Xenolith-derived mantle geotherms: whither the inflection?

Three possibilities exist for the geometry of the upper mantle geotherm determined from study of garnet lherzolite xenoliths from Cretaceous kimberlites of northern Lesotho, southern Africa: (1) the geotherm is inflected to a lower thermal gradient at greater depth (low-T inflection); (2) the geotherm is uninflected; and (3) the geotherm is inflected to a higher thermal gradient at greater depth (high-T inflection). In the past two years all three possibilities have been advocated. Finnerty and Boyd (1984, 1987) found that many independent thermometers yield similar P-T arrays, so that features of xenolith geotherms cannot be artifacts of the method of temperature estimation. Hence the current controversy centers on the barometers used for pressure estimation. Bertrand et al. (1986) calibrated a new aluminous-enstatite barometer using 50–100 kbar data of Yamada and Takahashi (1983), and presented a southern Africa geotherm displaying a low-T inflection. The high-P alumina solubility data are incompatible with lower-P data, however, with the result that the new barometer underestimates pressure: a diamond-bearing xenolith falls at least 5.7 kbar into the stability field of graphite. Thus, the Bertrand et al. (1986) barometer does not adequately test the reality of inflected geotherms. Carswell and Gibb (1987 a, b) modified the aluminous enstatite barometer of Nickel and Green (1985) to account for Jadeite molecule in orthopyroxenes containing relatively high concentrations of Na. When applied to xenoliths of northern Lesotho the apparent inflection is minimized but still evident. In this suite Na content of orthopyroxenes increases systematically with greater T or greater depth. Sodium correlates poorly with T (and depth) in a suite of xenoliths from Farm Louwrencia, Namibia, and application of the Nickel and Green (1985) barometer (with or without modification) destroys the correlation of T with P expected for a geotherm. The decorrelation of P from T in the Louwrencia suite is caused by errors in the Na correction. The minimization of the inflection in the northern Lesotho suite is caused by the correlation of Na with T (and depth) in that suite and does not result from an improved correction scheme for the aluminous enstatite barometer. Hence, the Carswell and Gibb (1987a, b) formulation of the barometer does not support the absence of an inflection in the northern Lesotho geotherm. Adams and Bishop (1986) recalibrated the olivine barometer (Finnerty and Boyd 1978) and presented a southern African P-T array that appears uninflected. Their barometer, however, underestimates pressure by 10–20 kbar: all xenoliths from the southern African diamond-bearing kimberlites plot well within the graphite stability field. This barometer is also too imprecise to judge whether an inflection is present or absent. Finnerty (1989), employing an empirical fit to data for Ca solubility in olivine and several different independent thermometers, presented northern Lesotho P-T arrays that satisfy the diamond-graphite constraint with minimal scatter and display high-T inflections. Because the inflection is evident with independent thermometers and independent barometers, it cannot be an artifact of the method of P-T estimation. The arguments contesting such an interpretation are flawed and so it is concluded that a high-T inflection is a fundamental property of the Cretaceous upper mantle geotherm beneath southern Africa.     

Petrology and geochemistry of peridotite xenoliths from the Letlhakane kimberlites, Botswana

