Amphibole Analyses Research Articles

AMPH-IMA97: a hypercard program to determine the name of an amphibole from electron microprobe and wet chemical analyses

In 1997, the International Mineralogical Association (IMA) revised the amphibole naming scheme making it necessary to amend the previous EMP-AMPH Macintosh program to the new rules. The AMPH-IMA97 program allows single input or automatic input of as many amphibole analyses as are available following a set input format. There are three options for calculation scheme of amphibole analysis: (1) complete chemical analyses can be calculated to 24(O, OH, F, Cl); (2) analyses with determined FeO and Fe 2O 3 but without H 2O can be calculated to 23(O) and (3) electron microprobe analyses with only total Fe determined and without H 2O can be calculated to 23(O) with IMA97-recommended normalization for Fe 3+ and Fe 2+ values. To test the program, over 500 analyses were calculated using the three options. The first comprehensive test of the third option successfully estimated Fe 2+ ( R 2=0.88), but became less successful for Fe 3+ ( R 2=0.42) estimation. Problems in the normalization of Mn 3+- and Mn 2+-bearing amphiboles and in the IMA97 sodic amphibole classification procedure emerged for Mn-rich amphiboles. This necessitated the proposal and implementation with the program of a simplified sodic amphibole classification scheme. A new coloured amphibole classification chart accompanies the program.

Halogen contents of eclogite facies fluid inclusions and minerals: Caledonides, western Norway

AbstractPrimary multiphase brine fluid inclusions in omphacite and garnet from low‐ to medium‐temperature eclogites have been analysed for Cl, Br, I, F, Li and SO4. Halogen contents and ratios provide information about trapped lower crustal fluids, even though the major element (Na, K, Ca) contents of inclusion fluids have been modified by fluid–mineral interactions and (step‐) daughter‐crystal formation after trapping. Halogens in the inclusion fluids were analysed with crush–leach techniques. Cl/Br and Cl/I mass ratios of eclogite fluids are in the range 31–395 and 5000–33 000, respectively. Most fluids have a Cl/Br ratio lower than modern seawater and a Cl/I ratio one order of magnitude lower than modern seawater. Fluids with the lowest Cl/Br and highest Cl/I ratios come from an eclogite that formed by hydration of granulite facies rocks, and may indicate that Br and I are fractionated into hydrous minerals. Reconstructions indicate that the inclusion fluids originally contained 500–4000 ppm Br, 1–14 ppm I and 33–438 ppm Li. Electron microprobe analyses of eclogite facies amphibole, biotite, phengite and apatite indicate that F and Cl fractionate most strongly between phengite (F/Cl mass ratio of 1469 ± 1048) and fluid (F/Cl mass ratio of 0.008), and the least between amphibole and fluid. The chemical evolution of Cl and Br in pore fluids during hydration reactions is in many ways analogous to Cl and Br in seawater during evaporation: the Cl/Br ratio remains constant until the aH2O value is sufficiently lowered for Cl to be removed from solution by incorporation into hydrous minerals.

Pre-tertiary blueschist facies metamorphism in the Hellenides evidenced by detrital blue amphiboles in the Paleocene flysch of the Othrys mountain

Analyses of detrital blue amphiboles from Paleocene flysch deposits of the western Othrys Mountain show a narrow compositional variability and are comparable with published analyses from the Cycladic Islands. In contrast, analyses from Pelagonian blueschists (Olympos, Ossa, Ambelakia) show a wide compositional variation that is rather due to the lithological variability of their parental rocks and incomplete overprinting by blueschist facies metamorphism. Combined with published evidence, the above features exclude the Pelagonian rocks from being the potential source of the detrital blue amphiboles. The latter were derived from a source with "Cycladic" affinities that (1) presently is not exposed, but may have been covered tectonically during the Tertiary and (2) has been already exposed to erosion in response to subduction at the Pindos oceanic realm at the active external Pelagonian margin during Maastrichtian/Paleocene times. The data demonstrate that blueschist facies metamorphism in the Hellenides started at Pre-Tertiary times.

