Large Plagioclase Research Articles

Origin of enclaves in S-type granites of the Lachlan Fold Belt

The more mafic S-type granites of the Lachlan Fold Belt contain a distinctive assemblage of lithic inclusions of deep crustal origin. Two types predominate, the schistose enclaves and the microgranular enclaves. There is also a small proportion of fragments of local country rock. All the features of the schistose enclaves are consistent with an origin as lithic restite fragments from the source. Their ubiquity in the mafic S-type granites and absence from both felsic S-type granites and I-type granites, conform with such an origin. The argument that these enclaves are not in chemical equilibrium with the host granite because they do not complement its composition is not valid, since they represent less fertile parts of the source that had different compositions that, as a result, melted to a lesser extent. Models that derived these enclaves from regions of the crust above the zones of partial melting, so that they are not restites, are complex, unnecessary, do not conform with their ubiquity in the more mafic S-type granites, and are not consistent with their chemical compositions.The origin of the microgranular enclaves is controversial. In most S-type granites these enclaves have been recrystallized to a similar mineral assemblage to the host granite with a higher proportion of biotite. In a few places, such material is seen to be forming by recrystallization of material in the core of the enclave, which has the assemblage quartz+calcic plagioclase+orthopyroxene±cordierite±biotite with accessory ilmenite, sulfide and apatite. All of the microgranular enclave cores show pseudo-doleritic texture in which calcic plagioclase crystals (>An60) and orthopyroxene crystals project into or are enclosed by quartz. Large plagioclase crystals are zoned, with some core compositions as calcic as An94. The low Na contents of the microgranular enclaves are not consistent with an igneous origin. We consider that these enclaves were derived by metamorphism of calcareous mudstones or argillaceous limestones. The pseudo-igneous textures result from the presence of a partial melt during metamorphism. An analysis of a calc-silicate lens of upper amphibolite grade from the Wilsons Group of Victoria Land, Antarctica is remarkably similar in composition to the microgranular enclaves from Lachlan Fold Belt S-type granites. We suggest that rocks equivalent to that Group, which is of appropriate age and general composition, were the source rocks for the S-type granites of the Lachlan Fold Belt.

Constraints on crystal storage timescales in mixed magmas: Uranium-series disequilibria in plagioclase from Holocene magmas at Mount Hood, Oregon

Uranium-series crystal ages, interpreted within a textural and geochemical framework, can provide insight into crystal storage timescales, especially in cases where crystals may derive from multiple sources. We report here 230Th–226Ra model ages of two distinct populations of plagioclase from low silica dacites from Mount Hood, Oregon, a volcano where previous studies show that the compositions of erupted magmas are controlled by magma recharge, mixing, and incorporation of plagioclase derived from mafic and silicic end-member magmas. We have measured trace element concentrations and 238U–230Th–226Ra disequilibria in four plagioclase size fractions from the Timberline (1500a) and Old Maid (215a) eruptive sequences. After correction for groundmass and apatite contamination, average 230Th–226Ra model ages of large (>500μm) plagioclase are >4.5ka (Timberline) and >5.5ka (Old Maid), with ages of cores that are >10ka in each case, indicating that plagioclase derived from silicic magmas crystallized thousands of years before eruptions. These model ages are longer than timescales of repose between eruptions, indicating that these crystals resided in the sub-surface over multiple eruptions, likely stored in a silicic crystal mush zone that periodically interacts with mafic recharge magmas, remobilizing a fraction of the large plagioclase crystals during each eruptive event. After correction for large plagioclase contamination, small (<500μm) plagioclase, derived from mafic magmas, have high (226Ra)/Ba relative to equilibrium with liquid proxies (groundmass and mafic inclusion), leading to 230Th–226Ra model ages that are <~3ka for Old Maid and undefined for Timberline separates. However, the preservation of significant 230Th–226Ra disequilibria require that the majority of crystals in the separate are young (<<10ka). The high (226Ra)/[Ba] could potentially be explained by rapid crystallization immediately prior to and/or during mixing events, consistent with evidence of rapid crystallization of rims. Rapid crystallization of mafic intrusions may trigger eruption at Mount Hood by producing a partially-crystalline mafic magma capable of mixing with a reheated silicic crystal mush.

