摘要:

This volume of theJournal of Geophysical Researchwas intended to be dedicated to Professor George C. Kennedy of the Institute of Geophysics and Planetary Physics, University of California, Los Angeles (UCLA), to commemorate his 60th birthday. However, George died of cancer on March 18, 1980, and this volume must now commemorate his career.On September 19 and 20, 1979, the Institute hosted a conference at UCLA to celebrate George's birthday, and most of the papers presented there, together with others by former coworkers, appear in this issue. All of the authors appreciate this opportunity to honor a man who made so many significant contributions to geochemistry and geophysics.

ISSN:0148-0227    

DOI:10.1029/JB085iB12p06901

年代:1980

数据来源: WILEY

摘要:

A new diapiric model for kimberlite genesis takes into account recent interpretations of peridotite‐CO2‐H2O melting relationships. A minor thermal perturbation at depth might trigger release of reduced vapors with major components C‐H‐O. Where these volatile components cross the estimated solidus boundary near 260 km, partial melting occurs, the density inversion causes diapiric uprise along adiabats, and the partially melted diapirs begin to crystallize at 100 to 80‐km depth, where they reach a temperature maximum (thermal barrier) on the solidus. The released vapor enhances the prospects for crack propagation through overlying lithosphere in tension, and this could produce an initial channel to the surface. Magma separation could then occur from progressively greater depths, releasing CO2‐under‐saturated kimberlitic magma for independent uprise through the established conduit, quite unaffected by the thermal barrier on the solidus of peridotite‐CO2‐H2O. Cooler diapirs would cross the solidus at somewhat greater depth, solidifying to phlogopite‐dolomite‐peridotite with the release of aqueous solutions. These solutions are likely candidates for the mantle metasomatism commonly considered to be a precursor for the generation of kimberlites and other alkalic magmas. According to this model the metasomatism is a consequence of kimberlite magmatism rather than

ISSN:0148-0227    

DOI:10.1029/JB085iB12p06902

年代:1980

数据来源: WILEY

摘要:

Studies of mineral inclusions in natural diamonds and rare diamondiferous xenoliths from kimberlites show that most diamonds are associated with a dunite or harzburgite paragenesis. The diamondiferous peridotites and dunites have predominantly coarse or tabular textures that suggest low‐temperature (<1100°C) equilibration. Application of theKDFe/Mg(Ga/Ol) geothermometer of O'Neill and Wood to analytical data for the minerals in these rocks shows that most have equilibrated below 1100°C. Application of this thermometer to pairs of olivine and garnet crystals included in individual diamonds indicates that the diamonds have crystallized in the range 900°–1300°C, with a majority of estimated equilibration temperatures falling in the range below 1150°C. Comparison of these estimates of equilibration temperature with the zone of invariant vapor composition solidus for kimberlite and garnet lherzolite determined by Eggler and Wendlandt suggests that many diamonds may have formed in subsolidu

ISSN:0148-0227    

DOI:10.1029/JB085iB12p06911

年代:1980

数据来源: WILEY

摘要:

New electron and ion microprobe analyses of 36 olivines, 9 pyroxenes, and 10 garnets included in diamond were obtained. Olivine inclusions from kimberlites with Precambrian ages have higher mean Cr2O3(850 versus 360 ppmw), Al2O3(200 versus 90 ppmw), CaO (490 versus 210 ppmw), Na2O (110 versus 30 ppmw), and Li (686 versus 466 counts per second) than olivine inclusions found in Mesozoic kimberlites, perhaps indicating a temporal change of either temperature or chemistry or both in the mantle source regions of diamonds. In contrast to lherzolitic olivines, aluminum and sodium in olivines from diamonds are insufficient to couple with trivalent chromium; perhaps divalent chromium is inferred. Some olivines show zoning to Ca‐rich edges. Temperature and pressure estimates for inclusions in four diamonds fall in the combined pressure‐temperature range of coarse and porphyroclastic garnet lherzolites, and temperature estimates for inclusions in seven diamonds lacking a pressure estimate cover most of the temperature range for lherzolites. Taken at face value, the pressure and temperature estimates for inclusions in diamonds indicate that some diamonds might have crystallized in the presence of a liquid containing H2O, CO2, or both. Other estimates fall in the subsolidus field even for H2O‐saturated peridotite. Although the pressure‐temperature estimates overlap for lherzolites and ultramafic‐type inclusions in diamonds, differences in trace element content of olivines require that the crystallization environments were not identical. Perhaps the Cr content of the olivine inclusions indicates that the host diamonds grew in a reducing environment not found in typical lh

