Compositional diversity and stratification of the Martian crust: Inferences from crystallization experiments on the picrobasalt Humphrey from Gusev Crater, Mars (original) (raw)
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The Dual Nature of the Martian Crust: Young Lavas and Old Clastic Materials
Visible and thermal infrared spacecraft datasets are used to gain insight into the nature of the surface materials and upper martian crust, revealing a distinct transition in the physical properties of martian crustal materials that occurred during the Hesperian era. Contrary to a prevailing view of the martian crust as primarily composed of lava flows, we find that most older regions of Mars have morphological and thermophysical properties consistent with poorly consolidated fine-particulate materials that may have a volcaniclastic origin. By contrast, younger surfaces contain blocky materials and thermophysical properties consistent with effusive lava flows. Explosive volcanism is likely to have been dominant on early Mars and these findings have implications for the evolution of the volatile content of the crust and mantle and subsequent development of the surface morphology. This dual nature of the crust appears to be a defining characteristic of martian history.
Lithos, 2016
Until recently, Mars was considered a basalt-covered world, but this vision is evolving thanks to new orbital, in situ and meteorite observations, in particular of rocks of the ancient Noachian period. In this contribution we summarise newly recognised compositional and mineralogical differences between older and more recent rocks, and explore the geodynamic implications of these new findings. For example the MSL rover has discovered abundant felsic rocks close to the landing site coming from the wall of Gale crater ranging from alkali basalt to trachyte. In addition, the recently discovered Martian regolith breccia NWA 7034 (and paired samples) contain many coarse-grained noritic-monzonitic clasts demonstrably Noachian in age, and even some clasts that plot in the mugearite field. Olivine is also conspicuously lacking in these ancient samples, in contrast to later Hesperian rocks. The alkali-suite requires low-degree melting of the Martian mantle at low pressure, whereas the later Hesperian magmatism would appear to be produced by higher mantle temperatures. Various scenarios are proposed to explain these observations, including different styles of magmatic activity (i.e. passive upwelling vs. hotspots). A second petrological suite of increasing interest involves quartzo-feldspathic materials that were first inferred from orbit, in local patches in the southern highlands and in the lower units of Valles Marineris. However, identification of felsic rocks from orbit is limited by the low detectability of feldspar in the near infrared. On the other hand, the MSL rover has described the texture, mineralogy and composition of felsic rocks in Gale crater that are granodiorite-like samples akin to terrestrial TTG (Tonalite-Trondhjemite-Granodiorite suites). These observations, and the low average density of the highlands crust, suggest the early formation of 'continental' crust on Mars, although the details of the geodynamic scenario and the importance of volatiles in their generation are aspects that require further work.
Alkali magmatism on Mars: an unexpected diversity
Comptes Rendus. Géoscience
Despite an apparent north/south topographic dichotomy that formed >4.0 Ga, the young Martian meteorites (<2.4 Ga) and first-order remote sensing observations revealed a surface of Mars that is uniformly basaltic. This simplistic vision has been challenged by the discovery of a brecciated meteorite and additional spacecraft data that all point to the presence of alkaline igneous rocks, thereby demonstrating an unexpected igneous diversity on Mars. In the present paper, we review a variety of effusive alkaline rocks (basalts to trachytes) recognized so far in the southern hemisphere of Mars as observed from a unique 4.47 Ga Martian meteorite, as well as ground, and orbital data. The complementary of effusive alkaline rocks and plutonic orthopyroxene-rich rocks in early Mars is discussed. We propose that mantle-derived magmas at high extent of melting at rather low pressure either erupted forming orthopyroxene-rich lavas, or crystallized at shallow crustal depths, fractionating orthopyroxene which sank to the bottom of the chamber and residual alkaline magmas which erupted at the surface of Mars. Widespread low pressure fractionation processes could also be related to heavy bombardment on the early Martian crust generating melt sheets that ultimately differentiated. The Noachian crust is more diverse than being merely basaltic.
