Olivine Microphenocrysts Research Articles

Origins of Marslike spectral and magnetic properties of a Hawaiian palagonitic soil

A palagonitic soil (HWMK1) from Mauna Kea, Hawaii, has Marslike spectral and magnetic properties. Reflectivity spectra of <20 μm size separates resemble spectra for Martian bright regions, and reflectivity spectra of coarser size separates have some spectral characteristics in common with spectra for Martian dark regions. The bulk soil is magnetic with a saturation magnetization of 0.98 A m2/kg of sample (∼10 A m2/kg of Fe) at 296 K. Petrographic observations show that the soil contains separate populations of black and orange tephra particles which are relatively unaltered fragments of tachylite (merocrystalline basalt) and extensively weathered (palagonitized) fragments of sideromelane (clear, brown basalt glass), respectively. Although both types of particles contain plagioclase and olivine microphenocrysts, their iron‐oxide mineralogies are substantially different, as documented by Mossbauer and static‐magnetic measurements. A strongly magnetic, opaque iron‐oxide (titanomagnetite) is associated with black particles and is responsible for the magnetic nature of the soil. A relatively weakly magnetic, superparamagnetic ferric‐oxide and minor hematite are associated with orange particles and are responsible for the color. The more highly weathered state (higher state of oxidation) of orange particles is a consequence of the faster weathering (palagonitization) rate for their sideromelane precursors than for tachylite. Increasing state of oxidation, decreasing saturation magnetization, and increasing near‐IR reflectivity with decreasing particle diameter of bulk soil are manifestations of the increasing proportions of palagonitic material as particle diameter decreases.

Geology and petrology of the Lower Tertiary plateau basalts of the Scoresby Sund region, East Greenland

Geology and petrology of the Lower Tertiary plateau basalts of the Scoresby Sund region, East Greenland

Petrographic constraints on modelling the crystallization of basalt magma, Cow Lakes, southeast Oregon

Thermodynamic calculations and models of olivine zoning profiles are used to estimate the crystallization history of a basaltic magma from Cow Lakes, southeast Oregon. The lava is an alkali olivine basalt containing olivine and plagioclase phenocrysts and microphenocrysts. The geometry and range of chemical zoning in the olivine phenocrysts have been delineated by laser interference microscopy and electron microprobe analysis. The olivine phenocrysts are characterized by homogeneous cores and rims that exhibit strong, continuous, normal zoning (ΔFo = 7–19 mol%).Thermodynamic modelling has been used to estimate the magmatic crystallization path of the Cow Lakes basalt on the basis of the phenocryst assemblage and mineral compositions. The calculated crystallization path begins at 1290 °C and 0.5 GPa ([Formula: see text]) with equilibrium crystallization of the olivine to 1265 °C. Plagioclase appears at 1225 °C, followed by clinopyroxene at 1205 °C. Intratelluric crystallization was terminated prior to crystallization of the clinopyroxene, which is seen in the groundmass but not as phenocrysts.The thermodynamic modelling provides a means to numerically simulate the zoning patterns in olivine defined by the laser interference microscopy. Simulated and observed zoning patterns both have compositionally flat cores and strongly zoned rims. The extent of zoning observed in the olivine phenocrysts is, however, approximately twice the predicted extent, and it appears that a significant proportion of olivine phenocrysts crystallized during ascent or upon eruption.

The Origins of Glass and Amphibole in Spined--Wehrlite Xenoliths from Foster Crater, McMurdo Volcanic Group, Antarctica

Vesicular interstitial glass in four kaersutite-bearing spinel–wehrlite xenoliths from Foster Crater, Antarctica has reacted with host olivine (Fo75–79) and clinopyroxene (Ca47 Mg45 Fe8) and contains a microphenocryst assemblage of spinel, olivine, and clinopyroxene together with later rhonite and plagioclase. Electron microprobe analyses of the glasses have low SiO2 (46–49 wt. per cent) and MgO (2.2–3.7) contents and high contents of alkalis, TiO2 (3.4–4.8), Al2O3 (18.1–20.6) and P2O5 (1.1–1.3). Olivine microphenocryst cores in glass are magnesian (up to Fo88) and must have precipitated from more primitive liquids; rim compositions are Fe-rich (Fo75) and in equilibrium with glass. Continuous core to rim zonation in the olivine microphenocrysts indicate that glass compositions have fractionated due to crystallization of the enclosed mineral assemblage. Mass balance addition calculations, using the compositions and proportions of the crystals in glass, produce melt compositions appropriate to primary alkali basaltic magmas. Glasses show light rare earth element (REE) enrichment relative to chondrites (Ce/YbN = 10.5) and, together with Ba, Rb, Cr, Hf, Ta, and Th are similar to many of the basanites from the Erebus Volcanic Province. Textural relationships of the kaersutite are complex owing to the instability of kaersutite in the presence of melt. However, in the association with glass we observe textural evidence such as olivine and clinopyroxene microphenocrysts, identical to the liquidus phases of the glasses, enclosed by kaersutite crystals. We believe that relationships such as this link the crystallization of kaersutite to mafic melt which infiltrated and reacted with the host wehrlite. Thus, the melt did not form in situ within the xenolith but originated elsewhere in the upper mantle.

