Spinel Lherzolite Research Articles

맨틀상부에서 암석의 변형 및 광물의 격자선호방향(LPO) 형성과 지진파 비등방성과의 연계성

감람석은 맨틀 상부의 주요 광물이고, 탄성적으로 매우 비균질하다. 맨틀암석이 고압고온에서 변형될 때, 이러한 감람석이 배열되어 격자선호방향(LPO, Lattice Preferred Orientation)이 형성된다. 감람석의 격자선호방향은 압력, 응력, 그리고 감람석 내부의 물(OH)의 양에 따라 크게 영향을 받을 수 있다. 이 논문에서는 물, 응력, 그리고 압력이 어떻게 감람석의 격자선호방향(LPO)의 형성에 영향을 주는지에 대해 그동안 발표된 논문들을 리뷰했고, 자연 맨틀암석에서 발견된 감람석의 격자선호방향들을 요약했으며, 감람석의 격자선호방향과 맨틀상부에서 나타나는 지진파의 비등방성과의 연계성에 대해 토의하였다. 그리고 최근에 맨틀암석에서 발견된 사방휘석의 네 가지 격자선호방향들을 기술하였다. Olivine is a dominant mineral in the upper mantle and is elastically very anisotropic. When olivine is deformed under stress at high pressure and high temperature, lattice preferred orientation (LPO) is formed. It is known that the LPO of olivine is affected by water, stress, and pressure. In this paper, I reviewed the papers dealing with the effects of water, stress, and pressure on the LPO of olivine, summarized the papers on the LPOs of olivine in natural mantle rocks, and discussed its implications for seismic anisotropy in the upper mantle. In addition, I also described four types of LPOs of orthopyroxene recently found in natural spinel lherzolite.

Fluorine in nominally fluorine-free mantle minerals: Experimental partitioning of F between olivine, orthopyroxene and silicate melts with implications for magmatic processes

We present new experimentally determined partition coefficients for fluorine (F) between nominally fluorine-free minerals and corresponding basaltic melts in the systems CaO–MgO–Al2O3–SiO2+F, Na2O–CaO–MgO–Al2O3–SiO2+F, and in natural compositions at pressures up to 2.5GPa and temperatures between 1285°C and 1445°C. Fluorine is incompatible in olivine with F partition coefficients DFol/melt varying between 0.0007 and 0.0033, whereas the F in orthopyroxene is more compatible with DFopx/melt between 0.031 and 0.037. These findings agree well with new analyses of F in natural olivines and orthopyroxene from spinel peridotites and oceanic basalts, and indicate that F is less incompatible in olivine and orthopyroxene than hydrogen (DF/DH∼3.3 for olivine, DF/DH∼2.4 for orthopyroxene). Based on high-resolution TEM images of one of the samples, we argue that the F incorporation into the olivine structure is mass balanced via oxygen defects. Furthermore, we present some analyses of Cl in olivine and orthopyroxene from spinel peridotites and basalts.Hence, fluorine, and to a lesser extend chlorine, may be effectively stored in nominally halogen-free mantle minerals such as olivine and orthopyroxene. Considering their high modal proportion in the upper mantle, both phases are the major hosts for F in the Earth's mantle, and must be taken into account when calculating the Earth's budget of halogens or global cycles of halogens in the deeper Earth. Other F-bearing minerals such as amphibole or phosphates are not needed to balance the F concentrations between crust and mantle.Applying our new partition coefficients to primitive olivine hosted melt inclusion data from mid-ocean ridge basalt (MORB), we calculate a concentration of 6–15μg/g F for the depleted MORB mantle source. Our results also show that typical spinel lherzolites from the subcontinental lithosphere show virtually identical concentrations of about 12μg/g. However, using the partition coefficients on primary melt inclusions from ocean island basalts (OIB), suggest slightly higher fluorine contents between 8 and 31μg/g for OIB mantle sources.

Deciphering the Trace Element Characteristics in Kilbourne Hole Peridotite Xenoliths: Melt–Rock Interaction and Metasomatism beneath the Rio Grande Rift, SW USA

