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Magmas with a high oxidation state (FMQ + 1 to + 2), a high-water content (>= 4 wt% H2O) and a high degree of fractionation are necessary to form porphyry copper deposits (PCDs). Different proxies, such as Fe3+/Fe2+ ratio of whole-rock, minerals indicative of high oxygen fugacity (fO(2)) and water content as well as geochemical signatures of whole-rock (e.g. high Sr/Y and V/Sc ratios) are commonly used to trace these magmatic conditions. However, these proxies are of limited use for rocks hosting porphyry copper mineralization because the hydrothermal fluids associated with ore deposits obliterate the primary minerals and their initial geochemical signatures. Zircon is a common accessory mineral of igneous rocks that retains its original chemical composition from the time of crystallization, therefore provides information related to parental magmas. Several studies on PCDs worldwide have investigated the geochemical characteristics of zircons (e.g., Hf concentration, Ce4+/Ce3+, Eu/Eu*) as indicators of a high oxidation state, high magmatic water content and high degree of magma fractionation. Here, the petrographic and geochemical characteristics of zircons collected from Chuquicamata, Mirador, Esperanza and Escondida PCDs, located in the upper Eocene to lower Oligocene metallogenic belt of northern Chile, were determined in order to validate the use of zircon as a tracer for porphyry fertility. Our results show that zircon collected from all studied PCDs crystallized from monzogranitic to granodioritic magmas at temperatures below 750 degrees C and show euhedral morphologies mainly characterized by prismatic forms of type {110} and pyramidal forms of type {101}. Most zircons from PCDs have Hf concentrations > 8,750 (ppm) and Ce/Nd > 1, Eu/Eu* > 0.4, 10,000x(Eu/Eu*)/Y > 1, (Ce/Nd)/Y > 0.01, Dy/Yb < 0.3, Th/U 0.1. From this group of geochemical values measured on zircon, we propose the Porphyry Indicator Zircon concept (PIZ), where each zircon from PCDs must comply with the previously mentioned geochemical framework. Our results show that studied PCDs have abundant zircons with PIZ characteristic (> 54%); at Mirador and Esperanza porphyries the PIZ proportion exceeding 70%. On the other hand, the non-mineralized intrusive bodies have no zircons with PIZ characteristics. Therefore, these results combined with previously published studies from other porphyry belts worldwide, validate zircons as an indicator of metallogenic fertility and its use as a porphyry indicator mineral and a potential exploration tool in search for PCDs.

Spinel is a ubiquitous mineral in mafic/ultramafic rocks. Spinel cores chemistry is extensively used as a petrogenetic proxy while their alteration phases, ferritchromite, and Cr-magnetite, are used as metamorphic grade indicators. However, the magnetic properties and composition of these phases are still ill-defined and no consensus exists concerning the metamorphic conditions involved in their formation. Here, we use the magnetic properties of these Cr-spinel alteration phases, via field-dependent parameters and observations with a magnetic microscope coupled with mineral chemistry and Mossbauer spectroscopy, to better constrain their composition. We identify Cr-magnetite by a Curie point of ca. 520 degrees C. We show that it is characterized by an n between 0.1 and 0.2 in the Fe-Cr spinel formula [Fe2+(Fe1-nCrn)(2)O-4], which corresponds to 6-13 wt.% of Cr2O3. The abundance of Cr-magnetite indicates a strong alteration of Cr-spinels that could reflect a significant hydrothermal activity rather than a high metamorphism grade. Normalized variation curves of the magnetic susceptibility during heating allow a relative quantification of the contributions of different magnetic phases to the magnetic susceptibility. This highlights a link between ferritchromite destabilization into maghemite at ca. 130 degrees C followed by the destabilization of this maghemite starting at 300 degrees C. We identify specific covariation trends between these two magnetic species characterizing different alteration processes. This study opens the door to magnetic monitoring of the Cr-spinel alteration state in mafic and ultramafic rocks. It constitutes a new, fast, and weakly destructive way to study the petrological history of both terrestrial and extraterrestrial rocks.

