Ferropericlase—a lower mantle phase in the upper mantle     

Gerhard P. Brey  Vadim Bulatov  Andrei Girnis  Jeff W. Harris  Thomas Stachel   《Lithos》2004,77(1-4):655-663

Experiments on compositions along the join MgO–NaA³⁺Si₂O₆ (A=Al, Cr, Fe³⁺) show that sodium can be incorporated into ferropericlase at upper mantle pressures in amounts commonly found in natural diamond inclusions. These results, combined with the observed mineral parageneses of several diamond inclusion suites, establish firmly that ferropericlase exists in the upper mantle in regions with low silica activity. Such regions may be carbonated dunite or stalled and degassed carbonatitic melts. Ferropericlase as an inclusion in diamond on its own is not indicative of a lower mantle origin or of a deep mantle plume. Coexisting phases have to be taken into consideration to decide on the depth of origin. The composition of olivine will indicate an origin from the upper mantle or border of the transition zone to the lower mantle and whether it coexisted with ferropericlase in the upper mantle or as ringwoodite. The narrow and flat three phase loop at the border transition zone—lower mantle together with hybrid peridotite plus eclogite/sediments provides an explanation for the varying and Fe-rich nature of the diamond inclusion suite from Sao Luiz, Brazil.  相似文献   

The Homestead kimberlite, central Montana, USA: mineralogy, xenocrysts, and upper-mantle xenoliths     

B. Carter Hearn Jr.   《Lithos》2004,77(1-4):473-491

The Homestead kimberlite was emplaced in lower Cretaceous marine shale and siltstone in the Grassrange area of central Montana. The Grassrange area includes aillikite, alnoite, carbonatite, kimberlite, and monchiquite and is situated within the Archean Wyoming craton. The kimberlite contains 25–30 modal% olivine as xenocrysts and phenocrysts in a matrix of phlogopite, monticellite, diopside, serpentine, chlorite, hydrous Ca–Al–Na silicates, perovskite, and spinel. The rock is kimberlite based on mineralogy, the presence of atoll-textured groundmass spinels, and kimberlitic core-rim zoning of groundmass spinels and groundmass phlogopites.

Garnet xenocrysts are mainly Cr-pyropes, of which 2–12% are G10 compositions, crustal almandines are rare and eclogitic garnets are absent. Spinel xenocrysts have MgO and Cr₂O₃ contents ranging into the diamond inclusion field. Mg-ilmenite xenocrysts contain 7–11 wt.% MgO and 0.8–1.9 wt.% Cr₂O₃, with (Fe⁺³/Fe^tot) from 0.17–0.31. Olivine is the only obvious megacryst mineral present. One microdiamond was recovered from caustic fusion of a 45-kg sample.

Upper-mantle xenoliths up to 70 cm size are abundant and are some of the largest known garnet peridotite xenoliths in North America. The xenolith suite is dominated by dunites, and harzburgites containing garnet and/or spinel. Granulites are rare and eclogites are absent. Among 153 xenoliths, 7% are lherzolites, 61% are harzburgites, 31% are dunites, and 1% are orthopyroxenites. Three of 30 peridotite xenoliths that were analysed are low-Ca garnet–spinel harzburgites containing G10 garnets. Xenolith textures are mainly coarse granular, and only 5% are porphyroclastic.

Xenolith modal mineralogy and mineral compositions indicate ancient major-element depletion as observed in other Wyoming craton xenolith assemblages, followed by younger enrichment events evidenced by tectonized or undeformed veins of orthopyroxenite, clinopyroxenite, websterite, and the presence of phlogopite-bearing veins and disseminated phlogopite. Phlogopite-bearing veins may represent kimberlite-related addition and/or earlier K-metasomatism.