Significant differences between bulk-rock lithophile trace element budgets and the sum of the contributions from the constituent minerals are common, if not ubiquitous, in peridotite xenoliths. Notwithstanding the possible contributions from fluid inclusions and grain-boundary glass and micro-phases, it is often difficult to reconcile the bulk-rock incompatible element budgets with those of the silicate phases. In the absence of modal metasomatism this discrepancy is often attributed to the ‘catch-all’yet often vague process of cryptic metasomatism.This study presents comprehensive petrological descriptions, major and trace element and Sr^Nd isotope data for variably metasomatized bulk-rock peridotites, host basalt, and constituent peridotite mineral phases from spinel lherzolite and harzburgite xenoliths from the Kilbourne Hole volcanic maar, New Mexico, USA. Similar measurements were also made on hand-picked interstitial glass from one of the most highly metasomatized samples in an attempt to unravel the sources, effects, and relative timings of multiple metasomatic events. Reaction textures around clinopyroxene grains are evident in the most metasomatized samples. These are accompanied by films of high-SiO2 interstitial glass, which is not in equilibrium with the primary silicate minerals. Trails of glassy melt inclusions terminate in these films against which the margins of the primary minerals appear partially resorbed. Furthermore, different styles of high field strength element fractionation [e.g. (Nb/Ta)N vs (Zr/Hf)N] are evident in the bulk-rocks and the clinopyroxenes that they host. In all of the Kilbourne Hole peridotites analysed, hand-picked, optically clean clinopyroxenes preserve a more unradiogenic Sr isotope signature than the corresponding bulk-rock. Hand-picked interstitial glass from KH03-16 reveals the most radiogenic Sr/Sr of any component recovered from these xenoliths (Sr/Sr1⁄4 0·708043 0·00009; [Sr]1⁄4 81ppm). Similarly, the Nd/Nd of the glass is amongst the most evolved of the peridotite components (Nd/Nd1⁄4 0·512893 0·000012; [Nd]1⁄4 10 ppm). However, the host basalt (Sr/Sr1⁄4 0·703953 0·00012; Nd/Nd1⁄4 0·512873 0·000013), similar in composition to that of the nearby, contemporaneous, Potrillo Volcanic Field basalts, contains nearly an order of magnitude more Sr and more than three times more Nd ([Sr]1⁄4 655 ppm; [Nd]1⁄4 34 ppm) than the interstitial glass. Despite the high Sr and Nd contents of the host basalt the evidence for basalt infiltration is scant, although the effects of melt^rock interaction, both in antiquity and more recently, are preserved in several xenoliths. Mixing between clinopyroxene and the host basalt cannot account for the full range of bulk-rock Sr^Nd isotope ratios; nearly half of the xenoliths require an additional component that could involve varying amounts of interstitial glass. In detail the behaviour of Sr and Nd isotopes has been decoupled,

Structure and composition of the subcontinental lithospheric mantle beneath the Sangilen Plateau (Tuva, southern Siberia, Russia): Evidence from lamprophyre-hosted spinel peridotite xenoliths

We present new data on spinel peridotite xenoliths hosted in Agardag alkaline lamprophyres from the Sangilen Plateau (Tuva, South Siberia, Russia), sampling at ~450Ma the subcontinental lithospheric mantle of the Tuva-Mongolian micro-continent that belongs to the accretionary Central Asian orogenic belt at the southern edge of the Siberian craton. Xenoliths are spinel lherzolites principally showing poikilitic and subordinately coarse granular and coarse equigranular textures. Geothermobarometric calculations for pyroxene yield a narrow range of equilibration temperature (ca. 1000–1100°C) that corresponds to lithospheric depths from 43 to 53km (1.3–1.6GPa) along a hot intracontinental geotherm. Variation of mean Mg# [100*Mg/(Mg+Fe)] of olivine (87.9–90.9) with mean Cr# [100*Cr/(Cr+Al)] of spinel (9.5–45.7) indicates that spinel lherzolites are mostly residues of up to 10% melting of a fertile peridotite source. In terms of normalized REE (Rare Earth Element) and incompatible trace element patterns of clinopyroxene, the Sangilen xenoliths can be classified into three types: Type I characterized by convex-upward REE patterns depleted in LREE (0.10≤La/YbN≤0.49), and with relative negative anomalies of Rb, Pb, Hf, Zr and Ti and positive spikes of U and Sr; Type II displaying variable LREE/HREE ratios (0.53<La/YbN<2.17) but generally flatter REE patterns) and similar abundances of other trace elements compared to Type I; and Type III showing a LREE enriched pattern [(La/Sm)N=2.22; (La/Yb)N=8.42], high REE contents and no relative anomalies of U and Sr. The elevated YbN concentration of one Type II clinopyroxene and the variable fractionation of LREE-MREE relative to HREE in most xenolith types indicate Sangilen xenoliths underwent variable metasomatic enrichment. This enrichment is well accounted by percolation‐reaction between depleted peridotite and small-melt fractions of alkaline mafic melts precursor to the Agardag alkaline lamprophyres. The lack of correlation with depth of modal variations, textural types, inferred degrees of melting and trace element patterns in xenoliths indicates the absence of a texturally or compositionally layered lithospheric mantle sampled by Ordovician lamprophyres beneath the Sangilen plateau. The observed compositional variations are better accounted by depleted lithosphere variably metasomatized along a network of percolating alkaline mafic melts heterogeneously distributed throughout the Sangilen lithospheric mantle section.

A thermodynamic model for silicate melt in CaO–MgO–Al2O3–SiO2 to 50 kbar and 1800 °C

AbstractA fully thermodynamic model for mafic melt in CaO–MgO–Al2O3–SiO2 (CMAS) has been calibrated, for calculation of melting equilibria in the pressure range 0–50 kbar. It is intended as a preliminary step towards a large‐system melt model, suitable for exploring melting, melt loss and crystallization processes in a wide range of natural rock compositions. Calibration was performed with attention to the model's behaviour in its compositional subsystems, as a rigorous test of model structure and parameterization. The model is consistent with the latest Holland &amp; Powell thermodynamic data set, and can therefore be used to calculate phase relations in conjunction with the many solid‐phase activity–composition models written for the data set. Model calculations successfully reproduce experimental melting reactions in CMAS spinel lherzolite and garnet lherzolite assemblages, as well as sapphirine‐ and kyanite‐bearing assemblages, at moderate to high pressure. Thermodynamically sensitive features, such as thermal divides are also recovered. However, some changes to the model structure will be required before the model can describe the full range of mafic and ultramafic melt compositions known from experiment at low pressures.

