Early crustal building processes on the moon: Models for the petrogenesis of the magnesian suite

The plutonic rocks of the magnesian suite (Mg-suite) represent the period of lunar basaltic magmatism and crustal growth (∼4.46 to 4.1 Ga) that immediately followed the initial differentiation of the Moon by magma ocean (LMO) formation and crystallization. The volume and distribution of the Mg-suite and its petrogenetic relationship to latter stages of lunar magmatism (mare basalts) remains obscure. These plutonic rocks exhibit a range of compositions and include ultramafics, troctolites, spinel troctolites, norites, and gabbronorites. A distinguishing characteristic of this suite is that they contain some of the most magnesium-rich phases (Fo_95-90) that had crystallized from lunar magmas, yet they also are significantly enriched in an incompatible element component referred to as KREEP (a late-stage product of LMO crystallization containing abundant potassium (K), rare earth elements (REE), phosphorous (P), uranium, and thorium). Ion microprobe analyses of individual mineral phases (olivine, pyroxene, and plagioclase) from the Mg-suite have shown some very unexpected characteristics that have profound implications on the origin of these basaltic magmas. Although the Mg-suite lithologies are typified by silicates with relatively high Mg′, early liquidus phases such as olivine are fairly low in Ni, Co, and Cr relative to more iron-rich olivines in the younger mare basalts. The high Y and Ti/Y in early phases such as olivine and orthopyroxene indicate that the parental basaltic melts were high in incompatible elements and contained an “ilmenite fractionation” signature. However, the Y in olivine from many of the troctolites and ultramafic lithologies are only slightly greater than that of the olivine in the mare basalts whereas olivine in the norites, gabbronorites, and Apollo 14 troctolites are exceedingly high. The KREEP component may have been added to the Mg-suite parent magmas by assimilation or mixing into the mantle source. The volume of KREEP required to be added to the parental magmas of the Mg-suite tends to favor the latter mechanism for KREEP incorporation. The extremely high abundances of KREEP in the norites and gabbronorites are a product of substantial crystallization (40% to 70%) of KREEP-enriched Mg-suite parental magmas. Basaltic magmatism associated with KREEP extended for over 1.5 billion years and appears to have changed over time. The early stages of this style of lunar magmatism (Mg-suite) appear to represent melting of early LMO cumulates with low abundances of Ni, Co, Cr, and V. Later stages of KREEP-rich basaltic magmatism seemed to clearly involve melting of a variety of LMO cumulate assemblages with higher incompatible element enrichment. It appears that the heat derived from the KREEP component was instrumental in at least initiating melting of the lunar mantle over this period of time.     

The stability and major element partitioning of ilmenite and armalcolite during lunar cumulate mantle overturn

Ilmenite has played an important role in the petrogenesis of lunar high-Ti picritic magmas, and armalcolite is another high-Ti oxide that was first discovered on the moon. In this study, we examined the thermodynamic stability of ilmenite and armalcolite in the context of lunar cumulate mantle overturn. Two starting compositions were explored, an ilmenite-bearing dunite (olivine + ilmenite) and an ilmenite-bearing harzburgite (olivine + orthopyroxene + ilmenite). Experiments were conducted using a 19.05 mm piston-cylinder apparatus at temperatures of 1235-1475 °C and pressures of 1-2 GPa. In runs with the ilmenite-bearing dunite mixture, ilmenite is stable in the subsolidus assemblage at least up to 1450 °C and 2 GPa. In runs with the ilmenite-bearing harzburgite starting mixture, ilmenite is stable at pressures greater than 1.4 GPa, and armalcolite is stable at lower pressures. Solidi for both starting compositions were determined, and the phase boundary between ilmenite- and armalcolite-bearing harzburgite was shown to have little dependence on temperature. During lunar cumulate overturn, sinking ilmenite formed near the end of lunar magma ocean solidification transforms into armalcolite when in contact with harzburgite cumulates at depths of less than 280 km in the lunar mantle. Inefficient overturn could leave isolated, inhomogeneously distributed pockets of armalcolite-bearing harzburgite in the upper lunar mantle, underlain by an ilmenite-bearing lower lunar mantle. These high-Ti oxide-bearing harzburgitic pockets can serve as potential sources for the generation of high-Ti magmas through partial melting or through assimilation of high-Ti minerals during transport of low-Ti picritic magmas in the lunar mantle.FeO-MgO exchange between olivine and either ilmenite or armalcolite was also examined in this study. We found the FeO-MgO distribution coefficient to be effectively independent of temperature for the pressures, temperatures, and compositions explored, with an average value of 0.179 ± 0.008 for olivine/ilmenite and 0.319 ± 0.021 for olivine/armalcolite. Given the bulk composition of an overturned lunar cumulate mantle, our measured FeO-MgO distribution coefficients can be used to estimate the Mg# of coexisting minerals in armalcolite- or ilmenite-bearing harzburgite and dunite in the overturned lunar mantle. Finally, the transformation from ilmenite-bearing harzburgite to armalcolite-bearing harzburgite results in a density increase of up to 2%. Large armalcolite-bearing cumulate bodies in the upper lunar mantle may be detectable in future lunar geophysical experiments.     

