the stability of plagioclase in the upper mantle subsolidus experiments on fertile and depleted lherzolite

Plagioclase peridotites are important markers of processes that characterize the petrological and tectonic evolution of the lithospheric mantle in extensional tectonic settings. Studies on equilibrated plagioclase peridotites have documented continuous chemical changes in mantle minerals in response to plagioclase crystallization, potentially tracing the re-equilibration of mantle peridotites up to very low pressure.This experimental study provides new constraints on the stability of plagioclase in mantle peridotites as a function of bulk composition, and the compositional and modal changes in minerals occurring within the plagioclase stability field as a function of P^T^bulk composition. Subsolidus experiments have been performed at pressures ranging from 0·25 to 1·0 GPa, and temperatures ranging from 900 to 12008C on fertile and depleted anhydrous lherzolites modelled in the system TiO2^Cr2O3^Na2O^CaO^FeO^ MgO^Al2O3^SiO2 (Ti,Cr-NCFMAS). In the fertile lherzolite (Na2O/CaO 1⁄4 0·08; XCr 1⁄4 0·07) a plagioclase-bearing assemblage is stable up to 0·7 GPa, 10008C and 0·8 GPa, 11008C, whereas in the depleted lherzolite (Na2O/CaO 1⁄4 0·09; XCr 1⁄4 0·10) the upper limit of plagioclase stability is shifted to lower pressure.The boundary between plagioclase lherzolite and spinel lherzolite has a positive slope in P^T space. In a complex chemical system, the plagioclase-out boundary is multivariant and sensitive to the XCr value [XCr 1⁄4 Cr/(Cr þAl)] of spinel.This latter is controlled by the reaction MgCr2O4 þ CaAl2Si2O8 1⁄4 MgCrAlSiO6 þ CaCrAlSiO6, which is a function of the Cr^Al partitioning between spinel and pyroxenes, and varies with the XCr value and chromite/ anorthite normative ratio of the bulk composition. Within the plagioclase stability field, the Al content of pyroxenes decreases, coupled with an increase in the anorthite content in plagioclase, and Ti and XCr in spinel with decreasing pressure; these chemical variations are combined with systematic changes in modal mineralogy governed by a continuous reaction involving both plagioclase and spinel. As a consequence, the composition of plagioclase varies significantly over a rather narrow pressure range and is similar at the same P^T conditions in the investigated bulk-rocks.This suggests the potential application of plagioclase composition as a geobarometer for plagioclase peridotites

Las facies canalizadas de la Secuencia Deposicional de Banastón (prov. Huesca)

En la Secuencia Deposicional de Banastón se pone en evidencia, la existencia a distintos niveles estratigráficos, de dos unidades canalizadas. La unidad inferior (áreas de Banastón y San Vicente) está caracterizada por cuerpos pequeños lenticulares de arenisca y conglomerados. Las paleocorrientes muestran una dirección al NW. La Unidad superior (área de Boltaña) muestra una gran potencia de depósitos residuales (conglomerados clast-supported) seguida por capas espesas de areniscas amalgamadas. La dirección de paleocorriente presenta un sentido hacia el N. Ello unido a la relación de onlap con los materiales carbonático-margosos del anticlinal de Boltaña sugieren que dicho anticlinal era un suave umbral en el momento de la sedimentación de la Secuencia Deposicional de Banastón.In the Ainsa-Boltaña region, two channel-fill units occur in the Banaston Sequence at different stratigraphic level. The lower unit (Banaston and San Vicente areas) is characterized by small, lenticular sandy and conglomeratic bodies. Paleocurrent directions are to the NW. The upper unit (Boltaña area) shows a sequence of lag deposits (clast supported conglomerates) overlain by thick-bedded sandstones with erosional features. Paleocurrent directions in this area are mainly to the N. Onlap relations with the Guara Limestones and paleocurrent trend suggest that the Boltaña anticline formed a gentle threshold during Banaston Sequence deposition

Facies de abanico fluvial en los afloramientos orientales de Formación Peraltilla

La Formación Peraltilla está constituida por una sucesión estratigráfica heterolitica, del Oligoceno inferior. El análisis de facies indica una asociación de ambientes de abanicos fluviales húmedos, formados por canales múltiples de baja sinuosidad, y una tendencia vertical a la progradación de los sistemas fluviales