Carbonate-rich melt infiltration in peridotite xenoliths from the Eurasian–North American modern plate boundary (Chersky Range, Yakutia)

A suite of mainly spinel peridotite and subordinate pyroxenite xenoliths and megacrysts were studied in detail, enabling us to characterize upper mantle conditions and processes beneath the modern North American–Eurasian continental plate boundary. The samples were collected from 37-Ma-old basanites cropping out in the Main Collision Belt of the Chersky Range, Yakutia Republic (Russian Far East). The spinel lherzolites reflect a mantle sequence, equilibrated at temperatures of 890–1,025 °C at pressures of 1.1–2 GPa, with melt extraction estimated to be around 2–6 %. The spinel harzburgites are characterized by lower P–T equilibration conditions and estimated melt extraction up to 12 %. Minor cryptic metasomatic processes are recorded in the clinopyroxene trace elements, revealing that percolating hydrous fluid-rich melts and basaltic melts affected the peridotites. One of the lherzolites preserves a unique melt droplet with primary dolomite in perfect phase contact with Na-rich aluminosilicate glass and sodalite. On the basis of the well-constrained P–T frame of the xenolith suite, as well as the rigorously documented melt extraction and metasomatic history of this upper mantle section, we discuss how a carbonated silicate melt infiltrated the lherzolite at depth and differentiated into an immiscible carbonate and silicate liquid shortly before the xenolith was transported to the surface by the host basalt. Decreasing temperatures triggered crystallization of primary dolomite from the carbonate melt fraction and sodalite as well as quenched glass from the Na-rich aluminosilicate melt fraction. Rapid entrainment and transport to the Earth’s surface prevented decarbonatization processes as well as reaction phenomena with the host lherzolite, preserving this exceptional snapshot of upper mantle carbonatization and liquid immiscibility.

Petrogenesis of fertile mantle peridotites from the Monte del Estado massif (southwest Puerto Rico): a preserved section of Proto-Caribbean lithospheric mantle?

The Monte del Estado massif is the largest and northernmost serpentinized peridotite belt in southwest Puerto Rico. It is mainly composed of spinel lherzolite and minor harzburgite with variable clinopyroxene modal abundances. Mineral and whole rock major and trace element compositions of peridotites coincide with those of fertile abyssal mantle rocks from mid ocean ridges. Peridotites lost 2-14 wt% of relative MgO and variable amounts of CaO by serpentinization and seafloor weathering. HREE contents in whole rock indicate that the Monte del Estado peridotites are residues after low to moderate degrees (2-15%) of fractional partial melting in the spinel stability field. However, very low LREE/HREE and MREE/HREE in clinopyroxene cannot be explained by melting models of a spinel lherzolite source and support that the Monte del Estado peridotites experienced initial low fractional melting degrees (~ 4%) in the garnet stability field. The relative enrichment of LREE in whole rock is not due to alteration processes but probably reflects the capture of percolating fluid/melt fractions or the crystallization of sub-percent amounts of hydrous minerals (e.g., amphibole, phlogopite) along grain boundaries or as microinclusions in minerals. We propose that the Monte del Estado peridotite belt represents a section of ancient Proto-Caribbean (Atlantic) lithospheric mantle originated by seafloor spreading between North and South America in the Late Jurassic- Early Cretaceous. This portion of oceanic lithospheric mantle was subsequently trapped in the forearc region of the Greater Antilles paleo-island arc generated by the northward subduction of the Caribbean plate beneath the Proto-Caribbean ocean. Finally, the Monte del Estado peridotites belt was emplaced in the Early Cretaceous probably as result of the change in subduction polarity of the Greater Antilles paleo-island arc without having been significantly modified by subduction processes

The Xinchang peridotite xenoliths reveal mantle replacement and accretion in southeastern China

The Xinchang mantle xenoliths, including garnet lherzolites, spinel lherzolites and harzburgites, have been studied for the nature and age of the lithospheric mantle beneath southeastern (SE) China. The spinel harzburgites have refractory compositions, with 0.95–1.73wt.% Al2O3, 0.65–1.47wt.% CaO, 0.03–0.16wt.% TiO2 and 2134–2422ppm Ni. The lherzolites (both spinel- and garnet-) have more fertile compositions, containing 2.4–5.43wt.% Al2O3, 2.17–3.22wt.% CaO, 0.1–0.38wt.% TiO2 and 1733–2055ppm Ni. Olivines in the harzburgites have Fo contents of 88.7–91.4, which is 88.4–90.2 for the lherzolites. Both clinopyroxene and orthopyroxene have lower Al2O3 but higher Cr2O3 contents than those in the lherzolites. Modeling of the Y and Yb contents in clinopyroxenes indicates that the spinel harzburgites have been subjected to ca. 20% degrees of partial melting. Reaction textures, in particular the sieve-texture in clinopyroxene, is widely developed in the Xinchang mantle xenoliths, which resulted from reaction with the host lavas. The sieve-textured clinopyroxenes show compositional zonation; the sieve-textured rims have lower Na2O and Al2O3 but higher Cr2O3 and CaO than the inclusion-free cores. The inclusion-free cores of clinopyroxene in the harzburgites show clearly enriched characteristics in trace elements, reflecting they have been metasomatized before the entrainment in the host lavas. Clinopyroxenes have high Ti/Eu but low La/Ya ratios, suggesting that the Xinchang mantle xenoliths were metasomatized by silicate melts. The spinel harzburgites have 187Os/188Os of 0.11999–0.12258, giving TRD age of 0.99–1.35Ga relative to the primitive upper mantle (PUM). In comparison, both spinel and garnet lherzolites have more radiogenic 187Os/188Os ratios (0.12424–0.12801) and younger model ages (0.22–0.75Ga). We explain that the spinel harzburgites represent the ancient lithosphere relic beneath the Xinchang region, whereas both spinel and garnet lherzolites represent the newly accreted mantle. This reflects that the lower portion of the Proterozoic mantle has been removed and replaced by younger, hotter and juvenile mantle that was accreted from the asthenosphere. The harzburgites have high equilibrium temperatures of 1112–1174°C, which are indistinguishable from those of the garnet lherzolites (1108–1198°C). This suggests that the lithosphere relic has been heated by the newly accreted asthenosphere. The mantle replacement and accretion is a common process in SE China and could be related to the lithospheric extension since the Mesozoic.