Constraints on the composition and petrogenesis of the Martian crust
J. Geophys. Res, 2003
Spectral interpretation that silicic rocks are widespread on Mars implies that Earth's differentiated crust is not unique. Evaluation of observations bearing on the composition of the Martian crust (Martian meteorite petrology and a possible crustal assimilant, analysis of Mars Pathfinder rocks, composition of Martian fines, interpretation of spacecraft thermal emission spectra, and inferred crustal densities) indicates that the crust can be either basalt plus andesite or basalt plus weathering products. New calculated chemical compositions for Thermal Emission Spectrometer (TES) global surface units indicate that surface type 1 has basaltic andesite composition and surface type 2 has the composition of andesite. If these materials represent volcanic rocks, their calc-alkaline compositions on a FeO*/MgO versus silica diagram suggest formation by hydrous melting and fractional crystallization. On Earth, this petrogenesis requires subduction, and it may suggest an early period of plate tectonics on Mars. However, anorogenic production of andesite might have been possible if the primitive Martian mantle was wet. Alternatively, chemical weathering diagrams suggest that surface type 2 materials could have formed by partial weathering of surface type 1 rocks, leading to depletion in soluble cations and mobility of silica. A weathered crust model is consistent with the occurrence of surface type 2 materials as sediments in a depocenter and with the alpha proton X-ray spectrometer (APXS) analysis of excess oxygen suggesting weathering rinds on Pathfinder rocks. If surface type 1 materials are also weathered or mixed with weathered materials, this might eliminate the need for hydrous melting, consistent with a relatively dry Martian mantle without tectonics.
Isotopic and petrographic evidence for young Martian basalts
Geochimica et Cosmochimica Acta, 2008
Radiometric age data for shergottites yield ages of 4.0 Ga and 180-575 Ma; the interpretation of these ages has been, and remains, a subject of debate. Here, we present new 39 Ar-40 Ar laser probe data on lherzolitic shergottites Allan Hills (ALH) 77005 and Northwest Africa (NWA) 1950. These two meteorites are genetically related, but display very different degrees of shock damage. On a plot of 40 Ar/ 36 Ar versus 39 Ar/ 36 Ar, the more strongly shocked ALH 77005 (45-55 GPa) does not yield an array of values indicating an isochron, but the data are highly scattered with the shock melts yielding 40 Ar/ 36 Ar ratios of 1600-2026. Apparent ages calculated from these extractions range from 374-8183 Ma, with 50% of the data, particularly from the shock melts, yielding impossibly old ages (>4.567 Ga). On the same plot, extractions from igneous minerals in the less shocked NWA 1950 (30-44 GPa) yield a fitted age of 382 ± 36 Ma. Argon extractions from the shock melts are well distinguished from minerals, with the melts exhibiting the highest 40 Ar/ 36 Ar ratios (1260-1488) and the oldest apparent ages. Laser step heating was also performed on maskelynite separates from NWA 1950 yielding ages of 1000 Ma at the lowest release temperatures, and ages of 360 and 362 Ma at higher temperature steps. Stepped heating data from previous studies have yielded ages of 500 and 700 Ma to 1.7 Ga for ALH 77005 maskelynite separates. If the ages obtained from igneous minerals represent undegassed argon from an ancient (4.0 Ga) rock, then the ages are expected to anticorrelate with the degree of shock heating. The data do not support this inference. Our data support young crystallization ages for minerals and Martian atmosphere as the origin of excess 40 Ar in the shock melts.
The petrological expression of early Mars volcanism
Crystallization products of liquids produced by partial melting of a possible Martian mantle for conditions covering the earliest Noachian era to the most recent Amazonian times have been modeled using the MELTS thermodynamic calculator. The results imply a transition from low-calcium pyroxene dominated assemblages in the Noachian to high-calcium pyroxene assemblages in the Hesperian and Amazonian, which is remarkably consistent with observations made by orbiting visible and near-infrared spectrometers. This transition is interpreted as the consequence of the thermal evolution of the mantle, with no need for exotic conditions, such as higher water content or nonchondritic Ca/Al ratio of the mantle source, to produce low-calcium pyroxene rich lithologies. Our results are compatible with numerical models of the thermal evolution of Mars that predict high production rates of crust on early Mars, implying that Noachian rocks exposed at the surface may be petrological expressions of this volcanism rather than being associated with mantle overturn following the crystallization of a magma ocean.