A revised program for microprobe-derived amphibole analyses using the IMA rules

A Microsoft QUICKBASIC program with the code name NEWAMPHCAL has been written to calculate the structural formulae of amphibole analyses taken from electron-microprobe, and to determine the classification parameters with the prefixes and modifiers according to the procedures proposed by the International Mineralogical Association (IMA) 1997 amphibole nomenclature scheme. The program, which came up with four Al-in hornblende geobarometers, is applicable to plutonic and its associated volcanic rocks including amphibole–biotite–quartz–plagioclase–ortochlase–sphene–FeTi oxides. An amphibole–plagioclase geothermometer for particular calcic amphiboles also is incorporated into the program structure. NEWAMPHCAL has been tested for determination and classification of numerous amphibole analyses compiled from literature. Depending on the evaluations of selected amphibole analyses by using NEWAMPHCAL program, it was determined that approximately 15% amphibole names were changed compared to the author’s IMA 1978 findings. The most important variation in amphibole names has been observed in the calcic amphibole group. This program permits the user data entry, their storage, and calculation and plotting of microprobe amphibole analyses. NEWAMPHCAL plots the calculated chemical parameters on one ternary and seventeen different binary diagrams for amphibole classification. This software, with well-designed screen forms, stores all the calculated results in comma delimited ASCII file format and thus permits the user to process the program outputs for further spreadsheet applications. A menu-driven and easy to use program runs on IBM-compatible microcomputers under DOS and Windows systems with the minimum hardware requirements. The compiled form of NEWAMPHCAL takes approximately 398 kilobytes together with the test data.

Trace element compositions of minerals in garnet and spinel peridotite xenoliths from the Vitim volcanic field, Transbaikalia, eastern Siberia

Peridotite xenoliths from the Bereya alkali picrite tuff in the Vitim volcanic province of Transbaikalia consist of garnet lherzolite, garnet–spinel lherzolite and spinel lherzolite varieties. The volcanism is related to the Cenozoic Baikal Rift. All peridotites come from pressures of 20–23 kbar close to the garnet to spinel peridotite transition depth, and the presence of garnet can be attributed to cooling of spinel peridotites, probably during formation of the lithosphere. The peridotites show petrographic and mineral chemical evidence for infiltration by an alkaline silicate melt shortly before their transport to the Earth's surface. The melt infiltration event is indicated petrographically by clinopyroxenes which mimic melt morphologies, and post-dates outer kelyphitic rims on garnets which are attributed to an isochemical heating event within the mantle before transport to the Earth's surface. Single-mineral thermometry gives reasonable temperature estimates of 1050±50°C, whereas two-mineral methods involving clinopyroxene are falsified by secondary components in clinopyroxene introduced during the melt infiltration event. Excimer Laser–ICP-MS analysis has been performed for an extensive palette of both incompatible and compatible trace elements, and manifests the most thorough dataset available for this rock type. Orthopyroxene and garnet show only partial equilibration of trace elements with the infiltrating melt, whereas clinopyroxene and amphibole are close to equilibration with the melt and with each other. The incompatible element composition of the infiltrating melt calculated from the clinopyroxene and amphibole analyses via experimental mineral/melt partition coefficients is similar to the host alkali picrite, and probably represents a low melt fraction from a similar source during rift propagation. The chemistry and chronology of the events recorded in the xenoliths delineates the series of events expected during the influence of an expanding rift region in the upper mantle, namely the progressive erosion of the lithosphere and the episodic upward and outward propagation of melts, resulting in the evolution of the Vitim volcanic field.

Proterozoic thermal activity in the Archaean basement of the Disko Bugt region and eastern Nuussuaq, West Greenland: evidence from K-Ar and 40Ar-39Ar mineral age investigations

