Plagioclase Grains Research Articles

Melt inclusions in rock-forming and accessory minerals of high-grade metamorphic rocks: Indicators of the nature and formation conditions of their protoliths

Segregations resembling melt inclusions (MI) are rather common in inclusions of mineral-forming substances from the rock-forming minerals of high-grade metamorphic rocks without signs of anatectic melting. Such inclusions were first noted by Khetchikov [1] who interpreted them as premetamorphic MIs that bear information on the nature of protoliths. Unfortunately, this discovery with a great scope for investigations into metamorphic geology remained ignored for a long time. Long-term thermobarogeochemical investigations of metamorphic rocks [2, 3, and others] yielded a great body of information on premetamorphic inclusions. At present, works devoted to this issue (mainly [4, 5] and others) have reliably confirmed the conservation of premetamorphic inclusions in zircons from high-grade metamorphic rocks. However, the existence of such inclusions in rock-forming minerals remains an open issue. Therefore, this paper presents summary characteristics of inclusions interpreted as premetamorphic MIs in rock-forming minerals and data in favor of such an interpretation. Our work is primarily based on results of the optical examination of inclusions in rockforming minerals from various complexes of the Aldan Shield (granulite facies and high-temperature subfacies of the amphibolite facies) coupled with the homogenization data on inclusions obtained with a Leica hightemperature heat chamber. Melt inclusions, 2‐20 µ m in size, occur as solitary inclusions or zonal (commonly, azonal) groups in central parts of clinopyroxene (Cpx), hypersthene (Opx), hornblende (Hbl), plagioclase (Pl), or quartz (Qtz) grains of rocks. The shape of vacuoles is oval and less commonly irregular or negative (Fig. 1). The presence of inclusions in the grains mentioned above is not an obligatory feature. The disposition of MIs and metamorphogenic fluid inclusions (FI) in minerals is a highly informative subject. One can often see intersection or enveloping of some MI parts by chains of metamorphogenic CO 2 -rich inclusions that are typical of the granulite-facies metamorphism. These inclusions are syngenetic (primary) segregations relative to the metamorphogenic part of minerals and the paragenesis of minerals in general. Such relationships indicate that the MI-containing part of the minerals is a relict substance that was not crystallized in the course of metamor

Petrology and mineralogy of granulite-facies mafic xenoliths (Sardinia, Italy): Evidence for KCl metasomatism in the lower crust

Here new mineralogical data is presented on the occurrence of K-feldspar in granulite-facies metagabbronorite xenoliths found in recent alkaline lavas from Western Sardinia, Italy. The xenoliths originated from the underplating of variably evolved subduction-related basaltic liquids, which underwent cooling and recrystallisation in the deep crust ( T = 850–900 °C, P = 0.8–1.0 GPa). They consist of orthopyroxene + clinopyroxene + plagioclase porphyroclasts (An = 50–66 mol%) in a granoblastic, recrystallised, quartz-free matrix composed of pyroxene + plagioclase (An = 56–72 mol%) + Fe–Ti oxides ± K-feldspar ± biotite ± fluorapatite ± Ti-biotite. Texturally, the K-feldspar occurs in a variety of different modes. These include: (1) rods, blebs, and irregular patches in a random scattering of plagioclase grains in the form of antiperthite; (2) micro-veins along plagioclase–plagioclase and plagioclase–pyroxene grain rims; (3) myrmekite-like intergrowths with Ca-rich plagioclase along plagioclase–plagioclase grain boundaries; and (4) discrete anhedral grains (sometimes microperthitic). The composition of each type of K-feldspar is characterized by relatively high albite contents (16–33 mol%). An increasing anorthite content in the plagioclase towards the contact with the K-feldspar micro-vein and myrmekite-like intergrowths into the K-feldspar along the plagioclase–K-feldspar grain boundary are also observed. Small amounts of biotite (TiO 2 = 4.7–6.5 wt.%; F = 0.24–1.19 wt.%; Cl = 0.04–0.20 wt.%) in textural equilibrium with the granulite-facies assemblage is present in both K-feldspar-bearing and K-feldspar-free xenoliths. These K-feldspar textures suggest a likely metasomatic origin due to solid-state infiltration of KCl-rich fluids/melts. The presence of such fluids is supported by the fluorapatite in these xenoliths, which is enriched in Cl (Cl = 6–50% of the total F + Cl + OH). These lines of evidence suggest that formation of K-feldspar in the mafic xenoliths reflects metasomatic processes, requiring an external K-rich fluid source, which operated in the lower crust during and after in-situ high- T recrystallisation of relatively dry rocks.

