Mantle xenoliths from the Kerguelen Archipelago record a complex multistage history involving a high degree (15 to 25%) partial melting that created a harzburgitic mantle completely stripped of Base ...Metal Sulfides (BMS), followed by pervasive melt-rock reaction with alkaline melts above the Kerguelen mantle plume. Subsequent reaction of the highly refractory protolith with small volumes of carbonate-rich silicate melts led to a re-enrichment in BMS (up to 0.05wt.%). Two BMS precipitation mechanisms are suggested: immiscibility from the silicate-carbonate melt and sulfidation reactions from a CO2-rich supercritical fluid. In-situ analyses of chalcophile and siderophile elements (major and trace levels) in the BMS shed new light on their origin. The BMS phases that precipitated via immiscibility are metal-rich sulfide melts which progressively evolved toward Ni and Cu-rich end-members by cumulate fractionation of monosulfide solid solution (mss) during percolation inside the peridotites. Some cumulate mss have elevated and fractionated IPGE contents (200–900× C1-Chondrite abundances), indicating random digestion of preexisting Os, Ir, Ru-rich PGM by the percolating sulfide melt. The BMS that precipitated by sulfidation reactions from a CO2-rich vapour phase are subsolidus exsolution products from Cu-bearing but Ni-poorer mss. They have the highest concentrations of PGEs and show selective enrichment in S, Pd, Pt and Os over Cu, S, Ir, Ru and Rh. Their PGE compositions confirm experimental data, which demonstrate that S, Pd, Pt and Os can be efficiently transported in a CO2-rich supercritical fluid. Superchondritic (S/Se), (Os/Ir) and (Pd/Pt) in both bulk-rocks and individual sulfides are inferred to be the geochemical fingerprints of sulfide crystallisation from a CO2-rich vapour exsolved from a highly evolved carbonate-rich metasomatic melt.
► Chalcophile and siderophile elements are reported for sulfides in mantle xenoliths. ► We examine fractionations of these elements during metasomatic addition. ► Different elemental fractionations depend on the sulfide precipitation mechanisms. ► PGE are best transferred during metasomatism involving a fluid/vapour-rich phase. ► PGE fingerprints may record dissolution of preexisting PGM during metasomatism.
We have carried out paleomagnetic sampling of a ∼750 m sequence of the Karoo large igneous province (Naude's Nek Pass, South Africa). K‐Ar dating (Cassignol‐Gillot) has been performed on four samples ...from the 650 m upper unit (mean age 179.2 ± 1.8 Ma) and a sample from the lower unit (184.8 ± 2.6 Ma). A succession of two phases of volcanism is suggested. The lower 25 flows (115 m thick) have recorded a reversed polarity; the next 23 flows (135 m thick) are transitional and contribute a detailed record of the “Van Zijl” (1962) Jurassic reversal. The upper 38 flows (500 m thick) have normal polarity. Directional groups (DGs) of lava flows with quasi‐identical remanence directions indicate eruption durations too short to have recorded geomagnetic secular variation and hence are interpreted as single eruptive events. Altogether, 19 DGs and 10 sheet lobes yield a sequence of 29 distinct directions. This could correspond to a total eruptive activity shorter than 3000 years, less than one per mil of the total duration over which the section was emplaced. We obtain a new paleomagnetic pole for South Africa at ∼180 Ma (λ = 75.2°N, ϕ = 276.4°E, A95 = 5.8°, N = 19), which is consistent with earlier reports.
Key Points
A 750 m thick detailed paleomagnetic and geochronologic section of Karoo lava
Paleomagnetism indicates 29 single eruptive events, a new pole is proposed
Two short volcanic phases (a few kiloyears long?) at 185 and 179 (much larger) Ma
Although tectonic plates are usually considered as rigid blocks, intraplate deformation such as lithospheric buckling or diffuse brittle deformation has been recognized for a long time. However, the ...origin of these deformations remains puzzling. Indeed, whereas the chronology of deformation at plate boundaries can be constrained by numerous methods (syntectonic sedimentary record, thermochronology, etc.), dating of brittle structures (faults, veins, and joints) in the far-field domains remains challenging, preventing a global interpretation of the system as a whole. In this contribution, we have combined a tectonic study with a synkinematical geochronological study of fault-related calcites of the Grands Causses intraplate domain, north of the Pyrenean orogeny. We show that these faults record a much longer history of deformation than previously thought. The Mesozoic extension, usually attributed to an early Jurassic Tethysian rifting event, probably lasted until the Barremian-Aptian epoch, in response to the Pyrenean basin’s opening. The so-called “Pyrenean deformation” of the Grands Causses domain, usually associated with the paroxysm of deformation in the belt during the late Eocene, began much earlier, around 100 Ma, and lasted for more than 60-70 Ma. This study demonstrates the high sensitivity of an intraplate domain (Grands Causses area) to record extensional or compressional deformations occurring at the edge of neighbouring plates.
