Tschermak Components Research Articles

Very-low-grade phyllosilicates in the Aravis massif (Haute-Savoie, France) and the di-trioctahedral substitution in chlorite

Abstract. Very-low-grade mineral veins investigated in the impure limestone massif of the Aravis (Haute-Savoie, France) yielded a rich mineral assemblage typified by dolomite, calcite, quartz, illitic mica, fluorite and three types of chlorite. The vein network extends over more than 5 km and was probably emplaced around peak burial of the limestone (∼7 km depth, 190 ∘C). The mineralogy has been investigated with electron microscopy, mass spectrometry and X-ray diffraction, with emphasis on chlorite types. The first chlorite type is a chamosite often interlayered with illitic mica. The second type is a Mg-rich, Al-depleted cookeite. The third type is a Li-rich sudoite. Presence of the three chlorite types shows limited solubility between di-trioctahedral chlorite phase components (sudoite and cookeite) and with tri-trioctahedral chlorite (chamosite). Departure of the Li-rich sudoite and Mg-rich cookeite from the ideal end-member compositions suggests solid solutions towards a Li-sudoite component. The associated illitic mica does not contain Li in significant proportion but shows pyrophyllitic and di-trioctahedral substitution, without a Tschermak component. These results call for systematic studies of the Li content of chlorite and for better modelling of di-trioctahedral substitution in chlorite and mica.

Pegmatitic granite fluid compositions and thermochronometry in the Seridó Belt, Borborema Province, Brazil: Insights from trace element advection-diffusion-partitioning halos in host schist and gneiss

At the Seridó Belt pegmatitic province, fluid advection from dykes and sills led to trace element diffusion and partitioning in biotite, producing sub metric concentration halos in host mica schist and gneiss. We analyzed trace element concentration vs. distance profiles in the host rock of pegmatitic granite intrusions showing enrichments in Li, F, Cl, Zn, Rb, Nb, Sn, Cs, Ta, U and depletions in Sr and Ba. We estimated fluid temperatures in the range 600 to 700 °C using methods of Ti in biotite, F-OH in biotite-apatite and Tschermak component in biotite-muscovite. We analytically modeled thermal profiles around pegmatitic planar intrusions and found that cooling by thermal flats in the first meters into the host rock is reached in 100 years for a 10 m thick sill. The model constraints sustained heat flow during pegmatitic melt emplacement into a colder host rock. The measured trace element profiles were fitted with solutions of differential equations modeling advection-diffusion-partitioning in porous media under isothermal and cooling conditions. Each trace element profile is equally fitted by values of pegmatitic fluid concentration vs. fluid flux duration plotting as a straight line. We obtained a theorical framework combining pegmatitic fluid trace element composition, fluid flux duration and velocity, rock/fluid trace element partition coefficient, and cooling rate. Using published fluid inclusion trace element composition, we obtained fluid flux duration between tens of years to several centuries for the studied profiles. In the case of low temperature fluid flux at 450 °C the modeled flux duration is increased by less than one order of magnitude. The results allow us to interpret the trace element halos as sub millennial hot isothermal fluid influx from the pegmatites into their host rock. Trace element partitioning from the fluid into host rock falls mostly within the range of 0.01 to 25 and decreases in the order Nb, Cu, Rb, Zn-Li, Cs-Sn. The model constraints fluid advection velocity between 1 × 10−11 m/s and 1 × 10−9 m/s, typical of regional metamorphism to shear focused fluid flow regimes. The trace element enrichment of biotite along the profiles reproduces the observed pattern for whole rock, indicating that biotite is their main sink during pegmatitic fluid influx. Using experimental F in biotite diffusivity we obtained a minimal planar diffusion distance, or the effective porosity, between 7 and 15 μm for the total F re-homogenization of metamorphic biotite by pegmatitic fluid. As shown from others fluid-present petrogenetic systems, the trace element contents of biotite in host rock at the contact with pegmatitic intrusions can be a potential tool for the assessment of their rare metal potential.

