Pyroxene-bearing Rocks Research Articles

Beyond the second-order magnetic anisotropy tensor: higher-order components due to oriented magnetite exsolutions in pyroxenes, and implications for palaeomagnetic and structural interpretations

SUMMARY Exsolved iron oxides in silicate minerals can be nearly ideal palaeomagnetic recorders, due to their single-domain-like behaviour and the protection from chemical alteration by their surrounding silicate host. Because their geometry is crystallographically controlled by the host silicate, these exsolutions possess a shape preferred orientation that is ultimately controlled by the mineral fabric of the silicates. This leads to potentially significant anisotropic acquisition of remanence, which necessitates correction to make accurate interpretations in palaeodirectional and palaeointensity studies. Here, we investigate the magnetic shape anisotropy carried by magnetite exsolutions in pyroxene single crystals, and in pyroxene-bearing rocks based on torque measurements and rotational hysteresis data. Image analysis is used to characterize the orientation distribution of oxides, from which the observed anisotropy can be modelled. Both the high-field torque signal and corresponding models contain components of higher order, which cannot be accurately described by second-order tensors usually used to describe magnetic fabrics. Conversely, low-field anisotropy data do not show this complexity and can be adequately described with second-order tensors. Hence, magnetic anisotropy of silicate-hosted exsolutions is field-dependent and this should be taken into account when interpreting isolated ferromagnetic fabrics, and in anisotropy corrections.

Magmatic differentiation at La Poruña scoria cone, Central Andes, northern Chile: Evidence for assimilation during turbulent ascent processes, and genetic links with mafic eruptions at adjacent San Pedro volcano

La Poruña (21°53′S; 68°30′W) is a 140 m high scoria cone composed of pyroclastic material and an extensive basaltic-andesite to andesite lava flow that is up to 8 km in length. Automated mineralogical analysis describes a suite of porphyritic mafic samples, comprising olivine- and pyroxene-bearing rocks. Well-defined major element compositional trends, as well as trace and rare earth element characteristics (e.g. Sr/Y < 47; Sm/Yb < 4), likely reflect magmatic differentiation at middle-upper crustal levels. Additionally, magma mixing and assimilation and fractional crystallization processes act on these La Poruña magmas within the thickened continental crust, which is typical in Andean volcanic systems. A remarkable compositional feature is the unusual reversed isotopic behaviour of increasing silica with decreasing 87Sr/86Sr compositions. In a process of crustal assimilation during turbulent magma ascent (ATA), the least differentiated rocks are the most contaminated ones since the turbulent hottest magmas effectively assimilate the crustal material. We relate the inverse Sr isotope trend to latter magmatic evolution involving ATA at shallow crustal levels prior to eruption, therefore differing from the broadly accepted Central Andean magmatic model. The older volcanics (>96 ka) from San Pedro volcano exhibit similar isotopic characteristics, therefore evidence of similar magmatic processes. This new dataset clearly defines magma compositional changes during the La Poruña eruption (ca. 100 ka), revealing an increase in crustal contamination at shallow crustal levels for the younger San Pedro lavas (<96 ka), likely controlled by increasing amounts of deep-sourced basaltic input over time.

