Petrological Evolution and Mass Redistribution in Basaltic Fault Zones: An Example From the Faroe Islands, North Atlantic Igneous Province

Fault rock petrology exerts an important influence on the permeability structure and mechanical properties of fault zones. Slip-related deformation on upper-crustal faults in basaltic rocks is closely associated with fluid-rock interaction, altering the distribution of physical properties within the fault. Here, we present quantitative descriptions of the geochemical and petrological evolution of basalt-derived fault rocks from three passively exhumed fault zones in the Faroe Islands. Fault-rock petrology is determined by optical petrography and automated phase identification based on micrometer-scale chemical maps from scanning electron microscope X-ray spectroscopy. Geochemical evolution is assessed from major and trace element composition measured by X-ray fluorescence. The fault rocks show intense fluid-mediated alteration from a tholeiitic basalt protolith in the damage zones, and mechanical mixing in the fault cores. Pervasive alteration occurs early during fault zone evolution, with incipient fault damage increasing permeability and allowing along-fault percolation of carbonated meteoric water, increasing fluid-rock ratios. Our results suggest that the only mobile species within the fault zones are Ca, Si, and Al, which are leached during the hydrolysis of volcanic glass and plagioclase, and CO2, which is added by percolating waters. These species are transported from the damage zones into the fault cores, where they precipitate as zeolite and calcite cement in veins and hydrothermal breccias. We propose that solutes are replenished by cement dissolution through pressure-solution during cataclastic creep, during repeated cycles of hydrofracture and cementation. The fault zones are natural reactors for fluid-mediated alteration by CO2 and water, while other species are redistributed within the fault zones

A snapshot of the transition from monogenetic volcanoes to composite volcanoes: Case study on the Wulanhada Volcanic Field (northern China)

The transition processes from monogenetic volcanoes to composite volcanoes are poorly understood. The Late Pleistocene to Holocene intraplate monogenetic Wulanhada Volcanic Field (WVF) in northern China provides a snapshot of such a transition. Here we present petrographic observations, mineral chemistry, bulk rock major and trace element data, thermobarometry, and a partial melting model for the WVF to evaluate the lithology and partial melting degree of the mantle source, the crystallization conditions, and pre-eruptive magmatic processes occurring within the magma plumbing system. The far-field effect of India-Eurasia collision resulted in a relatively high degree (10 %-20 %) of partial melting of a carbonate-bearing eclogite (~ 3 wt % carbonate; Gt/Cpx ≈ 2 : 8, where Gt denotes garnet and Cpx denotes clinopyroxene) followed by interaction with ambient peridotite. The primary melts ascended to the depth of the Moho (~ 33-36 km depth), crystallized olivine, clinopyroxene and plagioclase at the temperature of 1100-1160 °C with the melt water contents of 1.1 wt %- 2.3 wt %. Part of the primary melt interacted with the lithospheric mantle during ascent, resulting in an increase in the MgO contents and a decrease in the alkaline contents. The modified magma was subsequently directly emplaced into the middle crust (~ 23-26 km depth) and crystallized olivine, clinopyroxene and plagioclase at the temperature of 1100-1160 °C. The primary melts from the same mantle sources migrated upward to the twolevel magma reservoirs to form minerals with complex textures (including reverse and oscillatory zoning and sieve texture). Magma erupted along the NE-SW-striking basement fault and the NW-SE-striking Wulanhada- Gaowusu fault in response to the combined effects of regional tectonic stress and magma replenishment. The crustal magma reservoir in the WVF may represent a snapshot of the transition from monogenetic volcanoes to composite volcanoes. It is possible to form a composite volcano with large magma volumes and complex compositions if the magma is continuously supplied from the source and experiences assimilation and fractional crystallization processes in the magma plumbing system at crustal depth

Discovery of two new super-eruptions from the Yellowstone hotspot track (USA): is the Yellowstone hotspot waning?

Super-eruptions are amongst the most extreme events to affect Earth’s surface, but too few examples are known to assess their global role in crustal processes and environmental impact. We demonstrate a robust approach to recognize them at one of the best-preserved intraplate large igneous provinces, leading to the discovery of two new super-eruptions. Each generated huge and unusually hot pyroclastic density currents that sterilized extensive tracts of Idaho and Nevada in the United States. The ca. 8.99 Ma McMullen Creek eruption was magnitude 8.6, larger than the last two major eruptions at Yellowstone (Wyoming). Its volume exceeds 1700 km3, covering ≥12,000 km2. The ca. 8.72 Ma Grey’s Landing eruption was even larger, at magnitude of 8.8 and volume of ≥2800 km3. It covers ≥23,000 km2 and is the largest and hottest documented eruption from the Yellowstone hotspot. The discoveries show the effectiveness of distinguishing and tracing vast deposit sheets by combining trace-element chemistry and mineral compositions with field and paleomagnetic characterization. This approach should lead to more discoveries and size estimates, here and at other provinces. It has increased the number of known super-eruptions from the Yellowstone hotspot, shows that the temporal framework of the magmatic province needs revision, and suggests that the hotspot may be waning