Crystallization kinetics of clinopyroxene and titanomagnetite growing from a trachybasaltic melt: New insights from isothermal time-series experiments

In order to investigate the role of crystallization kinetics in mafic alkaline systems, textural measurements, mineral compositional changes and diffusion modelling calculations have been carried out on isothermal time-series experiments. The data were obtained at 400 MPa and 1100 °C under anhydrous (nominally 0 wt% H2O) and hydrous (2 wt% H2O added) conditions. A synthetic trachybasaltic melt was first heated up to the superliquidus temperature of 1300 °C and then rapidly cooled at 80 °C/min down to 1100 °C. The final target temperature was kept constant over variable dwell times in the range of 0.5–24 h. Results from textural analysis indicate the attainment of fast crystal growth kinetics at the shortest experimental run duration, with early achievement of stable crystal sizes for clinopyroxene and titanomagnetite. The surface area to volume ratio weakly decreases with increasing dwell time, according to the development of euhedral crystal morphologies. Crystal growth rates are also observed to progressively decrease from 0.5 to 24 h. Due to the effect of fast growth kinetics, the morphological maturation of clinopyroxene progresses by attachment of dendrite branches, infilling and overgrowth phenomena, leading to the formation of well-faced and euhedral crystals. The kinetically-controlled cation exchange (Si + Mg) → (TAl + Fe3+) controls the clinopyroxene compositional variation, expanding the stability of Tschermak component at the expense of diopside. Conversely, titanomagnetite is characterized by an almost constant composition that, however, is enriched in incompatible Al and Mg cations, as typically observed under rapid crystal growth conditions. Titanomagnetite crystals show always euhedral morphology that develops by heterogeneous nucleation on early-formed clinopyroxene dendrites. Overall, the effect of undercooling causes strong supersaturation phenomena in the trachybasaltic melt, resulting in enhanced nucleation kinetics and fast attainment of a high crystallinity. As the dwell time increases, the bulk system tends to minimize the interfacial free energy between crystals and surrounding melt. This results in the progressive replacement of the early dendritic shapes developed in a diffusion-limited growth regime, by the formation of euhedral morphologies typical of interface-limited regimes that still retain the chemical evidences of the dendritic stage as complex zoning patterns in clinopyroxene.

Metamorphic angrite Northwest Africa 3164/5167 compared to magmatic angrites

Northwest Africa (NWA) 3164 and 5167 are two angrites with a granulitic texture unlike that of other angrites, with a variable, up to millimeter grain size. Besides mineralogical and chemical similarities to other angrites, NWA 3164 and 5167 exhibit unique characteristics. Ca-rich olivine dominates (in NWA 3164: ∼49vol%, Fo57; NWA 5167: 40vol%, Fo59). Fassaitic clinopyroxene is the second major phase (in NWA 3164: 29vol%; in NWA 5167: 36vol%) with a significant Tschermak component. In addition, two Al-rich phases are present: plagioclase An99 and hercynitic spinel (∼6 and ∼7vol% respectively for NWA 3164; 17 and 4 vol% respectively for NWA 5167). Heavily weathered iron sulfide and kamacite, (9wt% in NWA 3164; 4wt% in NWA 5167) are the remaining minor phases, a unique feature among angrites. All mineral phases are homogeneous. Like other angrites, NWA 3164 and 5167 exhibit a superchondritic Ca/Al ratio, with negligible amounts of alkalis and very low silica content. The presence of metal results from the incorporation of exogenous iron following impact. Subsequent annealing resulted in the observed granulitic texture. Major element composition indicates that both NWA 3164 and 5167 are derived from a picritic angrite precursor after incorporation of metal and annealing.After correction for iron and Fo90 olivine incorporation, bulk rock REE abundances of both NWA 3164 and NWA 5167 appear lower than those of quenched angrites, showing the lowest absolute abundances among angrites. Laser ablation inductively coupled plasma mass spectrometry (LA-ICP-MS) analyses of clinopyroxene, olivine and anorthite indicate that difference of Rare Earth Element (REE) abundances between NWA 3164 and NWA 5167 can be explained by adding traces of phosphate to NWA 3164. NWA 3164/5167 trace elements abundances are similar, showing depletion in volatile elements and enrichment in refractory lithophile elements such as Ca, Ti, and Al. The most incompatible elements are depleted as well, unlike other angrites. This indicates that the source of these younger angrites was more depleted in incompatible elements when compared to the older magmatic angrites. The low Hf/W is understood as the result of exogenous iron incorporation and therefore the Hf/W and W isotopic heterogeneity of the angrite parent body (APB) mantle is secondary. Comparison with other angrites suggests that iron incorporation may be necessary to explain their low Hf/W and W isotopic compositions.