The role of Fe content on the Fe-Mg exchange reaction in augite

The study of the intracrystalline Fe-Mg exchange between M 1 and M 2 crystallographic sites in clinopyroxene on samples a variety of geological settings has provided a framework to understand the thermal history of pyroxene-bearing rocks. The Fe-Mg exchange reaction has successfully been exploited as a geothermometric tool in the study of orthopyroxene and pigeonite-bearing rocks, but relatively few data are available for clinopyroxene. A strong correlation between total iron content and the slope of the Fe 2+ -Mg equilibrium distribution coefficient ( k D ) as a function of temperature has been found for orthopyroxene and pigeonite, and we investigate this relationship in augite. We carried out new equilibrium annealing experiments at 800, 900, and 1000 °C followed by single-crystal X-ray diffraction and structural refinement to obtain a new geothermometric calibration for augite from a 120 m thick lava flow from Ontario, Canada [Theo’s Flow, En 49 Fs 9 Wo 42 hereafter also referred as Fs 9 where Fs = 100·ΣFe/(ΣFe+Mg+Ca) with ΣFe = Fe 2+ +Fe 3+ +Mn]. This new calibration enabled us to evaluate the compositional effects (mainly Fe content) by comparison with the data previously obtained on augite from MIL 03346 martian sample (En 36 Fs 24 Wo 40 hereafter referred to as Fs 24 ). The extremely good agreement observed between the data obtained on Theo’s Flow and Miller Range (MIL 03346) augite demonstrate that for the range of compositions between Fs 9 and Fs 24 , total iron content has a negligible or null influence on equilibrium behavior. Furthermore, linear regression of data from Theo’s Flow and MIL 03346 gave a single calibration equation: ln k D = − 4040 ( ± 180 ) / T ( K ) + 1 . 12 ( ± 0 . 17 ) ( R 2 = 0 . 988 ) . This new calibration describes the equilibrium behavior of augite and can be reliably used to determine the closure temperature ( T c ) of augite with composition ranging between Fs 9 and Fs 24 .

Mineralogy, morphology and stratigraphy of the light-toned interior layered deposits at Juventae Chasma

Juventae Chasma is a deep depression located north of Valles Marineris on Mars, with four bright mounds or light-toned interior layered deposits (ILDs) extending upwards from the Canyon floor. We present here the results of long-term imaging of Juventae Chasma including mounds A, B, C, and D using multiple datasets. Monohydrated sulfates (MHS) were deposited first on the canyon floor, followed by polyhydrated sulfates (PHS). The upper PHS-dominated units are largely eroded away at Juventae Chasma, but this material is still present in significant abundance at mound B. PHS are observed mixed with MHS in some areas of mounds A and C. Terraces are observed at the upper elevations of mound B that contain PHS at the steeper slopes and appear to be covered with dust on the horizontal surfaces. Current analyses of the MHS-rich unit indicate that kieserite (MgSO4⋅H2O) is the primary sulfate component, rather than szomolnokite (FeSO4⋅H2O) as previously thought. Formation of kieserite at Juventae Chasma likely required temperatures in the 150–200°C range. Geochemical modeling is most consistent with dissolution of mafic materials followed by precipitation of kieserite from solution. The dust exhibits ferric signatures and the sand is largely mafic material. Outcrops of olivine- and pyroxene-bearing rocks are best observed along the base of mound C and in the chaotic terrain surrounding mound D. This study summarizes the current understanding of Juventae Chasma and its ILDs using HRSC, HiRISE and CTX data, an expanded laboratory spectral library, and the latest calibrations available for CRISM.

The physical volcanology of Archaean komatiite sequences from Forrestania, Southern Cross Province, Western Australia

The Archaean supracrustal sequence at Forrestania contains at least five komatiitic belts which can be traced along strike for distances in excess of 30 km. Lithologies range from olivine cumulates, which crystallised at the interface between substrate and flowing lava, to spinifex-textured rocks, which underwent rapid crystallisation within ponded lava lobes. Characteristic associations of different rock types are interpreted in terms of variations in eruption rate, which presumably reflect proximity to the vent or to a major lava river. Thick sequences of olivine adcumulate to mesocumulate crystallised in a dynamic environment, where lava flow was continuous and eruptive velocities were high. Sheeted bodies of olivine cumulate formed from voluminous, turbulent flow of lava over relatively refractory substrates, such as basalt and oxide-facies banded iron-formation. Ribbon-shaped olivine-cumulate bodies formed in an equally dynamic environment within major lava channels, where the substrate was less refractory and consisted of quartz-rich sedimentary rocks which were susceptible to thermal erosion. Distributary lava channels in more distal settings are represented by thinner ribbon-like accumulations of olivine mesocumulate to orthocumulate. Unusual poikilitic pyroxene-bearing rocks are present at the base of some lava channels and appear to represent the crystallisation product of a komatiitic lava which has been contaminated by cherty sedimentary rocks. Pyroxene- and plagioclase-bearing cumulates are present at the top of other channel sequences and reflect late-stage ponding and in situ differentiation of komatiitic lava within the channel. The channel-facies rocks are flanked by sequences comprising multiple, thin, spinifex-textured flow-units, which formed from the episodic overflow of lava from the distributary channels. Similar rocks also occur stratigraphically above many of the thick olivine-cumulate bodies and reflect episodic emplacement of thin cooling units during the waning stages of the eruptive cycle.