Investigating the origin of anorthitic plagioclase through a combination of experiments and natural observations

The origin of anorthitic plagioclase in volcanic rocks worldwide remains an enigma. Arenal volcano, Costa Rica, provides a good example of the problem: up to An 96 plagioclase are found within moderately evolved basaltic andesites. We follow up a recent suggestion [Lundstrom, C.C., Boudreau, A., Pertermann, M., 2005. Diffusion-reaction in a thermal gradient: Implications for the genesis of anorthitic plagioclase, high alumina basalt and igneous mineral layering. Earth Planet. Sci. Lett. 237, 829–854] that diffusion-reaction processes occurring between ascending magmas and gabbroic wall rocks might provide a means of producing anorthitic plagioclase without requiring exotic, ultra-calcic melt compositions by integrating observations from three analytical methods: (1) laboratory diffusion-reaction experiments in the diopside–albite–anorthite (DAA) system; (2) transmission electron microscopy (TEM) of plagioclase grains from both a diffusion-reaction experiment and an Arenal lava; and (3) Sr isotope microstratigraphy of phenocrysts and megacrysts from natural lavas from the current eruption at Arenal. Two plagioclase saturated DAA melts, juxtaposed in experiments at either 1 bar pressure (anhydrous) or at 0.5 GPa pressure (hydrous) for 3–5 days at 1244 or 1125 °C, illustrate the dramatic effect of H 2O on facilitating plagioclase reaction. In the presence of wet melt, enrichment in anorthitic plagioclase occurs at the material interface while in the presence of the dry 1 bar melt, plagioclase shows no reaction. TEM analyses of a homogeneous anorthitic plagioclase from a diffusion-reaction experiment indicates heterogeneous columns of more anorthite-rich plagioclase at the < 100 nm scale while a homogeneous anorthitic plagioclase phenocryst from a 1968 lava from Arenal found much of the crystal to be composed of 20–50 μm diameter subgrains, having ∼ 1 μm diameter tubes filled with Fe, K, Si-rich glass. Large core–rim variations in 87Sr/ 86Sr occur within three plagioclase phenocrysts from both 1968 and 1999 Arenal lavas. Although core compositions vary widely (0.7038–0.7052), all three phenocrysts have similar 87Sr/ 86Sr rim compositions (0.7043). The variability in 87Sr/ 86Sr at the phenocryst scale but homogeneity in 87Sr/ 86Sr at the bulk scale in Arenal lavas [Ryder, C.H., Gill, J.B., Tepley III, F.J., Ramos, F., Reagan, M., this volume. Closed to open differentiation at Arenal volcano (1968–2003). J. Volcanol. Geotherm. Res.] could be explained by erupted lavas representing a near steady state balance between ascending magmas and a flux derived from diffusion-reaction processes occurring with surrounding crustal gabbros.

The first find of (Ba,V) micas in the Urals

(Ba,V) micas are rather rare [5]. Reports about theiroccurrence in the Urals are unknown to date [1, 8]. Wefound these micas in igneous and metamorphic rocks ofthe Il’menskie Gory (Ilmeny Mountains) Complex andits framework in the southern Urals.Two biotite varieties with different Ba contents wererevealed in boulders of the coarse-grained biotite–amphibole subalkaline gabbro in a serpentinitemelange sheet in the Osinovyi Mys (Promontory) areaon the eastern coast of Lake Greater Ishkul (Ilmenyreserve).The moderately Ba-rich biotite occurs in theinequigranular to porphyritic holocrystalline rock(sample IK-194-5). Biotite with a higher Ba content isobserved in another boulder of biotite–amphibole gab-bro with distinct ophitic texture. The composition,structure, and appearance of this rock indicate its intru-sive origin [3].Euhedral biotite flakes, 1–3 mm across, occur atboundaries of plagioclase and amphibole grains (Fig. 1)and as inclusions within these minerals. The inductionstriation developed in some cases testifies to the synchro-nous crystallization of biotite, amphibole, and plagio-clase. The content of biotite in the rock is less than 10%.Despite similar bulk mineral and chemical composi-tions of gabbro in the aforementioned two bouldersfrom the serpentinite melange in the Osinovyi Mysarea, they differ in compositions of feldspars, amphib-ole, apatite, magnetite, and biotite. The biotite is char-acterized by sharply variable BaO contents (Table 1). Interms of other parameters, both biotites represent theTi- and Mg-rich variety [7] with a significant role ofFe

Information about open-system magma chambers derived from textures in magmatic enclaves: the Kameni Islands, Santorini, Greece