K-Ar and 40Ar-39Ar analyses of amphiboles from Archaean amphibolites and gneisses show that Proterozoic tectono-thermal activity has played an important role in the metamorphic and structural development of the Precambrian rocks around north-eastern Disko Bugt and in eastern Nuussuaq. Proterozoic thermal activity lead to resetting of the K-Ar ages of amphiboles in eastern Nuussuaq, resulting in ages of c. 1750 to 1925 Ma; in the Disko Bugt area the effects are seen in total or partial resetting with K-Ar ages scattering mostly between 2750 and 1870 Ma. Resetting is caused either by total diffusion of earlier accumulated radiogenic argon or by complete recrystallisation of the amphiboles. Archaean 40Ar-39Ar ages obtained from mafic xenoliths within the Atâ tonalite show that not all parts of the area suffered argon loss during Proterozoic reheating. Incorporation of significant proportions of excess argon in some amphiboles is seen from 40Ar-39Ar mineral age spectra obtained for samples from supracrustal rocks and from mafic xenoliths in the Atâ tonalite. Phlogopite phenocrysts from a lamproite stock yielded a K-Ar age of 1764 ± 24 Ma, identical to a previously determined K-Ar age of the matrix phlogopite. These ages probably date the emplacement of the lamproite, and mark the time after which no tectono-thermal events affected the area.

Provenance of Cretaceous synorogenic sandstones in the Eastern Alps: constraints from framework petrography, heavy mineral analysis and mineral chemistry

The detrital components of Cretaceous sedimentary rocks in the Northern Calcareous Alps reflect the early Alpine geodynamic evolution of the Austroalpine microplate. Two contrasting source areas are distinguished on the base of light and heavy mineral analysis. The first source area is located at the southeastern margin of the Austroalpine and is composed of Palaeozoic sediments and metamorphic rocks, Mesozoic carbonate rocks, and ultrabasic rocks derived from the suture zone of the Vardar/Meliata Ocean. The second source area is located in the northwest in a Lower Austroalpine position near the transpressive plate margin that juxtaposes Austroalpine and Penninic units. This source area comprises Palaeozoic low-grade metamorphic rocks including high-pressure (HP) rocks, late Palaeozoic (meta)sediments, Mesozoic carbonate rocks, and ultrabasic rocks from obducted slices of Penninic oceanic crust. Chemical analyses of detrital white mica, amphibole and garnet support the discrimination between the two source areas. Tourmaline chemistry calls for a significant amount of metasedimentary rocks in the source area. Granitoid rocks and high-grade metamorphic rocks are rare. Blue amphibole, phengite, and chloritoid composition suggest the erosion of lower blueschist facies rocks in the northwestern source area. We suggest a modified model of Austroalpine Valanginian to Coniacian tectono-sedimentary evolution which is based on (1) an onset of subduction of the Penninic Ocean no earlier than Late Cretaceous, (2) deposition of the analyzed sedimentary rocks in piggyback basins, and (3) a reconstruction of provenance as proposed in this study.

Formula omitted] laserprobe ages of metamorphic hornblendes from the Coongan Belt, Pilbara, Western Australia

40Ar 39 Ar laser step-heating analyses of metamorphic amphiboles and a whole rock sample from metavolcanic rocks of the Archaean Coongan Greenstone Belt in the south-eastern Pilbara in Western Australia provide evidence that these rocks, with no previous age constraints, belong to the ca. 3460 Ma old Warrawoona Group. The western margin of the belt is formed by an east-dipping ductile shear zone; the eastern margin is undeformed and shows a narrow contact metamorphic zone. The Coongan Belt shows a metamorphic gradient from amphibolite facies at the western margin to greenschist facies in the centre of the belt. The rocks in the centre of the belt are transected by north-south striking retrograde shear zones, one of which has been constrained at ca. 2940 Ma and may be related to the ca. 2940 Ma intrusive event in the eastern Pilbara. Foliated granodiorites from the ductile shear zone along the western margin of the belt yield dates of ca. 3170 Ma, which are interpreted as the age of recrystallisation after deformation. Deformed blue-green hornblendes from the adjacent foliated amphibolites, however, give younger and inconsistent dates of ca. 3090 Ma and ca. 3020 Ma. These dates cannot be interpreted as meaningful ages and indicate an inhomogeneous distribution of argon in the shear zone. Age spectra from blue-green hornblendes from the western part of the Coongan Belt constrain the age of a regional metamorphic event at 3240 Ma, which compares well with the metamorphic hornblende ages of ca. 3200 Ma obtained in the Western Shaw Belt by Wijbrans and McDougall. The same event is indicated by complex age spectra of partially metamorphosed mafic volcanic rocks, which yield ca. 3260 Ma maximum ages in the higher temperature steps. The age spectrum of contact metamorphic hornblendes from the eastern margin indicates that the cooling age of the undeformed granite in the south-western Corunna Downs Batholith is ca. 3400 Ma, which is 70–90 Ma younger than the North Shaw granodiorites in the Shaw Batholith, but considerably older than previously assumed, that is ca. 3300 Ma. All obtained ages fall within the expected range of ca. 3.5-2.94 Ga, and generally increase from west to east through the Coongan Belt. The effects of excess argon on the age spectra seems to be rather considerable in some of the experiments.

