Fluid activity in the lower crust and upper mantle: mineralogical evidence bearing on the origin of amphibole and scapolite in ultramafic and mafic granulite xenoliths

1987 ◽  
Vol 51 (363) ◽  
pp. 719-732 ◽  
Author(s):  
A. J. Stolz
Keyword(s):  
Basaltic Magma ◽  
Olivine Basalt ◽  
Basalt Magma ◽  
Crust And Upper Mantle ◽  
Ultramafic Xenoliths ◽  
Basaltic Magmas ◽  
Alkali Olivine Basalt ◽  
Textural Evidence ◽  
Granulite Xenoliths ◽  
Mineralogical Evidence

AbstractXenoliths in an olivine nephelinite from the McBride Province, North Queensland, include Cr-diopside lherzolites, spinel and garnet websterites, felsic, 2-pyroxene and garnet granulites, and hornblendites. The spinel and garnet websterites are interpreted as crystal segregations from olivine basalt or alkali olivine basalt magma at ∼ 12 kbar followed by isobaric cooling (to approximately 900–1000°C) and subsolidus reequilibration. Garnet and 2-pyroxene granulites are mineralogically and texturally distinct and are considered to represent relatively large degrees of crystallization of basaltic magmas at comparable or slightly lower pressures (8–12 kbar). Mafic and ultramafic xenoliths have been modified to varying degrees following the relatively recent influx of a H2O- and CO2-bearing fluid. Variable amounts of amphibole and mica developed in response to the introduced fluid and it is argued that some hornblendites are the end-products of this process acting on spinel websterites. Felsic and 2-pyroxene granulite xenoliths display only minor evidence of increased PH2O. Mineralogical and textural evidence indicates high-sulphur Ca-rich scapolite in several garnet granulites did not form in response to the increased fluid activities. It is proposed the scapolite was a primary cumulate phase precipitated from alkali basaltic magma under elevated fo2 and fso2 conditions.

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

1988 ◽  
Vol 25 (4) ◽  
pp. 486-494
Author(s):  
J. K. Russell ◽  
G. T. Nixon ◽  
T. H. Pearce
Keyword(s):  
Basaltic Magma ◽  
Olivine Basalt ◽  
Significant Proportion ◽  
Thermodynamic Modelling ◽  
Interference Microscopy ◽  
Basalt Magma ◽  
Laser Interference ◽  
Chemical Zoning ◽  
Laser Interference Microscopy ◽  
Crystallization Path

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.

Exceptional REE-enrichment in apatite during the low pressure fractional crystallisation of alkali olivine basalt; an example from the British Tertiary Igneous Province

1999 ◽  
Vol 90 (4) ◽  
pp. 273-285 ◽  
Author(s):  
R.J. Preston ◽  
M.J. Hole ◽  
J. Still
Keyword(s):  
Olivine Basalt ◽  
Low Pressure ◽  
Basalt Magma ◽  
Accessory Phase ◽  
Igneous Province ◽  
Alkali Olivine Basalt ◽  
Typical Scheme ◽  
Common Variety ◽  
Fluor Apatite

AbstractThe Cnoc Rhaonastil dolerite boss on Islay, NW Scotland represents a body of alkali-olivine basalt magma which differentiated at low pressure and in situ, from dolerite through teschenite to minor nepheline-syenite. The syenites occur as isolated pods and pegmatitic schlieren within the leucodolerite, and have an exotic mineralogy including Zr-aegirine, Zr-arfvedsonite, Ca-catapleiite, zirconolite and aenigmatite. Fluor-apatite occurs as an accessory phase in the dolerite, but becomes more abundant within the teschenite and syenites. Total REE contents within apatites in the dolerites are typically low (σREE = 0·57–3·21 wt.% oxide), the highest REE contents occurring in irregular, deuterically altered rims and internal patches. The REE-enriched rims also have slightly elevated SiO2 contents at 0·81–0·95 wt.%, suggesting that the substitution scheme Ca2++P5+ ⇔ REE3++Si4+ was operating. These apatites have up to 0·08 wt.% Cl and 3·7 wt.% F, with most being almost pure end-member fluor-apatite. The majority of the teschenite apatites show the least REE-enrichment (σREE = 0·27–0·45 wt.%), coupled with low Na (<0·12 wt.%) and low SiO2 (<0·39 wt.%) contents. However, within the syenites two distinct populations of apatite exist. The first, most common, variety consists of unzoned, low-REE apatites (max. 3·1 wt.% σREE, again in irregular rims and patches), whereas the second variety is often complexly zoned, and has variably enriched zones up to a maximum σREE content of 42 wt.%; this is by far the most REE-enriched natural fluor-apatite so far reported from the British Isles. The REE-enriched zones are often less than 3 μm wide, and have Na content up to 5·4 wt.% Na2O, implying that the substitution scheme Na+ + REE3+⇔2Ca2+ dominated over the more typical scheme involving Si4+ which operated in the dolerites and teschenite. Other zones are either variably enriched in Y (up 2·1 wt.% Y2O3) or Th (up to 0·85 wt.% ThO2). However, there is no correlation between Y and REE contents, suggesting that crystallographic factors were involved in apatite Y and REE partitioning. The REE-rich apatites have very low Cl content (<0·04 wt.%), but high F concentrations (<2·8 wt.%). It is believed that these strongly enriched apatites crystallised under disequilibrium conditions from isolated, variably REE-enriched domains, within the fluid-rich residual syenitic magma. These domains may have been generated by the prior crystallisation of monazite, Ca-catapleiite or zirconolite, which can be found as small inclusions within albite and interstitial analcime. The dynamic process of slumping of the denser teschenite back into the leucodolerite crystal mush is believed to have played an important role in the release of deuteric fluids and the concentration of residual magmas.

