23 research outputs found

    Evidence for thermal fatigue on Mars from rockfall patterns on impact crater slopes

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    Individual block falls are one of the currently active surface processes on Mars. Similarly to Earth, clasts detach from upslope outcrops roll or bounce downslope, leaving a track on the substratum (Fig. 1). The trails show that the rockfalls are recent, as aeolian processes would infill topographic lows over time. Using rover-track erasure rates, these tracks are likely <100 ka. On Earth, slope instability is usually caused by phase changes of H2O [1]. However, solar-induced thermal stress could also play a key-role in rock breakdown leading to rockfalls [2]. Although liquid water is not stable at the surface of Mars today, sub-surface water ice is known to be present from mid- to high-latitudes [3]. Water ice and CO2 seasonal frost on shadowed pole-facing slopes may exist at latitudes down to 30° [4] or less [5]. On the other hand, insolation-related thermal stress has been used to explain fracture orientation patterns in martian boulders observed by the Mars Exploration Rovers [6] and other studies suggest that it could cause rock breakdown on airless bodies [7]. Therefore, both phase transitions and solar-induced thermal stress are plausible mechanisms for rock breakdown and preconditioning slopes for rockfalls on modern Mars. In this study we analyze distribution of rockfalls on impact crater walls to assess whether one of these mechanisms could be involved in local rock breakdown

    Evidence for thermal-stress-induced rockfalls on Mars impact crater slopes

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    Here we study rocks falling from exposed outcrops of bedrock, which have left tracks on the slope over which they have bounced and/or rolled, in fresh impact craters (1–10 km in diameter) on Mars. The presence of these tracks shows that these rocks have fallen relatively recently because aeolian processes are known to infill topographic lows over time. Mapping of rockfall tracks indicate trends in frequency with orientation, which in turn depend on the latitudinal position of the crater. Craters in the equatorial belt (between 15°N and 15°S) exhibit higher frequencies of rockfall on their N-S oriented slopes compared to their E-W ones. Craters >15° N/S have notably higher frequencies on their equator-facing slopes as opposed to the other orientations. We computed solar radiation on the surface of crater slopes to compare insolation patterns and rockfall spatial distribution, and find statistically significant correlations between maximum diurnal insolation and rockfall frequency. Our results indicate that solar-induced thermal stress plays a more important role under relatively recent climate conditions in rock breakdown and preconditioning slopes for rockfalls than phase transitions of H2O or CO2, at mid and equatorial latitudes. Thermal stress should thus be considered as an important factor in promoting mass-wasting process on impact crater walls and other steep slopes on Mars

    Thin crust and exposed mantle control sulfide differentiation in slow-spreading ridge magmas

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    Author Posting. © The Author(s), 2017. This is the author's version of the work. It is posted here under a nonexclusive, irrevocable, paid-up, worldwide license granted to WHOI. It is made available for personal use, not for redistribution. The definitive version was published in Geology 45 (2017): 935-938, doi:10.1130/G39287.1.Gabbroic veins enclosed in mantle peridotite from ocean core complexes next to oceanic transform faults demonstrate sub-crustal crystallization of silicate minerals from a MORB-like melt. Cooler lithosphere there may also affect sulfide crystallization and the metal budget of the lower and upper crust but the related sulfide behavior is poorly understood. Here, we use chalcophile elements to trace sulfide crystallization in a suite of MORB's erupted at the Kane Megamullion south of the Kane Fracture Zone along the Mid-Atlantic Ridge. Cool lithosphere there is inferred from a low magma supply, and lithostratigraphic evidence for thin crust with abundant mantle rock exposed to the seafloor (Dick et al., 2008). We show that the concentrations of Cu, Zn, As, Ga, Pb, Sb and Tl in the Kane Megamullion MORB's rise linearly with melt differentiation expressed by decreasing MgO and Ni content. The low-pressure fractional crystallization within the crust thus occurs at sulfur-undersaturated conditions. Sulfur-undersaturated MORB's are unusual. At the Kane Megamullion, however, the thin crust allows melt to more extensively interact with the shallow and serpentinized mantle. We argue that sulfur and chalcophile elements have been lost from the melt due to sulfide crystallization during melt-rock reaction in the shallow mantle.This research was funded by a Diamond Grant of the Polish Ministry of Science and Higher Education (DI2012 2057 42 to Ciazela), and partially supported by the European Association of Geochemistry (Early Career Science Ambassador grant to Ciazela) and the National Science Foundation (grant #’s OCE1434452 and OCE1637130 to Dick)

    Sulfide enrichment at an oceanic crust-mantle transition zone : Kane Megamullion (23°N, MAR)

