Anorthoclase Megacrysts Research Articles

40Ar/39Ar ages of alkali feldspar xenocrysts constrain the timing of intraplate basaltic volcanism

Abstract Intraplate continental basaltic volcanic provinces (ICBVPs) occur on all continents and represent some of the most enigmatic volcanic systems. Constraints on the origin(s) and evolution of ICBVPs are predicated on detailed knowledge of eruption histories, which are often poorly quantified. Although the 40Ar/39Ar geochronology method has been applied successfully to lava flows, age determinations on scoria cones and maars are more challenging. In this study, we test the potential of entrained anorthoclase megacrysts to yield accurate 40Ar/39Ar ages for scoria cones and maars from the Pliocene-Holocene Newer Volcanic Province (NVP) of south-eastern Australia. The NVP is an ICBVP containing >400 eruption centres with ages spanning 4.6 Ma – 5 ka. K-Ar and 40Ar/39Ar age data exist for a large number of lava flows, but age constraints for scoria cones and maars that lack associated lavas are rare. High-precision 40Ar/39Ar step-heating data were measured on anorthoclase megacrysts from five eruption centres in the NVP. From youngest to oldest, anorthoclase xenocrysts from Mount Noorat (n = 3), Mount Leura (n = 2), Mount Shadwell (n = 3), and The Anakies East Cone (n = 2) yielded reproducible mean inverse isochron ages of 101.75 ± 0.96 ka (2σ), 165.4 ± 1.6 ka (2σ), 353.8 ± 1.9 ka (95% CI), and 2.178 ± 0.005 Ma (95% CI), respectively. In contrast, Lake Keilambete anorthoclase (n = 2) produced discordant age spectra and incongruous isochron/mini-isochron ages of 433.5 ± 7.2 ka (2σ) and 413.4 ± 4.5 ka (2σ). A single anorthoclase megacryst from Mount Franklin gave an age of 126.3 ± 7.2 ka (95% CI). By directly comparing megacryst age results with recently published, high-precision (

40Ar/39Ar dating of alkali feldspar megacrysts from selected young volcanoes of the Newer Volcanic Province, Victoria

The Newer Volcanic Province (NVP) in Victoria, with extension into south-eastern South Australia, represents the youngest chapter of Cenozoic volcanism in south-eastern Australia. However, most ages have been determined by the potassium–argon (K–Ar) method, and the age data are not comprehensive. In addition, few ages exist for the array of scoria cone volcanoes in the NVP. Seven alkali feldspar samples, mostly anorthoclase megacrysts, from volcanic centres in the NVP were used for 40Ar/39Ar dating in the present study. In geochronological order, with ages quoting 95% confidence limits, locations are Mount Franklin near Daylesford (0.110 ± 0.014 Ma), Red Rock near Alvie (0.116 ± 0.048 Ma), Lake Bullenmerri at Camperdown (0.116 ± 0.019 Ma), Ridge Road Quarry near Daylesford (2.01 ± 0.11 Ma) and Mount Kororoit near Diggers Rest (3.74 ± 0.26 Ma). Two samples from The Anakies, near Bacchus Marsh, produced discordant results suggesting a maximum age of ca. 1.9 Ma. The analyses and reported ages in the present study not only provide new geochronological data for the province, but also elucidate the difficulties in dating very young basalts using the 40Ar/39Ar dating method. These results are consistent with the erosion levels of the scoria volcanoes sampled, and indicate a major episode of explosive volcanic activity at ca. 100 ka. In contrast, the more eroded Mount Kororoit is considered to be ca. 3.7 Ma in age. The age of The Anakies is more equivocal owing to the indicated presence of excess argon and a maximum age of ca. 1.9 Ma is suggested for this locality. Given the latter results and lack of precision obtainable from the younger samples, the possibility remains that other samples contained extraneous argon and that the ages generated are thus maximum eruption ages. Analyses of additional samples from these and other localities will be required to further resolve this issue.