ISSN:0148-0227    

DOI:10.1029/JB085iB12p06919

年代:1980

数据来源: WILEY

摘要:

A brief history of the many investigations of the pressure/temperature phase diagrams of elemental carbon is given. The strengths and weaknesses of the various proposed diagrams are discussed, and the areas where additional or better experimental data are needed are pointed out. Many questions still remain about linear solid carbons; about the low‐pressure/high‐temperature solid/liquid/vapor triple point; about the melting line of diamond; about the existence of a high‐pressure metallic phase; and about the existence of two liquid phases, one insulator and the other met

ISSN:0148-0227    

DOI:10.1029/JB085iB12p06930

年代:1980

数据来源: WILEY

摘要:

The system CaO‐MgO‐SiO2‐CO2‐H2O is an important model for many igneous and metamorphic processes, including the generation and differentiation of carbonatite and associated silicate magmas. We have experimentally established the vapor‐saturated solidus for the system CaO‐MgO‐CO2‐H2O from 595°C/1 kbar to<500°C/40 kbar, about 25°C lower than the corresponding temperatures for the CaO‐CO2‐H2O system. Brucite replaces periclase on the vapor‐saturated solidus at about 750 bars pressure, remaining as the stable phase to pressures of at least 40 kbar; no dolomite or magnesite was encountered. At a pressure between 35 and 40 kbar, the assemblage portlandite + brucite + aragonite + vapor changes to one containing phase W, a previously unreported Ca‐Mg carbonate. At 20 kbar, the vapor‐saturated liquid contains at least 24 wt % H2O. The vapor‐saturated solidus for the system CaO‐MgO‐SiO2‐CO2‐H2O ranges from 613°C/1 kbar to 565°C/10 kbar, experimentally indistinguishable from that for CaO‐MgO‐CO2‐H2O, about 10°C lower than that for CaO‐CO2‐H2O, and about 25°C lower than that for CaO‐SiO2‐CO2‐H2O. In the quinary system, monticellite is replaced by dellaite and an unidentified silicate on the vapor‐saturated solidus above 4.1 kbar. MgO‐poor liquids are similar in composition to the magma from which the Magnet Cove calcite carbonatite crystallized. More magnesian magmas wou

ISSN:0148-0227    

DOI:10.1029/JB085iB12p06937

年代:1980

数据来源: WILEY

摘要:

Phase equilibria in the system MgO‐SiO2‐H2O have been studied in the pressure range 85–160 kbar in the temperature range 750°–1200°C. In runs for a gel with composition of 64.1 wt % MgO, 31.9 wt % SiO2, 4.0 wt % H2O (atomic ratio Mg/Si = 3.0) a hydrous magnesian silicate, denoted phase B by Ringwood and Major, was found to exist from about 100 kbar to at least 160 kbar at temperatures 750°–1200°C. At pressures be1ow about 100 kbar the assemblage of phase A(Mg7Si2O14H6) + forsterite + periclase was stable at relatively low temperatures, while the assemblage of chondrodite (Mg5Si2O10H2) + periclase + forsterite was stable at relatively high temperatures. X ray diffraction shows that phase B is monoclinic (space group,P21/c) with cell constantsa= 10.600(2) Å,b= 14.098(2) Å,c= 10.092(2) Å, and β = 104.05(2)°. The chemical formula of phase B was deduced to be Mg23Si8O42H6. The observed density, 3.32±0.04 g/cm3, is the highest of the hydrous silicate phases reported in the system MgO‐SiO2‐H2O and is about 3% higher than that of forsterite. While this study indicates the stability of phase B for bulk compositions which are richer in MgO than accepted mantle models, it is regarded as a potential hydrous silicate phase in the upper mantle and transition zone in the depth range of 30