Richterite-bearing peridotites and MARID-type inclusions in lavas from North Eastern Morocco: mineralogy and D/H isotopic studies

K-richterite/phlogopite-bearing peridotite xenoliths and MARID inclusions have been found in Late Cretaceous (67±0.2 million years) monchiquites and an olivine nephelinite from North Eastern Morocco. It is the first evidence of MARID rocks and K-richterite/phlogopite-bearing peridotites outside the kimberlitic context. In the hydrous xenoliths, textural features suggest that K-richterite, phlogopite and Al-poor diopside are replacement minerals. K-richterites contain 2–5 wt% FeO, 0.1–1.5 wt% TiO2 and <0.5 wt% Cr2O3. Micas contain 5.4–7.4 wt% FeO and 0.3–2.2 wt% TiO2, with Cr and Ni contents <0.2 wt%. Diopsides are Al-poor (<0.2 wt% Al2O3) and contain 0.1–0.2 wt% TiO2, 0.9–1.1 wt% Na2O and 1.3–1.7 wt% Cr2O3. Compared to known K-richterites and micas from metasomatised peridotite nodules (PKP types), the Moroccan minerals are more Fe rich, K-richterites have higher Ti and micas less Cr and Ni. They are thus closer to MARID than to PKP minerals. K-richterites and mica from the MARID inclusions show typical characteristics, e.g. high FeO (4.3–4.7 wt% in richterite and 7.2 wt% in mica), low NiO and/or Cr2O3 and the incomplete filling of the tetrahedral site by Si+Al. Ion probe D/H analyses of amphiboles and micas from both xenolith types give high δD values ranging from –8 to –73, with large variations within single grains (up to 50‰). Both the D-enrichment and the δD variations are inherited from the mantle. The similar chemical composition and δD values of K-richterite/phlogopite from the hydrous peridotites and MARID minerals suggest a genetic link between the two types of xenoliths. The conditions required for producing MARIDs and K-richterite/phlogopite-bearing peridotites may thus exist in contexts other than stable cratonic settings. MARID rocks and the associated metasomites may result from a hydrous fluid interaction with a peridotite, the metasomatic agent being characterised by a high K and low Al signature and a high δD value. A D-rich source is involved in the metasomatic event producing the hydrous minerals, and the scatter observed in the δD values suggests a mixing between this source and another one with typical upper mantle D/H composition. As indicated by the low δD (–74) values of micas from the host lava, metasomatism predated and is unrelated to the alkaline volcanism.

Blueschist-to-Greenschist Transition and the P-T Path of Prasinites from the Lavrion Area, Greece

AbstractThe Lavrion area is part of the Attic-Cycladic massif. Blue amphibole analyses revealed that they are glaucophane or ferroglaucophane. Ca-amphiboles are characterized as actinolite or actinolitic hornblende. The Ps component of epidotes from the glaucophane-bearing rocks varies from 25.42–30.89%, whereas the Ps component of epidotes from the greenschist assemblages ranges from 23.81–26.88%. Chlorites show narrow compositional variations of ferromagnesian ratios (Xmg=0.48–0.53). Albites are almost pure Ab100. The Kfeldspar present has a low Ab content. The evolution of the prasinites studied is characterized by the progressive transformation of eclogite facies (?) rocks through epidote blueschists into greenschists. A P-T path for the prasinites is presented indicating epidote blueschist facies at P-T conditions of around 7.0–7.5 kbar and 300–340°C. Pressures of 4.0–4.5 kbar and temperatures around 340–360°C are estimated for the subsequent overprint in the greenschist facies. The path EBS to GS conditions followed under nearly isothermal uplift.