The Dialectics of Identity of the Modern and Postmodern Art

Ako pojam identiteta shvatimo u hegelijanskom smislu kao iskustvo što ga svijest stječe o sebi, onda se taj pojam nameće kao ključan u razmatranju (vizualne) umjetnosti XX. stoljeća. Prema Hegelu, moderna umjetnost transcendira mogućnost adekvatnog izražavanja svoga duhovnog sadržaja pukom osjetilnom reprezentacijom (koja je kao takvu određuje) te stoga zahtijeva pojmovnu refleksiju. Budući da je umjetnost uvijek i dio stvarnosti i o stvarnosti, propitivanje njezina vlastita pojma ide ruku pod ruku s ontološkom problematikom. Epistemološke promjene koje konstituiraju i modernu i postmodernu odražavaju se tako u dijalektici pojma moderne i postmoderne umjetnosti. Prema nekim autorima ta je dijalektika određena značajnim promjenama u teoriji subjekta, kulturalnim razlikama i tehnologiji.If the notion of identity is considered in the Hegelian sense as the experience of the consciousness about itself, then this notion becomes of key importance in reflecting upon the 20th-century (visual) art. Modern art, in Hegel’s view, transcends the possibility of an adequate expression of its spiritual content by its merely sensuous representation (that defines it as such) and hence calls for a reflection on its notion. Since art has always been both part of and about reality, the questioning of its own notion goes hand in hand with the ontological problematics. The epistemological changes that constitute both Modernism and Postmodernism thus reflect themselves in the dialectics of the notion of modern and postmodern art. According to some authors, such dialectics is determined by important changes which took place in the theory of the subject, in cultural differences as well as in technology

Permo-Triassic igneous rocks of Siberia, Russia

Widespread basaltic volcanism occurred in the region of the West Siberian Basin (WSB) and the Taimyr Peninsula in central Russia, and voluminous A-type magmatism within the Mongolian-Transbaikalian belt in southeast Siberia, during Permo-Triassic times. New 40Ar/39Ar age determinations on plagioclase grains from deep boreholes in the WSB reveal that the basalts were erupted at ~250 million years ago. This is synchronous with the main period of the Siberian Traps volcanism, which was located farther east. The age and geochemical data presented confirm that the WSB basalts are part of the Siberian Traps, and at least double the confirmed area of the volcanic province as a whole. The larger area of volcanism strengthens the link between the volcanism and the end-Permian mass extinction. Furthermore, it is argued that the WSB and Taimyr basalts are genetically related to the Siberian Traps basalts, especially the Nadezhdinsky Suite found at Noril’sk. This suite immediately preceded the main pulse of volcanism that extruded lava over large areas of the Siberian Craton. Magma volume and timing constraints strongly suggest that a mantle plume was involved in the formation of the Earth’s largest continental flood basalt province. The Mongolian-Transbaikalian granitoid belt covers over 600,000 km2 with over 350 single A-type plutons. New U-Pb geochronological data presented here demonstrate that no plutonic complex dated is 250 Ma old. Although mantle-derived material played a prominent role in the granitoid generation, these melts may have been generated by processes other than decompressional melting within the head of a mantle plume. The new U-Pb ages and other observations contradict the idea of a relation between the Siberian plume and magmatic activity in the territory of Transbaikalia. An alternative preferred model inducing up rise of asthenospheric material includes slab break-off after a long period of subduction

Cyanobacterial blooms tied to volcanism during the 5 m.y. Permo-Triassic biotic crisis

In a recent paper, Xie et al. (2010) elegantly demonstrate that cyanobacterial blooms recorded in Permian–Triassic (P–Tr) rocks are closely associated with local volcanic activity in South China, and with the development of large negative global carbon isotope excursions. Using a compiled data set of published U/Pb and 40Ar/39Ar ages obtained on volcanic ash layers from South China, and volcanic rocks of the Siberian Traps (ST), respectively, Xie et al. argue that the volcanism associated with the ST is predominantly younger than the P–Tr boundary age. Xie et al. note that the majority of ST 40Ar/39Ar ages (e.g., Reichow et al., 2009) are similar to U/Pb zircon ages for two Triassic boundaries, and consequently that ST volcanism was likely responsible for the prolonged stress in the Early Triassic ecosystems. However, the suggested age correlation is flawed, and the purpose of this Comment is to challenge the comparison based on ages obtained by different methodologies, and demonstrate that one of the conclusions drawn by Xie et al. is invalid