Post-caldera eruptions of Santorini, Greece, over the past 3000 years resulted in the formation of the Kameni Islands, which comprise a series of compositionally similar dacitic lava flows. Each lava flow has a distinct population of partially-crystalline mafic enclaves, which we propose were derived from the break-up of a layer of replenishing magma responsible for triggering the eruption. Five of the recent flows (erupted in 1570, 1939, 1940, 1941 and 1950) include enclaves of essentially identical andesitic bulk compositions, which formed by crystallization of originally aphyric melts prior to eruption. Detailed examination of angles subtended at the junctions between pairs of plagioclase grains demonstrates that enclaves from each flow have a characteristic textural signature, with distinct differences in the extent of quench-related modification of the original population of dihedral angles formed by impingement of growing grains. These variations suggest that the temperature difference between the host dacite and the replenishing andesite at the time of layer overturn and eruption differed between flows. The uniformity of major element compositions of both the replenishing magma and the host dacitic lava flows demonstrates that the critical parameter in determining the timing of layer-overturn is the pre-eruptive H2O content of both host dacite and replenishing magma. We suggest that the replenishing magma responsible for two of the three eruptions in the period 1939–1941 was significantly wetter than that responsible for the later 1950 eruption. The enclaves with the least amount of quench-related modification occur in the 1570 flow. We suggest that in this case the intruding magma was relatively dry.

40Ar/39Ar dating and cosmic‐ray exposure time of desert meteorites: Dhofar 300 and Dhofar 007 eucrites and anomalous achondrite NWA 011

Abstract— We performed high‐resolution 40Ar‐39Ar dating of mineral separates and whole‐rock samples from the desert meteorites Dhofar 300, Dhofar 007, and Northwest Africa (NWA) 011. The chronological information of all samples is dominated by plagioclase of varying grain size. The last total reset age of the eucrites Dhofar 300 and Dhofar 007 is 3.9 ± 0.1 Ga, coeval with the intense cratering period on the Moon. Some large plagioclase grains of Dhofar 007 possibly inherited Ar from a 4.5 Ga event characteristic for other cumulate eucrites. Due to disturbances of the age spectrum of NWA 011, only an estimate of 3.2–3.9 Ga can be given for its last total reset age. Secondary events causing partial 40Ar loss ≤3.4 Ga ago are indicated by all age spectra. Furthermore, Ar extractions from distinct low temperature phases define apparent isochrons for all samples. These isochron ages are chronologically irrelevant and most probably caused by desert alterations, in which radiogenic 40Ar and K from the meteorite and occasionally K induced by weathering are mixed, accompanied by incorporation of atmospheric Ar. Additional uptake of atmospheric Ar by the alteration phase(s) was observed during mineral separation (i.e., crushing and cleaning in ultrasonic baths). Consistent cosmic‐ray exposure ages were obtained from plagioclase and pyroxene exposure age spectra of Dhofar 300 (25 ± 1 Ma) and Dhofar 007 (13 ± 1 Ma) using the mineral's specific target element chemistry and corresponding 38Ar production rates.

Flow kinematics in the deeper part of a subduction channel, as inferred from inclusion trails in plagioclase porphyroblasts from the Sambagawa metamorphic rocks, south‐west Japan

AbstractThe subduction and exhumation of accretionary prism metasedimentary rocks are accompanied by large‐strain ductile deformations which may be recorded in microstructures. Porphyroblast microstructures have been a key to unravel the kinematics in such deformed belts. Shape‐preferred orientation (SPO) of epidote and amphibole inclusions that define S‐shaped trails in prograde cores of plagioclase porphyroblasts were analysed from the high‐P/T Sambagawa metamorphic rocks. Inclusions are found to be elongate parallel to the [010] and [001] directions, respectively, and their long‐axis orientations define an internal foliation Si (best‐fit great circle) and lineation Li (maximum on the Si). S‐shaped inclusion trails in the orthogonal sections do not exhibit the same geometries, but rather are grouped into two types, where the foliation intersection axes (FIAs) are nearly perpendicular and parallel to Li, respectively. These two types of S‐shaped inclusion trails are seen in the sections inclined at low and high angles to the Li, respectively. However, the latter type commonly consists of composite trails, where the Si is first rotated about an FIA perpendicular to the Li (i.e. unique axis), and then about an FIA parallel to the Li. The S‐shaped inclusion trails are interpreted to have formed by the successive overgrowth of matrix minerals and rotation of the plagioclase porphyroblast cores about a unique axis in non‐coaxial deformation. The rotation of Si about an FIA nearly parallel to the Li is perhaps an apparent rotation, caused by the deflection of foliation around the growing prismatic plagioclase grain prior to inclusion into the porphyroblast. This study has for the first time documented the 3‐D geometry of S‐shaped inclusion trails in porphyroblasts from accretionary prism metasedimentary rocks and identified their origin, which helps to understand the flow kinematics in the deeper part of a subduction channel.