Distribución y ordenación de Microcodium en la Formación Tremp: anticlinal de Campllong (Pirineos Orientales, provincia de Barcelona)

La abundancia excepcional de Microcodium en los sedimentos continentales de la Formación Tremp en el anticlinal de Campllong (Llinás de Berga, Provincia de Barcelona) es un rasgo característico en este tipo de sedimentos, igual que ocurre en otros puntos de los Pirineos. Diferentes facies litológicas (limos, calizas, areniscas y conglomerados) ricas en Microcodium se asocian en tres secuencias sedimentológicas de: a) somerización lacustre b) paleosuelos calcimorfos y c) relleno de canales fluviales. Los Microcodiums desarrollados in situ aparecen como individuos (colonias) dispersos y como agregados. Los agregados pueden ser masivos, rellenar cavidades ramificadas y formar a techo de capas entramados muy tupidos a modo de tapices. Los prismas disgregados y los fragmentos de individuos de Microcodium se presentan como componentes retrabajados y en ocasiones rellenan cavidades producidas por anélidos. El porcentaje de Microcodium en cada facies y a lo largo de cada secuencia varía considerablemente (5-100%). La abundancia excepcional de Microcodium sugiere una importante exposición sub aérea en la zona más distal de un abanico aluvial, donde tendría lugar el desarrollo de una cubierta vegetal, responsable de un posible proceso de calcitización de raíces

Origin of pyroxenites in the oceanic mantle and their implications on the reactive percolation of depleted melts

Pyroxenites are diffuse in fertile mantle peridotites and considered an important component in the mantle source of oceanic basalts. They are rarely documented in abyssal and ophiolitic peridotites representing residual mantle after melt generation, and few studies defining their origin are to date available. We present a field-based microstructural and geochemical investigation of the pyroxenite layers associated with depleted peridotites from the Mt. Maggiore ophiolitic body (Corsica, France). Field and petrographic evidence indicate that pyroxenite formation preceded the melt\u2013rock interaction history that affected this mantle sector during Jurassic exhumation, namely (1) spinel-facies reactive porous flow leading to partial dissolution of the pyroxenites, and (2) plagioclase-facies melt impregnation leading to [plagioclase + orthopyroxene] interstitial crystallization. Pyroxenes show major element compositions similar to abyssal pyroxenites from slow-spreading ridges, indicative of magmatic segregation at pressures higher than 7 kbar. Both the parental melts of pyroxenites and the melts involved in the subsequent percolation were characterized by Na2O-poor, LREE-depleted compositions, consistent with unaggregated melt increments. This implies that they represent the continuous evolution of similarly depleted melts leading to different processes (pyroxenite segregation and later melt\u2013rock interaction) during their upward migration. To support the genetic relation and the continuity between the formation of pyroxenites and the subsequent melt\u2013rock interaction history, we modeled all the documented processes in sequence, i.e.: (1) formation of single-melt increments after 6% mantle decompressional fractional melting; (2) high-pressure segregation of pyroxenites; (3) spinel-facies reactive porous flow, (4) plagioclase-facies melt impregnation. The early fractionation of pyroxenites leads to a decrease in pyroxene saturation that is necessary for the subsequent reactive porous flow process, without any significant change in the melt REE composition

Facies de abanicos fluvial en los afloramientos orientales de la formación peratalita

La Formación Peraltilla está constituida por una sucesión estratigráfica heterolitica, del Oligoceno inferior. El análisis de facies indica una asociación de ambientes de abanicos fluviales húmedos, formados por canales múltiples de baja sinuosidad, y una tendencia vertical a la progradación de los sistemas fluviales.The Peraltilla Formation consits of a heterogeneous succession of Early Oligocene age. Facies analysis revealed a characteristic paleoenvironmental association of wet fluvial fan deposits, formed by multiple low sinuosity channels. The stratigraphic succession evidences an evolution upward related to the progradation of the fluvial systems

Ligurian pyroxenite-peridotite sequences (Italy) and the role of melt-rock reaction in creating enriched-MORB mantle sources