Celotno besedilo
Dostopno za:
DOBA, IZUM, KILJ, NUK, PILJ, PNG, SAZU, UILJ, UKNU, UL, UM, UPUK
Many peridotite xenoliths sampled at ocean islands appear to have strongly refractory major element and modal compositions. To better constrain the chemistry, abundance and origin of these ...ultra-refractory rocks we compiled a large number of data for xenoliths from nine groups of ocean islands. The xenoliths were filtered petrographically for signs of melt infiltration and modal metasomatism, and the samples affected by these processes were excluded. The xenolith suites from most ocean islands are dominated by ultra-refractory harzburgites. Exceptions are the Hawaii and Tahiti peridotites, which are more fertile and contain primary clinopyroxene, and the Cape Verde suite, which contains both ultra-refractory and more fertile xenoliths. Ultra-refractory harzburgites are characterized by the absence of primary clinopyroxene, low whole-rock Al2O3, CaO, FeO/MgO and heavy rare earth element (HREE) concentrations, low Al2O3 in orthopyroxene (generally < 3 wt %), high Cr-number in spinel (0·3–0·8) and high forsterite contents in olivine (averages > 91·5). They are therefore on average significantly more refractory than peridotites dredged and drilled from mid-ocean ridges and fracture zones. Moreover, their compositions resemble those of oceanic forearc peridotites. The formation of ultra-refractory ocean island harzburgites requires potential temperatures above those normally observed at modern mid-ocean ridges, and/or fluid fluxed conditions. Some ultra-refractory ocean island harzburgites give high Os model ages (up to 3300 Ma), showing that their formation significantly pre-dates the oceanic crust in the area. A genetic relationship with the host plume is considered unlikely based on textural observations, equilibration temperatures and pressures, inferred physical properties, and the long-term depleted Os and Sr isotope compositions of some of the harzburgites. Although we do not exclude the possibility that some ultra-refractory ocean island harzburgites have formed at mid-ocean ridges, we favor a model in which they formed in a process spatially and temporally unrelated to the formation of the oceanic plate and the host plume. As a result of their whole-rock compositions, ultra-refractory harzburgites have a very high solidus temperature at a given pressure, low densities and very high viscosities, and will tend to accumulate at the top of the convecting mantle. They may be preserved as fragments in the convecting mantle over long periods of time and be preferentially incorporated into newly formed lithosphere.
Chalcophile and highly siderophile trace elements in Kerguelen mantle xenoliths monitor a multistage melt-depletion/metasomatic history in the lithospheric mantle. A high-degree partial melting event ...in a fast-spreading oceanic ridge setting generated protogranular harzburgites strongly depleted in S (<5 ppm), Se (<7 ppb), Cu (<5.1 ppm) and Pd (Pd/Ir
N<0.2; N=CI-chondrite normalised). This refractory protolith was subsequently percolated in a within-plate setting by large fractions of basaltic to highly alkaline melts, which produced poikiloblastic harzburgites. Their PGE concentrations (including the IPGE; Os, Ir, Ru) are depleted by factors of 2 to 5 (0.002–0.005×CI-chondrites) compared with those of the protogranular harzburgites. Spinel dunites sampled as wall-rocks to hornblendite dykes are similarly depleted. These strongly compatible elements form intergranular discrete phases rather than being locked in silicates and can be dissolved in the percolating melts.
After precipitation of amphibole and phlogopite in some harzburgites and the dunites, the highly alkaline melts evolved towards volatile-rich (carbonatite) small-melt fractions that ultimately unmixed into a highly modified, silica-rich and volatile-poor alkaline fraction and a carbonated fraction strongly enriched in C–O–H–S volatiles. The latter fluid affected the refractory protolith to various degrees by precipitating Pd–Pt-enriched sulfides of metasomatic origin. Bulk-rock S/Se, Cu/S, Os/Ir and Pd/Pt ratios combined with petrographic observations argue for two sulfide precipitation processes. A group of samples with broadly chondritic S/Se and Os/Ir ratios and no enrichment in Pd over Pt (Pd/Pt
N=1) displays Cu–Fe–Ni-rich sulfide melt droplets unmixed from metasomatic carbonate melt pockets. In contrast, some dunites with abundant Fe–Ni sulfides have superchondritic S/Se ratios (up to 10,000), coupled with superchondritic Os/Ir
N (2.7) and Pd/Pt
N (6). Such a sympathetic behaviour of Os, Pd and S suggests transport from a S- and Cl-bearing CO
2 vapour phase.