Late stage magmatic evolution of oceanic gabbros as a result of hydrous partial melting: Evidence from the Ocean Drilling Program (ODP) Leg 153 drilling at the Mid‐Atlantic Ridge

Petrographic observations within oceanic gabbros drilled by Ocean Drilling Program (ODP) at Leg 153 from the Mid‐Atlantic Ridge, Kane Fracture Zone (MARK) area (23°N) in combination with experimental findings suggest that the late stage magmatic evolution of these gabbros is characterized by pervasive hydrous partial melting. Water‐saturated melting experiments on a variety of natural gabbros between 900 and 1000°C at 200 MPa produced newly formed minerals forming a characteristic paragenesis consisting of plagioclase, orthopyroxene, and pargasitic amphibole ± clinopyroxene. The An content of the new plagioclases is uniformly higher than that of the protolith. Olivine and clinopyroxene primocrysts react to form neoblastic orthopyroxene and pargasitic amphibole. These features can also be observed in the gabbros from the MARK area. Here we found zones at plagioclase grain boundaries showing a strong enrichment in An component, with An contents up to 20 to 25 mol % higher than those of the host plagioclase. Primary olivines and clinopyroxenes in contact with such zones react to orthopyroxene and pargasitic amphibole. These phases rim olivine and clinopyroxene and grow “interstitially,” typical petrographic characteristics of a late stage magmatic phase. The observed late stage microstructures in the gabbros thus appear to be the results of partial melting processes triggered by water‐rich fluids and are not crystallization products of a percolating differentiated late melt. Here we demonstrate that hydrothermal circulation within the gabbroic layer starts at much higher temperatures (900°–1000°C) than up to now believed. Water‐rich fluids propagate on grain boundaries in a ductile regime, causing hydrous partial melting on a large scale. A cracks system, a prerequisite in current models for enabling hydrothermal circulation, is not necessary. The observed process has the potential for transfer of heat and mass between the upper and lower oceanic crust. Provided that the water‐rich fluids triggering the partial melting process are seawater derived, this process may have a significant influence on the cooling of the deep oceanic crust which is in concordance with new thermal models implying that high‐temperature hydrothermal circulation is regarded to play an important role in transport of heat in the deep oceanic crust.

田上花こう岩中のアルカリ長石の微細組織とその形成史

Alkali feldspars in the Tanakami granite, southwest Japan, contain a complicated fine-scale mixture of various micro-textures; lamellar crypto- and micro-perthites and micropore-rich patch perthites. Mineralogical examination mainly by TEM and SEM disclosed that there are two formational stages of these micro-textures, the subsolidus cooling stage and hydrothermal alteration stage. In the subsolidus cooling stage, lamellar crypto- and micro-perthite were formed by exsolution. Then nearly regularly arranged dislocations were formed at interfaces of Ab-rich lamellae and Or-rich feldspars. Hydrothermal alteration caused the dissolution of feldspar grains and re-precipitation of new feldspar grains mainly around the dislocations. These processes result in neo-formation of patch perthites that comprise irregular microcline, tweed orthoclase, numerous micropores and albite grains. Bulk compositions of cryptoperthite have excessive Or content (around Or90) in comparison to those expected for primary alkali feldspars. It is estimated that retrograde inter-crystalline Na-K exchange between alkali feldspars and plagioclases occurs in the subsolidus stage. On the contrary, bulk compositions of patch perthite have poor Or content (Or64-71) from those of the cryptoperthite. This feature indicates that the chemical compositions of alkali feldspars changed drastically as well as the micro-textures in the hydrothermal alteration stage. Coexisting plagioclases also underwent chemical change from the subsolidus cooling stage through the hydrothermal alteration stage. Especially in the hydrothermal alteration stage, alteration by percolation of hydrothermal water occurred in the internal areas of the plagioclase grains. These processes result in lower Or content (max Or2.7) and higher Ab content (>Ab90) of the plagioclases, and neo-formation of patchy zoning including micropores and fluorite subgrains. Summarizing all of these features, the relation between micro-textural development and chemical compositional changes in feldspars was discussed taking into consideration their formation processes such as inter-crystalline Na-K exchange and hydrothermal reaction.

Petrography of Lower Cretaceous sandstones on Spitsbergen

The sandstone petrography of sample suites from four sites spanning the Rurikfjellet (Hauterivian) to Carolinefjellet (Aptian–Albian) formations in central Spitsbergen was investigated. The sandstones show a distinct stepwise shift in composition from quartz arenites to sublitharenites and lithic arenites, typically within the upper part of the Helvetiafjellet Formation. This shift is related to the introduction of 10 - 25 % (grain %) plagioclase grains and volcanic lithics, and a notable increase in basement and sedimentary lithics. Quartz grain character also changes, and grain shapes become more varied. The shift is also associated with the transgressive arrival of marine sediments in the area, and the introduction of sands from the east-northeast by shore-parallel transport. Regional regression and subsequent transgression, and the change in sandstone composition is attributed to the development of the High Arctic Large Igneous Province in the region. The relative constancy of sand composition and volume of volcanic detritus within the Carolinefjellet Formation suggests long term (≈ 20 M) stability of the sediment system and a large volcanic source area, consistent with LIP (Large Igneous Province) derivation, along with signifi cant exposure of basement rocks. Sample spacing and sediment recycling and mixing do not allow detection of events that would have changed sandstone composition that were less than ≈ 1 M duration. Preservation of signifi cant amounts of plagioclase in a sediment-starved shelf can be explained by relatively cold climatic conditions.