Magnetic proxies for paleoclimate are tools widely used to understand climate variability, yet current proxies focus on loess-derived soils in humid to temperate climate zones, whereas coarse-grained sediments in arid-hyperarid climate zones remain poorly investigated. To test the potential paleoclimatic-environmental significance of the magnetic record of coarse-grained sediments deposited in a region with a mean annual precipitation (MAP) < 200 mm yr(-1), we selected three previously studied sedimentary sequences from the Atacama Desert (Centinela area) and explored their magnetic properties, paedogenic features, salt chemistry and mineralogy. These sequences were deposited under different climate-environmental conditions from the Eocene to the Miocene, and correspond, from oldest to youngest, to the Atravesados II gravels (Atli), the Arrieros gravels (Arr) and the Ratones sediments (Rat). The magnetic susceptibility values (k) obtained in the stratigraphic record of the gravels are mainly controlled by the concentration of detrital (titano)magnetite, which is concentrated in the finest fraction ( < 0.5 mm) of sediments. The values decrease in the coarse sediments from Atli to Arr following a climatic transition from arid to hyperarid conditions, an interpretation that is supported by the transition from carbonate-rich (MAP similar to 40 mm yr(-1)) to sulfate-rich paleosols (MAP < 10 mm yr(-1)), and changes in the tectonic conditions and/or sedimentary source. In contrast, the Rat fine-grained sediments record changes in paleo-wetland dynamics. The high frequency-dependent magnetic susceptibility (kfd%) values obtained from these layers are mainly controlled by the concentration of authigenic magnetite-maghemite and/or hematite crystals of superparamagnetic/single magnetic size. The increase in kfd% is linked to an increase in the authigenic degree, which is related to variations in the depth of the local water table. These results demonstrate the potential of magnetic proxies to reveal climatic/environmental signals in coarse-grained sediments deposited under desert climate conditions.

The Earth&rsquo, s continental crust represents the outermost envelope of the solid Earth, controlling exchanges within the geosphere and reflecting geodynamics processes. One of the fundamental issues of Earth Science aims to determine crustal thickness in past geodynamic environments in order to discuss the evolution of certain geodynamic processes through time. Despite presenting a continuing challenge, the evolution of crustal thickness during the last 3 billion years can be investigated using indirect clues yielded by the chemical signature of mafic magmas and associated ferromagnesian minerals (pyroxene, amphibole). Here, we present a new statistical assessment of a global database of magmatic and mineral chemical information. Analysis reveals the increasing occurrence of high-temperature pyroxenes and amphiboles growing in Ca-rich, Fe-poor magma since ~1 Ga, which contrasts with lower temperature conditions of minerals crystallization throughout the Meso- and Palaeoproterozoic times. This is interpreted to reflect temporal changes in the control of Earth&rsquo, s crust on mantle-derived magma composition, related to changes in lithospheric thickness and mantle secular cooling. We propose that thick existing crust is associated with deeper, hotter magmatic reservoirs, potentially elucidating the mineral chemistry and the contrasting iron content between primary and derivative mafic magmas. Based on both the chemical and mineral information of mafic magma, an integrated approach provides qualitative estimates of past crustal thickness and associated magmatic systems. Our findings indicate that the Proterozoic was characterized by thicker crustal sections (&gt, 40&ndash, 50 km) relative to the Phanerozoic and Archean (&lt, 35 km). This period of crustal thickening appears at the confluence of major changes on Earth, marked by the onset of mantle cooling and Plate Tectonics and the assembly of Columbia, the first supercontinent.