•A new record of 560 Ma banded iron formation in southwestern Asia.•BIF in association with volcanic rocks and glaciogenic metasediments suggest active-magmatic basin at ca. ∼560 Ma.•All evidence ...point to the suboxic conditions in Ediacaran seawater during deposition of iron formation.
The late Neoproterozoic Takab banded iron formation (BIF) occurs in one of the most important part of the Iranian plateau containing Precambrian crustal segments, now embedded in the Alpine-Himalayan orogenic system. The Takab BIF is hosted by low to medium grade metamorphic sequence including schist, quartzite and marble and its structure is dominated by bands or streaks. The ore body is mostly composed of quartz and iron oxides (magnetite, hematite, maghemite, goethite in different proportions), occurring as alternating iron- and silica-rich laminates. LA-ICP-MS U–Pb dating on zircons in schists from both the lower and upper layers of the orebody indicate a depositional age of ca. ∼560 Ma for iron deposits. Major, trace and rare earth elements of the ore body support the mixing of seawater and hydrothermal fluids through iron precipitation and up to ca. 20 % incorporation of terrigenous materials into the chemical precipitate of the original BIF. The provenance indicators as well Nd isotopic data, point towards the Neoproterozoic felsic-to-intermediate crystalline basement of the Takab area as the main sediment source. Fe isotope data together with Ce anomalies suggest that suboxic conditions prevailed in Ediacaran seawater at the time of deposition of the ore body. Combined with geochronological and geochemical data from other Neoproterozoic formations and crystalline rocks on the Iranian plateau, as well as supportive evidence for the occurrence of glacial sediments and bimodal volcanism in the studied area, we propose that the formation of the Takab BIF occurred in a back-arc basin environment, hence corresponding to a Rapitan-type iron ore. The co-occurrence of late Ediacaran BIFs and glaciogenic metasedimentary formations suggest the presence of active-magmatic marine anoxic basins at ca. ∼560 Ma, equivalent to the Ediacaran post-Gaskiers glaciation time.
With the aim to better understand the cause of the iron isotope heterogeneity of mantle-derived bulk peridotites, we compared the petrological, geochemical and iron isotope composition of four ...xenolith suites from different geodynamic settings; sub-arc mantle (Patagonia); subcontinental lithospheric mantle (Cameroon), oceanic mantle (Kerguelen) and cratonic mantle (South Africa). Although correlations were not easy to obtain and remain scattered because these rocks record successive geological events, those found between δ
57
Fe, Mg#, some major and trace element contents of rocks and minerals highlight the processes responsible for the Fe isotope heterogeneity. While partial melting processes only account for moderate Fe isotope variations in the mantle (<0.2 ‰, with bulk rock values yielding a range of δ
57
Fe ± 0.1 ‰ relative to IRMM-14), the main cause of Fe isotope heterogeneity is metasomatism (>0.9 ‰). The kinetic nature of rapid metasomatic exchanges between low viscosity melts/fluids and their wall-rocks peridotite in the mantle is the likely explanation for this large range. There are a variety of responses of Fe isotope signatures depending on the nature of the metasomatic processes, allowing for a more detailed study of metasomatism in the mantle with Fe isotopes. The current database on the iron isotope composition of peridotite xenoliths and mafic eruptive rocks highlights that most basalts have their main source deeper than the lithospheric mantle. Finally, it is concluded that due to a complex geological history, Fe isotope compositions of mantle xenoliths are too scattered to define a mean isotopic composition with enough accuracy to assess whether the bulk silicate Earth has a mean δ
57
Fe that is chondritic, or if it is ~0.1 ‰ above chondrites as initially proposed.