Petrography of Lower Cretaceous sandstones on Spitsbergen

The sandstone petrography of sample suites from four sites spanning the Rurikfjellet (Hauterivian) to Carolinefjellet (Aptian–Albian) formations in central Spitsbergen was investigated. The sandstones show a distinct stepwise shift in composition from quartz arenites to sublitharenites and lithic arenites, typically within the upper part of the Helvetiafjellet Formation. This shift is related to the introduction of 10 - 25 % (grain %) plagioclase grains and volcanic lithics, and a notable increase in basement and sedimentary lithics. Quartz grain character also changes, and grain shapes become more varied. The shift is also associated with the transgressive arrival of marine sediments in the area, and the introduction of sands from the east-northeast by shore-parallel transport. Regional regression and subsequent transgression, and the change in sandstone composition is attributed to the development of the High Arctic Large Igneous Province in the region. The relative constancy of sand composition and volume of volcanic detritus within the Carolinefjellet Formation suggests long term (? 20 M) stability of the sediment system and a large volcanic source area, consistent with LIP (Large Igneous Province) derivation, along with significant exposure of basement rocks. Sample spacing and sediment recycling and mixing do not allow detection of events that would have changed sandstone composition that were less than ? 1 M duration. Preservation of significant amounts of plagioclase in a sediment-starved shelf can be explained by relatively cold climatic conditions.

Response to Comment on "Bedout: A Possible End-Permian Impact Crater Offshore of Northwestern Australia"

Response to Comment on "Bedout: A Possible End-Permian Impact Crater Offshore of Northwestern Australia"

Mineralogy and petrology of Al-rich objects and amoeboid olivine aggregates in the CH carbonaceous chondrite North West Africa 739

The aluminum-rich (>10 wt% Al 2O 3) objects in the CH carbonaceous chondrite North West Africa (NWA) 739 include Ca,Al-rich inclusions (CAIs), Al-rich chondrules, and isolated mineral grains (spinel, plagioclase, glass). Based on the major mineralogy, 54 refractory inclusions found in about 1 cm 2 polished section of NWA 739 can be divided into hibonite-rich (16%), grossite-rich (26%), melilite-rich (28%), spinel-pyroxene-rich (16%) CAIs, and amoeboid olivine aggregates, (AOA's, 17%). Most CAIs are rounded, 25–185 μm (average=70 μm) in apparent diameter, contain abundant, tiny perovskite grains, and typically surrounded by a single- or double-layered rim composed of melilite and/or Al-diopside; occasionally, layers of spinel+hibonite and forsterite are observed. The AOAs are irregularly shaped, 100–250 μm (average=175 μm) in size, and consist of forsterite, Fe,Ni-metal, and CAIs composed of Al-diopside, anorthite, and minor spinel. One AOA contains compact, rounded melilite-spinel-perovskite CAIs and low-Ca pyroxene replacing forsterite. The Al-rich (>10 wt% bulk Al 2O 3) chondrules are divided into Al-diopside-rich and plagioclase-rich. The Al-diopside-rich chondrules, 50–310 μm (average=165 μm) in apparent diameter, consist of Al-diopside, skeletal forsterite, spinel, ±Al-rich low-Ca pyroxene, and ±mesostasis. The plagioclase-rich chondrules, 120–455 μm (average=285 μm) in apparent diameter, are composed of low-Ca and high-Ca pyroxenes, forsterite, anorthitic plagioclase, Fe,Ni-metal nodules, and mesostasis. The isolated spinel occurs as coarse, 50–125 μm in size, subhedral grains, which are probably the fragments of Al-diopside chondrules. The isolated plagioclase grains are too coarse (60–120 μm) to have been produced by disintegration of chondrules or CAIs; they range in composition from nearly pure anorthite to nearly pure albite; their origin is unclear. The Al-rich objects show no evidence for Fe-alkali metasomatic or aqueous alteration; the only exception is an Al-rich chondrule fragment with anorthite replaced by nepheline. They are texturally and mineralogically similar to those in other CH chondrites studied (Acfer 182, ALH85085, PAT91467, NWA 770), but are distinct from the Al-rich objects in other chondrite groups (CM, CO, CR, CV). The CH CAIs are dominated by very refractory minerals, such as hibonite, grossite, perovskite and gehlenitic melilite, and appear to have experienced very low degrees of high-temperature alteration reactions. These include replacement of grossite by melilite, of melilite by anorthite, diopside, and spinel, and of forsterite by low-Ca pyroxene. Only a few CAIs show evidence for melting and multilayered Wark-Lovering rims. These observations may suggest that CH CAIs experienced rather simple formation history and escaped extensive recycling. In order to preserve the high-temperature mineral assemblages, they must have been efficiently isolated from the hot nebular region, like some chondrules and the zoned Fe,Ni-metal grains in CH chondrites.

