar features as recent findings of ancient microbial remains elsewhere in the fractured Swedish Precambrian basement and may thus represent a more general feature in this scarcely explored habitat than previously known.

Fungal colonization of an Ordovician impact-induced hydrothermal system.

Impacts are common geologic features on the terrestrial planets throughout the solar system, and on at least Earth and Mars impacts have induced hydrothermal convection. Impact-generated hydrothermal systems have been suggested to possess the same life supporting capability as hydrothermal systems associated with volcanic activity. However, evidence of fossil microbial colonization in impact-generated hydrothermal systems is scarce in the literature. Here we report of fossilized microorganisms in association with cavity-grown hydrothermal minerals from the 458 Ma Lockne impact structure, Sweden. Based on morphological characteristics the fossilized microorganisms are interpreted as fungi. We further infer the kerogenization of the microfossils, and thus the life span of the fungi, to be contemporaneous with the hydrothermal activity and migration of hydrocarbons in the system. Our results from the Lockne impact structure show that hydrothermal systems associated with impact structures can support colonization by microbial life.

Alteration minerals in impact-generated hydrothermal systems – Exploring host rock variability

Impact-generated hydrothermal systems have been previously linked to the alteration of Mars’ crust and the production of secondary mineral assemblages seen from orbit. The sensitivity of the resultant assemblages has not yet been evaluated as a function of precursor primary rock compositions. In this work, we use thermochemical modeling to explore the variety of minerals that could be produced by altering several known lithologies based on martian meteorite compositions. For a basaltic host rock lithology (Dhofar 378, Humphrey) the main alteration phases are feldspar, zeolite, pyroxene, chlorite, clay (nontronite, kaolinite), and hematite; for a lherzolithic host rock lithology (LEW 88516) the main alteration phases are amphibole, serpentine, chlorite, clay (nontronite, kaolinite), and hematite; and for an ultramafic host rock lithology (Chassigny) the main minerals are secondary olivine, serpentine, magnetite, quartz, and hematite. These assemblages and proportions of phases in each of those cases depend on W/R and temperature. Integrating geologic, hydrologic and alteration mineral evidence, we have developed a model to illustrate the distribution of alteration assemblages that occur in different levels of an impact structure. At the surface, hot, hydrous alteration affects the ejecta and melt sheet producing clay and chlorite. Deeper in the subsurface and depending on the permeability of the rock, a variety of minerals – smectite, chlorite, serpentine, amphiboles and hematite – are produced in a circulating hydrothermal system. These modeled mineral distributions should assist with interpretation of orbital observations and help guide surface exploration by rovers and sample return assets.

Impact-generated hydrothermal systems on Earth and Mars

It has long been suggested that hydrothermal systems might have provided habitats for the origin and evolution of early life on Earth, and possibly other planets such as Mars. In this contribution we show that most impact events that result in the formation of complex impact craters (i.e., >2–4 and >5–10km diameter on Earth and Mars, respectively) are potentially capable of generating a hydrothermal system. Consideration of the impact cratering record on Earth suggests that the presence of an impact crater lake is critical for determining the longevity and size of the hydrothermal system. We show that there are six main locations within and around impact craters on Earth where impact-generated hydrothermal deposits can form: (1) crater-fill impact melt rocks and melt-bearing breccias; (2) interior of central uplifts; (3) outer margin of central uplifts; (4) impact ejecta deposits; (5) crater rim region; and (6) post-impact crater lake sediments. We suggest that these six locations are applicable to Mars as well. Evidence for impact-generated hydrothermal alteration ranges from discrete vugs and veins to pervasive alteration depending on the setting and nature of the system. A variety of hydrothermal minerals have been documented in terrestrial impact structures and these can be grouped into three broad categories: (1) hydrothermally-altered target-rock assemblages; (2) primary hydrothermal minerals precipitated from solutions; and (3) secondary assemblages formed by the alteration of primary hydrothermal minerals. Target lithology and the origin of the hydrothermal fluids strongly influences the hydrothermal mineral assemblages formed in these post-impact hydrothermal systems. There is a growing body of evidence for impact-generated hydrothermal activity on Mars; although further detailed studies using high-resolution imagery and multispectral information are required. Such studies have only been done in detail for a handful of martian craters. The best example so far is from Toro Crater (Marzo, G.A., Davila, A.F., Tornabene, L.L., Dohm, J.M., Fairèn, A.G., Gross, C., Kneissl, T., Bishop, J.L., Roush, T.L., Mckay, C.P. [2010]. Icarus 208, 667–683). We also present new evidence for impact-generated hydrothermal deposits within an unnamed ∼32-km diameter crater ∼350km away from Toro and within the larger Holden Crater. Synthesizing observations of impact craters on Earth and Mars, we suggest that if there was life on Mars early in its history, then hydrothermal deposits associated with impact craters may provide the best, and most numerous, opportunities for finding preserved evidence for life on Mars. Moreover, hydrothermally altered and precipitated rocks can provide nutrients and habitats for life long after hydrothermal activity has ceased.