On the Time Scales of Magma Genesis, Melt Evolution, Crystal Growth Rates and Magma Degassing in the Erebus Volcano Magmatic System Using the 238U, 235U and 232Th Decay Series

We investigate the time scales of magma genesis, melt evolution, crystal growth rates and magma degassing in the Erebus volcano magmatic system using measurements of 238U–230Th–226Ra–210Pb–210Po, 232Th–228Ra–228Th and 235U–231Pa–227Ac. These are the first measurements of 231Pa–227Ac in volcanic samples and represent the first set of data in a volcanic system to examine the entire suite of relevant 238U, 235U and 232Th decay series nuclides. Our sample suite consists of 22 phonolite volcanic bombs, erupted between 1972 and 2005, and five anorthoclase megacrysts separated from bombs erupted in 1984, 1989, 1993, 2004 and 2005. The 238U–230Th, 230Th–226Ra and 235U–231Pa systems are uniform over the 34 years examined. The anorthoclase megacrysts and phonolite glasses show complementary 226Ra/230Th disequilibria with (226Ra/230Th) ∼40 in the anorthoclase and ∼0·75 in the phonolite glass. In all samples, (210Pb/226Ra) is in radioactive equilibrium for both phases. In two phonolite glass samples (227Ac/231Pa) is unity. For the phonolite glasses (228Ra/232Th) is in equilibrium, whereas in the anorthoclase megacrysts it is significantly greater than unity. Instantaneous crystal fractionation, with magma residence times greater than 100 years and less than 10 kyr, can account for the measured 238U–230Th–226Ra–210Pb and 235U–231Pa–227Ac. However, the significant 228Ra/232Th disequilibria in the anorthoclase megacrysts preclude this simple interpretation. To account for this apparent discrepancy we therefore developed an open-system, continuous crystallization model that incorporates both nuclide ingrowth and decay during crystallization. This open-system model successfully reproduces all of the measured 238U and 232Th disequilibria and suggests that the shallow magma reservoir at Erebus is growing. The implication of this modeling is that when the time scale of crystallization is comparable with the half-life of the daughter nuclide of interest (e.g. 226Ra) the simple isochron techniques typically used in most U-series studies can provide erroneous ages. The observation that (210Pb/226Ra) and (227Ac/231Pa) are in radioactive equilibrium suggests that the residence time of the magmas is >100 years. When considering the effect of 222Rn degassing on 210Pb/226Ra, the data indicate that the majority of magma degassing is deep and long before eruption, consistent with melt inclusion data. Additionally, for the 2005 lava bomb, whose eruption date (16 December 2005) is known explicitly, 210Po was not completely degassed from the magma at the time of eruption. Incomplete degassing of 210Po is atypical for subaerially erupted lavas and suggests that the Erebus shallow magma degasses about 1% of its Po per day. The combined 238U and 232Th data further indicate that the pyroclasts ejected by Strombolian eruptions at Erebus have compositions that are close to what would be expected for a near-steady-state system, reflecting inmixing of degassed magmas, crystal fractionation, and aging.

Megacrysts and salic xenoliths in Scottish alkali basalts: derivatives of deep crustal intrusions and small-melt fractions from the upper mantle