Plagioclase from planetary basalts: Chemical signatures that reflect planetary volatile budgets, oxygen fugacity, and styles of igneous differentiation

Plagioclase grains from 15 basalt suites from the Earth, Moon, Mars, and 4 Vesta were studied by electron and ion microprobe techniques. The results demonstrate that chemical signatures in plagioclase record three planetary aspects that affect basaltic magmatism: (1) Volatile element depletions and enrichments in the planetary bodies are recorded by the systematics of Na and K in plagioclase. Terrestrial and Martian plagioclase are enriched in the volatile elements relative to plagioclase from the Moon and Vesta, a result that mimics the overall volatile budgets of these planets. Furthermore, the volatile-depleted nature of both the Moon and Vesta imply some similarities in planetary origin and/or evolution of these bodies. (2) The overall oxygen fugacity conditions on a planet may be realized by the Eu/Sm ratios of planetary plagioclase grains. Our data show that plagioclase from the Moon and 4 Vesta has higher Eu/Sm relative to plagioclase from the Earth and Mars. These data are consistent with low oxygen fugacity conditions on the Moon and Vesta, where divalent Eu is more compatible within the plagioclase structure than other trivalent REE elements. (3) The incompatible element systematics of K, REE, Ba, Y, and Sr in plagioclase show that styles of igneous differentiation affect basaltic magmatism on planets and more specifically, within tectonic settings on each planet. Terrestrial plagioclase grains from continental flood, continental rift, and Hawaiian basalt suites are enriched in the incompatible elements K, Ce, Ba, and Sr compared with Ocean Floor and Island Arc suites. The enrichments in the former are caused by continental crust interaction with basaltic magmas, and/or derivation from undepleted (fertile) mantle sources. Lunar KREEP basalt plagioclase enrichment in incompatible elements reflect mare basalt assimilation of a KREEP component, or partial melting of a KREEP-rich source. The Martian incompatible element data show that plagioclase from Shergotty is enriched in incompatible elements relative to plagioclase from QUE 94201. The data are in agreement with the idea that most Martian shergottites (except for QUE) have assimilated an incompatible element-rich crustal component on their way to eruption on the Martian surface.

Emplacement-related microstructures in the margin of a deformed pluton: the San José tonalite, Baja California, México

Abstract In the San Jose tonalite pluton, Baja California, Mexico, interior magmatic fabrics give way progressively to subparallel, solid-state fabrics with S–C structures near the margins. The outer rim (‘marginal northern unit’), approximately 1 km thick, is inferred to have been deformed during the emplacement of the inner parts of the magma chamber. The rocks show a transition from strongly deformed at the outer edge to progressively less deformed towards the inner edge of the marginal northern unit. A small amount of melt appears to have been present during the deformation, as indicated by (1) apparent contact melting between some grains of plagioclase, and (2) coarse-grained aggregates and single-grains of quartz, K-feldspar and/or more sodic plagioclase in fracture-controlled openings in primary plagioclase crystals. The more sodic plagioclase typically grew as rims on the walls of the openings in twin continuity with the primary plagioclase. The deformed rocks have a prominent foliation marked mainly by aggregates of biotite, with contributions by hornblende and titanite grains, as well as apparently recrystallized aggregates of quartz and/or feldspar, at least some of which may have originated as zones of fragmentation. Many primary grains of plagioclase, hornblende, biotite and titanite show evidence of brittle deformation and fragmentation. The shapes of aggregates in the folia, relationships between incipient folia and residual primary grains, and occurrences of many folia in narrow zones transecting strong minerals (plagioclase and hornblende), suggest that brittle processes may have controlled at least some of the foliation development. However, evidence of strain-induced grain-boundary migration and bulge nucleation of new grains of quartz and plagioclase indicates that dislocation creep also played a part in the deformation. Recrystallization of plagioclase, together with intrusion of the pluton at greenschist facies ambient temperatures, confirms high-temperature deformation, with heat derived from the cooling magma.