scholarly journals Igneous stratigraphy and internal structure of the Little Minch Sill Complex, Trotternish Peninsula, northern Skye, Scotland

1991 ◽  
Vol 128 (1) ◽  
pp. 51-66 ◽  
Author(s):  
Sally A. Gibson ◽  
Adrian P. Jones
Keyword(s):  
Internal Structure ◽  
Magma Chamber ◽  
Thermal Gradient ◽  
Olivine Basalt ◽  
Small Scale ◽  
Basalt Magma ◽  
Alkali Olivine Basalt ◽  
Modal Layering ◽  
Crustal Magma

AbstractDetailed sampling of the Little Minch Sill Complex reveals that it is composed of both single and multiple sills. These are formed of three main, genetically related units: picrite, picrodolerite and crinanite, which are the result of differentiation of an alkali-olivine basalt magma (approximately 10% MgO) in an upper-crustal magma chamber. Variations in igneous stratigraphy and the presence of internal chills in the Trotternish sills suggest that they were emplaced by multiple intrusion and subsequently differentiated in situ. Changes in petrography adjacent to pegmatite veins and textures within picrite units indicate compaction and filter-pressing were important processes after emplacement. Rhythmic layering (1 cm to 1 m thick) is conspicuous in the sills near contacts but does not involve cryptic mineral variation. Such modal layering may be more common than realised in relatively small-scale intrusions and maybe modelled in terms of in situ differentiation under conditions of significant undercooling in a changing thermal gradient at the synthetic for-sterite-diopside-anorthite eutectic.

A mechanism forming silicic segregations from basaltic magma discovered in igneous rocks of Western Sicily

1983 ◽  
Vol 120 (4) ◽  
pp. 321-329 ◽  
Author(s):  
G. Lucido
Keyword(s):  
Basaltic Magma ◽  
Liquid Immiscibility ◽  
Igneous Rocks ◽  
Silicate Liquid ◽  
Basaltic Magmas ◽  
Western Sicily ◽  
Basic Rocks ◽  
Silicate Liquid Immiscibility ◽  
Textural Evidence

Summary. Mechanisms forming silicic segregations from basaltic magmas are considered of primary importance when dealing with magmatic problems. However, the processes which give rise to silicic segregations from basaltic magmas are so far obscure. Fortunately, the discovery of spheroidal felsic masses in some basic rocks of Western Sicily throws light on this subject. To clarify the relationships between felsic and basic fractions particular attention has been paid to the interactions which occurred at their contact. Textural evidence indicates that the accretion mechanism of the Sicilian felsic segregations tends to obliterate the silicate liquid immiscibility effects and suggests that the formation of silicic segregations is a consequence of liquid unmixing phenomena.

The Petrology of Late Tertiary Dolerite Plugs in the South Cariboo Region, British Columbia

1973 ◽  
Vol 10 (2) ◽  
pp. 205-225 ◽  
Author(s):  
R. B. Farquharson
Keyword(s):  
British Columbia ◽  
Trace Element ◽  
Chemical Nature ◽  
Major Element ◽  
Basaltic Magma ◽  
Olivine Basalt ◽  
Coarse Grained ◽  
Basalt Magma ◽  
Late Tertiary ◽  
High Level

The petrology of four plugs of fresh, coarse-grained olivine dolerite, of Pliocene age, is described in this paper. Two plugs, which exhibit textural and mineralogical variations, are described in detail. Major element analyses and selected trace element analyses for fifteen samples illustrate the chemical nature of the dolerite as well as fractionation in small marginal portions of the plugs. Strontium isotope values indicate that the plugs represent basaltic magma that was not contaminated by crastal matter. Both the mineralogy and the chemistry of the plugs indicate that the magma was mildly alkaline, and suggest that it was a differentiate of a primary olivine basalt magma, perhaps derived through crystal fractionation in a high-level magma chamber.