Coupling, decoupling and metasomatism: Evolution of crust–mantle relationships beneath NW Spitsbergen

The Bockfjord area of NW Spitsbergen (Norwegian Arctic) exposes a long history of crustal evolution, culminating in the Caledonian (400–500Ma) orogeny; prior to the opening of the N. Atlantic Ocean, this area was part of the Laurentian (Greenland) side of the orogen. The N-striking Breibogen–Bockfjorden (BB) fault marks the western margin of a large graben filled with Devonian redbeds. West of the fault the basement consists of gneisses, schists and granites of the Hekla Hoek formation, inferred to represent a Caldeonian thrust sheet. U–Pb and Hf-isotope data for detrital zircons from this area show that the Hekla Hoek protoliths formed at ca 1.8Ga, but were heavily reworked ca 800–1000Ma ago, and again during the Caledonian orogeny.Quaternary alkali-basalt volcanism has provided abundant xenoliths of mantle and crustal rocks from both sides of the BB fault. Lower-crustal xenoliths are mainly mafic to intermediate granulites. Whole-rock Sr-, Nd- and Hf-isotope data for the granulites from both sides of the BB fault show significant disequilibrium, implying the removal of melts late in the evolution of the lower crust. Most zircons from eight xenoliths have Neoarchean/Paleoproterozoic and Paleozoic U–Pb ages; several also contain zircons with ages and/or Hf model ages >3.2Ga.The peridotite xenoliths are dominantly spinel lherzolites, metasomatized to varying extents. Xenoliths from basalts east of the BB fault commonly contain metasomatic amphibole, phlogopite and apatite; peridotites from west of the fault rarely display these phases. West of the fault, there is no clear correlation between cpx REE patterns and whole-rock Al contents; east of the fault there is a clear negative correlation between LREE enrichment and whole-rock Al2O3. In-situ Re–Os isotope analysis of sulfides in the peridotites shows another dichotomy. Xenoliths from west of the fault contain sulfides with Re depletion (TRD) model ages extending back to 3.3Ga, with major populations at 2.4–2.6Ga, 1.6–1.8Ga and 1.2–1.3Ga; the Caledonian orogeny is weakly represented. East of the BB fault the peaks in the TRD spectrum are at 900–1100Ma and 400–500Ma; only a few grains have TMA>2.5Ga.The data demonstrate a major disjunct, on both sides of the BB fault, between the Archean lower crust and a Proterozoic–Paleozoic upper crust; this suggests that the original Archean upper (and middle?) crust was detached and replaced by thrust sheets of younger material during the Caledonian orogeny. The striking differences in the sub-continental lithospheric mantle (SCLM) on either side of the BB fault suggest that major transcurrent movement has juxtaposed lithospheric sections that evolved independently. West of the fault, the presence of Archean lower crust overlying Archean SCLM suggests coupling of the crust and mantle for ≥3Ga. East of the BB, similar Archean lower crust overlies an apparently younger, more fertile SCLM. We suggest that pervasive melt-related metasomatism refertilized the SCLM east of the BB fault, largely obscuring its Archean origin. Typical Archean SCLM is 150–250km thick, whereas the Bockfjord volcanoes carry no samples from deeper than 80km. The detached lower part of the Archean SCLM may now lie beneath the Gakkel Ridge to the NW, as proposed by Goldstein et al. (2008) on the basis of basalt geochemistry. This proposition is supported by the mean TRD of the Sverrefjell sulfides (1740±718Ma), which is similar to the oldest whole-rock TRD values for peridotites from the Gakkel Ridge (1.8–2.2Ga; Liu et al., 2010).

The Pelona-Pico Duarte basalts Formation, Central Hispaniola: an on-land section of Late Cretaceous volcanism related to the Caribbean large igneous province