The diamondiferous Letlhakane kimberlites are intruded into the Proterozoic Magondi Belt of Botswana. Given the general correlation of diamondiferous kimberlites with Archaean cratons, the apparent tectonic setting of these kimberlites is somewhat anomalous. Xenoliths in kimberlite diatremes provide a window into the underlying crust and upper mantle and, with the aid of detailed petrological and geochemical study, can help unravel problems of tectonic setting. To provide relevant data on the deep mantle under eastern Botswana we have studied peridotite xenoliths from the Letlhakane kimberlites. The mantle-derived xenolith suite at Letlhakane includes peridotites, pyroxenites, eclogites, megacrysts, MARID and glimmerite xenoliths. Peridotite xenoliths are represented by garnet-bearing harzburgites and lherzolites as well as spinel-bearing lherzolite xenoliths. Most peridotites are coarse, but some are intensely deformed. Both garnet harzburgites and garnet lherzolites are in many cases variably metasomatised and show the introduction of metasomatic phlogopite, clinopyroxene and ilmenite. The petrography and mineral chemistry of these xenoliths are comparable to that of peridotite xenoliths from the Kaapvaal craton. Calculated temperature-depth relations show a well-developed correlation between the textures of xenoliths and P-T conditions, with the highest temperatures and pressures calculated for the deformed xenoliths. This is comparable to xenoliths from the Kaapvaal craton. However, the P-T gap evident between low-T coarse peridotites and high-T deformed peridotites from the Kaapvaal craton is not seen in the Letlhakane xenoliths. The P-T data indicate the presence of lithospheric mantle beneath Letlhakane, which is at least 150 km thick and which had a 40mW/m² continental geotherm at the time of pipe emplacement. The peridotite xenoliths were in internal Nd isotopic equilibrium at the time of pipe emplacement but a lherzolite xenolith with a relatively low calculated temperature of equilibration shows evidence for remnant isotopic disequilibrium. Both harzburgite and lherzolite xenoliths bear trace element and isotopic signatures of variously enriched mantle (low Sm/Nd, high Rb/Sr), stabilised in subcontinental lithosphere since the Archaean. It is therefore apparent that the Letlhakane kimberlites are underlain by old, cold and very thick lithosphere, probably related to the Zimbabwe craton. The eastern extremity of the Proterozoic Magondi Belt into which the kimberlites intrude is interpreted as a superficial feature not rooted in the mantle. Received: 19 March 1996 / Accepted: 16 October 1996     

Evaluation of mineral thermometers and barometers applicable to garnet lherzolite assemblages

We have tested the ability of some 12 different barometer and 20 different thermometer formulations to reproduce the experimental P-T equilibration conditions of natural multi-component garnet lherzolite assemblages. For natural rock compositions it is essential to take account of the influence of both Cr and Fe on the garnet-orthopyroxene Al exchange reaction customarily used as a barometer for such assemblages and accordingly our results demonstrate that the formulation of Nickel and Green (1985) is the most satisfactory. No single thermometer formulation was judged to be reliable throughout the P-T range of interest. In our view equilibration temperatures are best assessed by consideration of a combination of the most satisfactory thermometer formulations, based on the two-pyroxene solvus and the Fe²⁺-Mg²⁺ exchange reactions between mineral pairs. Our results further indicate that use of the barometer (MacGregor 1974) and thermometer formulations recommended by Finnerty and Boyd (1984 and 1986) will lead to inaccurate assessment of the temperatures and pressures of equilibration for most garnet lherzolite xenolith assemblages and hence to incorrect interpretation of their depth of derivation within the mantle.     

Single clinopyroxene thermobarometry for garnet peridotites. Part I. Calibration and testing of a Cr-in-Cpx barometer and an enstatite-in-Cpx thermometer

Experimental clinopyroxenes synthesized at 850–1500 °C and 0–60 kbar in the CMS and CMAS-Cr systems and in more complex lherzolitic systems have been used to calibrate a Cr-in-Cpx barometer and an enstatite-in-Cpx thermometer for Cr-diopsides derived from garnet peridotites. The experiments cover a wide range of possible natural peridotitic compositions, from fertile pyrolite to refractory, high-Cr lherzolite. The barometer is based on the Cr exchange between clinopyroxene and garnet. Pressure is formulated as a function of temperature and clinopyroxene composition:

Nd-isotopes in selected mantle-derived rocks and minerals and their implications for mantle evolution

The Sm-Nd systematics in a variety of mantle-derived samples including kimberlites, alnoite, carbonatite, pyroxene and amphibole inclusions in alkali basalts and xenolithic eclogites, granulites and a pyroxene megacryst in kimberlites are reported. The additional data on kimberlites strengthen our earlier conclusion that kimberlites are derived from a relatively undifferentiated chondritic mantle source. This conclusion is based on the observation that the _Nd values of most of the kimberlites are near zero. In contrast with the kimberlites, their garnet lherzolite inclusions show both time-averaged Nd enrichment and depletion with respect to Sm. Separated clinopyroxenes in eclogite xenoliths from the Roberts Victor kimberlite pipe show both positive and negative _Nd values suggesting different genetic history. A whole rock lower crustal scapolite granulite xenolith from the Matsoku kimberlite pipe shows a negative _Nd value of -4.2, possibly representative of the base of the crust in Lesotho. It appears that all inclusions, mafic and ultramafic, in kimberlites are unrelated to their kimberlite host.The above data and additional Sm-Nd data on xenoliths in alkali basalts, alpine peridotite and alnoite-carbonatites are used to construct a model for the upper 200 km of the earth's mantle — both oceanic and continental. The essential feature of this model is the increasing degree of fertility of the mantle with depth. The kimberlite's source at depths below 200 km in the subcontinental mantle is the most primitive in this model, and this primitive layer is also extended to the suboceanic mantle. However, it is clear from the Nd-isotopic data in the xenoliths of the continental kimberlites that above 200 km the continental mantle is distinctly different from their suboceanic counterpart.     

