Geothermobarometry in Four-phase Lherzolites II. New Thermobarometers, and Practical Assessment of Existing Thermobarometers   总被引：19，自引：4，他引：15  

BREY  G. P.; K?HLER  T. 《Journal of Petrology》1990,31(6):1353-1378

On the basis of experiments presented in Part I of this series,most of the published thermobarometers relevant to four-phaseperidotites are tested here for their ability to reproduce experimentalconditions. They were rejected if any systematic discrepancyin either pressure or temperature was discernible. This testcautions against the use of all published versions of thermometersbasad on the compositions of coexisting ortho- and clinopyroxenesand the use of existing barometers based on the Al content oforthopyroxene axxisting with garnet. Therefore, we formulatednew versions of the two-pyroxene thermometer and the Al-in-opxbarometer: with and is in degress Kelvin and P is in kilobars. Our new barometer is of the form (C₁–C₃) and site occupancies are given in the text. Temperatures may also be calculated from the Ca content of opxalone: This thermometer can be applied both to the CMS and the naturalsystem experiments, which may indicate that Fe and Na have counter-balancingeffects on the Ca content of opx. The partitioning of Na between opx and cpx can also serve asa useful thermometer, and was calibrated from natural rock data: where T is in degrees Kelvin, P is in kilobars, and D_Na=Na^opx/Na^cpx. The following three published thermobarometers based on furtherexchange reactions are capable of reprducing experimental conditions:

1. exchangeof Ca between olivine and clinopyroxene as a barometer(P_KB),
2. exchange of Fe and Mg between garnet and clinopyroxene asathermometer (T_Krogh),
3. exchange of Fe and Mg between garnetand olivine as a thermometer(T_O'Neiii).

Our tests also show that the most accurate pressure and temperatureestimates arc obtained from the following combinations of thermometersand barometers:

1. T_BKN+P_BKN,
2. T_BKN+P_KB,
3. T_Krogh+P_BKN,
4. T_O'Ne$$$ll+P_BKN.

  相似文献   

The Role of Continental Crust and Lithospheric Mantle in the Genesis of Cameroon Volcanic Line Lavas: Constraints from Isotopic Variations in Lavas and Megacrysts from the Biu and Jos Plateaux   总被引：1，自引：0，他引：1  

RANKENBURG  K.; LASSITER  J. C.; BREY  G. 《Journal of Petrology》2005,46(1):169-190

We present a combined Sr, Nd, Pb and Os isotope study of lavasand associated genetically related megacrysts from the Biu andJos Plateaux, northern Cameroon Volcanic Line (CVL). Comparisonof lavas and megacrysts allows us to distinguish between twocontamination paths of the primary magmas. The first is characterizedby both increasing ²⁰⁶Pb/²⁰⁴Pb (19·82–20·33)and ⁸⁷Sr/⁸⁶Sr (0·70290–0·70310), and decreasing_Nd (7·0–6·0), and involves addition of anenriched sub-continental lithospheric mantle-derived melt. Thesecond contamination path is characterized by decreasing ²⁰⁶Pb/²⁰⁴Pb(19·82–19·03), but also increasing ⁸⁷Sr/⁸⁶Sr(0·70290–0·70359), increasing ¹⁸⁷Os/¹⁸⁸Os(0·130–0·245) and decreasing _Nd (7·0–4·6),and involves addition of up to 8% bulk continental crust. Isotopicsystematics of some lavas from the oceanic sector of the CVLalso imply the involvement of a continental crustal component.Assuming that the line as a whole shares a common source, wepropose that the continental signature seen in the oceanic sectorof the CVL is caused by shallow contamination, either by continent-derivedsediments or by rafted crustal blocks that became trapped inthe oceanic lithosphere during continental breakup in the Mesozoic. KEY WORDS: crustal contamination; CVL; megacrysts; ocean floor; osmium isotopes  相似文献   

Geothermobarometry in Four-phase Lherzolites I. Experimental Results from 10 to 60 kb   总被引：12，自引：8，他引：4  

