M. Benoit

Late Miocene adakites and Nb-enriched basalts from Vizcaino Peninsula, Mexico: Indicators of East Pacific Rise subduction below southern Baja California?

A typical slab melt association was emplaced from 11 to 8 Ma in the Santa Clara volcanic field, Vizcaino Peninsula, Baja California Sur. It includes adakitic domes and associated pyroclastic flow deposits, together with lava flows of niobium-enriched basalts. The trace element and isotopic (Sr-Nd-Pb) signatures of adakites are consistent with melting of altered mid-ocean ridge basalts, and the sources of the Nb-enriched basalts contain an enriched mantle wedge component. Such associations commonly form at depths of 70–80 km during low-dip subduction of very young oceanic crust. However, the Santa Clara field is relatively close (100 km) to the paleotrench, which suggests that the genesis of its adakites and Nb- enriched basalts occurred in a very high thermal regime linked to the subduction of the then-active Guadalupe spreading center of the East Pacific Rise. Our data suggest that the asthenospheric window documented below northern Baja California also developed beneath the south of the peninsula during the Neogene. This hypothesis is consistent with the spatial distribution and the ages of adakites and magnesian andesites from this region.

Spatial and temporal evolution of basalts and magnesian andesites (“bajaites”) from Baja California, Mexico: the role of slab melts

Late Miocene to Quaternary basalts and associated magnesian basaltic andesites and andesites, locally referred to as ‘‘bajaites’’, occur in the central part of the Baja California (BC) Peninsula. They form five volcanic fields (Jaraguay, San Borja, San Ignacio, Santa Rosalia, La Purisima) delineating a 600-km-long array parallel to the Gulf of California. They range in age from Late Miocene to Pleistocene, and display very specific geochemical characteristics: SiO2=50% to 58%, high MgO contents, very low FeO*/MgO ratios usually less than 1.5, highly fractionated rare earth element patterns with low Yand heavy rare earth element, very high Sr (commonly between 2000 and 3000 ppm) and Ba (up to 2300 ppm) contents. The geochemical study and K–Ar dating of ca. 50 samples of these rocks allow us to show that most of their incompatible element ratios, which vary significantly in space and time, reflect source heterogeneities rather than partial melting, fractional crystallisation or crustal contamination effects. Their slab melt imprint increases from northwest to southeast and with time. It is best expressed in the geochemical signatures of Quaternary lavas from La Purisima volcanic field. These features reflect the origin of the ‘‘bajaites’’ by melting of mantle peridotites previously metasomatised by slab melts, in connection with the opening of an asthenospheric window below the Baja California Peninsula during Early and Middle Miocene in northern Baja California, and during Late Miocene in southern Baja California. Melting was initiated by the high thermal regime accompanying ridge subduction or slab tearing/breakoff, and later by Plio-Pleistocene thermal pulses linked to the opening of the Gulf of California. We show that the incongruent melting of metasomatic pargasitic amphibole, leaving a garnet-rich residue, accounts for most of the specific geochemical features of the magnesian andesite suite. This breakdown started at ca. 1000 jC at depths of 70–110 km, and amphibole was probably not entirely consumed during the melting process. D 2002 Elsevier Science B.V. All rights reserved.

Geochemical Diversity of Late Miocene Volcanism in Southern Baja California, Mexico: Implication of Mantle and Crustal Sources during the Opening of an Asthenospheric Window

Five main petrologic and geochemical groups can be identified among the Middle to Late Miocene lavas from the western part of southern Baja California: (1) calc‐alkaline and K‐rich andesites emplaced between 15.5 and 11.7 Ma; (2) adakites and (3) associated niobium‐rich basalts erupted between 11.7 and 8.5 Ma in the Santa Clara volcanic field, Vizcaino Peninsula; (4) 10.6–9.2 Ma tholeiitic basalts and basaltic andesites that form large tabular plateaus near San Ignacio; and (5) magnesian and basaltic andesites of adakitic affinity whose emplacement started at 11.7 Ma south of San Ignacio and between 9.7 and 8.8 Ma near La Purisima. These lavas, although spatially and temporally related, display very different geochemical signatures. Their trace elements and isotopic characteristics suggest that three different magma sources were involved in their genesis. Partial melts of subducting altered oceanic crust produced the adakites when erupted directly at the surface. These magmas were eventually trapped in the mantle wedge where they reacted with ultramafic lithologies. Such slab‐melt‐metasomatized mantle could then melt to produce niobium‐rich basalts or magnesian andesites, depending on the pressure that controlled the stability of garnet into the mantle wedge. The melting of fluid‐metasomatized mantle wedge led to the emplacement of andesites. In southern Baja California, the opening of a slab window following active ridge subduction resulted in the additional contribution of partial melts from the suboceanic mantle uprising through the tear in the slab. This process might be responsible for the occurrence of tholeiitic basalts and basaltic andesites near San Ignacio. The studied association can be considered as a modern analog of high‐thermal‐regime Archean subductions.