Located in Central Hispaniola, the Pelona-Pico Duarte basalts Formation (Fm.) offers an opportunity to study the Late Cretaceous Caribbean large igneous province magmatism on land. It is composed by a ~2.5km-thick pile of massive and monotonous submarine flows of basalts, locally intruded by synvolcanic dikes and sills of dolerite. The Pelona-Pico Duarte basalts Fm. was emplaced onto Turonian-Lower Campanian island-arc volcanic and sedimentary sequences, and is overlain by Maastrichtian platformal carbonates. Two 40Ar/39Ar plateau ages indicate both extrusive and intrusive magmatic activity at least during the 79-68Ma interval (Middle Campanian to Maastrichtian), so the magmas were in part coeval with the late phases of the Caribbean large igneous province. The basalts have a restricted major-and trace-element, and isotopic, compositional variation. For a range of 47.6- 50.2wt.% SiO2, the Pelona-Pico Duarte basalts Fm. has relatively high contents in TiO2 (1.5-3.6wt.%) and Fe2O3T (10.7-13.1wt.%). On the basis of MgO contents, samples can be classified into tholeiitic basalts ( 20 and Zr/Nb<10) are characteristic of transitional and alkalic oceanic-island basalts. In terms of Sr-Nd isotopic composition, the samples are homogeneous and enriched relative to older Caribbean large igneous province units in Hispaniola, with (87Sr/86Sr)i ratios between 0.70330 and 0.70348 for a very restricted range of (eNd)i values between +5.0 and +5.9 (where i=70Ma). The Pelona-Pico Duarte basalts Fm. are interpreted as partial melts of a plume-related, deep enriched source, which have not been contaminated by active subduction. Mantle melt modelling indicates that both high-Mg basalts and basalts formed by mixed melts of both garnet and spinel lherzolite in variable amounts. Melts incorporated at different mantle depths, most probably in relation to the melt column processes in an upwelling plume. The Pelona-Pico Duarte basalts Fm. has significantly different values of petrogenetic tracers compared to underlying arc-related lavas, indicating a fundamental change in the mantle sources. It has geochemical affinities with the mantle domain influenced by the Late Cretaceous Caribbean plume, suggesting that enriched mantle was flowing toward the NE, to the mantle wedge region of the Caribbean island-arc, in response to rollback of the SWdirected subduction of the proto-Caribbean slab.

Electrical conductivity of partial molten carbonate peridotite

In order to investigate the effect of carbonate-content in partial melt on bulk conductivity under high pressure, electrical conductivity measurements were performed on carbonate melt-bearing peridotites using a Kawai-type multi-anvil apparatus. The starting materials were composed of spinel lherzolite (KLB1) with small amounts of dolomite (1 and 3wt.%). To obtain various melt fractions, annealing experiments were performed at different temperatures above 1400K at 3GPa. At low temperatures (⩽1500K), the conductivity was distinctly higher than that of carbonate-free peridotite and close to that of the carbonatite melt-bearing olivine aggregates. Although the sample conductivity increased with increasing temperature, the rate at which the conductivity increases was small and the conductivity approached that of silicate melt-bearing peridotite. CO2 concentration in the partial melt decreased with increasing annealing temperature. Thus, the small increase of the conductivity with annealing temperature is attributed to a decrease of the melt conductivity due to a decrease in carbonate content in the partial melt. As the carbonate concentration in the melt decreases, the estimated melt conductivity approaches that of the basaltic melt. Therefore, conductivity enhancement by the carbonate-bearing melt is very effective at temperature just above that of the carbonate peridotite solidus.

Seismic properties of peridotite xenoliths as a clue to imaging the lithospheric mantle beneath NE Tasmania, Australia

Peridotite xenoliths from Northeast Tasmania, Australia, consist of spinel lherzolites and minor dunites that originated from the uppermost mantle at depths of less than 40km, based on the geothermal gradient beneath Tasmania, where the depth of the Moho is 29km. The crystal preferred orientations (CPOs) of olivine and pyroxene were measured in 5 of 13 collected samples (1 dunite and 4 lherzolites) that were large enough to enable measurements. Olivine CPOs indicate the dominance of (010) [100] slip. Geothermobarometric analyses of the xenoliths yield temperatures of 950–1050°C and pressures of 0.7–1.1GPa. Using these data, we calculated changes in the seismic properties of a rock with ideal volume fractions of olivine and pyroxene under conditions of 1000°C and 1GPa, for various orientations of the structural reference frame in the case of three geodynamic models (extension, shear, and upwelling). The model of horizontal extension performs best in explaining the structure of the uppermost lithospheric mantle beneath Northeast Tasmania.

A comparative study of melt-rock reactions in the mantle: laboratory dissolution experiments and geological field observations

Systematic variations in mineralogy and chemical composition across dunite-harzburgite (DH) and dunite-harzburgite-lherzolite (DHL) sequences in the mantle sections of ophiolites have been widely observed. The compositional variations are due to melt-rock reactions as basaltic melts travel through mantle peridotite, and may be key attributes to understanding melting and melt transport processes in the mantle. In order to better understand melt-rock reactions in the mantle, we conducted laboratory dissolution experiments by juxtaposing a spinel lherzolite against an alkali basalt or a mid-ocean ridge basalt. The charges were run at 1 GPa and either 1,300°C or 1,320°C for 8–28 h. Afterward, the charges were slowly cooled to 1,200°C and 1 GPa, which was maintained for at least 24 h to promote in situ crystallization of interstitial melts. Cooling allowed for better characterization of the mineralogy and mineral compositional trends produced and observed from melt-rock reactions. Dissolution of lherzolite in basaltic melts with cooling results in a clinopyroxene-bearing DHL sequence, in contrast to sequences observed in previously reported isothermal-isobaric dissolution experiments, but similar to those observed in the mantle sections of ophiolites. Compositional variations in minerals in the experimental charges follow similar melt-rock trends suggested by the field observations, including traverses across DH and DHL sequences from mantle sections of ophiolites as well as clinopyroxene and olivine from clinopyroxenite, dunite, and wehrlite dikes and xenoliths. These chemical variations are controlled by the composition of reacting melt, mineralogy and composition of host peridotite, and grain-scale processes that occur at various stages of melt-peridotite reaction. We suggest that laboratory dissolution experiments are a robust model to natural melt-rock reaction processes and that clinopyroxene in replacive dunites in the mantle sections of ophiolites is genetically linked to clinopyroxene in cumulate dunite and pyroxenites through melt transport and melt-rock reaction processes in the mantle.