Deep melt intrusion and melt-peridotite interaction may introduce small-scale heterogeneity in the MORB mantle. These processes generate pyroxenite-bearing veined mantle that represent potential mantle sources of oceanic basalts. Natural proxies of such veined mantle are very rare and our understanding of mechanisms governing the chemical modification of mantle peridotite by MORB-type pyroxenite emplacement is very limited. We report the results of detailed spatially-controlled chemical profiles in pyroxenite-peridotite associations from the Northern Apennine ophiolitic mantle sequences (External Liguride Units, Italy), and investigate the extent and mechanism driving the local modification of peridotite by the interaction with pyroxenite-derived melt. Pyroxenites occur as cm-thick layers parallel to mantle tectonite foliation and show diffuse orthopyroxene-rich reaction rims along the pyroxenite-peridotite contact. Relative to distal unmodified peridotites, wall-rock peridotites show i) modal orthopyroxene enrichment at the expense of olivine, ii) higher Al, Ca, Si contents and slightly lower XMg, iii) Al-richer spinel and lower-XMg pyroxenes. Clinopyroxenes from wall-rock peridotites exhibit variable LREE-MREE fractionation, always resulting in SmN/NdN ratios lower than distal peridotites. From the contact with pyroxenite layers, peridotite clinopyroxenes record a REE compositional gradient up to about 15\u202fcm marked by an overall REE increase away from the pyroxenite. Beyond 15\u202fcm, and up to 23\u202fcm, the MREE and HREE content decreases while the LREEs remain at nearly constant abundances. This REE gradient is well reproduced by a two-step numerical simulation of reactive melt percolation assuming variable amounts of olivine assimilation and pyroxene crystallization. Percolative reactive flow at decreasing melt mass and rather high instantaneous melt/peridotite ratio (initial porosity of 30%), combined with high extents of fractional crystallization (i.e. relatively low Ma/Mc ratio), accounts for the overall REE enrichment in the first 15\u202fcm. Change of melt-rock reaction regime, mostly determined by the drastic decrease of porosity (\u3a6i\u202f=\u202f0.01) due to increasing crystallization rates, results in more efficient chemical buffering of the host peridotite on the HREE composition of the differentiated liquids through ion-exchange chromatographic-type processes, determining the observed increase of the LREE/HREE ratio. Emplacement of thin (cm-sized) pyroxenite veins by deep melt infiltration is able to metasomatize a much larger volume of the host peridotite. Hybrid mantle domains made by pyroxenite, metasomatized peridotite and unmodified peridotite potentially represent mantle sources of E-MORB. Results of this work stress the key role of melt-peridotite reactions in modifying the upwelling mantle prior to oceanic basalts production

Melt migration and melt-rock reaction in the Alpine-Apennine peridotites: Insights on mantle dynamics in extending lithosphere

The compositional variability of the lithospheric mantle at extensional settings is largely caused by the reactive percolation of uprising melts in the thermal boundary layer and in lithospheric environments. The Alpine-Apennine (A-A) ophiolites are predominantly constituted by mantle peridotites and are widely thought to represent analogs of the oceanic lithosphere formed at ocean/continent transition and slow- to ultraslow-spreading settings. Structural and geochemical studies on the A-A mantle peridotites have revealed that they preserve significant compositional and isotopic heterogeneity at variable scale, reflecting a long-lived multi-stage melt migration, intrusion and melt-rock interaction history, occurred at different lithospheric depths during progressive uplift. The A-A mantle peridotites thus constitute a unique window on mantle dynamics and lithosphere-asthenosphere interactions in very slow spreading environments. In this work, we review field, microstructural and chemical-isotopic evidence on the major stages of melt percolation and melt-rock interaction recorded by the A-A peridotites and discuss their consequences in creating chemical-isotopic heterogeneities at variable scales and enhancing weakening and deformation of the extending mantle. Focus will be on three most important stages: (i) old (pre-Jurassic) pyroxenite emplacement, and the significant isotopic modification induced in the host mantle by pyroxenite-derived melts, (ii) melt-peridotite interactions during Jurassic mantle exhumation, i.e. the open-system reactive porous flow at spinel facies depths causing bulk depletion (origin of reactive harzburgites and dunites), and the shallower melt impregnation which originated plagioclase-rich peridotites and an overall mantle refertilization. We infer that migrating melts largely originated as shallow, variably depleted, melt fractions, and acquired Si-rich composition by reactive dissolution of mantle pyroxenes during upward migration. Such melt-rock reaction processes share significant similarities with those documented in modern oceanic peridotites from slow- to ultraslow-spreading environments and track the progressive exhumation of large mantle sectors at shallow depths in oceanic settings where a thicker thermal boundary layer exists, as a consequence of slow-spreading rate