The Mantle of Mars: Some Possible Geological Implications of Its High Density
1978
The density of the Martian mantle is estimated to be about 3.55 g/cm 3 (Reasenberg, 1977). Model mineral assemblages for the Martian mantle (at 30 kbar) were calculated using a modified CIPW norm scheme by adding FeO to model terrestrial mantle compositions. The density of the resulting mineral assemblages vary with increasing FeO content. With pyrolite starting compositions for the terrestrial mantle, the resulting model Martian mantle with density of 3.55 g/cm 3 is not garnet-lherzolite like the Earth; rather it is an assemblage properly called oxide-garnet wehrlite : oxide (periclase-wiistite) 2% ; garnet 11~ ; olivine 73% ; clinopyroxene 12~; with no orthopyroxene. Partial melting of such an assemblage would yield iron-rich, ultrabasic lavas, with extremely low viscosities. Specifically, model partial melts, assuming production from the quaternary eutectic (inferred to be near: OpT g4~ cpx43 OXs) yields an ultrabasic (SiO.2, 41 to 44%) picritic alkMi-basaltic melt (norm composition ne 2.5, plag 32, or 2.4, di 20, ol 37, mt 4.4 and ilm, tr), with a computed viscosity of about 12 P at 1200°C. This model for the composition of the Martian surface lavas (derived from geophysical data and petrologic arguments) is in remarkable agreement with a recently published model by Maderazzo and Huguenin (1977) (derived from reflection spectroscopy, experimental and theoretical nmdels h)r weathering in the Martian environment). The result also appears to be consistent with recent interpretations (Rasool arid Le Sergeant, 1977) of Viking atmospheric chemistry results, namely that the Martian crust is potassium poor. There are a number of geological implications which follow, including (1) superfluid lavas may account for some flood and erosional features observed on Mars; (2) the XIlF inorganic chemistry experiment on Vikings 1 and 2 (Baird, 1976) indeed may be measuring compositions approaching primary lavas, contrary to current interpretations which favor a rather mature (weathered) soil; (3) ultrabasic (ferrokimberlitic) ash might be a major constituent of the Martian soil, especially if cosmological models concerning the incorporation of a much volatile material within the early accreting Mars are correct-a matter of current debate; (4) a number of mineral assemblages riot previously considered are possible in the Martian mantle depending principally on the activity of volatile substances, (S, O, C, H) ; it is possible that some very unusual magmas are produced on partial melting; and (5) some fcrro-granite melts might be produced by liquid immiscibility.
Over the last ~ 3 years in Gusev Crater, Mars, the Spirit rover observed coherent variations in color, mineralogy, and geochemistry across Home Plate, an ~ 80 m-diameter outcrop of basaltic tephra. Observations of Home Plate from orbit and from the summit of Husband Hill reveal clear differences in visible/near-infrared (VNIR) colors between its eastern and western regions that are consistent with mineralogical compositions indicated by Mössbauer spectrometer (MB) and by Miniature Thermal Emission Spectrometer (Mini-TES). Pyroxene and magnetite dominate the east side, while olivine, nanophase Fe oxide (npOx) and glass are more abundant on the western side. Alpha Particle X-Ray Spectrometer (APXS) observations reveal that eastern Home Plate has higher Si/Mg, Al, Zn, Ni, and K, while Cl and Br are higher in the west. We propose that these variations are the result of two distinct alteration regimes that may or may not be temporally related: a localized, higher temperature recrystalliz...
Long-Term Evolution of the Martian Crust-Mantle System
Space Science Reviews, 2013
Lacking plate tectonics and crustal recycling, the long-term evolution of the crust-mantle system of Mars is driven by mantle convection, partial melting, and silicate differentiation. Volcanic landforms such as lava flows, shield volcanoes, volcanic cones, pyroclastic deposits, and dikes are observed on the martian surface, and while activity was widespread during the late Noachian and Hesperian, volcanism became more and more restricted to the Tharsis and Elysium provinces in the Amazonian period. Martian igneous ...