Putative fossil life in a hydrothermal system of the Dellen impact structure, Sweden

AbstractImpact-generated hydrothermal systems are commonly proposed as good candidates for hosting primitive life on early Earth and Mars. However, evidence of fossil microbial colonization in impact-generated hydrothermal systems is rarely reported in the literature. Here we present the occurrence of putative fossil microorganisms in a hydrothermal system of the 89 Ma Dellen impact structure, Sweden. We found the putative fossilized microorganisms hosted in a fine-grained matrix of hydrothermal alteration minerals set in interlinked fractures of an impact breccia. The putative fossils appear as semi-straight to twirled filaments, with a thickness of 1–2 μm, and a length between 10 and 100 μm. They have an internal structure with segmentation, and branching of filaments occurs frequently. Their composition varies between an outer and an inner layer of a filament, where the inner layer is more iron rich. Our results indicate that hydrothermal systems in impact craters could potentially be capable of supporting microbial life. This could have played an important role for the evolution of life on early Earth and Mars.

Impact-generated hydrothermal systems capable of forming phyllosilicates on Noachian Mars

The early history of the terrestrial planets was punctuated by a period of heavy bombardment. Frequent large impacts condensed in a short, 0.02–0.5-Ga-long time period resulted in heavily cratered planetary surfaces. In volatile-bearing planetary crusts, each of these large impacts created a volume of hot rock and melt that drove vast subsurface hydrothermal systems. In Mars' basaltic crust, these systems produced a variety of alteration phases, the nature of which we explored with thermochemical modeling. Using the computer code CHILLER, we found a variety of oxides, hydroxides, and hydrous and water-free silicates resulting from the hydrothermal alteration. The main hydrous silicates are serpentine, chlorite, nontronite, and other clay minerals. Some of the resulting assemblages contain up to 15 wt% bound water. Our results constrain the temperature and water rock ratio at the time of alteration and compare well to the mineral assemblages found on Mars by OMEGA (Observatoire pour la Mineralogie, l'Eau, les Glaces, et l'Activite) and CRISM (Compact Reconnaissance Imaging Spectrometer for Mars) and, therefore, provide an interpretative framework for analyzing Martian mineralogy from orbital data. Our results also provide a geological template needed to select landing locations for future missions that search for water and energy sources associated with potential habitats on early Mars.

Vitric Compositions in the Onaping Formation and Their Relationship to the Sudbury Igneous Complex, Sudbury Structure

The 1.85 Ga Sudbury structure is the largest, well-preserved, exposed example of a terrestrial impact crater; it contains world-class Ni-Cu-platinum-group element (PGE) ores associated with the igneous component. A new understanding of the geology and geochemistry of the vitric crater-fill sequence, the Onaping Formation, provides constraints on the evolution of 1.85 Ga igneous rocks in the Sudbury structure. The Onaping Formation has a well-defined mappable stratigraphy composed of three members, seven stratigraphic units, and two types of intraformational dikes that are overprinted by an impact-generated hydrothermal system. Stratigraphic and alteration mapping provided a basis for well-constrained sampling and allowed the definition of the geochemical characteristics of the less altered rocks from each unit, including vitric bombs and blocks and intraformational vitric dikes. Significantly, the identification of a least altered composition of the vitric Onaping Formation enabled an investigation of the link between the early formed vitric melt phase in the impact crater-fill sequence and the Sudbury Igneous Complex and offset dikes. The least altered vitric Onaping Formation, represented by the massive, xenolith-poor cores of vitric bombs, blocks, and intraformational dikes, is andesitic in composition, consistent with a crustally derived melt. The melt composition (61.6 wt % SiO 2 , 4.28 MgO, 60 ppm Ni) is remarkably uniform throughout the Onaping Formation with a typical enrichment of large ion lithophile and light rare earth elements and pronounced negative Nb and Ti anomalies. Incompatible trace element ratios and bulk concentrations of the Onaping Formation coincide with the main mass norite and offset quartz diorite compositions of the underlying Sudbury Igneous Complex but differ distinctly from the granophyre and quartz gabbro. Root mean squared differences in composition of the vitric Onaping Formation with the units of the Sudbury Igneous Complex and quartz diorite offset dikes show that the vitric Onaping Formation does not directly match any of these igneous units but more closely resembles the composition of the Ministic and Manchester offset dikes. The quartz diorite offset dikes have been proposed as representative of the bulk composition of the original melt of the Sudbury Igneous Complex. In detail, there are small compositional differences between North and South Range offset dikes and between these offset dikes and the concentric Manchester offset dike; however, such differences are lacking in the Onaping Formation. We propose that the least altered vitric composition of the Onaping Formation represents the best estimate of the bulk composition of an initial quenched melt, or shock melt, formed early in the evolution of the Sudbury structure and is a logical starting liquid to model the evolution of the Sudbury main mass and offset dikes. The offset dike compositions can be derived from Onaping Formation vitric shock melt by assimilation of progressively greater amounts of mafic material at deeper levels in the crater. This mafic-contaminated melt, or impact melt, developed at the base of the melt sheet and was injected into the offset dikes both prior to and contemporaneously with the earliest stages of sulfide precipitation from the Sudbury Complex. Fractionation of the mafic-contaminated melt by plagioclase and pyroxene accumulation produced the North Range felsic and mafic norite and the South Range norites. The chalcophile metal content of the Onaping Formation vitric phase closely reflects the overall metal ratios of the magmatic sulfide ores. The vitric Onaping Formation and unmineralized offset dikes have low liquidus temperatures (1,115°–1,125°C) and, if these units represent the source of metals in the sulfide deposits, unusually high mass distribution coefficients for Cu and Ni and high R factors are necessary to account for the metal endowment.