Geothermometry of garnet lherzolite nodules with special reference to those from the kimberlites of Northern Lesotho

Different published geothermometric methods have been tested on a large suite of garnet lherzolite nodules from the kimberlites of Northern Lesotho. Comparison of the various calculated equilibration temperature estimates indicates that several of these methods yield unreliable results and should therefore be rejected. However, there is little to choose between the values obtained from five other methods based on three different element exchange reactions. Accordingly we conclude that the petrogenesis of garnet lherzolite nodules is best discussed with reference to the mean temperature estimates derived from these five preferred methods. The late Cretaceous palaeogeotherm for Northern Lesotho is revised on this basis and found to be of a similar form to previous estimates but is significantly displaced to higher temperatures.     

汉诺坝地区上地幔尖晶石—石榴石相转变带温压条件

根据汉诺坝尖晶石石榴石二辉辉石岩包体矿物成分新资料和前人的尖晶石石榴石二辉橄榄岩包体矿物成分资料,运用斜方辉石Ｃａ溶解度温度计和斜方辉石－石榴石Ａｌ分配压力计,计算了尖晶石—石榴石相转变带的温度和压力条件,首次获得了汉诺坝地区的新生代古地温曲线。尖晶石石榴石二辉辉石岩包体代表的温度范围为９３０～９７８℃,压力范围为１４２～１６５ＧＰａ：尖晶石石榴石二辉橄榄岩包体代表的温度范围为９９１～１１１０℃,压力范围为１５８～２１６ＧＰａ,与最新的实验结果基本吻合。地温曲线沿大洋地温曲线（曲线方程为ｔ＝２５２４＋８９７４７８ｐ－１８３０８ｐ２）上方近平行延伸。推测汉诺坝玄武岩的起源深度至少为７０ｋｍ。二辉辉石岩包体来自较冷的岩石圈,而二辉橄榄岩来自岩石圈与软流圈的过渡带,并且表明后者的地温梯度以对流热地温梯度为主。     

Ultramafic Xenoliths From a Kamafugite Lava in Cenozoic Volcanic Field of West Qinling, China and Its Geological Implication

ULTRAMAFIC XENOLITHS FROM A KAMAFUGITE LAVA IN CENOZOIC VOLCANIC FIELD OF WEST QINLING, CHINA AND ITS GEOLOGICAL IMPLICATION     

Peridotitic mantle xenoliths from kimberlites on the Ekati Diamond Mine property, N.W.T., Canada: major element compositions and implications for the lithosphere beneath the central Slave craton

The composition, structure and thermal state of the lithosphere beneath the Slave craton have been studied by analysing over 300 peridotitic mantle xenoliths or multiphase xenocrysts entrained within kimberlites in the Lac de Gras area. These xenoliths are derived from seven kimberlites located on the Ekati Diamond Mine™ property and define a detailed stratigraphic profile through the central Slave lithosphere from less than 120 km down to 200 km. Two dominant peridotite types are present, namely garnet-bearing harzburgite and lherzolite with rare occurrences of chromite-facies peridotite, websterite and wehrlite. The pressures and temperatures (P–T's) defined by the entire data-set range from 28 to 62 kbar and 650 to 1250 °C, respectively, and approximately intersect the diamond stability field at 900 °C and 42 kbar. There is no apparent change in the geotherm with depth that is discernable beyond the resolution of the various thermobarometers. The peridotites can be divided into two compositional zones—a shallow layer dominated by garnet harzburgite that straddles the diamond–graphite boundary and a deeper layer that is strongly dominated by garnet lherzolite. Compositionally, the harzburgites (and to a lesser extent, the shallow lherzolites) are ultra-depleted relative to the more fertile deeper layer, irrespective of whether they reside within the graphite or diamond stability field. This ultra-depleted layer beneath Ekati continues to 150 km.     