AbstractCa-poor and typically Na-rich feldspar megacrysts are common associates of spinel lherzolitic and pyroxenitic xenoliths in Scottish alkalic basalts. Associated megacrysts and composite megacrysts and salic xenoliths include apatite, magnetite, zircon, biotite, Fe-rich pyroxene(s) and corundum. The salic xenoliths and related megacrysts are referred to collectively as the ‘anorthoclasite suite': the majority of the samples are inferred to derive from the disaggregation of coarse-grained, typically Na-rich, syenitic protoliths at depth. Rare occurrences of euhedral anorthoclase megacrysts, together with zircon dating, imply that the suite crystallized at, or very shortly before, their entrainment by the basaltic host magmas. Some evidence suggests that the anorthoclasite suite protoliths lie within ultramafic (pyroxenitic) domains in the deep crust. The latter are inferred to be pegmatites, crystallized from carbonated trachytic magmas with widely variable Ca, Na, K, Ba and trace-element contents, and to have ranged from metaluminous to peraluminous. Crystal zonation and resorption textures within the salic xenoliths imply that the crystallization of the parent magmas was complex. Confirmation of this comes from cathodoluminescence studies of the feldspars showing that early ('primary’) anorthoclases and potassian albites exhibit partial replacement by a more potassic feldspar. A third generation of potassic feldspar (enriched in an assortment of trace elements and deduced to have crystallized from a carbonated high-K melt) forms transecting zoned veins in which carbonate fills the axial zone.Whereas most of the anorthoclasite suite materials are inferred to have grown from metaluminous magmas, the occurrence of magmatic corundum in salic xenoliths indicates crystallization from magmas that were peraluminous. The corundum-bearing samples also contain Nb-rich oxide minerals and their associated feldspars have the highest rare-earth element (REE)contents. Accordingly, the peraluminous trachyte magmas are deduced to have been specifically enriched in high field-strength trace elements. It is proposed that formation of the anorthoclasite suite protoliths is a phenomenon closely related to that of salic glass ‘pockets', well known from spinel lherzolite xenoliths around the world. Not only are there compositional affinities, but both sets of phenomena appear to have closely pre-empted the ascent of alkali basalt (host) magmas. We propose that the two sets of phenomena are linked and that the anorthoclasite suite derived from coarse-grained sheets, generated by the aggregation of salic melt fractions rising from the shallow mantle and heralding the onset of basaltic magmatism.

Noble gases in corundum megacrysts from the basalts in Changle, Shandong Province, eastern China

This study presents noble gaseous data of the corundum megacrysts from the Cenozoic basalts in Changle, Shandong Province, eastern China. It is known that no noble gaseous data of corundum megacryst have been documented before. The 3He/4He ratios (1.13–7.37 Ra) of the corundums from Changle vary from atmosphere to MORB values; the 20Ne/22Ne (9.67–10.75) and 21Ne/22Ne (0.0280–0.0372) data define two linear trends on Ne three-isotope diagram, respectively, along the MFL and the correlation line between atmosphere and MORB; the 38Ar/36Ar (0.177–0.194) ratios, the 40Ar/36Ar (280.9–404.2) ratios and the 128–136Xe/132Xe ration with obvious 129Xe excess are generally higher than atmospheric component, but the 40Ar/36Ar ratios are much closer to atomospheric ratio. The isotopic compositions of noble gases (particularly for He and Ar) of the corundums are similar to those of pyroxene, anorthoclase megacrysts, and mantle-derived xenoliths from this area, and those of mantle-derived xenoliths from several areas in eastem China. Therefore, the noble gases trapped in the corundums probably are from mantle source, representing a ‘mixed fluid’ produced by the interaction between the lithospheric mantle and fluids releasing from the convective plate. Both the noble gas isotopic compositions and the oxygen isotopic compositions of the solid corundums are not the characteristics of crustal source. These suggest that the corundums crystallized from mantle-derived magmas with minimal crustal contamination.