Distribution of REE between clinopyroxene and basaltic melt along a mantle adiabat: effects of major element composition, water, and temperature

The distribution of rare earth elements (REE) between clinopyroxene (cpx) and basaltic melt is important in deciphering the processes of mantle melting. REE and Y partition coefficients from a given cpx-melt partitioning experiment can be quantitatively described by the lattice strain model. We analyzed published REE and Y partitioning data between cpx and basaltic melts using the nonlinear regression method and parameterized key partitioning parameters in the lattice strain model (D 0, r 0 and E) as functions of pressure, temperature, and compositions of cpx and melt. D 0 is found to positively correlate with Al in tetrahedral site (Al T ) and Mg in the M2 site (MgM2) of cpx and negatively correlate with temperature and water content in the melt. r 0 is negatively correlated with Al in M1 site (AlM1) and MgM2 in cpx. And E is positively correlated with r 0. During adiabatic melting of spinel lherzolite, temperature, Al T , and MgM2 in cpx all decrease systematically as a function of pressure or degree of melting. The competing effects between temperature and cpx composition result in very small variations in REE partition coefficients along a mantle adiabat. A higher potential temperature (1,400°C) gives rise to REE partition coefficients slightly lower than those at a lower potential temperature (1,300°C) because the temperature effect overwhelms the compositional effect. A set of constant REE partition coefficients therefore may be used to accurately model REE fractionation during partial melting of spinel lherzolite along a mantle adiabat. As cpx has low Al and Mg abundances at high temperature during melting in the garnet stability field, REE are more incompatible in cpx. Heavy REE depletion in the melt may imply deep melting of a hydrous garnet lherzolite. Water-dependent cpx partition coefficients need to be considered for modeling low-degree hydrous melting.

Seismic anisotropy of the uppermost mantle beneath the Rio Grande rift: Evidence from Kilbourne Hole peridotite xenoliths, New Mexico

Peridotite xenoliths from the Kilbourne Hole maar, New Mexico, consist of spinel lherzolite, harzburgite, and dunite. Because Kilbourne Hole erupted at approximately 10 ka, these xenoliths represent essentially current conditions beneath the Rio Grande rift. In this study, we present detailed petrofabric data and seismic properties obtained from peridotite xenoliths from Kilbourne Hole to illuminate the origin and significance of shear-wave splitting in the uppermost mantle beneath this active rift. Using phase relations and the temperature of equilibration, we infer that these xenoliths were derived from the uppermost mantle, from depths of 35–60 km. Their crystallographic preferred orientations indicate the preservation of olivine b-axis fiber fabrics with a strong concentration of [010] with girdles of [100] and [001]. We consider three geodynamic models for the source region of these xenoliths: horizontal extension, lateral shear, and upwelling. After calculating seismic properties using a volume fraction of olivine, orthopyroxene and clinopyroxene appropriate to each model, we conclude that these xenoliths are derived from a lateral shear zone (vertical foliation (XY plane) and horizontal lineation within the plane of the foliation (X-axis)). However, the degree of seismic anisotropy generated by peridotite xenoliths alone is limited, so that the existence of melt in thin cracks or dikes could be required to cause a significant increase; the orientation of such melt pockets parallel to the XY plane in either model would result in an increase in anisotropy. These results indicate that the shear-wave splitting observed in the Rio Grande rift is a reflection of the lithospheric fabric and the presence of melts as thin cracks or dikes.

Ultramafic xenoliths from Lake Nyos area, Cameroon volcanic line, West-central Africa: Petrography, mineral chemistry, equilibration conditions and metasomatic features

Spinel-bearing mantle xenoliths have been recovered in the pyroclastic breccia surrounding the Lake Nyos maar. These include spinel lherzolites, spinel harzburgites and olivine websterites. They exhibit coarse granular or protogranular to weakly porphyroclastic textures, and show variations in mineral chemistry, modal compositions and equilibrium temperature. The xenoliths consist of four mineral phases typical of upper mantle origin: olivine (Fo89–Fo91.5, NiO=0.29–0.38wt%, CaO=0.02–0.17wt%), enstatite (Mg#=90–92, Cr2O3=0.35±0.04wt%), Cr-diopside (Mg#=92–98, Cr2O3=0.7–1.65wt%, TiO2=0.26–0.6wt%) and spinel (high Mg# of 70–80, low TiO2≤0.4wt%). Spinels are aluminous (Cr#=9.7–11) in most lherzolites, and become increasingly chromiferous from websterites (Cr#Sp=15.3–19.8) to harzburgites (Cr#Sp=19–33.6). The lherzolites are composed of olivine (48–58%), orthopyroxene (22–30%), and clinopyroxene (8–15%). The harzburgites modes are olivine (60–81%), orthopyroxene (11–29%), and clinopyroxene (<5%). The websterites are mainly composed of pyroxene (∼62%) with variable amounts of olivine (23–31%). Temperatures of mineral equilibration in the xenoliths have been estimated from the two-pyroxene thermometer of Wells (1977) and range between 850 and 1050°C, corresponding to about 10–30kbar at a depth mantle of 30km at least. These P–T conditions show significant variations between different petrographical types, the maximum conditions being recorded in two spinel lherzolites (NY-05 and NY-23) that have atypical chemical compositions and textures suggesting that they were initially formed in an environment close to the garnet stability field, then re-equilibrated within the spinel stability field prior to their incorporation in the host magma. With the exception of minerals from these two lherzolite nodules, all the minerals exhibit depletion of light REE, a typical feature of abyssal peridotites implying that some xenoliths from the Cameroon volcanic line were probably sampled in a part of the sub-continental mantle that is chemically similar to sub-oceanic mantle. The variations observed in the mineral chemistry and modal compositions of xenoliths suggest that the spinel harzbugite nodules which represent residues of a significant degree of partial melting of lherzolitic mantle were affected by infiltration of alkali-enriched metasomatizing melts (or fluids) within the uppermost mantle to produce pargasitic amphiboles prior to their sampling by the host lava. The features of this metasomatism event occur in the rocks of all three petrographical facies xenoliths from Lake Nyos.