Garnet lherzolites from Somerset Island,Canada and aspects of the nature of perturbed geotherms

Garnet lherzolite xenoliths of similar petrography and mineralogy are found in the Elwin Bay, Nanorluk, and Amayersuk kimberlites. The xenoliths are either coarse equant to coarse tabular or porphyroclastic in texture. Compositions of coexisting pyroxenes indicates equilibration at 1000–1270° C at 34–41 kb (Wood-Banno/Wood method) or 865–1200° C at 29–36 kb (Wells/Wood method). No simple correlation exists between textural types and equilibration temperature. A primary spinel-bearing garnet lherzolite has equilibrated at 840° C at 21 kb (Wells/Wood) and provides the only known example of a xenolith with relatively high Cr/Cr+Al which has equilibrated at the spinel to garnet lherzolite transition along the continental geotherm. The pressure and temperature estimates for the xenoliths lie above those of the steady state geotherm and indicate that a perturbed geotherm existed in this region at the time of kimberlite intrusion. The formation of perturbed geotherms is discussed and it is considered that the upper high temperature limbs of inflected geotherms are transient pseudogeotherms generated in response to a thermal aureole about a rising mantle diapir and that the lherzolites which define such a geotherm represent a telescoped section of the mantle and include xenoliths derived from above and below the point of kimberlite liquid segregation. The lower temperature limbs of inflected geotherms are considered to be representative of the steady state geotherm and are sampled by the kimberlite which after segregation from the diapir rises at a much faster rate than the parent diapir and passes through material which is unaffected by the diapir thermal aureole.     

A pyroxene geotherm

Phase equilibria determined in high-pressure studies of the systems Mg₂Si₂O₆-CaMgSi₂O₆ and MgSiO₃-Mg₃Al₂Si₃O₁₂ can be used to estimate equilibration conditions of ultramafic rocks containing the assemblage enstatite + diopside + garnet. Garnet lherzolite nodules from kimberlites in northern Lesotho appear to have equilibrated in the upper mantle at depths ranging from 100 to 200 km and at temperatures in the range 900–1400°C. Temperature-depth points for these lherzolites form a trend that is interpreted as a segment of a fossil geotherm. The trend is inflected to higher temperatures at its deep end. Lherzolites that plot on the shallow limb of the geotherm have a granular texture whereas those that plot on the deep limb are intensely sheared. It is suggested that the shearing took place in response to plate movements during the break-up of Gondwanaland and that the sheared lherzolites were stress-heated as much as 300°C above their ambient, preshearing temperatures. The point of inflection of the geotherm may mark the top of the low-velocity zone beneath Lesotho in Late Cretaceous time.     

Petrochemistry of eclogites from the Koidu Kimberlite Complex,Sierra Leone

Petrography, mineral and bulk chemistry of upper mantle-derived eclogites (garnet and clinopyroxene) from the Koidu Kimberlite Complex, Sierra Leone, are presented in the first comprehensive study of these xenoliths from West Africa. Although peridotite-suite xenoliths are generally more common in kimberlites, the upper mantle sample preserved in Pipe Number 1 at Koidu is exclusively eclogitic, making this the fifth locality in which eclogite is the sole polymineralic xenolith in kimberlite. Over 2000 xenoliths were collected, of which 47 are described in detail that include diamond, graphite, kyanite, corundum, quartz after coesite, and amphibole eclogites. Grossular-pyrope-almandine garnets are chromium-poor (<0.72 wt% Cr₂O₃) and fall into two distinct groups based on magnesium content. High-MgO garnets have an average composition of Pyr₆₇Alm₂₂Gross₁₁, low-MgO garnets are grossular- and almandine-rich with an average composition of Gross₃₄Pyr₃₃Alm₃₃. Clinopyroxenes are omphacitic with a range in jadeite contents from 7.7 to 70.1 mol%. Three eclogites contain zoned and mantled garnets with almandine-rich cores and pyrope-rich rims, and zoned clinopyroxenes with diopside-rich cores and jadeite-rich rims, and are among a very rare group of eclogites reported on a world-wide basis. The bulk compositions of eclogites have ranges comparable to that of basalts. High-MgO eclogites (16–20 wt% MgO) have close chemical affinities to picrites, whereas low-MgO eclogites (6–13 wt% MgO) are similar to alkali basalts. High-MgO eclogites contain high-MgO garnets and jadeiterich clinopyroxenes. Low-MgO eclogites contain low-MgO garnets, diopside and omphacite, and the group of primary accessory phases (diamond, graphite, quartz after coesite, kyanite, and corundum); grospydites are peraluminous. Estimated temperatures and pressures of equilibration of diamond-bearing eclogites, using the diamond-graphite stability curve and the Ellis and Green (1979) geothermometer, are 1031°–1363° C at 45–50 kb.K _D values of Fe-Mg in garnet and clinopyroxene range from 2.3 to 12.2. Diamonds in eclogites are green, yellow, and clear, and range from cube to octahedral morphologies; the entire spectrum in color and morphology is present in a single metasomatized eclogite with zoned garnet and clinopyroxene. Ages estimated from Sm-Nd mineral isochrons range from 92–247 Ma. _Nd values range from +4.05 to 5.23. Values of specific gravity range from 3.06–3.60 g/cc, with calculated seismic Vp of 7.4–8.7 km/s. Petrographie, mineral, and bulk chemical data demonstrate an overall close similarity between the Koidu xenolith suite and upper mantle eclogites from other districts in Africa, Siberia and the United States. At least two origins are implied byP-T, bulk chemistry and mineral compositions: low-MgO eclogites, with diamond and other accessory minerals, are considered to have formed from melts trapped and metamorphically equilibrated in the lithosphere; high-MgO eclogites are picritic and are the products of large degrees of partial melting, with equilibration in the asthenosphere. Fluid or diluted melt metasomatism is pervasive and contributed here and elsewhere to the LIL and refractory silicate incompatible element signature in kimberlites and lamproites, and to secondary diamond growth.     