Mineralogy, whole-rock and Sr–Nd isotope geochemistry of mafic microgranular enclaves in Cretaceous Dagbasi granitoids, Eastern Pontides, NE Turkey: Evidence of magma mixing, mingling and chemical equilibration

Rocks of the Late Cretaceous Dagbasi Pluton (88–83 Ma), located in the eastern Pontides, include mafic microgranular enclaves (MMEs) ranging from a few centimetres to metres in size, and from ellipsoidal to ovoid in shape. The MMEs are composed of gabbroic diorite, diorite and tonalite, whereas the felsic host rocks comprise mainly tonalite, granodiorite and monzogranite based on both mineralogical and chemical compositions. MMEs are characterized by a fine-grained, equigranular and hypidiomorphic texture. The common texture of felsic host rocks is equigranular and also reveals some special types of microscopic textures, e.g., oscillatory-zoned plagioclase, poikilitic K-feldspar, small lath-shaped plagioclase in large plagioclase, blade-shaped biotite, acicular apatite, spike zones in plagioclase and spongy-cellular plagioclase textures and rounded plagioclase megacrysts in MMEs. Compositions of plagioclases (An 33–An 60), hornblendes (Mg#=0.77–1.0) and biotites (Mg#=0.61–0.63) of MMEs are slightly distinct or similar to those of host rocks (An 12–57; hbl Mg#=0.63–1.0; Bi Mg#=0.50–0.69), which suggest partial to complete equilibration during mafic–felsic magma interactions. The felsic host rocks have SiO 2 between 60 and 76 wt% and display low to slightly medium-K tholeiitic to calc-alkaline and peraluminous to slightly metaluminous characteristics. Chondrite-normalized rare-earth element (REE) patterns are fractionated (La cn/Lu cn=1.5–7.3) with pronounced negative Eu anomalies (Eu/Eu*=0.46–1.1). Initial ε Nd(i) values vary between −3.1 and 1.6, initial 87Sr/ 86Sr values between 0.7056 and 0.7067. Compared with the host rocks, the MMEs are characterized by relatively high Mg-number of 22–52, low contents of SiO 2 (53–63 wt%), low ASI (0.7–1.1) and low to medium-K tholeiitic to calc-alkaline, metaluminous to peraluminous composition. Chondrite-normalized REE patterns are relatively flat [(La/Yb) cn=1.4–3.9; (Tb/Yb) cn=0.9–1.5] and show small negative Eu anomalies (Eu/Eu*=0.63–1.01). Isotope signatures of these rocks ( 87Sr/ 86Sr (i)=0.7054–0.7055; ε Nd(i)=–1.0 to 1.9) are largely similar to the host rocks. Gabbroic diorite enclaves have relatively low contents of SiO 2, ASI; high Mg#, CaO, Al 2O 3, TiO 2, P 2O 5, Sr and Nb concentrations compared to dioritic and tonalitic enclaves. The geochemical and isotopic similarities between the MMEs and their host rocks indicate that the enclaves are of mixed origin and are most probably formed by the interaction between the lower crust- and mantle-derived magmas. All the geochemical data suggest that a basic magma derived from an enriched subcontinental lithospheric mantle, interacted with a crustal melt that originated from dehydration melting of the mafic lower crust at deep crustal levels. The existence of compositional and textural disequilibrium and the nature of chemical and isotopic variation in these rock types indicate that magma mixing/mingling between an evolved mafic and a granitic magma was involved in their genesis. Microgranular enclaves are thus interpreted to be globules of a more mafic magma probably from an enriched lithospheric mantle source. Al-in-amphibole estimates the pluton emplacement at ca. 0.3–3.8 kbar, and therefore, magma mixing and mingling must have occurred at 3.8 kbar or below this level.