Crystal-ring microstructures in Oceanographer Fracture Zone basalts

Pillow basalt fragments from the Oceanographer Fracture Zone contain spherical microstructures seen in section as rings of equant microphenocrysts of olivine±plagioclase±augite. They are uniformly ∼ 1 mm in diameter and have the same vitreous or crystalline material on the inside and outside. These structures occur in the same sections as vesicles and segregation vesicles and are clearly distinguishable from them.

Leucitites from Gaussberg, Antarctica

Leucitities of probable late Pleistocene age from the extinct volcano of Gaussberg on the coast of East Antarctica consist of microphenocrysts of olivine, diopside, and leucite in a largely glassy groundmass. Their extreme K contents and K/Na ratios, as well as high Rb, Ba, and possibly Ti, are compatible with an origin by small degrees of partial melting of phlogopite-rich mantle below the level of amphibole stability, whereas their enrichment in P, Sr, Zr, Nb, La, Ce, Pb, and Th indicates the presence of minor phases such as apatite in the source material. Although the Mg/(Mg+Fe) values (∼0.70) and relatively high Ni and Cr contents suggest that the Gaussberg lavas represent near-primary melts, some degree of fractionation involving an aluminous phase (probably garnet) may be necessary to produce liquids with atomic K>Al. Alternatively, an as yet poorly understood process such as wall-rock reaction, liquid immiscibility, or mantle metasomatism may have been a critical factor in the genesis of these unusual rocks. Gaussberg is situated on a passive continental margin and does not appear to be related to any other known area of Cainozoic volcanic activity.

Mineralogy and glass chemistry of recent volcanic ejecta from Mt Erebus, Ross Island, Antarctica

Abstract Bombs erupted in 1972 and recent flows of anorthoclase phonolite from Mt Erebus both contain phenocrysts of anorthoclase and microphenocrysts of olivine (Fa46Te3Fo51), clinopyroxene (Wo48Fs15En37), titanomagnetite (USP70), pyrrhotite, and apatite; the associated glass is peralkaline (agpaitic index = 1.06). The composition of glass inclusions in large anorthoclase (Ab65An16Or19) phenocrysts, which form a lag gravel on the summit cone of Mt Erebus, is generally more evolved than that from the groundmass of recent flows and bombs. Glass inclusions from between the core and rim of anorthoclase phenocrysts represent variations with time and show a small and systematic increase of Al2O3 and decrease of FeOT (total iron as FeO), possibly owing to reaction of the glass with the anorthoclase after entrapment. In contrast, whole rock-groundrnass trends show decreasing Al2O3 and increasing FeOT. The whole rock compositions of recent flows and ejecta are similar to older (up to 1 m.y.) anorthoclase phonolit...

Mantle composition derived from the chemistry of ultramafic lavas

ESTIMATES of the composition of the Earth's mantle have been based on both geochemical and geophysical studies (refs 1–3). As might be expected, there are some discrepancies between the estimates particularly for Na2O and TiO2. Such discrepancies in part arise from uncertainties concerning the degrees of melting which many volcanics represent and, possibly, the restricted tectonic settings of the ultramafic nodules studied. High MgO basalts and ultramafic lavas, some of which have been termed komatiites4 occur among the Archaean volcanic rocks. They represent exceptionally high degrees of melting (up to 50% or more) of peridotite source regions. Potentially such volcanics may be used to make a precise estimate of the composition of their mantle source region. High MgO lavas are well preserved in the Archaean Belingwe greenstone belt of southern Rhodesia, where they form part of a sequence deposited unconformably on granitic crust5. We assume that the occurrence of pillow lavas in this sequence containing both skeletal olivine spinifex (Fo92.5 in a rock with 28% MgO, E. G. Nisbet, M. J. Bickle and A. Martin, unpublished) and skeletal olivine microphenocrysts indicates that these particular lavas have a composition close to that of the liquid.

Petrological Features and Magnetic Properties of Pillow Lavas from the Thetford Mines Ophiolite (Quebec)

A detailed examination was made to evaluate the variations of the remanent magnetization and magnetic susceptibility through four ophiolitic pillow lavas of Cambrian age. These pillowed metabasalts, which are covered by a thin layer of cherty argillite, derive from an olivine tholeiite of probable oceanic origin. They have been metamorphosed in the greenschist facies under a regime of low pressure, moderate temperature, and in the presence of water but absence of significant stress. They still display their primary structural zoning characterized by a thin outer shell, a much wider globulitic intermediate zone, and a porphyritic core. Morphologies of quenched microphenocrysts of olivine and plagioclase are well preserved.The magnetization resides in very fine-grained magnetite and, in spite of a very low remanent magnetization, the primary magnetic imprint appears to be still discernible. Remanent magnetization and Koenigsberger ratio vary from the pillow margin to its center in a fashion similar to the magnetic signature of some recent and fresh oceanic basalts. The magnetic zoning matches the textural, mineralogical, and chemical zoning characteristic of these pillow lavas. We found also that the N.R.M. vector is consistent and relatively stable within the pillow inner part of the intermediate zone and the outer part of the core and therefore that samples from these zones can be used for a regional paleomagnetic study of the ophiolitic complex.