Genesis and evolution of the lithospheric mantle beneath the Buffalo Head Terrane, Alberta (Canada)

Mantle xenoliths and xenocrysts were retrieved from three of the 88–86 Ma Buffalo Hills kimberlites (K6, K11, K14) for a reconnaissance study of the subcontinental lithospheric mantle (SCLM) beneath the Buffalo Head Terrane (Alberta, Canada). The xenoliths include spinel lherzolites, one garnet spinel lherzolite, garnet harzburgites, one sheared garnet lherzolite and pyroxenites. Pyroxenitic and wehrlitic garnet xenocrysts are derived primarily from the shallow mantle and lherzolitic garnet xenocrysts from the deep mantle. Harzburgite with Ca-saturated garnets is concentrated in a layer between 135–165 km depth. Garnet xenocrysts define a model conductive paleogeotherm corresponding to a heat flow of 38–39 mW/m². The sheared garnet lherzolite lies on an inflection of this geotherm and may constrain the depth of the lithosphere–asthenosphere boundary (LAB) beneath this region to ca 180 km depth.

A loss of >20% partial melt is recorded by spinel lherzolites and up to 60% by the garnet harzburgites, which may be related to lithosphere formation. The mantle was subsequently modified during at least two metasomatic events. An older metasomatic event is evident in incompatible-element enrichments in homogeneous equilibrated garnet and clinopyroxene. Silicate melt metasomatism predominated in the deep lithosphere and led to enrichments in the HFSE with minor enrichments in LREE. Metasomatism by small-volume volatile-rich melts, such as carbonatite, appears to have been more important in the shallow lithosphere and led to enrichments in LREE with minor enrichments in HFSE. An intermediate metasomatic style, possibly a signature of volatile-rich silicate melts, is also recognised. These metasomatic styles may be related through modification of a single melt during progressive interaction with the mantle. This metasomatism is suggested to have occurred during Paleoproterozoic rifting of the Buffalo Head Terrane from the neighbouring Rae Province and may be responsible for the evolution of some samples toward unradiogenic Nd and Hf isotopic compositions.

Disturbed Re–Os isotope systematics, evident in implausible model ages, were obtained in situ for sulfides in several spinel lherzolites and suggest that many sulfides are secondary (metasomatic) or mixtures of primary and secondary sulfides. Sulfide in one peridotite has unradiogenic ¹⁸⁷Os/¹⁸⁸Os and gives a model age of 1.89±0.38 Ga. This age coincides with the inferred emplacement of mafic sheets in the crust and suggests that the melts parental to the intrusions interacted with the lithospheric mantle.