Isotopic-heterogeneity of the Thjorsa lava—Implications for mantle sources and crustal processes within the Eastern Rift Zone, Iceland

The Thjorsa lava in South Iceland is an early Holocene fissure lava within the Bardarbunga volcanic system—the most productive segment of the Eastern Rift Zone during the Quaternary and Holocene. The lava has a volume of 25 km 3 and is one of notably plagioclase-porphyritic early Holocene lava flows in Iceland. The possible genetic relationship between the large plagioclase phenocrysts and the host lava is investigated by analyzing carefully separated groundmass/plagioclase pairs. The groundmass was analyzed for major and trace elements and isotopic ratios of Sr, Nd and Pb. The large plagioclases were analyzed by microprobe and 87Sr/ 86Sr by mass spectrometry. The groundmass (host magma) of the host lava has a restricted range of elemental and isotopic composition. Both the groundmass and the separated plagioclases show a significant range of 87Sr/ 86Sr—but interestingly with only minor overlap between the two groups. Both are, however, within the range of values observed for the lavas of the Bardarbunga volcanic system. This lack of overlap suggests that the plagioclase and the host lava are genetically unrelated and derived from two distinct mantle sources. The suggested source for the large plagioclases is deep layered crustal gabbros formed in earlier intrusive events. The plagioclases are accompanied by minor amounts of Fo-rich olivine and occasional disintegrating Cr-diopside. There is a clear compositional gap between the An-rich (> An 80) plagioclases and the small groundmass plagioclase that appear to be in equilibrium with the groundmass composition. The parental magma of the anorthositic gabbro must have been relatively primitive, crystallizing additional olivine and Cr-diopside and with 87Sr/ 86Sr at the low end of the range observed in the Bardarbunga rift segment. During ascent of the host magma the gabbros were infiltrated and broken up by this more evolved host magma. With lowered pressures during ascent and lava effusion the Cr-diopside comes unstable with up to 5% being assimilated into the host magma mildly affecting isotopic ratios and those elements with clinopyroxene affinity. The two components—the host magma and the plagioclases are therefore genetically unrelated and with only minor mixing effect on the host magma. The results of the present study show that whole-rock chemical analyses of such rocks must be treated with caution as they may not represent actual magmatic liquids. At the beginning of the Holocene there was a large isostatic rebound in Iceland following the rapid ice melting. This was accompanied by a major pulse of basaltic eruptions and many of these eruptions, especially around Central Iceland, carry abundant large plagioclases. This is clearly not the only cause as there are a number of very plagioclase-porphyritic lavas in the Tertiary formations but may have facilitated plagioclase incorporation.

The petrology and geochemistry of Miller Range 05035: A new lunar gabbroic meteorite

Miller Range (MIL) 05035 is a lunar gabbroic meteorite. The mineralogy, Fe/Mn ratios in olivine and pyroxene, bulk-rock chemical composition and the bulk oxygen isotope values (δ 17O = 2.86–2.97‰ and δ 18O = 5.47–5.71‰) are similar to those of other mare basalts, and are taken as supporting evidence for a lunar origin for this meteorite. The sample is dominated by pyroxene grains (54–61% by area mode of thin section) along with large plagioclase feldspar (25–36% by mode) and accessory quartz, ilmenite, spinel, apatite and troilite. The bulk-rock major element composition of MIL 05035 indicates that the sample has a very low-Ti (VLT) to low-Ti lunar heritage (we measure bulk TiO 2 to be 0.9 Wt.%) and has low bulk incompatible trace element (ITE) concentrations, akin to samples from the VLT mare basalt suite. To account for these geochemical characteristics we hypothesize that MIL 05035’s parental melt was derived from a mantle region dominated by early cumulates of the magma ocean (comprised principally of olivine and orthopyroxene). MIL 05035 is likely launch paired with the Asuka-881757 and Yamato-793169 basaltic lunar meteorites and the basaltic regolith breccia MET 01210. This group of meteorites (Y/A/M/M) therefore may be a part of a stratigraphic column consisting of an upper regolith environment underlain by a coarsening downwards basalt lava flow.

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.

Geochemistry of mafic microgranular enclaves in the Tamdere Quartz Monzonite, south of Dereli/Giresun, Eastern Pontides, Turkey