Xenolith thermobarometry using published two-pyroxene and Al-in-opx methods suggest that garnet–spinel peridotites are derived from 1180 to 1390 °C and 3.6 to 4.7 GPa, close to the diamond–graphite boundary and above a 38 mW/m² shield geotherm. Low-Ca garnet–spinel harzburgites with G10 garnets fall in about the same T and P range. Most spinel peridotites with assumed 2.0 GPa pressure are in the same T range, possibly indicating heating of the shallow mantle. Four of 79 Cr diopside xenocrysts have P–T estimates in the diamond stability field using published single-pyroxene P–T calculation methods.  相似文献   

The trace element composition of silicate inclusions in diamonds: a review   总被引：1，自引：0，他引：1  

Thomas Stachel  Sonja Aulbach  Gerhard P. Brey  Jeff W. Harris  Ingrid Leost  Ralf Tappert  K.S. Viljoen 《Lithos》2004,77(1-4):1-19

On a global scale, peridotitic garnet inclusions in diamonds from the subcratonic lithosphere indicate an evolution from strongly sinusoidal REE_N, typical for harzburgitic garnets, to mildly sinusoidal or “normal” patterns (positive slope from LREE_N to MREE_N, fairly flat MREE_N–HREE_N), typical for lherzolitic garnets. Using the Cr-number of garnet as a proxy for the bulk rock major element composition it becomes apparent that strong LREE enrichment in garnet is restricted to highly depleted lithologies, whereas flat or positive LREE–MREE slopes are limited to less depleted rocks. For lherzolitic garnet inclusions, there is a positive relation between equilibration temperature, enrichment in MREE, HREE and other HFSE (Ti, Zr, Y), and decreasing depletion in major elements. For harzburgitic garnets, relations are not linear, but it appears that lherzolite style enrichment in MREE–HREE only occurs at temperatures above 1150–1200 °C, whereas strong enrichment in Sr is absent at these high temperatures. These observations suggest a transition from melt metasomatism (typical for the lherzolitic sources) characterized by fairly unfractionated trace and major element compositions to metasomatism by CHO fluids carrying primarily incompatible trace elements. Melt and fluid metasomatism are viewed as a compositional continuum, with residual CHO fluids resulting from primary silicate or carbonate melts in the course of fractional crystallization and equilibration with lithospheric host rocks.

Eclogitic garnet inclusions show “normal” REE_N patterns, with LREE at about 1× and HREE at about 30× chondritic abundance. Clinopyroxenes approximately mirror the garnet patterns, being enriched in LREE and having chondritic HREE abundances. Positive and negative Eu anomalies are observed for both garnet and clinopyroxene inclusions. Such anomalies are strong evidence for crustal precursors for the eclogitic diamond sources. The trace element composition of an “average eclogitic diamond source” based on garnet and clinopyroxene inclusions is consistent with derivation from former oceanic crust that lost about 10% of a partial melt in the garnet stability field and that subsequently experienced only minor reenrichment in the most incompatible trace elements. Based on individual diamonds, this simplistic picture becomes more complex, with evidence for both strong enrichment and depletion in LREE.

Trace element data for sublithospheric inclusions in diamonds are less abundant. REE in majoritic garnets indicate source compositions that range from being similar to lithospheric eclogitic sources to strongly LREE enriched. Lower mantle sources, assessed based on CaSi–perovskite as the principal host for REE, are not primitive in composition but show moderate to strong LREE enrichment. The bulk rock LREE_N–HREE_N slope cannot be determined from CaSi–perovskites alone, as garnet may be present in these shallow lower mantle sources and then would act as an important host for HREE. Positive and negative Eu anomalies are widespread in CaSi–perovskites and negative anomalies have also been observed for a majoritic garnet and a coexisting clinopyroxene inclusion. This suggests that sublithospheric diamond sources may be linked to old oceanic slabs, possibly because only former crustal rocks can provide the redox gradients necessary for diamond precipitation in an otherwise reduced sublithospheric mantle.  相似文献   

Partial Crystallization of Mid-Ocean Ridge Basalts in the Crust and Mantle   总被引：2，自引：2，他引：2  