Mineralogy and Chemistry of Glassy Basalts, Coastal Range Ophiolites, Taiwan

The allochthonous Coastal Range ophiolite suite of Taiwan is sporadically exposed within a sequence which includes Miocene chaotic turbidites and Miocene-Pliocene andesitic agglomerates. Unmetamorphosed basaltic rocks, directly resting on peridotite-gabbro masses, occur consistently in regular order from massive flows upward through brecciated flows to compact pillow lavas which in turn are interbedded with manganiferous shale. The glassy basalts at the top of the pillowed sheet are remarkably fresh except for minor palagonite on the outer surface and locally along small cracks. Some basalts contain as much as 98 volume percent glass with small euhedral olivine microphenocrysts (OL-bearing tholeiite). Others are typified by the intergrowth of olivine and plagioclase and spherulites of plagioclase + pyroxene + olivine (PL-bearing tholeiite). Delicate supercooling textures are well preserved, and all crystal morphologies of olivine and plagioclase described from oceanic tholeiites of the Mid-Atlantic Ridge are also common in the samples studied. Electron-microprobe analyses indicate that, except for spinel and palagonite, the chemical compositions of olivine, plagioclase, and glass in eight glassy basalts are very homogeneous. The glassy basalts are remarkably constant in bulk composition as regards K 2 O, TiO 2 , Na 2 O, and probably Fe 2 O 3 and H 2 O concentrations. The K 2 O concentrations are very low. The constancy holds both between different parts of a sample and between different samples. The slight variations in chemical composition, texture, and mineral content between the OL-bearing tholeiite and PL-bearing tholeiite are probably related to differences in the degree of differentiation of a single parent liquid. Olivine (Fo 87 ) was first to crystallize, later joined by calcic plagioclase (An 72 ) along the cotectic curve, and finally by spherulites of pyroxene + plagioclase + olivine, during rapid chilling. The field relations and all available petrological, textural, and chemical evidence suggest that the glassy basalts from Taiwan more nearly resemble recent oceanic tholeiite than any other modern volcanic rock type. It appears that the Coastal Range ophiolites in Taiwan are fragments of oceanic crust and upper mantle materials which were exposed by thrusting of the Philippine Sea plate over the continental crust.

Experimental petrology of Apollo 12 basalts: part 1, sample 12009

The lunar sample, 12009, is a rapidly quenched basalt with microphenocrysts of olivine (∼7%) and spinel in a cryptocrystalline matrix with many small microlites. The rock is olivine-normative (11%) and comparison of the olivine microphenocryst compositions with the experimentally determined liquidus olivine compositions shows that the rock was originally entirely liquid and that none of the observed olivine results from crystal accumulation. The magma (12009) began crystallizing olivine at ∼1230°C, spinel joined the olivine at ∼1210°C, and pigeonitic clinopyroxene would have appeared at ∼1190°C but sudden quenching of the magma occurred before this temperature was reached. Experimental studies at high pressure on 12009 magma show that olivine ceases to be a liquidus phase at pressures above 8kb and the liquidus clinopyroxene becomes more Ca and Al rich with increasing pressure. Although 12009 is not saturated with orthopyroxene at any pressure, a composition of 12009 + 10% olivine (Fo 75) has olivine and orthopyroxene as liquidus phases at 15kb. The data are used to infer partial melting of olivine pyroxenite [orthopyroxene + clinopyroxene + olivine, 100 Mg/Mg + Fe = 75–80] at depths > 200 km within the lunar interior, as the primary source of the maria-filling magmas.

V.—The Crawfordjohn Essexite and Associated Rocks

Another type, which is found in both quarries and which under the microscope has the aspect of a monchiquite or limburgite,. probably represents the actual marginal rock of the intrusion. It consists of small microphenocrysts of augite and olivine in a dark matrix, which can be resolved into granular augite and magnetite in a nearly isotropic base. The latter, which sometimes contains felspar microlites, seems to be mainly analcite with some nephelite, as it can be readily gelatinized and stained, while the refractive index is very low. The felspar microlites are small and sometimes have a rough trachytic structure resembling that of the mugearites. Following Harker's suggestion that augite-olivine rocks with an isotropic base should be classed as limburgites when the base is glass and as monchiquites when it is analcite, this rock can be included in the latter, group. The parts which are richer in felspar may be termed analcite-basalts.