PTMAFIC: Software package for thermometry, barometry, and activity calculations in mafic rocks using an IBM-compatible computer

PTMAFIC (v. 2.0) is a program for IBM-compatible personal computers with minimum hardware requirements, that can be used in thermometric and barometric calculations by petrologists concerned with the determination of P-T paths in mafic and ultramafic rocks. The program creates ASCII files of normalized chemical analyses of amphiboles, garnet, epidote, ilmenite, muscovite, pyroxenes (ortho- and clinopyroxene), olivine, plagioclase, and spinel. The program carries out a wide range of stoichiometric and thermodynamic calculations on these minerals, such as cation site-allocations at the different structural positions, the molar fractions and mole percent of the end-member terms, and the application of different thermodynamic models for calculation of activity coefficients and activities. The program contains calibrations published for 13 geothermometers and 12 geobarometers applicable to mafic and ultramafic rocks that cover metamorphic conditions, ranging from granulite and eclogite facies to green-schist facies.

Probe-AMPH—A spreadsheet program to classify microprobe-derived amphibole analyses

A Microsoft Excel™ spreadsheet program has been developed to calculate structural formulae of microprobe-derived amphibole analyses and determine classification parameters according to the International Mineralogical Association scheme for amphibole nomenclature. The program has been tested in the determination of formulae and classification of 138 amphibole analyses from the literature. Of these, the small percentage (14%) of discrepancies were determined mainly to be the result of (i) differences between measured (wet chemical) and estimated values for FeO and Fe2O3 and (ii) differences inherent in summing structural formula of microprobe data to 23 oxygens and wet chemical data to 24 oxygens. The program has been developed for the Apple Macintosh but a PC version also is available. Spreadsheet implementations of four published geobarometers based on the total aluminium content of certain calcic amphiboles also are presented. These geobarometers are applicable to metaluminous plutonic (and associated volcanic) rocks containing the assemblage amphibole-biotite-quartz-plagioclase-(orthoclase)-sphene-FeTi oxides and apply to a pressure range of 2.5–13 kbar.

ANALYSIS OF AMPHIBOLE ASBESTOS IN CHRYSOTILE AND OTHER MINERALS

Chrysotile asbestos and many other mineral raw materials contain amphibole minerals which may be asbestiform. There is currently no analytical method which will detect the presence of amphibole at sufficiently low limits to preclude the possibility of inadvertent exposure of persons handling these materials to hazardous airborne fibre concentrations. A method of chemical digestion of chrysotiles has been tested with regard to the determination of their tremolite contaminant content and this has been applied to a range of chrysotile and other minerals. The method improves the sensitivity of the amphibole analysis at least 10-fold giving detection limits of 0.01-0.05% in chrysotile by X-ray diffractometry (XRD). The difficulties arising from compositional and morphological variations are discussed in the context of the potential hazards from airborne fibres and the relative values of analyses by XRD, infrared spectrophotometry (IR) and electron microscopy. It is concluded that XRD and IR are useful as screening methods for the detection of amphibole in chrysotile but other materials should be analysed by optical or electron microscopy.

Coexistence and mixing of magmas in the late of cambrian Itaporanga Batholith, State of Paraíba, Northeastern Brazil

The Precambrian Cachoeirinha-Salgueiro Fold Belt (CSF) located in the western portion of the states of Pernambuco and Paraiba is intruded, in its northern portion, by several coarsely porphyritic potassic calc-alkalic .batholiths. These batholiths were syntectonicaly unplaced in relation to the Brasiliano cycle (=Pan-African) and are commonly associated with potassium diorites suggesting coexistence and mixing between felsic and mafic magmas. In the Itaporanga batholith three petrographic domains-were mapped. A hybrid zone characterized by intense mechanical mixing of granite to granodiorite and potassium diorite magmas is located towards the border of the batholith. A commingling zone where felsic porphyritic granite to granodiorite and potassium diorite rocks are individualized at outcrop scale is located towards the center of the batholith. Finally a felsic porphyritic fades occur in the hybrid zone. The intense mechanical mixing observed in the hybrid zone developed migmatite-like structures (stromatic and less frequently agmatic), pillov -like structures, mafic enclaves with irregular shape and cuspate contacts between mafic and felsic rocks, suggesting diffusion of chemical species across contact. Major and trace element plotted against SiO2 agree with a mixing model to explain the hybrid samples plotted at intermediate position between felsic and mafic rocks. Similarity among chemical analyses of amphiboles from potassium dioritic enclaves of the Itaporanga batholith and from the potassium diorite stock east of it suggest a common source for both magmas. This hypothesis is corroborated by similar REE patterns for potassium dioritic enclaves of the Itaporanga batholith and for the potassium diorite stock. The batholiths shows a well developed foliation which dips towards its core suggesting that the present level of exposure represents the root zone of a diapir, where intense interaction between felsic and mafic magmas took place.