Geochemistry and paleomagnetism of Late Cretaceous mafic dikes in Kerala, southwest coast of India in relation to large igneous provinces and mantle plumes in the Indian Ocean region

Research Article| January 01, 2012 Geochemistry and paleomagnetism of Late Cretaceous mafic dikes in Kerala, southwest coast of India in relation to large igneous provinces and mantle plumes in the Indian Ocean region T. Radhakrishna; T. Radhakrishna † 1Center for Earth Science Studies, Trivandrum 695 031, India †E-mail: tradha1@rediffmail.com Search for other works by this author on: GSW Google Scholar Mathew Joseph Mathew Joseph 2Geological Survey of India, Trivandrum 695 013, India Search for other works by this author on: GSW Google Scholar GSA Bulletin (2012) 124 (1-2): 240–255. https://doi.org/10.1130/B30288.1 Article history received: 05 Mar 2010 rev-recd: 10 Oct 2010 accepted: 14 Oct 2010 first online: 08 Mar 2017 Cite View This Citation Add to Citation Manager Share Icon Share Facebook Twitter LinkedIn MailTo Tools Icon Tools Get Permissions Search Site Citation T. Radhakrishna, Mathew Joseph; Geochemistry and paleomagnetism of Late Cretaceous mafic dikes in Kerala, southwest coast of India in relation to large igneous provinces and mantle plumes in the Indian Ocean region. GSA Bulletin 2012;; 124 (1-2): 240–255. doi: https://doi.org/10.1130/B30288.1 Download citation file: Ris (Zotero) Refmanager EasyBib Bookends Mendeley Papers EndNote RefWorks BibTex toolbar search Search Dropdown Menu toolbar search search input Search input auto suggest filter your search All ContentBy SocietyGSA Bulletin Search Advanced Search Abstract New geochemical and paleomagnetic results are presented on two Late Cretaceous dikes of the 85–90 Ma leucogabbroic and doleritic dikes and the 65–70 Ma dolerites in Kerala, India. The dikes are rich in incompatible elements, have fractionated patterns with light rare-earth element enrichment and are akin geochemically to Cretaceous basalts on the east coast of Madagascar. The magmas were formed at garnet lherzolite depths above the Marion plume, constituting part of a large igneous province in Madagascar. In contrast, the 65–70 Ma dolerites are moderately depleted in incompatible elements, with almost flat, rare-earth element patterns and resemble the upper formations of the Deccan Traps and the tholeiitic dikes of the Seychelles. These dolerites were formed by melting of spinel lherzolite over the Reunion plume. Paleomagnetic data from the dikes and the other coeval igneous units from south India provide the 90 Ma pole (latitude: 24°; longitude: 293°; A95 = 5.9; N = 18 sites) for India. The 65–70 Ma dolerites possess both normal and reverse polarities, and the mean pole (latitude: 36°; longitude: 283°; A95 = 5.7°; N = 10 sites) compares well with the Deccan superpole. Paleolatitude estimates indicate ∼5° southward migration for the Marion plume and a northward migration for the Reunion plume, in conformity with global mantle-circulation models; however, distinguishing migration of the Reunion plume from the effects of true polar wander is difficult. Furthermore, the geodynamic reconstructions extending the shear zones of southern Madagascar into south India are not tenable. You do not have access to this content, please speak to your institutional administrator if you feel you should have access.