The physical volcanology of Archaean komatiite sequences from Forrestania, Southern Cross Province, Western Australia

The Archaean supracrustal sequence at Forrestania contains at least five komatiitic belts which can be traced along strike for distances in excess of 30 km. Lithologies range from olivine cumulates, which crystallised at the interface between substrate and flowing lava, to spinifex-textured rocks, which underwent rapid crystallisation within ponded lava lobes. Characteristic associations of different rock types are interpreted in terms of variations in eruption rate, which presumably reflect proximity to the vent or to a major lava river. Thick sequences of olivine adcumulate to mesocumulate crystallised in a dynamic environment, where lava flow was continuous and eruptive velocities were high. Sheeted bodies of olivine cumulate formed from voluminous, turbulent flow of lava over relatively refractory substrates, such as basalt and oxide-facies banded iron-formation. Ribbon-shaped olivine-cumulate bodies formed in an equally dynamic environment within major lava channels, where the substrate was less refractory and consisted of quartz-rich sedimentary rocks which were susceptible to thermal erosion. Distributary lava channels in more distal settings are represented by thinner ribbon-like accumulations of olivine mesocumulate to orthocumulate. Unusual poikilitic pyroxene-bearing rocks are present at the base of some lava channels and appear to represent the crystallisation product of a komatiitic lava which has been contaminated by cherty sedimentary rocks. Pyroxene- and plagioclase-bearing cumulates are present at the top of other channel sequences and reflect late-stage ponding and in situ differentiation of komatiitic lava within the channel. The channel-facies rocks are flanked by sequences comprising multiple, thin, spinifex-textured flow-units, which formed from the episodic overflow of lava from the distributary channels. Similar rocks also occur stratigraphically above many of the thick olivine-cumulate bodies and reflect episodic emplacement of thin cooling units during the waning stages of the eruptive cycle.

Cosmogenic 3He production rates from 39°N to 46°N latitude, western USA and France

The in-situ production rate of cosmogenic 3He was determined in seven different olivine- or pyroxene-bearing rocks between the latitudes 39°N and 46°N. The integrated production rate over the last 17,500 years is about 115 ± 4 atoms/gram/yr for high latitude at sea level. The variation in the integrated production rate between 2000 and 18,000 years is less than 30% and is probably on the order of 20% or less

Petrology of the Turnagain ultramafic complex, northwestern British Columbia

The Turnagain ultramafic complex is an Alaskan-type complex of possible Late Triassic age. It has a central zone of dunite and an outer zone of wehrlite, olivine clinopyroxenite, clinopyroxenite, dunite, hornblendite (rare), and basic hornblende and plagioclase-bearing rock (rare). The complex is largely fault-bounded. The central dunite intrudes the pyroxene-bearing rocks in one area; elsewhere the two zones are gradational. Small-scale layering occurs locally, and is most common in the outer zone. Layering dips steeply due to folding during regional deformation. The ultramafic rocks are generally only partly serpentinized, and consist largely of olivine, clinopyroxene, pargasite, phlogopite, and chrome spinel. The outer zone contains sporadic concentrations of iron–nickel–copper sulfides. Olivine is most magnesian (Fo94.9) in the central dunite, and most iron-rich (Fo80.2) in olivine clinopyroxenite. Clinopyroxene is diopside and follows an iron-enrichment trend. Minor-element contents of minerals indicate that crystallization caused magmatic impoverishment in Ni and Cr, and enrichment in Ti and Al. The primitive Turnagain magma was probably ultrabasic, extremely magnesian, and poor in Al and Ti. Differentiation followed an alkalic trend, possibly at relatively low oxygen fugacity compared to other Alaskan-type complexes. The complex may have formed in a subvolcanic magma chamber, and thus be related to nearby Upper Triassic volcanic rocks.