Megacrysts in the Cenozoic basalt of the Tuoyun Basin, Southwest Tianshan

Abundant megacrysts of clinopyroxene, amphibole, anorthoclase, and phlogopite are found together with deep-seated xenoliths in the Cenozoic basalt of the Tuoyun Basin, Southwest Tianshan. The megacrysts are mainly in the cone sheet formed at the early stage of the volcanic activity. Clinopyroxene megacrysts are located in the lower part of the profile, with amphibole and phlogopite megacrysts in the middle part and anorthoclase megacrysts in the upper part. The crystal integrity, absence of deformation fabric and their relation to the host basalt suggest that they were crystallized from the host magma and quickly transported to the surface. The mineralogical studies imply that the clinopyroxene megacrysts are of Al-augite with higher Al2O3 (>9%). Amphibole megacrysts are kaersutite rich in TiO2 (>4.5%). Sulfide inclusions such as pyrrhotite occur in some clinopyroxene and amphibole megacrysts. Thermodynamic calculations reveal that pyroxene megacrysts formed under the temperature of 1185.85–1199.85°C and the pressure between 1.53 and 1.64 GPa comparable to the crust-mantle boundary and amphibole megacrysts crystallized under the pressure of around 0.85 GPa, temperature about 1000°C comparable to the depth of 30 km. Anorthoclase megacrysts crystallized under the pressure between 0.8–1 GPa, temperature about 900°C. The absence of Ti-rich inclusions such as rutile can be considered as an evidence of quick magma ascending. The P-T conditions estimated via pyroxene megacrysts and phenocrysts compose a P-T path with a steep slope. It can be considered as another evidence of quick magma ascending. However, the estimated temperatures for amphibole megacrysts are markedly lower than those for pyroxene megacrysts given the same pressure. It probably shows that the amphiboles have crystallized at the vanguard of magma and under the volatile-rich condition. Thus, we can conclude that the Cenozoic basalts are produced in an extensional tectonic setting and the processes governing crystallization and ascending of the megacrysts are very complex.

An FTIR study of hydrogen in anorthoclase and associated melt inclusions

High-resolution Fourier transform infrared (FTIR) spectroscopy has been used to document the presence of hydrogen, to estimate its concentration, and to document its oxygen speciation in anorthoclase crystals and associated melt inclusions from Mount Erebus, Antarctica. Synchrotron-generated infrared radiation, 100 to 1000 times brighter than globar-generated infrared radiation, permits rapid collection of maps that depict relative intensities of a chosen FTIR band across the mapped area. Spectra and/or compositional maps showing variations in water concentration were collected from anorthoclase megacrysts and melt inclusions in the megacrysts. Studies of anorthoclase megacrysts involved collection of spectra from three mutually perpendicular sections cut from the crystals. FTIR spectra of anorthoclase crystals are characterized by a broad absorption band at approximately 3200 cm{sup -1} in the mid-IR range. The universal mass absorption coefficient for mid-IR range feldspar spectra, established by Johnson and Rossman (2003), was used for quantitative estimates of water concentrations in the feldspar crystals based on integrated area under the 3200 cm{sup -1} band. Water concentration in the anorthoclase sample was approximately 126 ppm, with an overall error of approximately {+-}30%. FTIR spectra of melt inclusions are characterized by a broad asymmetric absorption band at {approx}3550 cm{sup -1} that was used tomore » calculate total water concentration. The absence of a band at 1630 cm{sup -1} suggests that water in the melt inclusions occurs as OH{sup -} rather than as molecular H{sub 2}O. Absorption coefficients established by Mandeville et al. (2002) for H species in glass were used to calculate water concentrations in the melt inclusions. Melt inclusions in the Mt. Erebus anorthoclase have water concentrations ranging from 0.12 to 0.39 wt%, with an overall error of approximately {+-}15%. The ratio of water in anorthoclase crystals to water in the melt from which the crystals formed, based on this study, and at these low melt water concentrations, is approximately 1:10. However, water concentration varies significantly from one melt inclusion to another, possibly suggesting initial melt water heterogeneity. Maps of water concentration show that variations in water concentration within melt inclusions are associated with fractures that cut the melt inclusions and in some cases do not extend out into surrounding crystals or into crystal inclusions. Thin ({approx}50 {micro}m thick) zones of elevated water concentrations on the boundaries of the crystals in contact with melt inclusions suggest that water has diffused into the crystals from the melt inclusions.« less

Petrochemical study of the Jingpohu Holocene alkali basaltic rocks, northeastern China.