Garnet exsolution in pyroxene from clinopyroxenites in the Moldanubian zone: constraining the early pre‐convergence history of ultramafic rocks in the Variscan orogen

AbstractExsolution lamellae of garnet in clinopyroxene and orthopyroxene porphyroclasts from garnet pyroxenites in the Moldanubian zone were studied to elucidate the pressure–temperature conditions of the exsolution process and to reconstruct the burial and exhumation path of ultramafic rocks in the Variscan orogen. The porphyroclasts occur in a fine‐grained matrix with metamorphic fabrics, which consists of clinopyroxene and small amounts of garnet, orthopyroxene and amphibole. The clinopyroxene porphyroclasts contain garnet + orthopyroxene lamellae as well as ilmenite rods that have orientation parallel to (100) planes of the porphyroclasts. Orthopyroxene porphyroclasts host garnet and clinopyroxene lamellae, which show the same lattice preferred orientation. In both cases, lamellar orthopyroxene, clinopyroxene and garnet were partially replaced by secondary amphibole. Composition of exsolution phases and that of host pyroxene were reintegrated according to measured modal proportions and demonstrate that the primary pyroxene was enriched in Al and contained 8–11 mol.% Tschermak components. Conventional thermobarometry and thermodynamic modelling on the reintegrated pyroxene indicate that primary clinopyroxene and orthopyroxene megacrysts crystallized at 1300–1400 °C and 2.2–2.5 GPa. Unmixing and exsolution of garnet and a second pyroxene phase occurred in response to cooling and pressure increase before the peak pressure of 4.5–5.0 GPa was reached at ∼1100 °C. This scenario is consistent with a burial of hot upper‐mantle ultramafics into a cold subcratonic environment and subsequent exhumation through 900 °C and 2.2–3.3 GPa, when the pyroxenites would have partially recrystallized during tectonic incorporation into eclogites and felsic granulites.

Sapphirine + quartz assemblage in contrasting textural modes from the Eastern Ghats Belt, India: Implications for stability relations in UHT metamorphism and retrograde processes

Stability of the assemblage sapphirine + quartz in Mg–Al-rich granulites implies ultrahigh temperature (UHT) condition of metamorphism but their direct contact is rarely preserved in natural rocks. The present study shows contrasting textural relations between sapphirine and quartz in different parts of the same occurrence of a Mg–Al-rich granulite, Eastern Ghats Belt, India. Textural data suggest stabilization of the assemblage sapphirine + quartz with orthopyroxene and cordierite during the metamorphic peak. Thermometric estimates yield temperature exceeding 950 °C for the stability of this assemblage. Most of such sapphirine grains (Spr 1) are texturally separated from quartz and cordierite grains by double corona of sillimanite + orthopyroxene that results due to isobaric cooling during the post-peak stage. Sapphirine (Spr 2) also forms a symplectic intergrowth with quartz and orthopyroxene at the fringe of coarse orthopyroxene. This textural feature can be explained by the breakdown of (Fe, Mg)-Tschermak components of orthopyroxene during the same isobaric cooling episode from UHT peak condition. The preservation of grain contact of this intergrown sapphirine and quartz can be attributed to a problem in reaction kinetics. In the other mode, sapphirine (Spr 3) occurs with quartz with a thin skin of cordierite near a quartz vein. Such texture could result from isothermal decompression of the cooled crust. Alternatively and more possibly, cordierite could form from ingress of CO 2–H 2O rich fluid during terminal stage of cooling. Finally, sapphirine (Spr 4) and quartz show direct contact close to the quartz vein. Direct contact of such sapphirine and quartz represents textural disequilibrium as this particular quartz is introduced as a vein much later than the peak metamorphism but prior to the major foliation-forming deformation. Coarse sapphirine and vein quartz, therefore, accidentally came in contact with each other and persisted metastably. Therefore, though coexistence of sapphirine and quartz is considered to be a strong evidence for ultrahigh temperature condition, care should be taken to decipher their stable coexistence. Different types of textural relations involving this mineral pair could originate in a single rock, probably in different stages of its metamorphic history.