Commingling and mixing of S-type peraluminous, ultrapotassic and basaltic magmas in the Cayconi volcanic field, Cordillera de Carabaya, SE Peru

The Cayconi district of the Cordillera de Carabaya, SE Peru, exposes a remnant of an upper Oligocene–Lower Miocene (22.2–24.4 Ma) volcanic field, comprising a diverse assemblage of S-type silicic and calc-alkaline basaltic to andesitic flows, members of the Picotani Group of the Central Andean Inner Arc. Basaltic flows containing olivine, plagioclase, clinopyroxene, ilmenite and glass, and glassy rhyolitic agglutinates with phenocrystic quartz, cordierite, plagioclase, sanidine, ilmenite and apatite, respectively exhibit mineralogical and geochemical features characteristic of medium-K mafic and Lachlan S-type silicic lavas. Cordierite-bearing dacitic agglomerates and lavas, however, are characterized by dispersed, melanocratic micro-enclaves and phenocrysts set in a fine-grained quartzo-feldspathic matrix. They contain a bimodal mica population, comprising phlogopite and biotite, as well as complexly zoned, sieve-textured plagioclase grains, sector-zoned cordierite, sanidine, quartz, irregular patches of replaced olivine, clinopyroxene and orthopyroxene and accessory phases including zircon, monazite, ilmenite and chromite. The coexistence of minerals not in mutual equilibrium and the growth/dissolution textures exhibited by plagioclase are features indicative of magmatic commingling and mixing. Trachytic-textured andesite flows interlayered with olivine+plagioclase–glomerophyric, calc-alkaline basalts have a phenocrystic assemblage of resorbed orthopyroxene and plagioclase and exhibit melanocratic groundmass patches of microphenocrystic phlogopite, Ca-rich sanidine, ilmenite and aluminous spinel. The mineralogical and mineral chemical relationships in both the dacites and the trachytic-textured andesites imply subvolcanic mixing between distinct ultrapotassic mafic melts, not represented by exposed rock types, and both the S-type silicic and calc-alkaline mafic magmas. Such mixing relationships are commonly observed in the Oligo-Miocene rocks of the Cordillera de Carabaya, suggesting that the S-type rocks in this area and, by extension, elsewhere derive their unusually high K 2O, Ba, Sr, Cr and Ni concentrations from commingling and mixing with diverse, mantle-derived potassic mafic magmas.

Isotopic composition of surface-correlated chromium in Apollo 16 lunar soils

We have analyzed by thermal ionization mass spectrometry (TIMS) the isotopic composition of Cr in five progressive etches of size-sorted plagioclase grains separated from lunar soils 60601 and 62281. Aliquots of the etch solutions were spiked for isotopic dilution (ID) analysis of Cr and Ca. The Ca ID data indicate that the initial etch steps represent dissolution of an average 0.1 to 0.2 μm depth from the grain surfaces, the approximate depth expected for implanted solar wind. The Cr/Ca ratio in the initial etches is several fold higher than that expected for bulk plagioclase composition, but in subsequent etches decreases to approach the bulk value. This indicates a source of Cr extrinsic to the plagioclase grains, surface-correlated and resident in the outermost fraction of a μm, which we provisionally identify as solar wind Cr. The surface-correlated Cr is isotopically anomalous and by conventional TIMS data reduction has approximately 1 permil excess 54Cr and half as great excess 53Cr. In successive etches, as the Cr/Ca ratio decreases and approaches the bulk plagioclase value, the magnitude of the apparent anomalies decreases approaching normal composition. If these results do indeed characterize the solar wind, then either the solar wind is enriched in Cr due to spallation in the solar atmosphere, or the Earth and the various parent bodies of the meteorites are isotopically distinct from the Sun and must have formed from slightly different mixes of presolar materials. Alternative interpretations include the possibility that the anomalous Cr is meteoritic rather than solar or that the observed (solar) Cr is normal except for a small admixture of spallation Cr generated on the Moon. We consider these latter possibilities less likely than the solar wind interpretation. However, they cannot be eliminated and remain working hypotheses.