A younger metasomatic event is indicated by the occurrence of sulfide-rich melt patches, unequilibrated mineral compositions and overgrowths on spinel that are Ti-, Cr- and Fe-rich but Zn-poor. Subsequent cooling is recorded by fine exsolution lamellae in the pyroxenes and by arrested mineral reactions.

If the lithosphere beneath the Buffalo Head Terrane was formed in the Archaean, any unambiguous signatures of this ancient origin may have been obliterated during these multiple events.     

Mineral Chemistry of Peridotites from Paleozoic, Mesozoic and Cenozoic Lithosphere: Constraints on Mantle Evolution beneath Eastern China

Major- and trace-element data on the constituent minerals ofgarnet peridotite xenoliths hosted in early Paleozoic (457–500Ma) kimberlites and Neogene (16–18 Ma) volcanic rockswithin the North China Craton are compared with those from thepre-pilot hole of the Chinese Continental Scientific DrillingProject (CCSD-PP1) in the tectonically exhumed Triassic (220Ma) Sulu ultrahigh-pressure (UHP) terrane along its southernmargin. P–T estimates for the Paleozoic and Neogene peridotitexenoliths reflect different model geotherms corresponding tosurface heat flows of 40 mW/m² (Paleozoic) and 80 mW/m² (Neogene).Garnet peridotite xenoliths or xenocrysts from the Paleozoickimberlites are strongly depleted, similar to peridotites fromother areas of cratonic mantle, with magnesium olivine (meanFo_92.7), Cr-rich garnet and clinopyroxene with high La/Yb. Garnet(and spinel) peridotite xenoliths hosted in Neogene basaltsare derived from fertile mantle; they have high Al₂O₃ and TiO₂contents, low-Mg-number olivine (mean Fo_89.5), low-Cr garnetand diopside with flat rare earth element (REE) patterns. Thedifferences between the Paleozoic and Neogene xenoliths suggestthat a buoyant refractory lithospheric keel present beneaththe eastern North China Craton in Paleozoic times was at leastpartly replaced by younger, hotter and more fertile lithosphericmantle during Mesozoic–Cenozoic times. Garnet peridotitesfrom the Sulu UHP terrane have less magnesian olivine (Fo_91.5),and lower-Cr garnet than the Paleozoic xenoliths. The diopsideshave low heavy REE (HREE) contents and sinusoidal to light REE(LREE)-enriched REE patterns. These features, and their highMg/Si and low CaO and Al₂O₃ contents, indicate that the CCSD-PP1peridotites represent a moderately refractory mantle protolith.Details of mineral chemistry indicate that this protolith experiencedcomplex metasomatism by asthenosphere-derived melts or fluidsin Mesoproterozoic, and subsolidus re-equilibration involvingfluids/melts derived from the subducted Yangtze continentalcrust during UHP metamorphism in the early Mesozoic. Tectonicextension of the subcontinental lithospheric mantle of the NorthChina Craton and exhumation of the Sulu UHP rocks in the earlyMesozoic induced upwelling of the asthenosphere. Peridotitessampled by the Neogene basalts represent newly formed lithospherederived by cooling of the upwelling asthenospheric mantle inJurassic–Cretaceous and Paleogene time. KEY WORDS: garnet peridotite xenoliths; North China Craton; lithospheric thinning; Sulu UHP terrane; UHP lithosphere evolution; mantle replacement     

Petrogenesis of spinel lherzolite and pyroxenite suite xenoliths from the Koolau shield,Oahu, Hawaii: Implications for petrology of the post-eruptive lithosphere beneath Oahu

Three major types of xenoliths, namely, dunite, spinel lherzolite, and pyroxenite suites, occur. The spinel lherzolite suite [ol: Fo_86–92] is more refractory than the pyroxenite suite [Fo_71–85], and is composed of olivine, orthopyroxene, Cr-diopside, and spinel. Spinel lherzolites represent metasomatically modified mantle residues that constitute the lithosphere underneath Oahu. Metasomatism has induced significant heterogeneity in terms of [Na]^cpx in the spinel lherzolitic lithosphere: compared to other vents, Salt Lake xenoliths are anomalously high in [Na]^cpx. The fluids responsible for such a process may have been released after crystallization of the hydrous phases in pyroxenite suite veins intrusive into the spinel lherzolites.The pyroxenite suite rocks range from clinopyroxenites, wehrlites, websterites, to lherzolites and a rare dunite. Garnet generally occurs as a secondary phase forming reaction rims around spinel or exsolved blebs in clinopyroxene. Phlogopite and amphibole are common. The garnet-bearing pyroxenite suite rocks last equilibrated in the mantle at 1000°–1150° C and 16–25 kb (50–75 kms depth). Similar temperature range is recorded by the spinel lherzolite suite and rare plagioclase lherzolites. This P-T path is significantly hotter than a calculated conductive geotherm indicating that the lithosphere was substantially warmed up by passing Hawaiian magmas.Contribution No. 585, Geosciences Program, University of Texas at Dallas     