Garnet-spinel intergrowths in ultrahigh-temperature granulite, Eastern Ghats, India: Possible evidence of an early Tschermak-rich orthopyroxene during prograde metamorphism

UHT metamorphosed aluminous granulites from the Eastern Ghats Belt, India, contain contrasting mineral assemblages in closely spaced domains. Reaction textures and mineral chemistry in one domain are consistent with prograde metamorphism. A unique intergrowth texture is found in garnet porphyroblasts from the other domain. Lamellar to vermicular intergrowth of spinel solid-solution occurs in garnet, particularly in the central part of the porphyroblast. Locally, sapphirine, overgrowing spinel, is also intergrown with garnet. Three possible mechanisms are discussed to account for the garnet-spinel intergrowth: (1) from early stabilized orthopyroxene + sillimanite; (2) spinel forming pseudomorphs after early sillimanite inclusions in garnet; and (3) from an early stabilized hypothetical, highly aluminous orthopyroxene involving a complex reaction between the (Fe,Mg)-Tschermak and Fe3+-Tschermak components. Sapphirine is produced by a reaction involving spinel and aluminous pyroxene. Regardless of the exact mechanism, available thermodynamic data suggest that all such reactions presumably occurred due to loading and/or heating during prograde metamorphism consistent with the conclusion from the other domain. Such an intergrowth texture has not been reported previously in aluminous granulite parageneses, but these could provide crucial information regarding the prograde segment of the metamorphic evolution of deep crustal rocks.

Silica precipitates in omphacite from eclogite at Alpe Arami, Switzerland: evidence of deep subduction

AbstractObservations of oriented SiO2 precipitates in omphacite from eclogite with tholeiitic basalt protolith bordering the Alpe Arami garnet peridotite massif, Ticino, Switzerland, and petrological studies of the eclogitic mineral assemblages, suggest that this rock was subjected to higher‐pressure metamorphism than previously realized. We employed various calibrations of the Fe2+ − Mg exchange thermometer and calculations of equilibria with thermodynamic data, considering the calcium–Tschermak's component (CaAl2SiO6), of garnet‐pyroxene pairs. From these calculations, it is concluded that the eclogitic lenses have recorded at least four stages of mineral growth corresponding to the following: Stage I (prograde) c. 2.4 GPa; 700 °C; Stage IIa (maximum recorded grade) c. 7.0 GPa; 1100 °C; Stage IIb (retrograde) c. 3.7 GPa; 900 °C; Stage III (retrograde) c. 2.1 GPa; 750 °C. Because of the preservation of Stage I, a relatively rapid subduction and exhumation of Alpe Arami eclogite is suggested. The exhumation path of the eclogitic rock is in good agreement with most exhumation paths inferred for the Alpe Arami garnet lherzolite proposed previously by several authors based upon a variety of different observations, although the eclogite and peridotite exhumation paths may diverge at depths greater than 120 km.