Northwest Africa 428: Impact‐induced annealing of an L6 chondrite breccia

Abstract— Northwest Africa (NWA) 428 is an L chondrite that was successively thermally metamorphosed to petrologic type‐6, shocked to stage S4–S5, brecciated, and annealed to approximately petrologic type‐4. Its thermal and shock history resembles that of the previously studied LL6 chondrite, Miller Range (MIL) 99301, which formed on a different asteroid. The petrologic type‐6 classification of NWA 428 is based on its highly recrystallized texture, coarse metal (150 ± 150 μm), troilite (100 ± 170 μm), and plagioclase (20–60 μm) grains, and relatively homogeneous olivine (Fa24.4 ± 0.6), low‐Ca pyroxene (Fs20.5 ± 0.4), and plagioclase (Ab84.2 ± 0.4) compositions. The petrographic criteria that indicate shock stage S4–S5 include the presence of chromite veinlets, chromite‐plagioclase assemblages, numerous occurrences of metallic Cu, irregular troilite grains within metallic Fe‐Ni, polycrystalline troilite, duplex plessite, metal and troilite veins, large troilite nodules, and low‐Ca clinopyroxene with polysynthetic twins. If the rock had been shocked before thermal metamorphism, low‐Ca clinopyroxene produced by the shock event would have transformed into orthopyroxene. Post‐shock brecciation is indicated by the presence of recrystallized clasts and highly shocked clasts that form sharp boundaries with the host. Post‐shock annealing is indicated by the sharp optical extinction of the olivine grains; during annealing, the damaged olivine crystal lattices healed. If temperatures exceeded those approximating petrologic type‐4 (˜600–700°C) during annealing, the low‐Ca clinopyroxene would have transformed into orthopyroxene. The other shock indicators, likewise, survived the mild annealing. An impact event is the most plausible source of post‐metamorphic, post‐shock annealing because any 26Al that may have been present when the asteroid accreted would have decayed away by the time NWA 428 was annealed. The similar inferred histories of NWA 428 (L6) and MIL 99301 (LL6) indicate that impact heating affected more than 1 ordinary chondrite parent body.

The Melt Character of Partial Molten Gabbro During Plastic Deformation Process

AbstractWe study the microstructure and chemical composition of melt formed in process of plastic deformation of gabbro using transmission electron microscope (TEM). The result shows that initial melt formed at 950°C~1 000°C is distributed in a film‐like shape along the boundary between two plagioclase grains or in the tubules of triangle between three plagioclase grain section. SiO2 content of the early melt is about 60%. At 1 050°C, with increasing melting degree, the melt appears not only between boundaries of plagioclases, but also between boundaries of plagioclase and pyroxene or magnetite, and its SiO2 content is lower than 50%. The melt films between boundaries of plagioclase grains widen with increasing melt, and connect the isolated melt tubules of triangle into local connected network which is oriented melt tubules of triangles in cross‐section. Maybe this is a result of migration of melt toward lower stress under the effect of differential stress. Many dark “small balls” about 0.1~0.2 μm in diameter are evenly distributed in the melt. Silicate melt around the dark “small balls” is rich in SiO2 and poor in FeO, whose SiO2 content is higher than 70%. But the dark “small balls” are poor in SiO2 and rich in FeO, whose SiO2 content is lower than 40%. It indicates that the melt is differentiated. The FeO‐rich and SiO2‐poor melt locally crystallized into dendritic small magnetite. The separation of iron from the melt is an important factor for forming magnetite ore.

Lunar regolith breccia Dhofar 287B: A record of lunar volcanism

Abstract— Dhofar 287 (Dho 287), a recently found lunar meteorite, consists in large part (95%) of low‐Ti mare basalt (Dho 287A) and a minor, attached portion (˜5%) of regolith breccia (Dho 287B). The present study is directed mainly at the breccia portion of this meteorite. This breccia consists of a variety of lithic clasts and mineral fragments set in a fine‐grained matrix and minor impact melt. The majority of clasts and minerals appear to have been mainly derived from the low‐Ti basalt suite, similar to that of Dho 287A. Very low‐Ti (VLT) basalts are a minor lithology of the breccia. These are significantly lower in Mg# and slightly higher in Ti compared to Luna 24 and Apollo 17 VLT basalts. Picritic glasses constitute another minor component of the breccia and are compositionally similar to Apollo 15 green glasses. Dho 287B also contains abundant fragments of Mg‐rich pyroxene and anorthite‐rich plagioclase grains that are absent in the lithic clasts. Such fragments appear to have been derived from a coarse‐grained, Mg#‐rich, Na‐poor lithology. A KREEP component is apparent in chemistry, but no highlands lithologies were identified.The Dho 287 basaltic lithologies cannot be explained by near‐surface fractionation of a single parental magma. Instead, magma compositions are represented by a picritic glass; a low‐Ti, Na‐poor glass; and a low‐Ti, Na‐enriched source (similar to the Dho 287A parental melt). Compositional differences among parent melts could reflect inhomogeneity of the lunar mantle. Alternatively, the low‐Ti, Na‐poor, and Dho 287A parent melts could be of hybrid compositions, resulting from assimilation of KREEP by picritic magma. Thus, the Dho 287B breccia contains lithologies from multiple magmatic eruptions, which differed in composition, formational conditions, and cooling histories. Based on this study, the Dho 287 is inferred to have been ejected from a region located distal to highlands terrains, possibly from the western limb of the lunar nearside, dominated by mare basalts and KREEP‐rich lithologies.