BREY  G. P.; K?HLER  T.; NICKEL  K. G. 《Journal of Petrology》1990,31(6):1313-1352

Experiments were carried out on primitive natural lherzoliticcompositions from 10 to 60 kb and to 1400?C. Mineral compositionswere reversed by using mixes of finely ground (<20µm)minerals from natural rock samples with different initial compositions.The minerals were mixed in proportions to give a primitive uppermantle composition. Further starting materials were a syntheticmineral mix and a sintered oxide mix. Equilibrium was thus approachedfrom different chemical directions, and equilibrium mineralcompositions were inferred from the overlap of microprobe analyses.Fe-loss problems were avoided by using single-crystal olivinesas sample containers. Samples were placed into holes drilledinto cylindrically shaped olivine crystals, which in turn werewelded into Pt capsules. A further advantage of this methodis that oxygen fugacities seem to be buffered by the Ni precipitationcurve. This curve lies very close to the iron-w?stite (IW) buffercurve for mantle olivines. Oxygen fugacities calculated fromcompositions of experimentally produced spinels give valuesslightly above the IW buffer curve. Therefore, practically noFe³⁺ should be present in our experimental charges. Ortho- and clinopyroxene compositions were strictly reversedwith respect to Al Cr, and Na (only cpx) contents and theirmutual amounts of enstatite and diopside components. The datashow the fundamental influence of Na in cpx on two-pyroxenethermometry. Garnet compositions were reversed with respectto Ca, Al, Cr, Ti, and Mn. Other than in CMAS, Ca in garnetis mostly dependent on temperature in the natural system andnot so much on pressure. This behaviour of Ca will most stronglyinfluence barometers based on the Al content of orthopyroxenecoexisting with garnet. The mg-numbers of coexisting phaseswere not strictly reversed. The homogeneity of olivine compositionsand internal consistency of the data set give us confidencethat equilibrium values of mg-number have been reached in almostall cases. The experimental results presented in this paperoffer the unique possibility of simultaneously testing manydifferent thermobarometers which are based on dfferent exchangereactions and which were calibrated in different laboratories.  相似文献   

Reconstruction of the exhumation path of the Alpe Arami'ql garnet-peridotite body from depths exceeding 160 km     

F. E. BRENKER  & G. P. BREY 《Journal of Metamorphic Geology》1997,15(5):581-592

Chemical disequilibrium exists between all phases of the Alpe Arami garnet-peridotite body (Ticino, Switzerland) which hampers the evaluation of P–T  conditions of origin, yet disequilibrium offers the inherent possibility to derive a P–T–t path for this mantle slice. We tried to tackle this problem by carrying out new mineral analyses and taking diffusion rates and bulk-rock compositional effects into consideration. Peak metamorphic conditions from mineral core compositions were estimated as 1120±50 °C/5±0.2 GPa. These values are significantly higher than previously published results and were determined from a combination of the O'Neill & Wood (1979) Fe/Mg garnet–olivine exchange thermometer and the Al-in-orthopyroxene barometer (Brey & Köhler, 1990), and are supported by the Ca/Cr ratios in garnet, which are in accord with these conditions. Details of the exhumation path were derived from (1) rim compositions of minerals that yield a first retrograde stage of 720±50 °C/2±0.25 GPa (2) a spinel lherzolite assemblage in narrow shear zones (tectonic phase F0', after Möckel, 1969) which documents a second retrograde stage at 500–600 °C/0.8–1.5 GPa. The Ca content in olivine (Köhler & Brey, 1989) can be used to evaluate further P–T  conditions along the retrograde path. We measured very low values (30–40  ppm Ca) in the cores of olivine and a remarkable increase towards the rim (120  ppm). The low core values may reflect an equilibrium stage during the main Alpine metamorphism. The increasing values towards the olivine rims probably represent a late-stage heating event. The initial cooling rates for the peridotite body are between 2700 and 5100 °C Ma⁻¹, depending on which diffusion data are used.  相似文献