Rocks of the Late Cretaceous Tamdere Quartz Monzonite, constituting a part of the Eastern Pontide plutonism, include mafic microgranular enclaves (MMEs) ranging from spheroidal to ellipsoidal in shape, and from a few centimeters to decimeters in size. The MMEs are composed of diorite, monzodiorite and quartz diorite, whereas the felsic host rocks comprise mainly quartz monzonite, granodiorite and rarely monzogranite on the basis of both mineralogical and chemical compositions. The common texture of felsic host rocks is equigranular. MMEs are characterized by a microgranular texture and also reveal some special types of microscopic textures, e.g. antirapakivi, poikilitic K-feldspar, small lath-shaped plagioclase in large plagioclase, blade-shaped biotite, acicular apatite, spike zones in plagioclase and spongy-cellular plagioclase textures. The distribution of major, trace and RE elements apparently reflect exchange between the MMEs and the felsic host rocks mainly due to thermal, mechanical and chemical interactions between coeval felsic host magma and mafic magma. The most evident major element transfer from felsic host magma to mafic magma blob is that of alkalis such as Na and K. LILEs such as Rb, Sr, Ba and some HFSEs such as Nb, Y, Zr and Th have been migrated from felsic host magma to MMEs. Apart from these major and trace elements, the other element transfer from felsic host magma to mafic one concerns REE contents. Such a transfer of REEs has evidently increased the LREE contents of MMEs. Enrichments in alkalis, LILEs, HFSEs and REEs could have been achieved by diffusional processes during the solidification of magma sources. The felsic and mafic magma sources behave as Newtonian and visco-plastic materials. In such an interaction, small MMEs behave as a closed system due to immediate rapid cooling, whereas the bigger MMEs suffer greater diffusion from the Newtonian felsic host magma due to slow cooling.

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"

Magmatic inclusions and felsic clasts in the Dar al Gani 319 polymict ureilite

Abstract— Magmatic inclusions occur in type II ureilite clasts (olivine‐orthopyroxene‐augite assemblages with essentially no carbon) and in a large isolated plagioclase clast in the Dar al Gani (DaG) 319 polymict ureilite. Type I ureilite clasts (olivine‐pigeonite assemblages with carbon), as well as other lithic and mineral clasts in this meteorite, are described in Ikeda et al.(2000). The magmatic inclusions in the type II ureilite clasts consist mainly of magnesian augite and glass. They metastably crystallized euhedral pyroxenes, resulting in feldspar component‐enriched glass. On the other hand, the magmatic inclusions in the large plagioclase clast consist mainly of pyroxene and plagioclase, with a mesostasis. They crystallized with a composition along the cotectic line between the pyroxene and plagioclase liquidus fields.DaG 319 also contains felsic lithic clasts that represent various types of igneous lithologies. These are the rare components not found in the common monomict ureilites. Porphyritic felsic clasts, the main type, contain phenocrysts of plagioclase and pyroxene, and their groundmass consists mainly of plagioclase, pyroxene, and minor phosphate, ilmenite, chromite, and/or glass. Crystallization of these porphyritic clasts took place along the cotectic line between the pyroxene and plagioclase fields. Pilotaxitic felsic clasts crystallized plagioclase laths and minor interstitial pyroxene under metastable conditions, and the mesostasis is extremely enriched in plagioclase component in spite of the ubiquitous crystallization of plagioclase laths in the clasts. We suggest that there are two crystallization trends, pyroxene‐metal and pyroxene‐plagioclase trends, for the magmatic inclusions and felsic lithic clasts in DaG 319. The pyroxene‐metal crystallization trend corresponds to the magmatic inclusions in the type II ureilite clasts and the pilotaxitic felsic clasts, where crystallization took place under reducing and metastable conditions, suppressing precipitation of plagioclase. The pyroxene‐plagioclase crystallization trend corresponds to the magmatic inclusions in the isolated plagioclase clast and the porphyritic felsic clasts. This trend developed under oxidizing conditions in magma chambers within the ureilite parent body.The felsic clasts may have formed mainly from albite component‐rich silicate melts produced by fractional partial melting of chondritic precursors. The common monomict ureilites, type I ureilites, may have formed by the fractional partial melting of alkali‐bearing chondritic precursors. However, type II ureilites may have formed as cumulates from a basaltic melt.

ORIGIN OF PLAGIOCLASE LHERZOLITE FROM THE NIKANBETSU PERIDOTITE COMPLEX, HOKKAIDO, NORTHERN JAPAN

ORIGIN OF PLAGIOCLASE LHERZOLITE FROM THE NIKANBETSU PERIDOTITE COMPLEX, HOKKAIDO, NORTHERN JAPAN

Pétrologie et éochimie des basaltes de Murta : une éruption sous-glaciaire dans les Andes patagoniennes (46° lat. S.), Chili. Relation avec la subduction de la ride du Chili

The Holocene Murta basalts represent a small volume monogenetic centre located in the northern Patagonian Andes at the latitude of the Chile Triple Junction. These basalts show morphological evidence of subglacial eruption. They are locally porphyritic with large plagioclase feldspar, clinopyroxene and olivine megacrysts. The major- and trace-element concentrations, REE patterns, and isotopic data, distinguish them clearly from the other Holocene arc basalts or the Palaeocene back-arc lavas. Their asthenospheric signature is in accordance with a geodynamic context characterised, since the Miocene, by the subduction of the Chile Ridge below the South American Plate and the progressive development of a slab window.

