Ca In Olivine Research Articles

The univariant curve liquid = forsterite + anorthite + diopside in the system CMAS at 1 bar: solid solutions and melt structure

One atmosphere liquid-present experiments were carried out in the CMAS system using an ordinary quench furnace apparatus. The runs, including reversal and duplicate experiments, describe the univariant curve l=fo+an+ di between the invariant points Q: l+fo+an=di+sp and F: l+fo=an+di+oen, located respectively at 1245±1° C and 1244±1° C. The thermal divide on this curve M3: l= fo+an+di is located at 1275±1° C and plots well within the silica-saturated field, in agreement with Longhi's (1987) experiments. Along the univariant curve l=fo+an+di, liquid composition evolves away from the thermal divide either toward invariant points Q or F and pierces the silica saturation plane, i.e., the join Di-An-En, in the silica saturated field. In this compositional range, the Al solubility in clinopyroxene changes drastically from one side of the thermal divide to the other, with great increase of Al solubility in the silica-undersaturated field. Four endmembers must be used to describe the complex solid solution of anorthite: CaAl2Si2O8, CaMgSi3O8, MgAl2Si2O8 and [] Si4O8. The last two of these are present only within the silica-saturated field. Unlike clinopyroxene, the Mg content of anorthite is insensitive to the thermal barrier but is only sensitive to silica-saturation plane. Olivine composition can be described by a binary solid solution of forsterite and monticellite with no Ca in the M2 site. As with anorthite, olivine compositions exhibit a marked change with crossing of the silica-saturation plane. The above features imply that the solubility of minor elements in crystalline phases (Al in clinopyroxene, Ca in olivine and Mg in anorthite) selectively respond to only one or another of these particular plane. Results have many important consequences. One is the likelihood of changes in melt speciation depending on position with respect to the thermal divide and the silica-saturation plane.

Ca in olivine as a geobarometer for lherzolites

Ca in olivine as a geobarometer for lherzolites

ELEMENTAL COMPOSITIONS OF MAJOR SILICIC PHASES IN CHONDRULES OF UNEQUILIBRATED CHONDRITIC METEORITES

Elemental compositions of olivine, low‐Ca pyroxene and mesostasis in chondrules from type‐3 ordinary chondrites (OC), CV3, CO3, CM2 and EH3 chondrites were compiled in a search for mineral compositional differences among chondrules of different chondrite groups. Such differences are demonstrated. A few elements occur in silicic phases in amounts proportional to their bulk chondrule concentrations: e.g., Mn in OC chondrules, Ti in CV chondrules, Cr in EH chondrules. However, OC chondrules have higher bulk Cr than CM‐CO chondrules, higher Cr in mesostasis, but lower Cr in olivine and low‐Ca pyroxene. The higher oxidation state of OC chondrules implies that Cr is more likely to be trivalent, and thus, less likely to enter the olivine crystal structure and more likely to concentrate in pyroxene and mesostasis. CV and OC chondrules have similar high bulk Fe and mesostasis Fe, but OC chondrules have much more FeO in olivine and low‐Ca pyroxene. The remaining Fe in CV chondrules is reduced and occurs as metal blebs in the mesostasis. Relative to OC chondrules, EH chondrules have lower bulk Ca, lower Ca in pyroxene and mesostasis, but higher (by a factor of 2) Ca in olivine. EH chondrules may have been incompletely melted, preserving relict refractory lithophile‐rich olivine nuclei. OC chondrules are richer than EH chondrules in FeO; they have a lower melting temperature and may have been more completely melted during chondrule formation.

Upper mantle rocks from basalts of the northern Hessian depression (NW Germany)

Whole rock and mineral analyses of about 30 spinel lherzolite and harzburgite xenoliths from Tertiary alkali olivine basalts, limburgites and olivine nephelinites have been performed. After testing various geothermometers, a few pyroxene solvus calibrations, the Wells geothermometer and the calibration of Ca in olivine were favoured. “Wells-temperatures” range from 1100°C to less than 900°C. Several low temperature xenoliths lack equilibrium documented in heterogeneous mineral grains and deviations from regular Al, Cr partition between coexisting minerals. P-T correlations are derived from a continental geotherm corresponding to a surface heat flow density of 65 mW/m2. Several spinel peridotites must have existed outside the field of spinel stability. All xenoliths exhibit a coarse-grained granular texture without recrystallization from shearing in diapiric mass movements. The average modal composition of 73 vol.-% olivine, 18 vol.-% orthopyroxene, 7 vol.-% clinopyroxene and 1.3 vol.-% spinel is close to xenoliths from the Eifel and from worldwide sampling. Host basalts are most probably products of partial melting from phlogopite bearing peridotites, the phlogopite being the result of interactions between peridotite and metasomatic mantle fluids.

Experimental investigation of Ca [sbnd]Mg exchange between olivine, orthopyroxene, and clinopyroxene: potential for geobarometry

The distribution of Ca and Mg among coexisting olivine, clinopyroxene and orthopyroxene has been studied in a piston-cylinder apparatus in the temperature range 1100–1300°C and pressure range 9–41 kbar. Ca in olivine decreases with increasing pressure and decreasing temperature. The pressure effect is the result of Ca going into the higher-coordination M2 site in clinopyroxene as pressure is increased. For the Ca Mg exchange reaction between olivine and clinopyroxene, ΔV°=0.249J bar −1 mole −1; this is sufficient for pressure estimates accurate to ±3kbar if temperatures of equilibration are independently known. Ca Mg exchange between olivine and orthopyroxene is not sufficiently pressure dependent to be used as a geobarometer. Application of the olivine-clinopyroxene geobarometer to coarse garnet lherzolites from southern Africa gives P-T results consistent with a continental geotherm. For spinel lherzolites from southwestern United States, the geotherm appears to be displaced to higher temperatures indicating oceanic affinities. Application of the geobarometer to natural systems requires assumptions about activity relationships in clinopyroxene which should be checked by experiment.