Holocene volcanoes in the Jingpohu region are situated in the “Crater Forest” and “Frog Pool” areas along the northwest side of Jingpohu Lake, northeastern China. Dating of three charcoal samples from the first and second volcanic cycles shows that the ages of the first and second cycles are 3430~3490 and 2470 years respectively. The lavas from the Jingpohu area consist of basanites (BSN), alkali olivine basalts (AOB) and tephrites (TP). Crystal fractionation models are consistent with the generation of AOB and TP from a basanitic parent. Minor fractionation of olivine, augite, magnetite and Cr-spinel is required to produce AOB compositions whereas the generation of TP requires extensive fractionation of kaersutite, phlogopite and anorthoclase with minor olivine, augite, magnetite and leucite. The presence of kaersutite, phlogopite and anorthoclase megacrysts and mantle xenoliths suggest a fractionation history occurring at high pressure in the mantle. Although all basaltic rocks contain many granitic xenoliths, their geochemical characteristics show that they have not undergone any contamination of upper crust en route to the surface, but some alkali basalts were suffered from the contamination of lower crust. Relatively unradiogenic isotope ratio (compared with Bulk Earth), steep chondrite-normalized REE patterns and strong incompatible element enriched patterns suggest that the magmas are derived from a mixture of an incompatible element depleted anhydrous lherzolite asthenospheric mantle source and an enriched, amphibole-phologite(apatite-)bearing lherzolite continental lithospheric mantle source. We propose that the basanites are the products of very low degree partial melts (<1%) of this source under high extension strength.

Ba partitioning and the origin of anorthoclase megacrysts in basalitic rocks

Ba partitioning and the origin of anorthoclase megacrysts in basalitic rocks

A reconnaissance of trace elements in anorthoclase megacrysts

AbstractMajor, minor, and trace elements were measured with electron and ion microprobes for eighteen anorthoclase megacrysts from five locations. Comparisons with concentration ranges tabulated by Smith (1974) show excellent agreement except for consistently lower values of Mg and B. The discrepancy for Mg probably reflects Mg-rich impurities contaminating samples analysed by bulk methods. The cause of the discrepancy for B is unknown and deserves further study. Samples from Mt. Erebus, Antarctica, contain glass inclusions from which the following crystal/liquid partition coefficients ensue: Mg, 0.008; P, 0.04; Ti, 0.08; Fe, 0.04; Ba, 5.2; Sr, 8; Rb, 0.27.

Polysynthetic twin width distributions in anorthoclase

The widths of just under 7000 Albite and Pericline lamellae were measured in a suite of 20 polysynthetically twinned anorthoclase megacrysts collected at two sites in New Mexico. The majority of the twins were formed during the inversion of the anorthoclase crystals from their high-temperature monoclinic modification to their present triclinic form. Textural evidence suggests that a few of the lamellae postdate the transformation and are mechanical in origin. Twin density is exceedingly inhomogeneous on all scales and the related black and white lamella sets are unequally developed within even a single crystal. This type of inhomogeneity is probably a common feature of all inversion twinning since it arises as a natural consequence of the complex and rapidly changing conditions within a crystal undergoing transformation.

Geochronological evidence for phased volcanic activity in Fife and Caithness necks, Scotland

AbstractIn an attempt to establish a chronology for volcanic neck emplacement and so elucidate petrogenesis, isotopic studies have been carried out on various cumulate inclusions, blocks and megacrysts which occur chiefly in association with tuffs infilling several Scottish vents. K-Ar ages of 13 samples of low-pressure cumulate minerals (biotite, hornblende and pyroxene) from necks in East Fife indicate crystallisation at shallow depth at 314 Ma. U-Pb analyses of zircons are concordant at 318 Ma suggesting they are also members of this suite and their formation is penecontemporaneous with the Namurian volcanic activity which is welldocumented stratigraphically. By 295 Ma crystallisation of anorthoclase megacrysts had been completed, perhaps from the fractionated residuum. An eruption from considerable depth (within the stability field of garnet precipitation) then broke through to the surface bearing high-pressure megacrysts. This penetrated and disrupted the early cumulates carrying them to the surface and producing the diverse vent assemblages. K-Ar dating of basanites suggest that the Duncansby Ness neck in Caithness was emplaced around 270 Ma in the early Permian. For two Fife necks the balance of evidence favours an age of 290 Ma (Stephanian) for this final explosive activity associated with vent formation.