Experimental study of the join tremolite-tschermakite: A reinvestigation

The inability to synthesize pure tschermakite over the range of 6–15 kbar and 700–850° C, the optimal conditions reported by Oba (1978), prompted a reinvestigation of the join tremolite-tschermakite which was originally investigated by Jasmund and Schafer (1972). The results of syntheses performed in 10 mol% increments along this join at 12 kbar and 850° C indicate that solid solution exists only in the composition range or TS0TR100 to TS50 TR50. This was confirmed by direct chemical analyses of the synthetic amphiboles and by monitoring the shift in the d310. Syntheses along this same join at 3 kbar and 850° C reveal an even more limited range of solid solution, namely from TS0TR100 to TS10TR90, which was also confirmed by direct analyses and variation in the d310 values. Comparison of the results at 3 kbar and 12 kbar indicates a pressure dependency of Al solubility in tremolite corresponding to 0.17 Al (total)/kbar on the basis of 23 oxygens. An attempt to re-equilibrate tremolite and an Al-rich amphibole at 6 kbar and 850° C to an intermediate composition resulted in the Al-rich amphibole displaying the best evidence of re-equilibration, which was to a lower Al content. This observation supports the results obtained from the synthesis experiments by showing that decreased pressure decreases the Al solubility. It is emphasized that the Al content of natural calcic amphiboles, whose compositions are approximately modeled by the join tremolite-pargasite, is probably a function of both pressure and temperature and must ultimately be calibrated in a fashion analogous to the pyroxenes.

紀伊半島中央部の三波川帯中の含アルカリ角閃石片岩

Alkali amphibole-bearing schist is found in the Sambagawa metamorphic terrane, central Kii Peninsula where the zone of the glaucophane schist facies has never been recognized. Alkali amphibole is rimmed or included by actinolite and coexists with epidote, chlorite, albite and quartz, occasionally associated with stilpnomelane, muscovite, sphene and calcite. Microprobe analyses of the alkali amphiboles reveal that they fall into the two compositional fields of glaucophane and crossite to magnesioriebeckite. The petrological evidence shows that the metamorphic grade in the study area has attained to the glaucophane schist facies; actinolite enclosing the alkali amphibole is regarded as being formed during a later probably low-pressure phase of the metamorphism.

Discussion

Dr M. D. Max &amp; Dr J. C. Roddick write: Dallmeyer and Gibbons (1987) have provided evidence on the timing of blueschist metamorphism in eastern Anglesey. We present a further 40Ar/39Ar amphibole mineral analysis that supports this timing and examine the wider implications of the data. Figure D1 is an age spectrum for amphibole from a massive crossite-epidote rock (Geological Survey of Ireland specimen 77–1745) taken from a quarry along the northern side of the old Bangor-Holyhead road about 250m to the ESE of the Marquess of Anglesey’s Monument (537 715 on 1 : 50 000 sheet 114, Anglesey). This is about 1 km from the localities sampled by Dallmeyer and Gibbons. The rock was composed of about 70% felted, schistose, light blue to light violet crossite, up to (rarely) 1 mm in length, with about 22% epidote group minerals and about 8% chlorite. There was neither apparent retrogression nor intergrowth with other amphibole species. The analysed separate consisted of 74 mg of 100-250 pm grains of essentially pure crossite. The age spectrum is broadly similar to that of sample 1 of Dallmeyer and Gibbons, giving a maximum age of 550 Ma. For the final 20% gas the ages decrease to 517 Ma, a phenomenon seen in other amphibole analyses (Harrison &amp; McDougall 1981) and possibly related to the degassing of a high Ca region within the crossite, as reflected in this case by the low K/Ca ratio (Fig. Dl). The peak age of 550 Ma is considered to provide