The occurrence of Zr-bearing amphiboles and their relationships with the pyroxenes and biotites in the teschenite and nepheline syenites of a differentiated dolerite boss, Islay, NW Scotland

2000 ◽  
Vol 64 (3) ◽  
pp. 459-468 ◽  
Author(s):  
R. J. Preston ◽  
M. J. Hole ◽  
J. Still
Keyword(s):  
Trace Element ◽  
Late Stage ◽  
Nepheline Syenite ◽  
Olivine Basalt ◽  
Local Environment ◽  
Basalt Magma ◽  
Single Body ◽  
Small Scales ◽  
Alkali Olivine Basalt

AbstractThe Cnoc Rhaonastil minor dolerite intrusion on Islay, NW Scotland represents a single body of alkali olivine basalt magma which differentiated in situ, from leucodolerite, through teschenite to minor nepheline syenite. The syenites occur as isolated nests and pegmatitic schlieren within the leucodolerite, and schlieren of gabbroic pegmatite also occur at the margin of the teschenite. The differentiated rocks contain pyroxene, amphibole and biotite of variable compositions which reflect both primary fractionation processes and late-stage deuteric alteration and reaction.Mafic phases within the gabbroic pegmatite, teschenite and syenite are typically rimmed and speckled with biotite, the composition of which is controlled by the local environment of crystallization. The nepheline syenites contain primary ferro-kaersutite which, where in contact with interstitial patches, has been altered to arfvedsonite, which occasionally contains up to 1.2 wt.% ZrO2. The occurrence of Zr-arfvedsonite (and of Zr-aegirine) in interstitial patches suggests that variably trace element-enriched domains existed within the residual melts on very small scales.

Motukoreaite, a new hydrated carbonate, sulphate, and hydroxide of Mg and Al from Auckland, New Zealand

1977 ◽  
Vol 41 (319) ◽  
pp. 389-390
Author(s):  
K. A. Rodgers ◽  
J. E. Chisholm ◽  
R. J. Davis ◽  
C. S. Nelson
Keyword(s):  
New Zealand ◽  
Olivine Basalt ◽  
Unit Cell ◽  
Rock Fragments ◽  
Alkali Olivine Basalt ◽  
Beach Rock ◽  
Wet Chemical ◽  
O 19 ◽  
Olivine Sand ◽  
Sand And Gravel

Motukoreaite occurs as relatively abundant, white, clay-like cement in both beach-rock and basaltic volcanic tuffs on the flanks of a small, extinct, late Pleistocene, basaltic cone at Brown's Island (Motukorea), within Waitemata Harbour, Auckland, New Zealand (36° 50′ S., 174° 35′ E.). The occurrence was originally recorded by Bartrum (1941) as ‘beach limestone’ found at two places of the island's shore. The beach-rock consists of a grain-supported fabric of poorly sorted, well-rounded, alkali-olivine basalt pebbles and granules, subangular to sub-rounded fresh olivine sand and abraded sand- and gravel-sized bioclasts in a colourless to pale yellow-green aphanocrystalline matrix of motukoreaite. Additional detritals include quartz, feldspar, and sedimentary rock fragments. Stereoscan examination of the surface of pieces of the cement prised from the beach-rock showed a box-work of plate-like crystals with a hexagonal form in which individuals measured about 3×3×0·02 microns (fig. 1).Wet-chemical analysis of a separate of the cement containing some 5 % quartz and traces of calcite and goethite gives SiO2 5·55, Al2O3 17·87, Fe2O3 0·73, CaO 0·92, MgO 22·98, MnO 0·70, ZnO 0·56, Na2O 0·71, K2O 0·10, CO2 9·32, SO3 10·00, H2O+ 19·62, H2O- 10·35, sum 99·41 %. The unit-cell formula using obtained unit-cell constants and measured specific gravity 1·43) is (Na0·73K0·07)∑0·80(Mg18·13Mn0·32Zn0·21)∑18·66Al11·15(CO3)6·22(SO4)3·97 (OH)51·1927·20H2O. Of several idealized formulae that may be proposed NaMg19Al12(CO3)6.5 (SO4)4(OH)54·28H2O is preferred.

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