Sm-Nd,K-Ar and petrologic study of some kimberlites from eastern United States and their implication for mantle evolution

We provide new data on Sm-Nd systematics, K-Ar dating and the major element chemistry of kimberlites from the eastern United States (mostly from central New York State) and their constituent mineral phases of olivine, clinopyroxene, garnet, phlogopite and perovskite. In addition, we report Nd-isotopes in a few kimberlites from South Africa, Lesotho and from the eastern part of China. The major element compositions of the New York dike rocks and of their constituent minerals including a xenolith of eclogite are comparable with those from the Kimberley area in South Africa. The K-Ar age of emplacement of the New York dikes is further established to be 143 Ma.We have analyzed the Nd-isotopic composition of the following kimberlites and related rocks: Nine kimberlite pipes from South Africa and Lesotho, two from southern India; one from the U.S.S.R., fifteen kimberlite pipes and related dike rocks from eastern and central U.S. and two pipes from the Shandong Province of eastern China. The age of emplacement of these kimberlites ranges from 1300 million years to 90 million years. The initial Nd-isotopic compositions of these kimberlitic rocks expressed as _Nd ^I with respect to a chondritic bulk-earth growth-curve show a range between 0 and +4, with the majority of the kimberlites being in the range 0 to +2. This range is not matched by any other suite of mantle-derived igneous rocks. This result strengthens our earlier conclusion that kimberlitic liquids are derived from a relatively primeval and unique mantle reservoir with a nearly chondritic Sm/Nd ratio.     

The origin of garnet and clinopyroxene in “depleted” Kaapvaal peridotites

A detailed petrographic, major and trace element and isotope (Re–Os) study is presented on 18 xenoliths from Northern Lesotho kimberlites. The samples represent typical coarse, low-temperature garnet and spinel peridotites and span a P–T range from 60 to 150 km depth. With the exception of one sample (that belongs to the ilmenite–rutile–phlogopite–sulphide suite (IRPS) suite first described by [B. Harte, P.A. Winterburn, J.J. Gurney, Metasomatic and enrichment phenomena in garnet peridotite facies mantle xenoliths from the Matsoku kimberlite pipe, Lesotho. In: Menzies, M. (Ed.), Mantle metsasomatism. Academic Press, London 1987, 145–220.]), all samples considered here have high Mg# and show strong depletion in CaO and Al₂O₃. They have bulk rock Re depletion ages (T_RD) >2.5 Ga and are therefore interpreted as residua from large volume melting in the Archaean. A characteristic of Kaapvaal xenoliths, however, is their high SiO₂ concentrations, and hence, modal orthopyroxene contents that are inconsistent with a simple residual origin of these samples. Moreover, trace element signatures show strong overall incompatible element enrichment and REE disequilibrium between garnet and clinopyroxene. Textural and subtle major element disequilibria were also observed. We therefore conclude that garnet and clinopyroxene are not co-genetic and suggest that (most) clinopyroxene in the Archaean Kaapvaal peridotite xenoliths is of metasomatic origin and crystallized relatively recently, possibly from a melt precursory to the kimberlite.

Possible explanations for the origin of garnet are exsolution from a high-temperature, Al- and Ca-rich orthopyroxene (indicating primary melt extraction at shallow levels) or a majorite phase (primary melting at >6 GPa). Mass balance calculations, however, show that not all garnet observed in the samples today is of a simple exsolution origin. The extreme LREE enrichment (sigmoidal REE pattern in all garnet cores) is also inconsistent with exsolution from a residual orthopyroxene. Therefore, extensive metasomatism and probably re-crystallization of the lithosphere after melt-depletion and garnet exsolution is required to obtain the present textural and compositional features of the xenoliths. The metasomatic agent that modified or perhaps even precipitated garnet was a highly fractionated melt or fluid that might have been derived from the asthenosphere or from recycled oceanic crust. Since, to date, partitioning of trace elements between orthopyroxene and garnet/clinopyroxene is poorly constrained, it was impossible to assess if orthopyroxene is in chemical equilibrium with garnet or clinopyroxene. Therefore, further trace element and isotopic studies are required to constrain the timing of garnet introduction/modification and its possible link with the SiO₂ enrichment of the Kaapvaal lithosphere.     

