EMI Signature Research Articles

Two million years of evolution of the southern central andes retroarc, Payenia Volcanic Province, Argentina, from the study of the Cerro Nevado Volcanic Complex

New K–Ar ages, major and trace element analyses, and Sr and Nd isotopic data for the Cerro Nevado Volcanic Complex (CNVC) coupled with those previously published allow us to better document the hot thermal anomaly which affected the upper mantle beneath the Andean back-arc Payenia Volcanic Province and generated alkaline magmatism. These data, along with previously published ones from this and neighbor volcanic centers, suggest that this anomaly stagnated there, during at least the last 2 million years. Geochemical features from the CNVC basaltic magmas indicate that they were sourced within an OIB-type mantle with an EMI signature, similar to that underlying the neighboring Auca Mahuida Shield Volcano to the south, and attributed to the rise of a mantle thermal anomaly. We identify at least five events of partial melting and seven eruptive phases from 1.88 to 0.75 Ma. Continental crust contamination in the San Rafael Block crustal basement occurred coupled with fractional crystallization processes, probably in magma chambers and across several cycles, producing the trachybasaltic, trachyandesitic and trachytic magmas of the Cerro Nevado volcanic cone (1.34–1.22 Ma).

Gate Oxide Degradation Condition Monitoring Technique for High-Frequency Applications of Silicon Carbide Power MOSFETs

Gate oxide degradation, which considerably affects turn- <sc xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">on</small> /- <sc xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">off</small> dynamics of the switch, embraces a large percentage of chip-related failure modes both in silicon and silicon carbide power MOSFETs. The gate oxide layer is thinner in silicon carbide power MOSFETs in comparison to their silicon-based counterparts. Consequently, the problem of gate oxide degradation has become a more crucial impediment in achieving reliable performance in silicon carbide power MOSFETs. This problem is even more severe in high-frequency applications due to higher EMI signature and complicated and costly measurement. In this article, a reliable fully analog cost-effective gate oxide degradation condition monitoring technique is proposed and validated. High-order harmonics magnitudes of drain–source voltage are used to produce a dc signal as the aging precursor of the gate oxide region. Using a dedicated degradation setup, the credibility of the developed condition monitoring technique was examined at different rates of gate oxide degradation for 650-V/22-A silicon carbide discrete MOSFET. In 200-kHz, 217-V switch operation, the proposed precursor showed 68% change in comparison to its initial value. This brings a high-resolution assessment on the reliability level of the switch gate oxide region.

Geochronology and Petrogenesis of Early Pleistocene Dikes in the Changbai Mountain Volcanic Field (NE China) Based on Geochemistry and Sr-Nd-Pb-Hf Isotopic Compositions

Changbai Mountains intraplate volcanism (NE China) developed above the 500 km deep stagnant portion of the Pacific slab in the last 20 Ma. The more recent activity includes a shield-forming stage (2.8–0.3 Ma), the Tianchi cone construction stage (1.5–0.01 Ma), and a caldera-forming stage (0.2 Ma-present). Detailed studies on the petrogenesis of the volcanic products between the first two stages and the possible role of geodynamics and local tectonics in controlling the volcanism, however, are lacking. Here, we present structural and whole-rock geochemical and zircon Hf isotopic data on Pleistocene dikes of the Changbai Mountains at the transition from the shield-forming to the Tianchi stage with the aim to constrain their age and the source(s) of their parental magma. The dikes represent the shallower feeding system of monogenetic cones and have a NW-SE strike, which is also the preferred strike of the major fault affecting the area and along which the Changbai Mountains monogenetic scoria cones align. The dikes have a potassic affinity and a trachybasaltic composition. Their zircon U–Pb age is 1.19–1.20 Ma (Calabrian). The trachybasalts are enriched in Rb, Ba, Th, U, Nb, Ta, K, Pb, and LREE and slightly depleted in Sr, Zr, Hf, Ti, and HREE with a weak negative Eu/Eu* (δEu = 0.96–0.97). Trace elements and isotopic compositions are compatible with an OIB-type source with an EMI signature. The calculated (87Sr/86Sr)i(=0.705165–0.705324), (143Nd/144Nd)i(= 0.512552–0.512607, εNd(t) = −0.58 to −1.65), and Hf model ages (TDM2) of 1768–1562 Ma suggest that the trachybasaltic dikes were contaminated by a Mesoproterozoic, relatively basic lower crust. The source of the Calabrian trachybasalts consists of asthenospheric melts modified by a subcontinental lithospheric mantle. These melts upwell from depth and stop at the crust-mantle interface where underplating processes favor the assimilation of ancient lower crust material. During the ascent to the surface along deep-seated crustal discontinuities, these magmas weakly differentiate.