Previous paleo-glacial studies on Kerguelen showed a singular pattern of Holocene glacier evolution on this archipelago in comparison with other southern mid-latitude glacier records. In this study, ...we aim to test this singularity on a longer timescale, based on 26 new in situ-produced 36Cl ages from pre-Holocene glacio-geomorphic features. Samples from moraine boulders and glacially polished bedrock were extracted at six different sites, located near the Port-aux-Français scientific station (PAF site), on Longue Island, Australia Island, on the Port-Jeanne d’Arc Peninsula (PJDA site), on the Gallieni Peninsula at Baie Larose (BLR site) and the McMurdo Island. The moraine ages indicate that glacier culminations occurred during Marine Isotopic Stage 3 (MIS 3) at 42.2 ± 4.9 ka on the PAF site, and during the global Last Glacial Maximum (gLGM) at 21.5 ± 3.2 ka on the PJDA site and at 21.4 ± 3.7 ka and 19.4 ± 2.6 on Baie Larose site. This is the first time that Late Pleistocene glacier culminations are evidenced on Kerguelen by direct moraine dating, thus allowing comparison with other moraine records from the southern mid-latitudes. While it remains speculative whether or not the MIS 3 glacial maximum at ⁓42.2 ka is in phase with other glaciers at this latitude (due to high age uncertainties), the gLGM glacial maximum is synchronous with that in other southern mid-latitude regions. 36Cl CRE ages of glacially polished bedrock surfaces sampled in different locations of the archipelago vary from ⁓39 ka to ⁓19 ka. We interpret these results as reflecting periods of deglaciation that occurred in between the two glacier culminations and right after the gLGM on Kerguelen. These ages also suggest that some places of the archipelago were free of ice at least since ⁓ 39 ka. The presence of a MIS 3 moraine at PAF site that has not been obliterated by a gLGM advance suggests that the ⁓ 42.2 ka glacier extent was at least as large as gLGM glacial maxima on the archipelago. The glacier culmination during MIS 3 being larger than that during the gLGM on the Kerguelen Archipelago matches observations in other southern mid-latitude regions. Late Pleistocene glacier culminations on Kerguelen may have been in phase with cold temperatures recorded in SST records, which suggest a cooling around Kerguelen. However, climate drivers responsible for the larger MIS 3 glacier culmination on Kerguelen still remain unclear even if we hypothesize that changes in precipitation may have superimposed on temperature changes.
•We investigate glacier evolution on the Kerguelen Archipelago for the past 45 ka.•Glacier chronologies are based on 26 new 36Cl ages from moraines and bedrocks.•Preserved moraines attest to glacier culminations at ∼42 ka and during the LGM.•Evidence of larger MIS 3 than gLGM glacier culmination.•This pattern matches records from other southern mid-latitude regions.
Tephrochronology studies the deposits of explosive volcanic eruptions in the stratigraphic record. The Southern (SVZ, 33–46° S) and Austral (AVZ, 49–55° S) Volcanic Zones of the Andes are two very ...active volcanic zones where tephrochronology is of great use. There, it can be used to improve chronologies of paleoenvironmental records in Patagonia, an area providing valuable records at global scale; as well as to identify areas likely to be affected by volcanic eruptions in the future, essential for producing volcanic hazard maps. The close proximity of many volcanic centers with recurrent explosive activity, which have very similar geochemical compositions, and very often poor age constraints, represent a challenge for the study of tephrochronology in the region. In addition to this, the ever-growing amount of tephrochronological information in the area, dispersed in different types of publications which vary greatly in format, makes the integration of the data produced by different actors, and consecutively its interpretation, increasingly difficult. Here we address this issue by compiling the BOOM! dataset, which integrates ∼30 years of research on 32 active volcanic centers and 132 different eruptions, which took place during the last 20,000 years. To help users find and reuse data in the large dataset, we developed an online platform which provides user-friendly tools for exploring it, and helps users download subsets of it. To integrate this very heterogeneous information, special attention was given to include information which allows users to evaluate data quality and comparability, as well as to provide tools in the explorer for users to filter data by different criteria. The integration of this dataset opens new perspectives for the development of novel visualizations of tephrochronological data, for example, to better understand the multidimensional uncertainties associated with it. For example, uncertainties associated with analytical precision, with age estimates of both tephra deposits and volcanic eruptions, and of tephra classification. Additionally, it allows for the use of robust statistical tools to correlate tephra deposits, including those based on machine learning algorithms, which are here explored.
•Unprecedented compilation of tephrochronological data of the Southern and Austral Andes.•Novel online explorer of tephrochronological data of the Southern and Austral Andes.•Good performances of machine learning algorithms for tephra correlation.•Data on 32 active volcanic centers, 132 eruptions and 79 different publications.
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