A FORTRAN Program for tabulating and naming amphibole analyses according to the International Mineralogical Association Scheme

Amphibole analyses input to program AMPHTAB (optionally with H2O, F, Cl, minor elements and one or both of Fe2O3, FeO), are output in conventional column tables, with formula units, complete IMA names, and extensive messages indicating the quality of each analysis. AMPHTAB accepts data-files ranging from a single amphibole analysis to (in principle) unlimited batched data-e.g., multiple analyses from each of numerous probe sections. Options are provided to reallocate total Fe in probe data between Fe3 and Fe2, to allow for common analytical problems (e.g., low H2O values), and to enable the number of oxygens in the formula unit to be either preset or assigned automatically. Results for 57 probe analyses are compared with an unpublished BASIC program, which uses a different Fe-reallocation method. Three-quarters of the IMA names resulting from the two programs are essentially identical, despite somewhat different estimated formula units. The remainder are borderline cases, where incidental changes in formula units effect disproportionate differences in name.

The Mont Saint Hilaire plutonic complex: occurrence of excess 40Ar and short intrusion history

Mont Saint Hilaire is an approximately 10 km2 alkaline plutonic complex in the Monteregian petrographic province of Quebec. The complex consists of an older, western half of alkali gabbros and an eastern portion of nepheline syenites and magmatic breccias. The intrusives were emplaced at depths of no more than a few kilometres into Paleozoic sedimentary rocks through the underlying Grenville basement. 40Ar/39Ar analyses of amphibole yield complex age spectra in which the apparent ages decrease with increasing heating temperature. Conventional K–Ar analyses of amphibole, pyroxene, and feldspar yield anomalously old dates. These data indicate the presence of small yet significant amounts (up to 8 × 10−11 mol/g) of excess 40Ar. The quantities of excess argon are variable between rocks and minerals from the same rock. In contrast, biotites yield concordant plateaus and consistent ages ranging only from 124.1 to 124.6 Ma. These data define the age of the complex (124.4 ± 1.2 Ma) and imply a short time span of emplacement of about 0.5 Ma or less. The restricted time interval suggests the various lithologies are cogenetic. The nature of the amphibole 40Ar/39Ar spectra indicates that excess 40Ar was acquired after crystallization. The 40Ar could have been present in the initial magma either from crustal assimilation or from a mantle source. Alternatively, it could have been derived from outgassing of older crustal rocks. The occurrence of excess 40Ar in epizonal plutons may be a more widespread phenomenon than commonly recognized.

Pétrologie du massif volcanique du Guilliz (Maroc oriental). Cristallisation fractionnée, mélanges de magmas et transferts de fluides dans une série shoshonitique

The Guilliz volcano is composed of upper miocene and pliocene suites (8.0–4.9 Ma) of shoshonitic and alkaline affinity. The shoshonitic suite is comprised of absarokites, shoshonites, latites and trachytes. Lavas with intermediate compositions are characterized by the presence of biotite and K-feldspar. Alkaline lavas are, for the most part, hawaiites. The hawaiites are found as aphanitic inclusion in the shoshonitic series. Amphibole bearing absarokites and shoshonites erupted in the pliocene have compositions similar to that of the hawaiite inclusions. Electron microprobe analyses of oxides, olivines, pyroxenes, amphiboles, biotites, feldspars and glasses indicate a multi-stage evolutionary process. Shoshonitic lavas appear to differentiate through crystal fractionation, leading to a zoned magma chamber. An attempt has been made to calculate the mineral assemblages and the proportion involved in the fractionation process. The existence of an episode of magma mixing is evidenced by geological observations, and corroborated by chemical evolution of the lavas. The mixing occurs between shoshonitic and hawaiitic liquids. The mixing liberates a K-rich vapor phase from the hawaiitic magma, and this vapor contributes to the K-enrichment of the liquids present in the upper parts of the magma chamber.