Progressive sequence of reactions of the Ryoke metamorphism in the Yanai district, southwest Japan: the formation of cordierite

The compositions of biotite and muscovite were examined in terms of the paragenesis and the metamorphic grade in low‐ to medium‐grade pelitic rocks of the Ryoke metamorphism in the Yanai district, southwest Japan. The biotite and muscovite that coexist with K‐feldspar have a higher K component in an A′KF diagram than those in rocks lacking K‐feldspar. This fact reflects an increase in the K2O content in muscovite, but in biotite it reflects an increase of not only the K2O content but also of the octahedral vacancy.At higher metamorphic grade beyond the cordierite isograd, where cordierite coexists with neither chlorite nor K‐feldspar, the biotite shows an increase in illite, K Aliv □xii−1 Si−1, and Tschermak components, Alvi Aliv R+−1 Si−1, where □xii and R+ denote the interlayer vacancy and (Fe+Mg+Mn), respectively. A reaction to define the cordierite isograd is proposed by treating this chemical change as being responsible for the first appearance of cordierite, i.e. K,Al‐poor biotite+phengitic muscovite=K,Al‐rich biotite+cordierite+quartz+water .By treating this as a key reaction in medium‐grade metamorphism, a set of reaction in a progressive metamorphism is established for the Ryoke metamorphism, a typical low‐pressure type metamorphism. Some textures in one of the high‐grade areas, the K‐feldspar‐cordierite zone, suggest that a further two prograde reactions have taken place, i.e. andalusite+biotite+quartz=cordierite+K‐feldspar+waterand andalusite=sillimanite.quartz=cordierite+K‐feldspar+waterThis implies that this zone probably has a P–T path involving isobaric heating.

Compositional evolution in Ca‐amphibole in the Karmutsen metabasites, Vancouver Island, British Columbia, Canada

Abstract The 6‐km‐thick Karmutsen metabasites, exposed over much of Vancouver Island, were thermally metamorphosed by intrusions of Jurassic granodiorite and granite. Observations of about 800 thin sections from the Campbell River and Buttle Lake area show that the metabasites provide a complete succession of mineral assemblages ranging from the zeolite to pyroxene hornfels facies around the intrusion. The most important observations are as follows. (1) The compositional change of Ca‐amphiboles with increasing metamorphic grade is not straightforward. The tremolite component decreases from the prehnite–actinolite facies to the greenschist facies with a compensating tschermak component increase, but the tendency is not clear thereafter. Instead, the edenite component increases from the amphibolite facies to the pyroxene hornfels facies. (2) The most pargasitic Ca‐amphibole occurs in high‐Fe2+/Mg metabasite from the greenschist/amphibolite transition zone. (3) The reasons for such irregular compositional trends, even in the rather uniform MORB‐like composition of the Karmutsen metabasites, are non‐ideal solid solutions of Ca‐amphibole at low temperature and the effective control by bulk rock composition in the amphibolite facies. (4) The data from this study support, but do not prove, a transition loop for the actinolite–hornblende compositional gap rather than a solvus. If the gap is a solvus, its shape is asymmetric, and is highly dependent on the other compositional parameters such as Fe3+/Al and Fe2+/Mg. (5) The XNaA/XA±XAb) ratios between Ca‐amphibole and plagioclase are most useful as an indicator of metamorphic grade even within the amphibolite facies, and these change systematically from 0.2 to 0.5 from the greenschist to pyroxene hornfels facies. (6) The compositional trend of Ca‐amphibole from the Karmutsen metabasites indicates a typical low‐P/T metamorphic facies series on a Rbk–Gln–Tr–Ts diagram.