Stability of sanidine + forsterite and its bearing on the genesis of potassic magmas and the distribution of potassium in the upper mantle

Experimental determination of the reaction KAlSiO 4 + 4MgSiO 3 ⇌KAlSi 3 O 8 + 2Mg 2 SiO 4 between 17.5 and 30 kbar and 900–1300°C defines an average slope of 33 bar/°C. The reaction delineates the maximum pressure stability of sanidine in a forsterite-bearing assemblage; sanidine and anorthoclase megacrysts in alkalic magmas must have crystallized at lower pressures. The reaction occurs at slightly lower pressures than the transition from silicate liquids (extremely silica-undersaturated) to carbonate-normative liquids as observed in partial melting studies of the systems KAlSiO 4 MgO SiO 2 CO 2 and KAlSiO 4 MgO SiO 2 H 2 O CO 2 . Both the reaction and the change in melt compositions reflect the rapid expansion of the enstatite stability volume with increasing pressure. Suggestions that potassium resides in sanidine or a hollandite structure in the deep upper mantle need to be revised. The stable K Al silicate at high pressures in anhydrous lherzolite compositions is kalsilite.

Anorthoclase megacrysts: physical conditions of formation

SummaryPrevious interpretations relating to precipitation of anorthoclase megacrysts from alkaline basaltic magmas under high-pressure conditions are contradicted. It is proposed that these feldspars were crystallized above the solvus from hydrous melts at shallow depths, under low-pressure equilibrium conditions. These melts had been considerably enriched with volatiles of sodium, strontium, and barium, and to some extent also of rubidium.

Strontium isotope geochemistry of megacrysts and host basalts from southeastern Australia

Strontium isotopic data for megacrysts and lavas from six eruptive centers within the Newer Basalts province of southeastern Australia show that megacrysts of clinopyroxene are in isotopic equilibrium with associated basalts, but that megacrysts of kaersutite, ferrokaersutite, orthopyroxene and anorthoclase may exhibit slight disequilibrium with their host basalts. Furthermore, the anorthoclase megacrysts may be either more or less radiogenic than their hosts. The 87Sr/ 86Sr ratios for 14 basalts from throughout the province vary from 0.7035 to 0.7045 and it is proposed that anorthoclase, amphibole and orthopyroxene megacrysts which crystallized in isotopic equilibrium with one magma may have been caught up in a pulse of a later magma of a different isotopic composition. The variations in the 87Sr/ 86Sr ratios for the basalts are attributed to variations in the isotopic composition of their source regions. Such isotopic heterogeneity is supported by published data for ultramafic xenoliths which occur in the Newer Basalts lavas.

Origin of anorthoclase megacrysts in alkali basalts

Anorthoclase megacrysts commonly occur with low pressure cumulate nodules (olivine + clinopyroxene + kaersutite ± oligoclase) in alkali basalts and their differentiates. The absence of anorthoclase from the cumulate nodules indicates that anorthoclase remained suspended in the magma while the other minerals sank, forming the cumulates, assuming a congeneric origin for megacrysts and nodules. On this basis, density calculations indicate that anorthoclase crystallised from a magma of approximately trachyandesitic composition, while the anorthoclase megacrysts usually occur in magmas more basic than trachybasalt. Thus, the anorthoclase megacrysts and the associated cumulate nodules did not crystallise from the host magma, but were picked up from a high-level partly crystallised magma pool containing a more evolved alkali basaltic differentiate by a later surge of more basic liquid which then carried both anorthoclase megacrysts and fragmented cumulates to the surface.