ISSN:0148-0227    

DOI:10.1029/JB085iB12p06944

年代:1980

数据来源: WILEY

摘要:

The thermal decomposition (PT) curve of dolomite has been extended in piston‐cylinder apparatus from a point at 9 kbar and 1130°C to 1450°C. The piston‐cylinder runs, in which unsealed Pt sample capsules were embedded in glass pressure media, were compatible with earlier gas system determinations. At approximately 11 kbar and 1175°C the curve delineating the onset of decomposition of CaMg(CO3)2deviates strongly from any extrapolated extension of the PT curve up to 10 kbar and 1150°C, and the dolomite composition is stabilized to significantly higher temperatures than those predicted from low‐pressure data. At 1450°C, Pco2is less than 14 kbar. The striking increase in thermal stability at pressures above 10 kbar is the result of rapidly shifting phase boundaries in TX sections at temperatures above the calcite‐dolomite solvus, and probably of an entropy increase in CaMg(CO3)2, due to a variety of disorderin

ISSN:0148-0227    

DOI:10.1029/JB085iB12p06949

年代:1980

数据来源: WILEY

摘要:

Phase relationships in the systems NaAlSi3O8‐KAlSi3O8‐H2O and NaAlSi3O8‐KAlSi3O8‐SiO2‐H2O to 10 kbars pressure are discussed using pressure‐temperature projections and isobaric, polythermal projections. Complexities in these systems result at about 4.2 and 2.5 kbar, respectively, as the result of critical endpoints generated by the intersections of the vapor‐saturated solidi and the feldspar solvi, previously investigated experimentally by Seek and by Morse. A little‐known method derived by Hillert is used to distinguish eutectic and peritectic reactions involving alkali feldspars and water‐under

ISSN:0148-0227    

DOI:10.1029/JB085iB12p06955

年代:1980

数据来源: WILEY

摘要:

The Al2O3solubility in orthopyroxene (OPx) coexisting with pyrope has been determined at severalP‐Tconditions in the system MgO‐Al2O3‐SiO2(MAS), using synthetic crystalline starting mixtures and approaching from both high‐ and low‐alumina sides. These results and the reversed experimental data of Danckwerth and Newton (1978) on Al‐OPx + spinel + forsterite equilibrium have been treated thermodynamically, using the solution model of Al‐OPx proposed by Ganguly and Ghose (1979), to express Al2O3solubility in OPx as a function ofPandTin both assemblages. The calculated alumina isopleths are in very good agreement with all available reversed experimental data. In addition, the univariant boundary between spinel peridotite and garnet peridotite in the MAS system, which is defined by the intersection of complementary alumina isopleths in these two fields, is in excellent agreement with experimental reversals of the boundary by Danckwerth and Newton. The spinel field isopleths have somewhat steeperdT/dPslopes than those calculated by previous workers but are still not sufficiently sensitive to pressure to be useful as a practical geobarometer. The garnet field isopleths are nearly equally sensitive to bothPandTbut are somewhat different from those of MacGregor (1974) and show mild curvature, concave toward the pressure axis. The univariant boundary shows moderate curvature, convex toward the temperature axis, with a pressure minimum of 19 kbar at 840°C. The ‘pyroxene geotherm’of Boyd (1973) has been reexamined in the light of the results of this study and the garnet‐clinopyroxene geothermometer of Ganguly (1979). Boyd's conclusion that the sheared garnet lherzolite nodules in kimberlite pipes have formed at higherP‐Tconditions than the granular ones is reinforced; however, the inferredP‐Tconditions do not seem to suggest an inflection of the geotherm between the two groups of nodules. The pyroxene or nodule geotherm overlaps the shield geotherm of Clark and Ringwood but progressively diverges from the latter toward higher temperatu

ISSN:0148-0227    

DOI:10.1029/JB085iB12p06963

年代:1980

数据来源: WILEY