Ophiolite metamorphism in northeast Borneo

The Darvel Bay ophiolite consists of a conformable sequence of mantle harzburgite, a gabbro layer of about 2 km thickness, a basalt layer and associated chert-spilite association of Late Cretaceous to Eocene age, and overlying Miocene melange and olistostrome deposits. The ophiolite is an extension into Borneo of the recently inactive Sulu Archipelago non-volcanic arc. Generally the gabbro layer has been converted to banded plagioclase-amphibole gneisses and the basalt layer to crudely foliated epidote amphibolite. Igneous relicts occur sporadically, especially in the lower gabbro horizons. They have either cumulus granular or gabbroic textures and contain diopsidic augite, bronzite, and a dark brown pargasitic hornblende. The labradorite is occasionally clouded. The partitioning of Fe/Mg between the two pyroxenes and the hornblende suggests an igneous paragenesis and the former allocation of these pyroxene-bearing rocks to the granulite facies is now rejected. The basites have undergone metamorphic re-equlibration, frequently sporadic and imperfec, through amphibolite facies, characterized by hornblende, to greenschist facies, characterized by actinolite. Igneous labradorite frequently persists to the lower grades. The partitioning of Ca/Na between amphibole and plagioclase may be correlated with metamorphic grade. Sub sea-floor retrogressive metamorphism, combined with large scale metasomatism resulting from the circulation of hot brine, appears to be the most appropriate mechanism to explain the sodium enrichment in the basalt layer and the imperfect metamorphic re-equilibration of the ophiolite generally.

Visible and near-infrared diffuse reflectance spectra of pyroxenes as applied to remote sensing of solid objects in the solar system

Spin-allowed Fe2+ absorption bands occur in the visible and near-infrared diffuse reflectance spectra of most pyroxenes. The wavelengths of the bands centered near 1 μm and 2 μm vary as functions of pyroxene composition, making possible mineralogical and chemical deductions based on spectral reflectance curves. Typically, pyroxene bands are well developed in relation to absorption features in the spectra of other rock-forming minerals and glass; thus pyroxene often dominates the spectral curves of pyroxene-bearing rocks. Telescopic spectra of the moon and some other solar system objects contain absorption features that can be interpreted in terms of pyroxenes mixed with other minerals and/or glass.

A Local Potassic Modification of the Ballachulish Granodiorite

AbstractThe development of potash-rich pyroxene-bearing rocks around inclusions derived from the Appin limestone is associated with the transfer of magmatic silicon, aluminium, iron, and sodium. Calcium lost to the magma, by combining with further silicon, aluminium, and sodium to form a marginal zone rich in plagioclase, has left the magma enriched in potassium.

The Rocks of Pulau Ubin and Pulau Nanas (Singapore)

I. I ntroduction . I n the January number of the ‘Geological Magazine’ for 1909 a short note of mine appeared on the igneous rocks of Singapore, giving the results of observations made on occasional visits to Singapore from the Malay States during the years 1904 to 1908. Not many weeks after the paper had been despatched, a further opportunity presented itself of visiting some of the localities mentioned; and, as perhaps has happened to others, I found that the fresh information available in the quarries was so extensive and so significant that my remarks in the note mentioned appeared very meagre. It proved too late, however, to withdraw the paper, and therefore the following remarks must be considered in some sense supplementary to those in the earlier paper; but, at the same time, they embody other new information that will, I think, be found to be of considerable interest to those who have studied the geology of the East Indies. My object in the present paper is to describe the granite of Pulau Ubin, veins of rhombic pyroxene-bearing rocks traversing this granite, as well as ‘basic masses’ in the granite of the same island; also to describe fragments of granite in tuffs on Pulau Nanas, and to show that these are derived from a granite mass distinct from and much older than that forming Pulau Ubin, and therefore also older than the tin-bearing granite that now forms the backbone of the Peninsula. Further, the mutual relations of the granite, the veins, and