Exsolution of Cummingtonite from Glaucophane: a New Orientation For
Total Page：16
File Type：pdf, Size：1020Kb
American Mineralogist, Volume 76, pages 971-984, 1991 Exsolution of cummingtonitefrom glaucophane:A new orientation for exsolution lamellae in clinoamphiboles EugeneA. Smelik,David R. Veblen Department of Earth and Planetary Sciences,The Johns Hopkins University, Baltimore, Maryland 21218, U.S.A. Arsrnlcr Samples of glaucophane from eclcgite assemblagesin northern Vermont have been studied using transmission and analytical electron microscopy (TEM, AEM). The results show that the glaucophaneis exsolved on a submicroscopicscale and contains abundant lamellae of cummingtonite. AEM analyses indicate that the present pair of coexisting sodic-ferromagnesianamphiboles differs significantly from all previously reported pairs and representsthe first report of coexistingglaucophane-cummingtonite from normal eclo- gitic assemblages. The lamellae are disc shapedand are coherently intergrown with the host. They occur in two symmetrically related orientations, nearly parallel to (281) and (28I) planes of the host. Optimal phaseboundary calculationsindicate that theseorientations representplanes of best dimensional fit between the glaucophaneand cummingtonite lattices. This orien- tation differs significantly from previously reported lamellar orientations for monoclinic amphiboles,which generallyoccur nearlyparallel to (I0l) and (100) planesfor the C2/m unit-cell seltins. INrnooucrroN tation for exsolution lamellae in monoclinic amphiboles, rationalized by optimal phase boundary During the last two decades,there has been a consid- which can be erable effort made toward understandingmiscibility gaps arguments' between different amphibole series.Research has shown SETTTNGOF SAMPLES that compositional gapsexist betweenall pairs of the four Gnolocrc (sodic, sodic-calcic, calcic, and ferromagnesian) major The glaucophanesamples examined in this study are amphibole subgroupsof kake (1978) (Ghose, 198l; partofatypeCeclogiteassemblagedescribedbyBothner Robinson et al., 1982: Smelik and Veblen, 1989).Evi- and t aird (1987).These eclogites occur at a localityknown dencecited for miscibility gapsbetween major amphibole as Eclogite Brook in north-central Vermont (seeFig. 5 in end-members includes coexisting amphibole grains that Bothner and Laird, 1987). They belong to the Belvidere grew under equilibrium conditions and the observation Mountain Amphibolite Member of the Hazens Notch of oriented exsolution lamellae of one amphibole in an- Formation (nomenclature from Doll et al., l96l) and are other. Exsolution among calcic and ferromagnesianam- structurally related to the more abundant outcrops of phiboles and in the orthoamphibole (gedrite-anthophyl- blueschist that occur several kilometers to the south at lite) systemhas been widely reported,and thesemiscibility Tillotson Peak (Bothner and Laird, 1987). gapshave been delineatedby many workers (e.g.,Vernon, Evidence of high-pressuremetamorphism in the mafic 1962;Robinson, 1963;Jaffe et al., 1968;Robinson et al., schistsat Tillotson Peak was first recognizedby Laird 1969;Ross et al., 1969;Robinson and Jaffe,1969; Rob- (1977). Detailed electron microprobe analysesof the inson et al., l97la Stout 1970, 1971,1972 Gittos et al., mineralsin theserocks werepresented by I-aird and Al- 1974, 1976;Spear, 1980, 1982).Exsolution between so- bee (l98la), who reportedthe occurrenceofthree differ- dic and ferromagnesianamphiboles has been reported by ent amphiboles: glaucophane,actinolite, and barroisite Klein (1966, 1968) in metasedimentaryrocks from Lab- (classification of Leake, 1978). In a subsequentpaper, rador and by Ghose et al. (1914) and Shau et al. (1989) Laird and Albee (l98lb) used changing mineral compo- in Mn-rich rocks from Tirodi. Maharashtra. India. Other sitions in mafic schistsfrom Tillotson Peak and a variety authors have reported coexistingcoarse-grained sodic and ofother locations to help unravel the complicated poly- ferromagnesianamphiboles (Black, 1973; Kimball and metamorphic history of Vermont. From their analysis, Spear, 1984). they have identified four major periods of metamorphism This paper describes new evidence for immiscibility affecting the Paleozoic rocks of Vermont (seeLaird and betweensodic and ferromagnesianamphiboles and dis- Albee, l98lb, Table 3, p. 160-16l). cusses(l) exsolution of cummingtonite in common eclo- Two major events (Ol, 02) affectingCambrian to Or- gitic glaucophaneand (2) a new crystallographic orien- dovician samples occurred during the Ordovician (Ta- 0003-o04x/9l/0506-097 l $02.00 971 972 SMELIK AND VEBLEN: EXSOLUTION IN CLINOAMPHIBOLES Fig. l. I-ow-magnification TEM image of exsolved glauco- Fig. 2. TEM imageof cummingtonitelamellae in glauco- phane grain. Note that lamellae of cummingtonite