Mafic magmas from Mount Baker in the northern Cascade arc, Washington: probes into mantle and crustal processes

Five mafic lava flows located on the southern flank of Mount Baker are among the most primitive in the volcanic field. A comprehensive dataset of whole rock and mineral chemistry reveals the diversity of these mafic lavas that come from distinct sources and have been variably affected by ascent through the crust. Disequilibrium textures present in all of the lavas indicate that crustal processes have affected the magmas. Despite this evidence, mantle source characteristics have been retained and three primitive endmember lava types are represented. These include (1) modified low-K tholeiitic basalt (LKOT-like), (2) typical calc-alkaline (CA) lavas, and (3) high-Mg basaltic andesite and andesite (HMBA and HMA). The Type 1 endmember, the basalt of Park Butte (49.3–50.3 wt% SiO2, Mg# 64–65), has major element chemistry similar to LKOT found elsewhere in the Cascades. Park Butte also has the lowest overall abundances of trace elements (with the exception of the HREE), indicating it is either derived from the most depleted mantle source or has undergone the largest degree of partial melting. The Type 2 endmember is represented by the basalts of Lake Shannon (50.7–52.6 wt% SiO2, Mg# 58–62) and Sulphur Creek (51.2–54.6 wt% SiO2, Mg# 56–57). These two lavas are comparable to calc-alkaline rocks found in arcs worldwide and have similar trace element patterns; however, they differ from each other in abundances of REE, indicating variation in degree of partial melting or fractionation. The Type 3 endmember is represented by the HMBA of Tarn Plateau (51.8–54.0 wt% SiO2, Mg# 68–70) and the HMA of Glacier Creek (58.3–58.7 wt% SiO2, Mg# 63–64). The strongly depleted HREE nature of these Type 3 units and their decreasing Mg# with increasing SiO2 suggests fractionation from a high-Mg basaltic parent derived from a source with residual garnet. Another basaltic andesite unit, Cathedral Crag (52.2–52.6 wt% SiO2, Mg# 55–58), is an Mg-poor differentiate of the Type 3 endmember. The calc-alkaline lavas are least enriched in a subduction component (lowest H2O, Sr/PN, and Ba/Nb), the LKOT-like lavas are intermediate (moderate Sr/PN and Ba/Nb), and the HMBA are most enriched (highest H2O, Sr/PN and Ba/Nb). The generation of the LKOT-like and calc-alkaline lavas can be successfully modeled by partial melting of a spinel lherzolite with variability in composition of slab flux and/or mantle source depletion. The HMBA lavas can be successfully modeled by partial melting of a garnet lherzolite with slab flux compositionally similar to the other lava types, or less likely by partial melting of a spinel lherzolite with a distinctly different, HREE-depleted slab flux.

Alkaline lamprophyric province of Central Iran

AbstractPaleozoic lamprophyres exhibit good exposures in the western part of the Central–East Iranian microcontinent. These rocks crop out as volcanoes, dykes, and plugs. The constituent minerals are amphibole, clinopyroxene, plagioclase, K‐feldspar, olivine, Cr‐spinel, titanite, biotite, and ilmenite. The main textures in volcanic lamprophyres are porphyritic, trachytic, microlithic, and variolitic, whereas in dykes and plugs, intergranular texture is common. These lamprophyres are regionally metamorphosed in some areas. Petrographical and geochemical characteristics of the studied rocks suggest that they are classified as alkaline lamprophyres and camptonites. They are enriched in alkalis (Na2O + K2O), large ion lithophile elements, and light rare earth elements, and the features of trace element concentrations are similar to those of within‐plate basalts. This study suggests that the lamprophyres were derived from different degrees of partial melting of metasomatized amphibole‐bearing spinel lherzolite. Subduction of Paleo‐Tethys oceanic crust from the Early to late Paleozoic resulted in enrichment in fluids in the mantle, and lamprophyric magmatism occurred along the minor and major faults. This limited but typical lamprophyric magmatism in a broad area of Central Iran suggests that, in spite of the long length of the Paleozoic (∼250 my), it was a relatively calm era from the viewpoint of magmatism in Central Iran.

Ancient sub-continental lithospheric mantle (SCLM) beneath the eastern part of the Central Asian Orogenic Belt (CAOB): Implications for crust–mantle decoupling

Potassic basaltic lavas erupted during the Cenzoic are widespread in Wudalianchi, Erkeshan and Keluo (WEK) areas, northeast China, and contain abundant mantle xenoliths. Mantle xenoliths in this study are selected from the Keluo potassic lavas, and mainly comprise spinel lherzolites and harzburgites with minor dunites and wehrlites. Modal metasomatism is evident in some Keluo xenoliths from the presence of phlogopite and rutile. Mineral compositions indicate that the Keluo xenoliths represent mantle residues after variable degrees of melting, i.e., 1–5% for lherzolites and 3–11% for harzburgites, with subsequent refertilization. Both bulk-rock and clinopyroxene data show enrichment in light rare earth elements (LREE) and some other incompatible elements (e.g. Ba, U). Clinopyroxenes from some of the Keluo xenoliths have radiogenic Sr isotopes, but unradiogenic Nd–Hf isotopes relative to the Cenozoic mantle xenoliths from other localities of the NE China. The Keluo mantle xenoliths have variable 187Os/ 188Os ratios ranging from 0.11458 to 0.13194. Although most Keluo mantle xenoliths have been affected by Re addition, their Os isotope compositions have not been significantly modified. The unradiogenic 187Os/ 188Os ratios of the refractory harzburgites give Re depletion ages (T RD) of ~ 1.9–2.1 Ga. This suggests that the sub-continental lithospheric mantle (SCLM) beneath the Keluo area was formed during the Paleoproterozoic, which is older than the age of the overlying crust that was mainly formed since the Neoproterozoic. Therefore, the SCLM beneath the Kelou area is temporally decoupled from the overlying crust. It is unlikely that the Cenozoic SCLM beneath the Keluo area was newly accreted from the asthenosphere, in which ancient mantle domains were preserved. We propose that the ancient mantle beneath the Keluo region is extraneous and has been emplaced from other locations.