A multiple-system study of the geochemical evolution of the mantle with force-balanced plates and thermochemical effects

Here, multiple isotope systems are tracked simultaneously in models of mantle convection and it is show that this can provide powerful constraints on the role of oceanic crust recycling in the development of isotopic end-member compositions. The dynamical models are based on high-resolution cylindrical calculations with force-balanced plates and variable chemical density. The dynamic results span a parameter space of variable realistic excess crustal density compared to experimental estimates and convective vigor measured by plate velocities and surface heat flow. Isotope geochemistry is then modeled for the U–Th–Pb, Sm–Nd, Rb–Sr, and Re–Os isotope systems. The role of a dense crustal layer in development of a HIMU-isotope signature is confirmed. The extraction of continental crust is found to be essential for the formation of all isotope compositional end-members, including HIMU. This extraction is implemented as an ad-hoc process secondary to partial melting at mid-ocean ridges and constrained by estimated isotopic abundances in the present-day crust. Whereas previous studies generated mantle isotopic arrays that spanned DMM–HIMU, the additional isotope systems in this analysis indicate that enrichment purely from ancient oceanic crust may also generate an EM-I component without invoking the subduction of sediment. In this case, the EM-I signature may be indicative of mantle enriched by oceanic crust produced before 2.25 Byr, while the HIMU signature indicates enrichment by oceanic crust extracted more recently. However, it is found to be difficult to maintain a true DMM isotopic end member in Sr–Nd isotope space when significantly enriched end-members are present. This may highlight the sensitivity of the Rb–Sr system to mass exchange between the upper and lower mantle.

PETROLOGY, GEOCHEMISTRY AND Sr-Nd ISOTOPES OF THE PALEOPROTEROZOIC MAFIC DYKES FROM THE GOIÁS-CRIXÁS ARCHEAN BLOCK, GOIÁS STATE, BRAZIL

Paleoproterozoic mafic dyke swarm that intruded the Goias-Crixas Archean Block comprises two groups of dykes: the high Ti (HTi - TiO2 >1.5%) and the low Ti (TiO2 < 1.5%) tholeiites. HTi and LTi have mg# values ranging from 0.49 to 0.31 and from 0.33 to 0.18 respectively. HTi is enriched in LILE (Rb, Ba, K) and LREE (La, Ce) with respect to LTi. HTi incompatible trace elements (IE) patterns and ratios are broadly intermediate between E-MORB and OIB, whereas LTi is closer to E-MORB, but exhibits some N-MORB features. However Nb anomalies are always negative. The studies on the origin of HTi and BTI reveled that mantle heterogeneity is the only possible explanation. The initial 87Sr/86Sr (Sri) ranges from 0.700 to 0.705 (eSr –21.89 to + 46.52) and the initial 143Nd/144Nd (Nd i) varies from 0.5092 to 0.5120 (eNd – 4.84 to + 0.53) at 2.49 Ga. Isotopic and geochemical data suggest a mixed parental source constituted by DMM and EMI components. It is suggested that the EMI signature originates from an oceanic slab containing eclogites and sediments, subducted during Paleoproterozoic or Archean times. Metasomatic fluids and/or melts from the eclogites account for greater and lesser IE mantle enrichment, giving rise to HTi and LTi respectively. The Nb depletion relative to LILE and LREE is due to retention by rutile of eclogites, and characterizes continental and IAB tholeiites. The positive spikes of Ba in HTi can be attributed to pelagic sediments of the slab. The similarity between geochemical and isotopic data of the Goias dykes and others from the Sao Francisco and Amazonic cratons suggest a similar continental environment. Keywords: mafic dykes, Goias, mantle, metasomatism.