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If magnetite is a common serpentinization product, centimetric, massive and pure magnetite veins are rarely observed in serpentinites. Unique example, in the Ait Ahmane ultramafic unit (Bou Azzer Neoproterozoic ophiolite, Anti-Atlas, Morocco) allows to assess the hydrothermal processes that prevailed at the ending Precambrian. In this study, rock magnetism, petrography, mineral and bulk chemistry are combined to assess iron behavior in these meta-ultramafics, in order to constrain the serpentinites alteration and magnetite veins formation processes. Very high Cr#, low Mg#, high Fe3+# and low Ti content of Cr-spinels cores reflect a supra-subduction zone origin for the Alt Ahmane serpentinites precursor. Typical lizardite/chrysotile pseudomorphic texture in fresh serpentinites reveals an initial oceanic-like serpentinization, involving temperature < 350 degrees C while the abundance of magnetite (up to 10.14 wt%) in these unaltered serpentinites attests of a relatively high serpentinization temperature > 200 degrees C. Magnetic measurements reveal a lower magnetite content in hydrothermalized serpentinites hosting the magnetite veins, with lowest values (down to 0.58 wt%) for bleached serpentinites constituting the wall rock of the veins. These magnetic data are consistent with bulk rock chemistry showing a lower total iron content in hydrothermalized serpentinites. Hysteresis parameters and thermomagnetic measurements denote a magnetic grains size that increases with the alteration, associated with the emergence of a new magnetic phase (Cr-magnetite) produced by Cr-spinels alteration. A new proxy, based on thermomagnetic measurements, the CrM/M ratio, provides a quantification of its contribution to the magnetic susceptibility. Mineral chemistry allowed to identify the Cr-spinels alteration sequence and reveals an important chlorine enrichment in serpentine phases from hydrothermalized serpentinites. These results suggest that a Cl-rich acidic hydrothermal event involving temperatures below 350 degrees C produced an intense magnetite leaching in the host serpentinite and an advanced Cr-spinels alteration to ferritchromite and Cr-magnetite. Iron provided by this leaching have led to the formation of unique magnetite veins in the Alt Ahmane ultramafic unit. Two different settings are proposed for the hydrothermal event: (1) a continental hydrothermal system as advanced for the Co-Ni-As ores in the Bou Azzer inlier or (2) an oceanic black smoker type hydrothermal vent field on the Neoproterozoic seafloor.

To better constrain Baltica's position within Pangea, we conducted a palaeomagnetic study of Permo-Triassic dykes from the Oslo Graben, as a follow-up to an initial, but rather limited, study by Torsvik and colleagues in 1998. The age of these so-called Lunner dykes had previously been determined as similar to 240 Ma in that study, but details in their analyses and new Ar-40/Ar-39 ages reveal that there may have been some argon loss in the initially dated dyke minerals and that a combined (weighted mean) age of 271 +/- 2.7 (2 sigma) Myr for the dykes is preferable. We find two major components of magnetization in our samples: one carried by an Fe-sulphide (likely pyrrhotite) and the other carried by low-Ti magnetite; these magnetization components may be found together (superposed) in a given sample or they may occur apart. Micronmetre-sized crystals of Ti-Fe oxides, observed with a scanning electron microscope (SEM) show exsolution lamellae, formed upon cooling from intrusion temperatures. Assuming that the submicronmetre-sized (Ti)-magnetite grains that carry a stable remanence are of the same generation as the observed larger grains, we interpret the magnetite remanence in the dykes as of primary, thermoremanent origin. The sulphide remanence appears to be slightly younger, as seen by the SEM observations of pyrite framboids and a Fe-sulphide grain invading a Ti-magnetite grain. Moreover, the sulphide mineralization is likely of region-wide hydrothermal origin. The magnetizations carried by the pyrrhotite and magnetite have nearly identical directions and so, must be nearly of the same age. For this study, we sampled 56 sites including 39 dykes, 10 baked-contact rocks and 7 host rocks removed from the immediate dyke contacts. The dykes and the contact rocks have the same SW and up directions of magnetization, and contain the Fe-sulphide or the magnetite magnetization or both, as diagnosed by their relative unblocking temperatures. However, all the sampled carbonate and igneous host rocks far away from the dykes also have the same directions. Thus, all of the 10 originally planned contact tests are inconclusive. The new palaeopoles of this study are a few degrees apart; the magnetite pole (from dykes only, N = 25) is located at 51 degrees N, 164 degrees E, K = 69, A(95) = 3.5 degrees, whereas the pole calculated from iron sulphide magnetic directions (all rock types, N = 20) is at 54 degrees N, 166 degrees E, K = 112, A(95) = 3.1 degrees. All directions are of reversed polarity, suggesting that the magnetization was acquired during the Kiaman Reversed Superchron. The palaeomagnetic mean result from the magnetite-bearing sites implies a palaeolatitude of Oslo of 23 degrees N, whereas the palaeolatitude calculated from the pyrrhotite magnetizations is 25-27 degrees N, depending on choice of host lithologies. As noted in many previous publications, the palaeomagnetic poles for the late Palaeozoic and Early-Middle Triassic are in conflict with classical Pangea reconstructions. The poles with ages of 250 +/- 10 Ma, in particular, previously showed a discrepancy of some 25 degrees or more, when the Gondwana and Laurussia continents are restored to their juxtapositions in the Pangea-A fit, before the opening of the Atlantic Ocean. Proposed solutions to this conundrum have been controversial, involving doubts about (1) the geocentric coaxial dipole field model, (2) the reliability of the palaeomagnetic results or their ages, or (3) the validity of the Pangea-A reconstruction, leading to proposals of a Pangea B reconstruction in which Gondwana is displaced some 3500 km to the east with respect to Laurussia. The significance of our new result for this Pangea controversy resides in its improved age within an early Guadelupian (mid to late Permian) time interval where few results exist from well-dated igneous rocks in either Baltica or Laurentia. There are quite a few results from sedimentary rocks, but these may be suspected to suffer inclination shallowing, and are therefore less suitable to settle a palaeolatitudinal argument.