Evidence of Magma Mixing and Hybrid Source on Calc-Alkaline Sarihan (Bayburt) Granitoid, NE Turkey

The study area is in the South Zone of Eastern Pontides (NE Turkey). The basement is Pre-PermoCarboniferous aged Pulur Massif which contains greenschist, micaschist, para-amphibole schist, ortoamphibolite, gneiss, marble, quarzite and intruding Middle-Carboniferous aged Saraycik Granodiyorite. Liassic aged Hamurkesen Formation overlying unconformably the massif begins with Dikmetas Conglomerate and continues with volcano-sedimentary rocks containing tuff, sandstone and andesite, all of which are cut by diabase dykes. Maim-Lower Cretaceous aged Hozbirikyayla Formation overlies conformably Hamurkesen Formation and contains limestone and sandy limestones. Hozbirikyayla Formation is overlied by conformably Otlukbeli Melange containing red-limestone, pyroclastics, serpentinite, radiolarite, limestone olistoliths and breciated basalt. Campanien aged Sarihan Granitoid cuts all these lithologies. Sarihan Granitoid covering an area of km 2 contains quartz-monzodiorite (60%), granodiorite (35%) and quartz-diorite (5%), and is cut by 5 to 10 cm width aplite dykes. These rocks show similar petrographical features. In the center of intrusion, 0.5-1 cm width cracks were filled by black tourmaline. The rocks of the intrusion shows medium-grained, poikilitic, anti-rapakivi and sometimes myrmekitic textures, and contains 43-64% plagioclase, 7-15% orthoclase, 10-29% quartz, 5-20% hornblende, 1-8% biotite and 1-6% opaque oxides. Plagioclase is subhedral, mainly oligoclase and rare andesite in composition (Anls-37 Ab55-79 Or22_9). Large crystals show generally oscillatory zoning and albite twinning. Crystals with oscillatory zoning range in composition from An33 in the core to Anls in the rim. Mirmekitic textm'es developed at the contact of orthoclase. Large plagioclase may contain plagioclase, apatite, magnetite and rare hornblende inclusions.

Emplacement and petrological evolution of the andesitic dome of Galeras volcano, 1990–1992

Since the reactivation of Galeras volcano in 1988, the emplacement of a lava dome in the main crater of the volcano has been its most significant magmatic activity. Changes leading to the emplacement of the dome were present as early as June–July 1990, when the trend of degassing for the major species (mainly SO 2 and CO 2) in the summit fumaroles changed drastically, and small vulcanian explosions become more frequent. Moreover, after August 1990, an inflationary trend as measured by an electronic tiltmeter was observed to increase over background levels, and the temperature of a newly formed fumarole (Besolima) increased from 130 to 738 °C within a month. The dome was seen for the first time in early October 1991 at the base of the main crater. The dome grew for about a month to a total volume of ≈ 4 × 10 5 m 3. During this time, the growth rate was calculated to be 12 × 10 3 m 3/day or 0.14 m 3/s. An explosive eruption on 16 July 1992 destroyed most of the dome. The chemical composition of the dome ranges from 59.4 to 61.1 wt.% SiO 2 (anhydrous). The presence of (1) large plagioclase phenocrysts with strongly inversely zoned rims and (2) small euhedral plagioclase crystals with normal zoning and Ca-rich cores indicates that a magma mixing event occurred early in the history of dome emplacement. Large amounts of crystal fractionation then took place. About 70% fractionation can explain the mineral suite and the major- and trace-element compositions of the glasses. The uppermost part of the dome, degassing freely at the surface for ten months, was able to isolate the magmatic system, producing a highly evolved melt in the dome interior. The July 1992 eruption tapped not only the uppermost part of the dome but also a deeper H 2O-saturated magma. Analyses from these samples show that most of the H 2O was degassed while Cl and B contents were higher than in the uppermost part of the dome. However, the F content is lower in this deeper part of the conduit, while it increases in the matrix glass of the degassed upper part of the dome.