三波川帯,八幡浜地域のざくろ石一単斜輝石角閃岩類

Garnet-clinopyroxene amphibolites from the Kawamai tectonic mass emplaced in the Sanbagawa metamorphic terrain of the Yawatahama district, southwestern Shikoku have been studied chemically. The amphibolites are characterized by the assemblage brown hornblende-garnet-clinopyroxene-calcic plagioclase indicating, at least, the amphibolite facies.Their bulk rock compositions are always undersaturated with silica. The ferromagnesian minerals are saute with appreciable amounts of acmite and Tschermak's components, Ca-rich almandine and ferroan pargasite. From chemical comparison of clinopyroxene and of hornblende the formation temperature seems to be higher in the Kawamai amphibolites than in the Iratsu amphibolites. The partition coefficient of Fe and Mg between garnet and clinopyroxene suggests that the Kawamai amphibolites equilibrated at a temperature higher than or similar to that of the Higashi-Akaishi-yama garnet clinopyroxenites.It can be concluded that the Kawamai mass suffered the Sanbagawa metamorphism corresponding to the amphibolite facies of which Ps-T conditions were similar to those of the Higashi-Akaishi-yama mass, but prevailing PH2O must have been different between the two. Retrogressive reactions have been caused by the subsequent uplift of the Kawamai mass during the Sanbagawa metamorphism.

The metamorphic history of LL-group ordinary chondrites

A study of feldspar grain size and microprobe analysis of coexisting orthopyroxene, olivine, diopside, and metal in twenty equilibrated LL-group chondrites and clasts defines the conditions of metamorphism, indicates the relative position of samples within the parent body, and suggests an accretionary-metamorphic history. Variation of mineral chemistry indicates a range of equilibrium temperature and parallels textural variation, providing two continuous relative temperature scales, based on mean feldspar grain size and the calcium content of orthopyroxene coexisting with diopside. Maximum equilibrium temperatures range from less than 600° up to 950°C. The total Tschermak's component in orthopyroxene indicates that maximum pressure was less than 1 kbar, and a pressure gradient correlates directly with temperature. A reduction sequence, evident from systematic variation of iron in ferromagnesian silicates and metal, correlates inversely with temperature. The maximum permissible sampling depth for the formation of surface breccias (2/10 r) also limits pressure to less than 1 kbar. The LL parent body had a minimum radius of 200 km, determined using the permissible sampling depth and published cooling rates. The data are broadly consistent with simple reheating of colder material within a completely accreted body. This model may have problems with volatile depletion, a required lack of impact sampling during metamorphism, and the timing of the event. The data are more consistent with accretion of colder material on a hot but cooling surface. Both models apparently require a recycling process during accretion to account for the reduction sequence.

Pyroxenes from lherzolite inclusions of Itinome-gata, Japan

The lherzolites have recrystallized to plagioclase lherzolites consisting of olvine, pyroxenes, chromian spinel, plagioclase and pargasite at a depth of 20 to 25 km in the uppermost part of the mantle. It is believed that the garnet lherzolites and spinel lherzolites were originally derived from depths of 50–75 km and 30–50 km respectively. The clinopyroxenes contained about 10 mol. % of jadeite and Tschermak's molecules, respectively and the orthopyroxenes also included about 5–10% of Tschermak's component. Transported upward, the garnet was transformed through pyroxene-spinel symplectite to olivine, plagioclase and spinel aggregates, and most of the jadeite amd some Tschermak's components in the pyroxenes formed secondary pyroxenes and pargasite, and finally plagioclase under isochemical conditions.

PARGASITES IN LHERZOLITE AND WEBSTERITE INCLUSIONS FROM ITINOME-GATA, JAPAN

Pargasite occupies up to 4 volume per cent of lherzolite inclusions and up to 8 per cent of websterite inclusions. It occurs as discrete primary crystals and as interstitial grains among other minerals averaging less than 2 volume per cent. New analyses are presented for six representative pargasites. They are characterized by high Cr2O3 and wide variation of K2O and Mg/(Mg+Fe) ratio. The following origin of the pargasites is hypothsized from the mineralogy and chemistry of the host rocks. Original garnet lherzolite and websterite were located at a depth of up to 50 to 75km. During upward transport, to 25km deep in the uppermost part of the mantle, the garnet was finally transformed to olivine+plagioclase+spinel and both orthopyroxene and clinopyroxene were recrystallized toward a relatively rich Tschermak's molecule. Jadeite and Tschermak's components separated from pyroxenes and silica liberated from the breakdown of garnet produced plagioclase, pargasite and orthopyroxene. However, minor newly-formed minerals were not always in equilibrium with major constituents. It is demonstrated that the lherzolite mantle just below the M-discontinuity contains only up to 0.04 per cent water.