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    Author Posting. © The Author(s), 2018. This is the author's version of the work. It is posted here under a nonexclusive, irrevocable, paid-up, worldwide license granted to WHOI. It is made available for personal use, not for redistribution. The definitive version was published in Geochimica et Cosmochimica Acta 230 (2018): 155-189, doi:10.1016/j.gca.2018.03.027.The Kane Megamullion oceanic core complex located along the Mid-Atlantic Ridge (23°30′N, 45°20′W) exposes lower crust and upper mantle directly on the ocean floor. We studied chalcophile elements and sulfides in the ultramafic and mafic rocks of the crust-mantle transition and the mantle underneath. We determined mineralogical and elemental composition and the Cu isotope composition of the respective sulfides along with the mineralogical and elemental composition of the respective serpentines. The rocks of the crust-mantle transition zone (i.e., plagioclase harzburgite, peridotite-gabbro contacts, and dunite) overlaid by troctolites are by one order of magnitude enriched in several chalcophile elements with respect to the spinel harzburgites of the mantle beneath. Whereas the range of Cu concentrations in spinel harzburgites is 7–69 ppm, the Cu concentrations are highly elevated in plagioclase harzburgites with a range of 90–209 ppm. The zones of the peridotite-gabbro contacts are even more enriched, exhibiting up to 305 ppm Cu and highly elevated concentrations of As, Zn, Ga, Sb and Tl. High Cu concentrations show pronounced correlation with bulk S concentrations at the crust-mantle transition zone implying an enrichment process in this horizon of the oceanic lithosphere. We interpret this enrichment as related to melt-mantle reaction, which is extensive in crust-mantle transition zones. In spite of the ubiquitous serpentinization of primary rocks, we found magmatic chalcopyrites [CuFeS2] as inclusions in plagioclase as well as associated with pentlandite [(Fe,Ni)9S8] and pyrrhotite [Fe1−xS] in polysulfide grains. These chalcopyrites show a primary magmatic δ65Cu signature ranging from −0.04 to +0.29 ‰. Other chalcopyrites have been dissolved during serpentinization. Due to the low temperature (<300 °C) of circulating fluids chalcophile metals from primary sulfides have not been mobilized and transported away but have been trapped in smaller secondary sulfides and hydroxides. Combined with the Cu deposits documented in the crust-mantle transition zones of various ophiolite complexes, our results indicate that the metal enrichment, increased sulfide modes, and potentially formation of small sulfide deposits could be expected globally along the petrological Moho.This research was funded by a Diamond Grant of the Polish Ministry of Science and Higher Education (DI2012 2057 42 to J. Ciazela), and partly supported by grants of the U.S. National Science Foundation (OCE1434452 and OCE1637130 to H.J.B. Dick), and the German Science Foundation (Bo2941/4-1 to R. Botcharnikov)

    Sulfide enrichment along igneous layer boundaries in the lower oceanic crust: IODP Hole U1473A, Atlantis Bank, Southwest Indian Ridge