Petrogenesis of lavas from the AMAR Valley and Narrowgate region of the FAMOUS Valley, 36°-37°N on the Mid-Atlantic Ridge

Basaltic lavas from the AMAR Valley and the Narrowgate region of the FAMOUS Valley on the Mid-Atlantic Ridge (36° to 37° N) range in texture from aphyric to highly plagioclase phyric (>25% large plagioclase phenocrysts). Based on 87Sr/86Sr and 143Nd/144Nd ratios, most of these lavas can be subdivided into two distinct, isotopically homogeneous, groups: Group I has lower 87Sr/86Sr (0.70288±1) and higher 143Nd/144Nd (0.51312±1) ratios; Group II has higher 87Sr/86Sr (0.70296±1) and lower 143Nd/144Nd (0.51309±2) ratios. Most Group II lavas are aphyric, whereas Group I lavas are primarily plagioclase phyric. Lavas from both groups show a wide range in incompatible element abundance ratios (e.g., Zr/Nb =6–29; (La/Sm)n=0.6–1.7). Aphyric lavas have relatively constant Sc (40±1.5 ppm) abundances and CaO/Al2O3 ratios (0.80±0.02). Group I lavas are confined primarily to the AMAR rift valley floor whereas Group II lavas are found along the east and west marginal highs. We interpret the isotopic differences between the two groups as reflecting a temporal change in the upwelling mantle beneath this region of the Mid-Atlantic Ridge which is south of the Azore Islands. For each group, a petrogenetic model consistent with the geochemical data is multi-stage decompression melting of an initially enriched, homogeneous, mantle source region. If the early derived, incompatible-element enriched, melt increments are not always pooled with subsequent increments, the erupted magma batches may have the major element characteristics of melts derived by 10 to 20% melting, but with incompatible element abundance ratios reflecting the change from an enriched to depleted source during the incremental melting process. In this process an initially homogeneous source can generate primary magmas with the required range in incompatible element abundance ratios shown by each group. The nearly constant CaO/Al2O3 ratios and Sc contents of the aphyric lavas with decreasing Mg ? reflects subsequent polybaric fractionation of clinopyroxene, plagioclase and olivine over the pressure interval 8–6 kbar (24–18 km), followed by rapid transport to the surface and eruption. There is no geochemical evidence for a crustal magma chamber beneath this section of the Mid-Atlantic Ridge.

Anatexis of interlayered amphibolite and pelite at 10 kbar: effect of diffusion of major components on phase relations and melt fraction

We have conducted H2O-undersaturated melting experiments on charges consisting of a layer of a sillimanite-bearing metapelite and a layer of garnet-bearing amphibolite, with the goal of studying how layered metamorphic rocks interact during anatexis. Experiments with the layered charges were performed at 10 kbar (1 GPa) at 900 °C (334 h), 925 °C (341 h), and 950 °C (50, 161, 332 h). Dehydration-melting of the amphibolite alone and of the pelite alone were studied at the same P-T conditions. When run alone at 900 °C, the pelite and the amphibolite yielded ca. 40 vol.% and 15 vol.% melt, respectively. Corresponding melt fractions at 950 °C were ca. 50 vol.% and 30 vol.%. When run side by side, melt abundance in the pelite is fairly uniform and ca. 20 vol.% higher than in the solo runs at all temperatures studied. In contrast, complex zoning patterns parallel to the interface develop in the amphibolite layer. Far from the interface the melt fraction is 5–10 vol.% lower than in the amphibolite-solo runs. Closer to the interface the melt fraction in the amphibolite layers are comparable to, or somewhat higher than, in the amphibolite-solo runs, and both clinopyroxene and plagioclase react-out and are replaced by orthopyroxene and quartz. Melt fraction closest to the interface reaches 65–70 vol.%, and the only crystalline phase present is garnet with hollow cores. The increase in melt fraction in the pelite layers of the layered runs is caused by transfer of Na from the amphibolite to the pelite, which forces biotite to be eliminated at lower temperature along the biotite-garnet reaction boundary. K is transferred from the pelite to the amphibolite, and the lower melt fraction in the distal end of the amphibolite layers is caused by faster diffusion of Na compared to K, which results in a net depletion of alkalies. Ca is transferred from the amphibolite to the pelite, and the resulting changes in CaO concentrations affect the stabilities of plagioclase, pyroxenes, garnet and quartz and cause the observed zoning in the amphibolite layer. Ca entering the pelite layer induces crystallization of large plagioclase neoblasts radiating from the interface into the pelite layer. These results show that diffusion of components between contiguous lithologies undergoing anatexis can cause profound changes in phase relations, melt fractions and restite compositions.