The mineralogy of the glaucophane schists and associated rocks from �le de Groix, Brittany, France

Ninety-seven mineral phases consisting of ten chloritoids, fifteen epidotes, sixteen garnets, four sphenes, seven rutiles, seven pyroxenes, thirteen blue amphiboles, two green amphiboles, eleven phengites, two paragonites, a mariposite, seven chlorites, and two specimens of albite were obtained from the metamorphic rocks of Ile de Groix, and their chemical, physical, optical and X-ray properties determined. The chloritoids are all optically positive, monoclinic polymorphs with large 2V, moderate refractive indices and characterized by high densities. Their fluorine contents have been used to propose a new upper limit for OH→F substitution in the chloritoid structure, suggesting that partial pressure of fluorine might modify the stability of chloritoids from that determined in pure H2O. The epidotes belong to the Al-Fe epidote series and are “epidote” sensu stricto. The almandine-rich garnets and the chloromelanites are metastable relics in the glaucophane schists. The grossular contents of the calcareous schist garnets are believed to have become depressed under high CO2 pressure and the low Tschermak's contents of the pyroxenes are to be explained by equilibria involving epidote at high \(P_{{\text{H}}_{\text{2}} {\text{O}}} \) and low temperature when the Tschermak's components will break down to epidote group minerals. The sphenes contain appreciable amounts of combined water, fluorine substituting for oxygen and aluminium substituting for silicon and titanium. The presence of H3O+ is suspected in a specimen of blue amphibole. The barroisite has a composition between glaucophane and hornblende. On account of its high Fe3+ content it is believed to have formed under higher PO2 than the blue amphiboles. The paragonites which occur in the ohloritoid veins are unstable in the potassium-rich aluminous schists. The phengites show a tendency towards sericitic composition due to post-glaucophanisation readjustments under the lower pressure conditions of the greenschist facies. Some of the Fe3+ contents of the chlorites are interpreted as due to oxidation of ferrous iron, e.g. 2 [Fe(OH)2]→2FeOOH + H2. The minerals show strong chemical control of the host rock and their Mn contents are directly related to those of the minerals from which they have evolved through retrogression.

Some clinopyroxenes from ultramafic inclusions in Dreiser Weiher, Eifel

Eight clinopyroxenes from wehrlites and clinopyroxenites and three clinopyroxenes of crystal lapilli in tuff of Dreiser Weiher in Eifel, Germany, have been separated and chemically analysed. One hornblende and two phlogopites from a wehrlite and clinopyroxenites have also been analysed. The rocks enclosing these inclusions are alkali basalts of basanite composition. The analysed clinopyroxenes contain considerable amounts of Al2O3 (3.87–10.84 wt%). The calculated Tschermak's component ranges from 5.9 to 18.4 mol per cent. All of the analysed clinopyroxenes are clearly different from chromian diopsides in lherzolite inclusions in basaltic rocks in Dreiser Weiher and other localities; the former has higher contents of total FeO, CaO and TiO2 and lower contents of MgO and Cr2O3 than the latter. Two clinopyroxenes separated from apatite-bearing clinopyroxenites show high contents of Fe2O3 with about 2 per cent of Na2O, indicating the presence of considerable amounts of acmite component in addition to Tschermak's component. The relative proportions of Al in the tetrahedral site and that in the octahedral site in the analysed clinopyroxenes are clearly different from those of the common igneous clinopyroxenes and eclogites, and similar to those of the clinopyroxenes from other inclusions in basaltic rocks and granulites. It is suggested that all the analysed clinopyroxenes and their host inclusions have crystallized from alkali basalt magmas in relatively deep levels of the continental crust.