HERZBERG  CLAUDE 《Journal of Petrology》2004,45(12):2389-2405

Pressures at which partial crystallization occurs for mid-oceanridge basalts (MORB) have been examined by a new petrologicalmethod that is based on a parameterization of experimental datain the form of projections. Application to a global MORB glassdatabase shows that partial crystallization of olivine + plagioclase+ augite ranges from 1 atm to 1·0 GPa, in good agreementwith previous determinations, and that there are regional variationsthat generally correlate with spreading rate. MORB from fast-spreadingcenters display partial crystallization in the crust at ridgesegment centers and in both mantle and crust at ridge terminations.Fracture zones are likely to be regions where magma chambersare absent and where there is enhanced conductive cooling ofthe lithosphere at depth. MORB from slow-spreading centers displayprominent partial crystallization in the mantle, consistentwith models of enhanced conductive cooling of the lithosphereand the greater abundance of fracture zones through which theypass. In general, magmas that move through cold mantle experiencesome partial crystallization, whereas magmas that pass throughhot mantle may be comparatively unaffected. Estimated pressuresof partial crystallization indicate that the top of the partialmelting region is deeper than about 20–35 km below slow-spreadingcenters and some ridge segment terminations at fast-spreadingcenters. KEY WORDS: MORB; olivine gabbro; partial crystallization; partial melting; ridge segmentation; fracture zones; crust; mantle; lithosphere  相似文献   

Rheological Properties of Partially Molten Lherzolite   总被引：2，自引：1，他引：2  

ZIMMERMAN  MARK E.; KOHLSTEDT  DAVID L. 《Journal of Petrology》2004,45(2):275-298

Lherzolite samples synthesized from fine-grained powders preparedfrom a natural xenolith were deformed at P = 300 MPa and 1373  相似文献   

An Archean(?) to Paleozoic Evolution for a Garnet Peridotite Lens with Sub-Baltic Shield Affinity within the Seve Nappe Complex of Jamtland, Sweden, Central Scandinavian Caledonides   总被引：2，自引：0，他引：2  

BRUECKNER  HANNES K.; VAN ROERMUND  HERMAN L. M.; PEARSON  NORMAN J. 《Journal of Petrology》2004,45(2):415-437

Mineralogical, isotopic, geochemical and geochronological evidencedemonstrates that the Friningen body, a garnet peridotite bodycontaining garnet pyroxenite layers in the Seve Nappe Complex(SNC) of Northern Jämtland, Sweden, represents old, certainlyProterozoic and possibly Archean, lithosphere that became incorporatedinto the Caledonian tectonic edifice during crustal subductioninto the mantle at c. 450 Ma. Both garnet peridotite and pyroxenitecontain two (M₁ and M₂) generations of garnet-bearing assemblagesseparated by the formation of two-pyroxene, spinel symplectitearound the M₁ garnet and the crystallization of low-Cr spinel_1Cin the matrix. These textures suggest initial high-pressure(HP) crystallization of garnet peridotite and pyroxenite succeededby decompression into the spinel stability field, followed byrecompression into the garnet peridotite facies. Some pyroxenitelayers appear to be characterized solely by M₂ assemblages withstretched garnet as large as several centimeters. Laser ablationmicroprobe–inductively coupled plasma mass spectrometryRe–Os analyses of single sulfide grains generally definemeaningless model ages suggesting more than one episode of Reand/or Os addition and/or loss to the body. Pentlandite grainsfrom a single polished slab of one garnet peridotite, however,define a linear array on an Re–Os isochron diagram that,if interpreted as an errorchron, suggests an Archean melt extractionevent that left behind the depleted dunite and harzburgite bodiesthat characterize the SNC. Refertilization of this mantle bymelts associated with the development of the pyroxenite layersis indicated by enriched clinopyroxene Sr–Nd isotope ratios,and by parallel large ion lithophile-enriched trace elementpatterns in clinopyroxene from pyroxenite and the immediatelyadjacent peridotite. Clinopyroxene and whole-rock model Sm–Ndages (T_DM = 1·1–2·2 Ga) indicate that fertilizationtook place in Proterozoic times. Sm–Nd garnet₂–clinopyroxene₂–wholerock ± orthopyroxene₂ mineral isochrons from three pyroxenitelayers define overlapping ages of 452·1 ± 7·5and 448 ± 13 Ma and 451 ± 43 Ma (2  相似文献   

Nature of the Source Regions for Post-collisional, Potassic Magmatism in Southern and Northern Tibet from Geochemical Variations and Inverse Trace Element Modelling   总被引：20，自引：0，他引：20  

WILLIAMS  H. M.; TURNER  S. P.; PEARCE  J. A.; KELLEY  S. P.; HARRIS  N. B. W. 《Journal of Petrology》2004,45(3):555-607

Neogene potassic lavas in northern and southern Tibet have differentisotopic (  相似文献