High spatial resolution [formula omitted] investigations using an ultra-violet laser probe extraction technique

Ultra-violet laser extraction offers a significant new approach to 40Ar 39Ar dating. The technique surmounts two important drawbacks to the existing visible and near IR laser probe 40Ar- 39Ar techniques; the difficulty in analysing minerals such as quartz, plagioclase, and K-feldspar which are transparent at these wavelengths, and the spatial resolution (~50 μm) limited by argon loss due to beam reflection/ refraction into the surrounding minerals. Three experiments demonstrate the advantages of UV laser ablation using a quadrupled Nd-YAG laser (λ = 266 nm) in applications to geological problems. 1. (1) A traverse across a biotite/quartz boundary demonstrates that UV laser ablation extracts negligible amounts of argon from beyond the visible laser pit, even in quartz. 2. (2) Analyses of a large plagioclase phenocryst from the Paraná volcanic field, Brazil, differentiated pristine and altered areas. Analysis of the feldspar grain margin caused no detectable argon loss from groundmass only 20 μm distant. 3. (3) A depth profile in K-feldspar produced by repeatedly rastering the UV laser beam across an area achieves yet higher spatial resolution. Argon diffusion profiles around 20 μm deep, were measured with a spatial resolution of 2–3 μm in a sample of gem quality K-feldspar, thermally treated at 700°C and 2 kbar argon pressure.

Petrological significance of zoned plagioclase in Eldjurti granite and mafic microgranular enclaves, North Caucasus, Russia

Plagioclase phenocrysts from mafic enclaves and plagioclase from its host granite possess a pattern of complex zonation. A plagioclase phenocryst can generally be divided into three parts: an oscillatory, locally patchy zoned core (An47-19), a ring with dusty, more calcic plagioclase (An64-20) and a normally zoned rim composed of sodic plagioclase (An22-3.3). Major discontinuities in zoning coincide with resorption surfaces that are overgrown by the more calcic plagioclase. The cores of large plagioclase phenocrysts from mafic enclaves and host granite show similar zoning patterns and similar compositions, indicating their crystallization under the same conditions. Steep normal zoning of the rims of plagioclases both from host granite and mafic enclaves illustrates a drastic decrease in An content which is considered to have resulted from the continuous differentiation of hybrid magma and efficient heat loss because of the upward emplacement of the residual magma. Wide rims of plagioclases from the host granite against the discrete rims of plagioclases from mafic enclaves indicate that differentiation and cooling lasted much longer in the host granite than in the mafic enclaves.

Plagioclase xenocrysts and mafic magmatic enclaves in some granitoids of the Sierra Nevada Batholith, California

Plagioclase phenocrysts from mafic magmatic enclaves and plagioclase crystals from host granitoids of some plutons of the central part of the Sierra Nevada Batholith are complexly zoned and commonly divided into three neatly distinct parts: an oscillatory, locally patchy zoned, andesine or more calcic core, a ring with dusty calcic plagioclase, and a normally zoned rim of sodic plagioclase. Although aspect of the calcic rings and width and zoning of the rims may slightly vary from the enclaves to the hosts, cores of both phenocrysts and large plagioclase crystals show similar zoning and composition. The andesine or more calcic cores are interpreted to have crystallized in the felsic or the mafic magma, respectively, and been incorporated into the coeval magma when the two magmas mixed. Introduction of the xenocrystic cores into a magma where they were not in equilibrium resulted in partial dissolution, development of abundant patchy zoning, and coating with dusty calcic plagioclase. In both granitoids and mafic magmatic enclaves, composition and zoning contrast between cores and rims of the plagioclase crystals reflect drastic changes in conditions of crystallization before and after the mechanical mixing event. Mixing of two magmas with contrasted compositions is suggested to be the major mechanism for generating complexly zoned plagioclase xenocrysts in granitoids and mafic magmatic enclaves. This hypothesis is consistent with many recent models in which mixing of two contrasting components is proposed to play a fundamental part in the generation of calc‐alkaline granitoids and mafic magmatic enclaves of the Sierra Nevada Batholith. Plagioclase xenocrysts may also provide information on the timing of the different mixing processes and on the magmatic evolution of the plutons.