Trace element and isotopic characterization of mafic cumulates in a fossil mantle diapir (Oman ophiolite)

This paper is devoted to an exploratory geochemical study (trace elements, SrNd isotopes) of a suite of cumulate features cropping out in the mantle harzburgites of Oman. The cumulates are concentrically distributed around a fossil mantle diapir. From the centre to the periphery of the diapir, their mineralogy becomes more and more evolved, from troctolite to olivine gabbro and gabbronorite, and their textural and field characteristics point to injection in a cooler environment. Nd isotopic data are consistent with a mantle origin for all these cumulates. The mantle source is heterogeneous on a small scale (6.09 <ϵNd < 1015) but this heterogeneity and the average ϵrmNd (≈8) are comparable to those of Indian Ocean mid-ocean ridge basalts (MORBs). An origin of this cumulate suite, largely by fractional crystallization from liquids with MORB characteristics, is supported by major and compatible trace element variations. However compatible elements show that it is unlikely that all these cumulates are derived by different degrees of fractional crystallization from the same liquid. This conclusion is corroborated by incompatible trace element data. The calculated liquids in equilibrium with these cumulates have MORB-like REE patterns. However, variations in REE patterns from one lithological group to the next, and within each lithological group, imply a pronounced heterogeneity in the REE content of the equilibrium liquids that clearly does not result from a simple fractional crystallization process. Liquids in equilibrium with the troctolites have a higher range in [La]n/[Yb]n than liquids in equilibrium with the olivine gabbros. The most evolved cumulates (gabbronorites) have REE patterns that might imply a more complex origin for their parental melt involving mixing of MORB-like liquids with melts from a LREE-depleted source.

The remelting of hydrothermally altered peridotite at mid-ocean ridgesby intruding mantle diapirs

Most gabbroic cumulates found at ocean spreading centres are thought to have been generated by the fractional crystallization of melts with the composition of mid-ocean ridge basalt (MORB). There are exceptions, however, including some cumulates which appear to have come from melts that contain more silica than MORB and are much more depleted in the incompatible elements (those elements that do not readily substitute into the main mineral phases). These unusual rocks bear witness to relatively deep petrological processes that are not accessible through the study of melts erupted on the sea floor, and their origin is still debated. Fortunately, the same lithologies can be studied in detail in ophiolites (sections of oceanic crust accreted to a continent). In a fossil mantle diapir of the Oman ophiolite, we have observed the same dichotomy between a suite of ‘normal’, MORB-type, cumulates (‘N-cumulates’) and a suite of cumulates issued from silica-enriched but incompatible-element-depleted melts (‘D-cumulates’). While the N-cumulates crystallized inside the diapir, the D-cumulates occur essentially as intrusions surrounding the diapir. The combination of silica enrichment, extreme depletion in incompatible elements, and seawater isotopic signature indicates that the D-cumulates were formed by the remelting at low pressure of hydrated residual peridotites left after MORB extraction at the ridge axis. The distribution of the D-cumulates relative to the N-cumulates suggests that such depleted melts are produced episodically at ridge axes when the lithospheric mantle is reheated by a new diapiric pulse.

Tectonic setting for the genesis of oceanic plagiogranites: evidence from a paleo-spreading structure in the Oman ophiolite

Abstract In the Oman ophiolite, detailed structural and lithological mapping of the 60 × 45 km Maqsad area has revealed several plagiogranite intrusions, ranging from metre scale dykes to hectometre scale bodies. These plagiogranites are spatially related to kilometre scale mafic plutons, displaying generally pegmatitic textures and locally deformed in amphibolite facies conditions along shear zones up to a few hundred metres thick. Isotope data demonstrate the mantle origin of these pegmatites and plagiogranites: ϵ Nd ranges from 6.3 to 9.9 and has an average value (26 samples) of 8.1, within the field of present-day MORBs, and similar to the average ϵ Nd (7.8) of the Oman ophiolite primary igneous sequence. These intrusions are not restricted to a given structural level: their paleovertical extent exceeds 5–6 km, from 2 km below the Moho up to the base of the sheeted dyke complex. Their modal composition evolves up section, from predominantly pyroxenites and gabbro norites in the mantle harzburgites to gabbro norites, diorites and quartz diorites at crustal level. Petrography, whole rock and mineral chemistry show that this vertical succession of lithologies can be simply explained by low pressure fractional crystallization of a hydrated basaltic melt. Plagiogranites, although more abundant at upper crustal level, are not restricted to this horizon. Plagioclase composition in plagiogranites presents a remarkable evolution with paleodepth, from highly calcic (70–95% an ) in the mantle section, intermediate and highly scattered in the lower crust (10–85% an ), to quite sodic at upper crustal level (5–20% an ). Highly calcic plagioclases in quartz-bearing rocks is a puzzling character, inconsistent with fractional crystallization. Even if high degrees of fractional crystallization of a hydrated basaltic melt remain, the simplest way to account for the chemistry of the upper crustal plagiogranites, processes such as fluid-induced remelting or assimilation of country rocks must be invoked to account for the chemistry and field characteristics of the deeper plagiogranites. Mantle and crustal structures in the Maqsad area indicate that the pegmatites and plagiogranites emplaced in an ocean ridge setting, when a mantle diapir, soaked with basaltic melt, intruded a cool, hydrothermally altered, lithosphere. They were eventually transposed in an off-axis setting as the ascension of the diapir progressed up to Moho level. It is concluded that the formation of large volumes of buoyant leucocratic rocks is possible at ocean ridges, provided periods of amagmatic spreading are long enough to induce the growth and alteration of an axial lithospheric lid. This is likely more frequent at slow spreading centres where mantle upwelling is discontinuous in space and time.