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    © The Author(s), 2022. This article is distributed under the terms of the Creative Commons Attribution License. The definitive version was published in Pieterek, B., Ciazela, J., Boulanger, M., Lazarov, M., Wegorzewski, A., Pańczyk, M., Strauss, H., Dick, H. J. B., Muszyński, A., Koepke, J., Kuhn, T., Czupyt, Z., & France, L. Sulfide enrichment along igneous layer boundaries in the lower oceanic crust: IODP Hole U1473A, Atlantis Bank, Southwest Indian Ridge. Geochimica et Cosmochimica Acta, 320, (2022): 179–206, https://doi.org/10.1016/j.gca.2022.01.004.Reactive porous or focused melt flows are common in crystal mushes of mid-ocean ridge magma reservoirs. Although they exert significant control on mid-ocean ridge magmatic differentiation, their role in metal transport between the mantle and the ocean floor remains poorly constrained. Here we aim to improve such knowledge for oceanic crust formed at slow-spreading centers (approximately half of present-day oceanic crust), by focusing on specific igneous features where sulfides are concentrated. International Ocean Discovery Program (IODP) Expedition 360 drilled Hole U1473A 789 m into the lower crust of the Atlantis Bank oceanic core complex, located at the Southwest Indian Ridge. Coarse-grained (5–30 mm) olivine gabbro prevailed throughout the hole, ranging locally from fine- (30 mm). We studied three distinct intervals of igneous grain size layering at 109.5–110.8, 158.0–158.3, and 593.0–594.4 meters below seafloor to understand the distribution of sulfides. We found that the layer boundaries between the fine- and coarse-grained gabbro were enriched in sulfides and chalcophile elements. On average, sulfide grains throughout the layering were composed of pyrrhotite (81 vol.%; Fe1-xS), chalcopyrite (16 vol.%; CuFeS2), and pentlandite (3 vol.%; [Ni,Fe,Co]9S8), which reflect paragenesis of magmatic origin. The sulfides were most commonly associated with Fe-Ti oxides (titanomagnetites and ilmenites), amphiboles, and apatites located at the interstitial positions between clinopyroxene, plagioclase, and olivine. Pentlandite exsolution textures in pyrrhotite indicate that the sulfides formed from high-temperature sulfide liquid separated from mafic magma that exsolved upon cooling. The relatively homogenous phase proportion within sulfides along with their chemical and isotopic compositions throughout the studied intervals further support the magmatic origin of sulfide enrichment at the layer boundaries. The studied magmatic layers were likely formed as a result of intrusion of more primitive magma (fine-grained gabbro) into the former crystal mush (coarse-grained gabbro). Sulfides from the coarse-grained gabbros are Ir-Platinum Group Element-rich (PGE; i.e., Ir, Os, Ru) but those from the fine-grained gabbros are Pd-PGE-rich (i.e., Pd, Pt, Rh). Notably, the sulfides from the layer boundaries are also enriched in Pd-PGEs, and therefore elevated sulfide contents at the boundaries were likely related to the new intruding melt. Because S concentration at sulfide saturation level is dependent on the Fe content of the melt, sulfide crystallization may have been caused by FeO loss, both via crystallization of late-precipitating oxides at the boundaries, and by exchange of Fe and Mg between melt and Fe-bearing silicates (olivine and clinopyroxene). The increased precipitation of sulfide grains at the layer boundaries might be widespread in the lower oceanic crust, as also observed in the Semail ophiolite and along the Mid-Atlantic Ridge. Therefore, this process might affect the metal budget of the global lower oceanic crust. We estimate that up to ∼20% of the Cu, ∼8% of the S, and ∼84% of the Pb of the oceanic crust inventory is accumulated at the layer boundaries only from the interaction between crystal mush and new magma.This research was funded by National Science Centre Poland (PRELUDIUM 12 no. 2016/23/N/ST10/00288), Graduate Academy of the Leibniz Universität Hannover (60421784), and ECORD Research Grant to J. Ciazela, as well as Deutsche Forschungsgemeinschaft (KO1723/23-1) to J. Koepke and H. Strauss. J. Ciazela is additionally supported within the START program of the Foundation for Polish Science (FNP). This is CRPG contribution No. 2813

    Dynamic Accretion Beneath a Slow‐Spreading Ridge Segment: IODP Hole 1473A and the Atlantis Bank Oceanic Core Complex

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    809 deep IODP Hole U1473A at Atlantis Bank, SWIR, is 2.2 km from 1,508-m Hole 735B and 1.4 from 158-m Hole 1105A. With mapping, it provides the first 3-D view of the upper levels of a 660-km2 lower crustal batholith. It is laterally and vertically zoned, representing a complex interplay of cyclic intrusion, and ongoing deformation, with kilometer-scale upward and lateral migration of interstial melt. Transform wall dives over the gabbro-peridotite contact found only evolved gabbro intruded directly into the mantle near the transform. There was no high-level melt lens, rather the gabbros crystallized at depth, and then emplaced into the zone of diking by diapiric rise of a crystal mush followed by crystal-plastic deformation and faulting. The residues to mass balance the crust to a parent melt composition lie at depth below the center of the massif—likely near the crust-mantle boundary. Thus, basalts erupted to the seafloor from >1,550 mbsf. By contrast, the Mid-Atlantic Ridge lower crust drilled at 23°N and at Atlantis Massif experienced little high-temperature deformation and limited late-stage melt transport. They contain primitive cumulates and represent direct intrusion, storage, and crystallization of parental MORB in thinner crust below the dike-gabbro transition. The strong asymmetric spreading of the SWIR to the south was due to fault capture, with the northern rift valley wall faults cutoff by a detachment fault that extended across most of the zone of intrusion. This caused rapid migration of the plate boundary to the north, while the large majority of the lower crust to spread south unroofing Atlantis Bank and uplifting it into the rift mountains

    Recent rockfalls on Mars

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    International audienceMars is known to have a plethora of active surface processes at the present day, and the role of water is vigorously debated. Here we study rocks falling from exposed outcrops of bedrock, which have left trails on the slope over which they have bounced and/or rolled. The presence of these trails shows that these rocks have fallen relatively recently because aeolian processes are known to infill topographic lows over time (estimations from rover-track erasure rates date these trails at 20 • N/S have notably higher frequencies on their equator-facing slopes as opposed to the other orientations. These trends suggest that insolation plays a key role in determining the modern rockfall rate, indicating that thermal stress is playing a more important role than ice-presence in rock break down on modern Mars. To this end we use a Global Climate Model to assess the timescales over which these stresses are generated (diurnal, seasonal, etc)
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