occur in two phane.Several smaller lamellae have grown in areasbetween orientations and vary in size, reachinga maximum thicknessof coarserlamellae, indicating progressive stages ofexsolution with about nm. 60 The electron beam is parallel to [102] ofthe host. decreasingtemperature. Note that somelamellae bulge slightly at the approachof other lamellae(arrow). Coherency strain is alsoevident at someof the lamellartips. The orientationis the sameas Figure I with b verticaland [20I]* horizontal.The elec- tron beamis parallelto u02l ofthe host. conic Orogeny). Each of these is characterizedby high- pressurefacies metamorphism in some areasand by me- dium-pressure facies in other areas (Laird and Albee, SpncrunN DEscRrprroN AND ExpERTMENTAL l98lb). The 02 event has been dated by Lanphereand TECHNIQUE Albee (1974) and assignedan isotopic age of 463 + 43 Specimensfor transmissionelectron microscopy (TEM) m.y. No absolute agehas been determined for the earlier experiments were made from 30 pm thin sectionstaken event,Ol. from Eclogite Brook samples (see locality 2 of Bothner Two metamorphic events (Dl, D2) also occurred dur- and Laird, 1987, Fig. 5). The glaucophanegenerally oc- ing the Devonian (Acadian Orogeny). The first (Dl) is curs in clots adjacent to layers ofgarnet porphyroblasts. characterized by low-pressure facies metamorphism in The glaucophaneis usually lavender to blue in color and Devonian rocks and by medium-pressure facies meta- is sometimes strongly pleochroic. Although some grains morphism in Cambrian to Ordovician samples.The sec- are zoned with darker-blue cores, none of the glauco- ond is characterized by low-pressure facies metamor- phane crystals show any evidence of exsolution lamellae phism in Cambrian to Devonian samples (Laird and when examined with the petrographic microscope. Albee, l98lb). Several TEM samples were prepared by ion thinning The minerals in the Eclogite Brook rocks reflect high- selectedareas of petrographic thin sections.Petrographic pressure metamorphism assigned by Laird and Albee examination of ion-thinned edgesrevealed subtle refrac- (l98lb) to the Ordovician events.The coresof discon- tive index discontinuities that might reflect inhomogene- tinuously zoned amphibole grains are thought to have ities in the samples.Electron microscopy was performed grown during the Ol event, and the rims plus optically with a Philips EM420 transmission electron microscope continuous grains have been assignedto the 02 event. operated at 120 keV. Both a Supertwin (ST) objective SMELIK AND VEBLEN: EXSOLUTION IN CLINOAMPHIBOLES 9'73 Fig. 3. HRTEM image of host-lamellar interfaces.The (020) lattice fringesfor the glaucophane(=0.89 nm) and cummingtonite (=0.907 nm) are coherent acrossthe interface with very little distortion at the phaseboundary (viewed best at a low angJe).The electron beam is parallel to [02] ofthe host. lens (sphericalaberration coefficient C": 1.2 mm, chro- betweencoarse lamellae (Fig. 4), similar to the GP zones matic aberrationcoefficient C.: 1.2 mm) and a Twin observedin some pyroxenesamples (e.g., Nord, 1980). (T) objective lens (C" : 2.0 mm, C. : 2.0 mm) were The wide rangeof lamellar sizesin thesesamples suggests employed. For HRTEM imaging, the objective aperture progressivestages ofexsolution in the glaucophaneas the diameter either matched the point resolution of the mi- rocks cooled. croscope(0.30 nm for the ST lensand 0.34 nm for the T In general,interfaces between the lamellae and host are lens) or was smaller in order to eliminate irrelevant high- close to planar and are coherent (Fig. 3). TEM imagesof frequency information from the images. fine-scaleexsolution lamellae reveal the lamellae to be Energy dispersive X-ray spectrawere collectedwith an disc-shaped precipitates that taper at both ends when EDAX SiLi detector and processed with a Princeton viewed in cross section. It is also common to observe Gamma-Tech model 4000 analvzer.as describedbv Livi bulging of one lamella as it is approachedby others (see and Veblen(1987). Figs. l, 2, and 5). This bulging of lamellae is commonly observed in pyroxene (e.g., Rietmeijer and Champness, DrsrnrnurroN AND MoRpHot,ocy oF 1982; Kitamura, l98l) and may be caused,in part, by EXSOLUTION LAMELLAE strain field interactions between the growing lamellae (Livi As noted above, there was no obvious evidence of and Veblen. 1989). exsolution lamellae in the glaucophanecrystals when ex- amined with the petrographic microscope,although vari- Auprlror-r cHEMTcAL RELATToNS ations in optical properties were observedin ion-thinned edges.When examined with the TEM, the glaucophane Cummingtonite crystalswere found to contain abundant, submicroscopic Using a fine probe (diameter t 10-20 nm), analyses exsolution lamellae of a secondphase (Figs. I and 2). The of both the host and lamellae