An Internally Consistent Thermodynamic Model for the System CaO-MgO-Al2O3-SiO2 Derived Primarily from Phase Equilibrium Data

An internally consistent thermodynamic model for the subsolidus system CaO-MgO-Al2O3-SiO2 (CMAS) was developed and refined using primarily data from phase equilibrium experiments. The solution properties of pyroxenes and garnet were approximated with an ionic model, with independent mixing on adjacent crystallographic sites. This approach simplified the calculation of phase relations by allowing sequential calculation of the site occupancies. Enthalpy, entropy, and volume differences, nominally at 970 K, were derived for all participating phases by matching as closely as possible the experimentally observed phase relations. Although thermochemical measurements were not used directly in the refinement, the results were continuously monitored and compared with the thermochemical data to achieve a close match. The new model can be used to calculate phase diagrams for the CMAS system and its subsystems in the whole pressure range of the upper mantle. Simple empirical corrections for the effects of Na, Fe, Cr, etc., could potentially be introduced to make the model applicable to the thermobarometry of chemically complex mantle materials. Application of the new model to garnet lherzolite xenoliths from northern Lesotho and garnet peridotites from Norway supports the proposals for higher temperatures of the continental lithosphere.     

Upper-Mantle Fragments from Beneath the Sierra Nevada Batholith: Partial Fusion, Fractional Crystallization, and Metasomatism in a Subduction Related Ancient Lithosphere

Upper-mantle xenoliths in volcanic pipes cutting the axis ofthe Sierra Nevada batholith contain predominantly spinel-bearingperidotites (with sporadic garnet) and garnet websterites. Inspite of the enormous thickness of the Sierran crust, the Sierranupper mantle has not attained the garnet peridotite stabilityfield. The peridotites have forsteritic (Fo_88–92) olivines,Cr-diopsides, Cr-spinels, and magnesian orthopyroxenes (En_88–92).Their texture and compositional characteristics of the coexistingphases indicate that these are fragments of the upper mantlethat had undergone various degrees of partial fusion. The Pconditions of reequilibration and mineralogical characteristicssuggest that the partial fusion was accompanied by diapiricuprise. The REE distribution patterns are nearly chondritic.Garnet websterite xenoliths also contain magnesian and Cr-richphases. Their bulk chemical compositions are like pyroxenitecumulates. The garnet websterites from Big Creek differ fromthose occurring at Pick and Shovel in having more Fe-rich phasesand occasional hydrous minerals. The Pick and Shovel garnetwebsterites are interpreted to be pyroxene-rich, garnet-freecumulates formed by fractional crystallization of melts generatedby partial melting of subcontinental lithosphere at depth 60km. The REE abundance of these xenoliths is consistent withthis mode of origin. Presence of jadeitic clinopyroxenes andF-rich phlogopites, and the LREE- and ⁸⁷Sr/⁸⁶Sr-enriched characterof the garnet websterites from Big Creek may suggest their originas metasomatized upper-mantle garnet peridotites. The latestP-T conditions of equilibration of all garnet-bearing samplesshow that they lie along a nearly adiabatic gradient in therange of 900–1000 C and 18–32 kbar. An isotopically heterogeneous, old (1 b.y.) subcontinental lithosphere,characterized by high ⁸⁷Sr/⁸⁶Sr (0.7044–0.7082), radiogenic²⁰⁶Pb/²⁰⁴Pb (18.86–20.04), ²⁰⁷Pb/²⁰⁴Pb (15.64–15.69)and ²⁰⁸Pb/²⁰⁴Pb (38.69–39.11), and moderate ¹⁴³Nd/¹⁴⁴Nd(0.51234–0.51260; E_Nd–0.35 to –5.8) is consideredto be the source of these rocks. There was a fluid influx froma subducted slab carrying Ba, K, Rb, U, Th, and radiogenic Pbinto the overlying ancient lithosphere.