Newly identified segments of the Pacific–Australia plate boundary along the North Fiji transform zone

Abstract The North Fiji transform zone, a 1500 km long and 200 km wide transform segment of the Pacific–Australia plate boundary, is one of the major transform fault systems of the Earth. New data collected during the ALAUFI cruise (March 2000) on board the R/V L’Atalante make it possible to define more accurately the geometry and kinematics of this transform plate boundary. Three spreading centers or extensional zones (the North Cikobia spreading center, the Futuna spreading center and the southeast Futuna volcanic zone) and a strike-slip fault zone (the Futuna transform fault) have been discovered over a distance of 500 km along the eastern North Fiji transform zone, from the north of the Fiji platform to the east of the Futuna archipelago. The Futuna transform fault oriented 100° has been mapped over a distance of 250 km. It must be considered to be an important tectonic element of the transform plate boundary. Pure strike-slip as well as transpression and transtension motions are responsible for the complex morphology of this feature. The uplifted Futuna–Alofi ridge represents a major compressional relay along the Futuna transform fault. The Futuna spreading center trending 20–30° is composed of a series of en echelon left-stepping spreading segments. It represents a 200 km long extensional relay between the Futuna transform fault and the western part of the North Fiji transform zone, the Fiji transform fault, which bounds the Fiji platform to the north. The opening rate at the Futuna spreading center is estimated at 4 cm/yr. Although the North Cikobia spreading center and the southeast Futuna volcanic zone have been only partly mapped, bathymetric and reflectivity data clearly reveal that active extension also takes place along these two features. A spreading rate of 2 cm/yr is inferred at the North Cikobia spreading center. Therefore, the North Fiji transform zone appears to be composed of two main overlapping transform segments relayed by parallel extensional zones. The three active extensional zones have an ENE–WSW to NNE–SSW orientation, while compressive features along the Futuna transform fault are NW–SE to NNW–SSE oriented, in accordance with the present-day left-lateral transform motion along this part of the Pacific–Australia plate boundary.

Planar pixel sensors for the ATLAS upgrade: beam tests results

The performance of planar silicon pixel sensors, in development for the ATLAS Insertable B-Layer and High Luminosity LHC (HL-LHC) upgrades, has been examined in a series of beam tests at the CERN SPS facilities since 2009. Salient results are reported on the key parameters, including the spatial resolution, the charge collection and the charge sharing between adjacent cells, for different bulk materials and sensor geometries. Measurements are presented for n+-in-n pixel sensors irradiated with a range of fluences and for p-type silicon sensors with various layouts from different vendors. All tested sensors were connected via bump-bonding to the ATLAS Pixel read-out chip. The tests reveal that both n-type and p-type planar sensors are able to collect significant charge even after the lifetime fluence expected at the HL-LHC.

A prototype hybrid pixel detector ASIC for the CLIC experiment

A prototype hybrid pixel detector ASIC specifically designed to the requirements of the vertex detector for CLIC is described and first electrical measurements are presented. The chip has been designed using a commercial 65 nm CMOS technology and comprises a matrix of 64 × 64 square pixels with 25 μm pitch. The main features include simultaneous 4-bit measurement of Time-over-Threshold (ToT) and Time-of-Arrival (ToA) with 10 ns accuracy, on-chip data compression and power pulsing capability.

Simulation of Radiation Damage Effects on Planar Pixel Guard Ring Structure for ATLAS Inner Detector Upgrade

We use ac and dc technology computer assisted design simulations to study the effects of radiation damage in the planar pixel sensors of the ATLAS inner detector upgrade, in particular on the active area and breakdown protection of different multiguard rings structure. Using a model of defect introduction into silicon band gap that agrees well with simulation up to fluences of 1015neq/cm2, we simulated how high radiation damage phenomena like space-charge sign inversion and double-junction formation affects the efficiency of different models of multiguard ring structures in n-type and p-type bulk. Depletion potential, potential distribution on guard rings, breakdown voltage, and leakage current were extracted from simulations to compare the behavior of the different models.