LITHOSPHERIC MANTLE EVOLUTION IN THE WESTERN-CENTRAL MEDITERRANEAN AREA: EVIDENCE FROM WITHIN-PLATE CENOZOIC VOLCANISM AND ASSOCIATED MANTLE XENOLITHS

Extension-related within-plate Cenozoic volcanism in the central-western Mediterranean mostly occurs in three provinces: Veneto, Adria plate, i.e. African northern promontory; Iblei (Sicily), African lithosphere; and Sardinia, a drifted fragment of the European lithosphere. In Veneto (Paleogene) and Iblei (Neogene-Quaternary), transtensional rift volcanism developed as foreland reaction to collisional processes along the Alpine and Maghrebian chains respectively, generating prevailing basic magmas - from tholeiites to Na-alkali basalts and nephelinites. In Sardinia, the Pliocene-Quaternary volcanism - related to general tensional tectonics of the central Mediterranean – produced comparatively more potassic magmas from subalkaline basalts, alkali basalts/trachybasalts to basanites, locally associated with rhyolitic and phonolitic differentiates. Integrated petrogenetic study, based on incompatible element and Sr-Nd-Pb isotope systematics for the three volcanic provinces, leads to the following constraints: 1) the primary magmas, from tholeiites, alkali basalts to basanites and nephelinites, were generated by decreasing melting degrees of progressively deeper lithospheric mantle sources (ca. 30 to 100 km depth); 2) extensive and multiple enrichment processes by alkali-silicate metasomatizing components widely affected all mantle sources, which invariably require 5 – 10% metasomatic volatile-bearing phases, mainly amphibole for Veneto and Iblei, and phlogopite + amphibole for Sardinia; 3) a previously depleted lithospheric mantle (DM) is enriched by prevailing HIMU (with subordinate EMII) geochemical components in both Veneto and Iblean Provinces, whereas in Sardinia distinctly more potassic EMI signature predominates, in addition to HIMU; 4) in all the investigated provinces, the more alkaline deeper basic magmas (compared to sub-alkaline basalts) show relatively more marked HIMU signatures, also with involvement of carbonatitic components in the lowest lithospheric sources where nephelinites were generated. Mantle xenoliths associated to alkaline lavas of the three volcanic provinces, generally represent shallow portions of the lithospheric mantle column (< 40-50 km depth, i.e. in the Mechanical Boundary Layer, MBL) according to thermobarometric estimates and rheologic characteristics (e.g. grain size). Petrological data invariably indicate that these mantle xenoliths underwent a complex compositional evolution, characterized by depletions due to extraction of basic magmas (mostly occurring in the pre-Palaeozoic), and multiple metasomatic enrichments reflected in compositional variations of the constituent mineral phases, particularly pyroxenes, and the variable occurrence of pyrometamorphic textures and new phases, including glass + feldspar + amphibole + phlogopite. The resulting isotopic signatures conform well to those of their host magmas, being dependent on the variable contributions of the DM, HIMU, and EM components: a prevailing HIMU imprint is recorded for both Veneto and Iblean mantle materials, whereas a predominant EMI component, in addition to HIMU, is observed for the Sardinian mantle. A comparison at a circum-Mediterranean scale reveals that the geochemical features of Sardinia are also observed in the Cenozoic anorogenic magmas and associated mantle xenoliths located within the Variscan basement of centralwestern Europe. Peridotite massifs, tectonically emplaced during the Alpine orogeny, are similarly characterized (in addition to HIMU) by a prevalent EMI signature, which could thus have been active at least since the pre-Mid Mesozoic. By contrast, EMI signature is lacking in Veneto and Iblei, as well as in the mantle xenoliths and host alkaline lavas from other occurrences of the North African domain, where the HIMU component predominates. The widespread presence of HIMU in the European/North African domains may be related to the existence of a Cenozoic plume region extending from the eastern Atlantic to Central Europe and Central-Western Mediterranean, also suggested by the seismic tomographic data. Accordingly, the more recent HIMU metasomatizing agents rising from the convecting mantle appear to have been effectively accumulated in the lower lithospheric portions (i.e. Thermal Boundary Layer, TBL), whereas older metasomatic components (e.g. EMI for Europe) may have been better preserved in the upper, more rigid lithospheric mantle (i.e. MBL).