P>Large igneous provinces (LIPs) have recently been suggested to originate at the edges of low-velocity zones on the core mantle boundary (Plume Generation Zones). If true, LIPs can potentially be used to constrain paleolongitude in plate tectonic reconstructions. To validate the hypothesis, it is essential to study LIPs of which the paleolongitude can be constrained by other methods, such as hotspot reference frames. An ideal candidate to this end is the early Cenozoic North Atlantic Igneous Province (NAIP). Despite being the largest volcanic unit of the British Tertiary Igneous Province (BTIP, part of the NAIP), the age and paleoposition of the Antrim Lava Group (ALG) in Northern Ireland, which is key to the NAIP as a whole, was hitherto poorly constrained. In this paper, we therefore present an integrated high-resolution paleomagnetic and geochronological study. The ALG is divided into three formations: the Lower Basalt Formation (LBF), Interbasaltic Formation (IBF) and the Upper Basalt Formation (UBF). The IBF is mostly lateritic and encloses the Tardree rhyolite. We offer new age constraints from all three formations using the 40Ar/39Ar method and propose that 62.6 +/- 0.3, 61.3 +/- 0.3 and 59.6 +/- 0.3 Ma (1 Sigma, internal uncertainties) are sound estimates of the age of emplacement of the LBF, Tardree rhyolite (IBF) and UBF, respectively. This constrains the nominal duration of emplacement of the ALG to 3 +/- 0.6 Ma (1 Sigma). This reevaluation of the magnetic signature in the ALG revealed reverse polarity remanence in all three formations and an overall paleomagnetic north pole at latitude 78.9 degrees N, longitude 167 degrees E (A95 = 6.3; age similar to 61 Ma) in the European reference system. This appears consistent with paleomagnetic poles from the rest of the NAIP; both in Europe and Greenland, as well as predictions from modern apparent polar wander paths. The new radiometric ages span magnetochron C26r, C27n and C27r. The normal polarity chron C27n most probably occurred during the IBF hiatus, explaining why no normal polarity remanence was detected in the paleomagnetic investigation. Emplacement of the LBF falls in magnetochron C27r, making this one of the oldest lava sequence in the NAIP; older than the C27n lava pile in Western Greenland. The 60 Ma position of the NAIP in a paleomagnetic reference frame, puts it close to the northern edge of the African large low shear wave velocity anomaly at the core-mantle boundary and therefore in the line with the Plume Generation Zone hypothesis. However, the back-projected Icelandic hotspot, normally considered to have formed the NAIP, is located similar to 1500 km north of the latitude at which the NAIP erupted. The northward motion of the north Atlantic lithosphere since the late Cretaceous challenging the existing correlation of the NAIP to the Icelandic hotspot, normally used to explain the observed pre- and syn-breakup North Atlantic magmatism (63-55 Ma), and either an additional plume located further south in the North Atlantic may be invoked to create the NAIP, or the Icelandic hotspot must have undergone a northward motion together with the North Atlantic lithosphere (which according to present mantle flow models seems unlikely).