Small‐scale disequilibrium in a magmatic inclusion and its more silicic host

Basaltic andesite inclusions and their host dacite from the Purico‐Chascon complex in northern Chile are isotopically distinct. Textural characteristics of the inclusions are typical of those resulting from magma mingling. Serial sectioning across the interface of an inclusion and its host dacite, complemented by microdrill sampling and detailed microprobe work, has enabled us to examine the scale of mixing and chemical (isotopic and trace element) disequilibrium. The results of this work show that (1) the composition of the inclusion is relatively homogeneous; (2) the dacite host is generally higher in 87Sr/86Sr and lower in 143Nd/144Nd than the enclave, but it is heterogeneous on a small scale, and probably a “hybrid”; (3) the isotopic composition in the marginal zone, apparently on both host and inclusion sides of the weakly chilled interface, actually shows the highest 87Sr/86Sr and lowest 143Nd/144Nd; (4) large plagioclase crystals in the inclusions and host are xenocrystic, with higher 87Sr/86Sr than any of the other samples. These observations are reconciled with a model of magma evolution in a crustal magma chamber. In such a scenario the mafic magma is overlain by a cap of rhyolite—a partial melt of the dacitic ignimbrites which now underlie the Purico‐Chascon complex. Overturn of such a magma system gives rise to a hybrid dacite containing discrete mafic inclusions.

Gabbroic xenoliths from the Northern Gorda Ridge: Implications for magma chamber processes under slow spreading centers

Abundant gabbroic xenoliths in porphyritic pillow basalt were dredged form the northern Gorda Ridge. The host lava is a moderately fractionated, normal mid‐ocean ridge basalt with a heterogeneous glass rind (Mg numbers 56–60). Other lavas in the vicinity range from near primary (Mg number 69) to fractionated (Mg number 56). On the basis of textures and mineral compositions, the xenoliths are divided into five types: Type A xenoliths consist of large clinopyroxene oikocrysts, enclosing euhedral to subhedral plagioclase. Olivine and glass are present only in trace amounts. Type B xenoliths consist of olivine and plagioclase (±spinel) with abundant intersertal glass. Partially resorbed olivine with deeply embayed margins in places subophitically encloses small plagioclase crystals. Olivine and large plagioclase crystals often contain glass inclusions and/or spinel. Type C xenoliths are a composite of types A and B, with areas of large clinopyroxene oikocrysts adjoining areas of loosely joined clusters of olivine and plagioclase in glass. Type D is represented by a single large xenolith that, except for containing a larger proportion of crystals, appears identical to the host lava. Type E is represented by a single small xenolith that is mineralogically similar to type B but with a distinct, fine‐grained, tightly interlocking texture, formed by small olivine oikocrysts enclosing euhedral plagioclase, in a small amount of intersertal glass. Many single crystals and glomerocrysts in the host lava appear to be xenocrysts. Some forsteritic olivine and anorthitic plagioclase contain glass inclusions more primitive than the host lava, while rare glass inclusions in skeletal olivine suggest entrapment of melt more evolved than the host lava. The overabundance of plagioclase crystals and their textural features suggest accumulation. The xenoliths are not cognate to the host lava, but they are genetically related. Chemistry of mineral phases in conjunction with textural features suggests that the xenoliths formed in different parts of a convecting magma chamber that underwent a period of closed system fractionation. The chamber was filled with a large proportion of crystalline mush when new, more primitive, and less dense magma was injected and mixed incompletely with the contents in the chamber, forming the hybrid host lava. Plagioclase‐rich leucogabbro layers in ophiolites attest to similar processes in magma chambers of past spreading centers. However, xenoliths in ferrobasalt from the southern Juan de Fuca Ridge differ significantly from the Gorda Ridge xenoliths and suggest that the magma chamber at the southern Juan de Fuca Ridge is located at shallower depth and in cooler crust than the one at the northern Gorda Ridge.