Geochemical and isotopic characteristics of the Kinan Seamount Chain in the Shikoku Basin.

The Kinan Seamount Chain lies close to the extinct spreading center of the Shikoku Basin, one of the largest marginal basins in the western Pacific region. Geochemical and isotopic analyses reveal that basalts from the Kinan Seamount Chain have the following characteristics that differ from the Shikoku Basin tholeiite: 1) more incompatible emelent enriched compositions, 2) lower abundance of the heavy rare earth elements (HREE), and 3) higher 87Sr/86Sr and lower 143Nd/144Nd ratios. These results indicate: 1) a more enriched mantle source for basalts from the Kinan Seamount Chain, 2) existence of clinopyroxene and/or amphibole in their residual mantle, and 3) mixing of magmas from a normal MORB source with the EMI signature and an ocean-island basalt (OIB) source with the EMII signature; and 4) the latter has a more significant contribution to the Kinan Seamount Chain basalts than the Shikoku Basin tholeiite.

Sr, Nd, and Pb isotopes of ultramafic xenoliths in volcanic rocks of Eastern China: enriched components EMI and EMII in subcontinental lithosphere

The U-Th-Pb, Sm-Nd, and Rb-Sr isotopic systematics of mafic and ultramafic xenolithic rocks and associated megacrystic inclusions of aluminous augite and garnet, that occur in three alkalic volcanic suites: Kuandian in eastern Liaoning Province, Hanluoba in Hebei Province, and Minxi in western Fujian Province, China are described. In various isotopic data plots, the inclusion data invariably fall outside the isotopic ranges displayed by the host volcanic rocks, testifying to the true xenolithic nature of the inclusions. The major element partitioning data on Ca, Mg, Fe, and Al among the coexisting silicate minerals of the xenoliths establish their growth at ambient mantle temperatures of 1000–1100°C and possible depths of 70–80 km in the subcontinental lithosphere. Although the partitioning of these elements reflects equilibrium between coexisting minerals, equilibria of the Pb, Nd, and Sr isotopic systems among the minerals were not preserved. The disequilibria are most notable with respect to the 206Pb/ 204Pb ratios of the minerals. On a Nd-Sr isotopic diagram, the inclusion data plot in a wider area than that for oceanic basalts from a distinctly more depleted component than MORB with higher 143Nd/ 144Nd and a much broader range of 87Sr/ 86Sr values, paralleling the theoretical trajectory of a sea-water altered lithosphere in Nd-Sr space. The garnets consistently show lowerμ andκ values than the pyroxenes and pyroxenites, whereas a phlogopite shows the highestμ andκ values among all the minerals and rocks studied. In a plot ofΔ207 andΔ208, the host basalts for all three areas show lowerΔ207 and higherΔ208 values than do the xenoliths, indicating derivation of basalts from Th-rich (relative to U) sources and xenoliths from U-rich sources. The xenolith data trends toward the enriched mantle components, EMI and EMII-like, characterized by high 87Sr/ 86Sr andΔ207 values but with slightly higher 143Nd/ 144Nd. The EMI trend is shown more distinctly by the host basalts. The EMII mantle domain may be present in the Chinese continental lithosphere just above the EMI domain of the basalt source at the lower part of the lithosphere. We argue that the ancient depleted continental lithosphere was metasomatized, imparting the EMI signature, in earlier times ( > 1000 m.y.), and U migrated upward, resulting in highTh/U ratios in the lower portion of the lithosphere. Observed highTh/U,Rb/Sr, 87Sr/ 86Sr andΔ208, lowSm/Nd ratios, and a large negativeɛ Nd in phlogopite pyroxenite with a depleted mantle model age of 2.9 Ga, support our contention that metasomatized continental lower mantle lithosphere is the source for the EMI component. We also suggest that the EMII signature may have been introduced later (less than ∼ 500 Ma) by another metasomatic event during the subduction of an oceanic plate, which was partially responsible for some of the observed inter-mineral isotopic disequilibria.