Petrological Evolution and Mass Redistribution in Basaltic Fault Zones: An Example From the Faroe Islands, North Atlantic Igneous Province

Fault rock petrology exerts an important influence on the permeability structure and mechanical properties of fault zones. Slip‐related deformation on upper‐crustal faults in basaltic rocks is closely associated with fluid‐rock interaction, altering the distribution of physical properties within the fault. Here, we present quantitative descriptions of the geochemical and petrological evolution of basalt‐derived fault rocks from three passively exhumed fault zones in the Faroe Islands. Fault‐rock petrology is determined by optical petrography and automated phase identification based on micrometer‐scale chemical maps from scanning electron microscope X‐ray spectroscopy. Geochemical evolution is assessed from major and trace element composition measured by X‐ray fluorescence. The fault rocks show intense fluid‐mediated alteration from a tholeiitic basalt protolith in the damage zones, and mechanical mixing in the fault cores. Pervasive alteration occurs early during fault zone evolution, with incipient fault damage increasing permeability and allowing along‐fault percolation of carbonated meteoric water, increasing fluid‐rock ratios. Our results suggest that the only mobile species within the fault zones are Ca, Si, and Al, which are leached during the hydrolysis of volcanic glass and plagioclase, and CO2, which is added by percolating waters. These species are transported from the damage zones into the fault cores, where they precipitate as zeolite and calcite cement in veins and hydrothermal breccias. We propose that solutes are replenished by cement dissolution through pressure‐solution during cataclastic creep, during repeated cycles of hydrofracture and cementation. The fault zones are natural reactors for fluid‐mediated alteration by CO2 and water, while other species are redistributed within the fault zones.</p

Distinguishing and Correlating Deposits from Large Ignimbrite Eruptions Using Paleomagnetism: the Cougar Point Tuffs (Mid-Miocene), Southern Snake River Plain, Idaho, USA

In this paper, we present paleomagnetic, geochemical, mineralogical, and geochronologic evidence for correlation of the mid-Miocene Cougar Point Tuff (CPT) in southwest Snake River Plain (SRP) of Idaho. The new stratigraphy presented here significantly reduces the frequency and increases the scale of known SRP ignimbrite eruptions. The CPT section exposed at the Black Rock Escarpment along the Bruneau River has been correlated eastward to the Brown's Bench escarpment (six common eruption units) and Cassia Mountains (three common eruption units) regions of southern Idaho. The CPT records an unusual pattern of geomagnetic field directions that provides the basis for robust stratigraphic correlations. Paleomagnetic characterization of eruption units based on geomagnetic field variation has a resolution on the order of a few centuries, providing a strong test of whether two deposits could have been emplaced from the same eruption or from temporally separate events. To obtain reliable paleomagnetic directions, the anisotropy of anhysteretic remanence was measured to correct for magnetic anisotropy, and an efficient new method was used to remove gyroremanence acquired during alternating field demagnetization

Tracing Deep Carbon Cycling by Zinc Isotopes in a Peralkaline‐Carbonatite Suite

Sedimentary carbonates are known to be carried into the deep mantle by subducted slabs, and studies on mantle‐derived magmas have attempted to trace the recycled carbonate in their mantle source. However, the final depth of storage of recycled carbonate and the role of recycled carbonate in the partial melting of mantle remain controversial. Peralkaline‐carbonatite suites are considered to have been derived from a carbonated mantle source and are windows to evaluate carbon in the mantle. In this study, we report the Zn isotopic compositions of a peralkaline‐carbonatite suite from the Tarim Large Igneous Province (Tarim LIP). The peralkaline‐carbonatite suite has heavier δ66Zn than normal mantle with δ66Zn of 0.34–0.40 ‰ for nephelinite, 0.35–0.47 ‰ for aillikite, 0.51–0.55 ‰ for nepheline syenite, 0.58–0.67 ‰ for calciocarbonatite and 0.38–0.56 ‰ for magnesiocarbonatite. The heavy Zn isotopic compositions of the peralkaline‐carbonatite suite in the Wajilitag complex suggest the incorporation of recycled carbonate‐bearing materials into the deep mantle. We infer that the calciocarbonatite was produced by the initial partial melting of subducted MgSiO3/MgO + C‐bearing carbonated eclogite, whereas the magnesiocarbonatite, aillikite, and nephelinite are considered as reacted melts between carbonated eclogite‐derived melts and peridotite. The heavy Zn isotopic compositions of the nepheline syenite are attributed to fractional crystallization from nephelinite magma in the magma reservoir. Our study highlights the incorporation of carbonated eclogite as an important agent of recycled carbon in the deep mantle and interactions between carbonated eclogite‐derived melts and peridotite lead to the complex lithological heterogeneities in the peralkaline‐carbonatite suite in Tarim LIP.</p