Relationships between tectonite and cumulate in ophiolites: the Miyamori ultramafic complex,Kitakami Mountains,northeast Japan

The Miyamori ultramafic complex forms the basal ultramafic portion of an ophiolite. The complex consists of a tectonic member which is composed dominantly of harzburgite and dunite, and a cumulate member which is composed of interlayered wehrlite, dunite and clinopyroxenite. The tectonite member is overlain by the cumulate member and characterized by tabular granular or porphyroclastic textures, a strong lineation and magnesian olivine (Mg/Mg + Fe = 0.88–0.93). In contrast, the cumulate member exhibits igneous textures and shows no evidence of a penetrative deformation. The olivine is less magnesian than that of the tectonite member (Mg/Mg + Fe = 0.82–0.89). At the boundary of the two members, harzburgite xenoliths have been found in wehrlite of the cumulate member. The minerals at the core of a few large harzburgite xenoliths preserve the compositional characteristics of typical harzburgites in the tectonic member. The occurrence of the harzburgite xenolith in vehrlite and the structural and textural features of the two members indicate that the tectonite member had already been deformed before a magma intruded into the tectonite member and formed a magma chamber in which cumulates were deposited together with harzburgite fragments on the floor of the tectonite. The xenoliths show a fine grained mosaic texture, which may be attributed by the heat of the intruded magma. This hiatus implies that the magma which made the cumulate member did not originate directly from the underlying harzburgite.     

Lunar Magma Ocean crystallization revisited: Bulk composition, early cumulate mineralogy, and the source regions of the highlands Mg-suite

Crystallization of the Lunar Magma Ocean (LMO) has been numerically modeled and its products inferred from sample observations, but it has never been fully tested experimentally. This study is a reexamination of the LMO hypothesis by means of the first experimental simulation of lunar differentiation. Two end-member bulk Moon compositions are considered: one enriched in refractory lithophile elements relative to Earth and one with no such enrichment. A “two-stage” model of magma ocean crystallization based on geophysical constraints is simulated and features early crystal suspension and equilibrium crystallization followed by fractional crystallization of the residual magma ocean. An initially entirely molten Moon is assumed. Part 1 of this study, presented here, focuses on stage 1 of this model and considers the early cumulates formed by equilibrium crystallization, differences in mantle mineralogy resulting from different bulk Moon compositions, and implications for the source regions of the highlands Mg-suite.Refractory element enriched bulk Moon compositions produce a deep mantle that contains garnet and trace Cr-spinel in addition to low-Ca pyroxene and olivine. In contrast, compositions without refractory element enrichment produce a deep dunitic mantle with low-Ca pyroxene but without an aluminous phase. The differences in bulk composition are magnified in the residual melt; the residual LMO from the refractory element enriched composition will likely produce plagioclase and ilmenite earlier and in greater quantities. Both compositions produce Mg-rich early cumulate piles that extend from the core-mantle boundary to ∼355 km depth, if 50% equilibrium crystallization and whole Moon melting are assumed. These early LMO cumulates provide good fits for the source regions for a component of the high-Mg^∗, Ni- and Co-poor parental magmas of the Mg-suite cumulates, if certain conditions are called upon. The olivine in early LMO cumulates produced by either bulk Moon composition is far too rich in Cr to be reasonable for the source regions of the Mg-suite, meaning either core formation in the presence of S and/or C must be invoked to deplete the LMO and the crystallizing olivine in Cr, or that current estimates of the bulk lunar Cr content are too high. We infer that melts meeting the criteria of the Mg-suite parents could be produced from early LMO cumulates by solid state KREEP and plagioclase hybridization near the base of the crust and subsequent partial melting. Additionally, we propose a revised model for Mg-suite petrogenesis.     

Environments of Crystallization and Compositional Diversity of Mauna Loa Xenoliths

Two picrite flows from the SW rift zone of Mauna Loa containxenoliths of dunite, harzburgite, lherzolite, plagioclase-bearinglherzolite and harzburgite, troctolite, gabbro, olivine gabbro,and gabbronorite. Textures and olivine compositions precludea mantle source for the xenoliths, and rare earth element concentrationsof xenoliths and clinopyroxene indicate that the xenolith sourceis not old oceanic crust, but rather a Hawaiian, tholeiitic-stagemagma. Pyroxene compositions, phase assemblages and texturalrelationships in xenoliths indicate at least two different crystallizationsequences. Calculations using the pMELTS algorithm show thatthe two sequences result from crystallization of primitive MaunaLoa magmas at 6 kbar and 2 kbar. Independent calculations ofolivine Ni–Fo compositional variability in the plagioclase-bearingxenoliths over these crystallization sequences are consistentwith observed olivine compositional variability. Two parentsof similar bulk composition, but which vary in Ni content, arenecessary to explain the olivine compositional variability inthe dunite and plagioclase-free peridotitic xenoliths. Xenolithsprobably crystallized in a small magma storage area beneaththe rift zone, rather than the large sub-caldera magma reservoir.Primitive, picritic magmas are introduced to isolated rift zonestorage areas during periods of high magma flux. Subsequenteruptions reoccupy these areas, and entrain and transport xenolithsto the surface. KEY WORDS: xenolith; Hawaii; volcano plumbing; mineral composition; picrite     

新疆洪古勒楞蛇绿岩套中堆积杂岩的地球化学特征及成因

新疆和布克赛尔蒙古族自治县境内的洪古勒楞蛇绿岩块,出露于西准噶尔海西褶皱带的西部。该蛇绿岩块的层序发育完好,尤其是保存了具有完整层序的堆积杂岩相。本文描述了堆积杂岩的矿物化学成分演化以及岩石化学、微量元素和稀土元素的地球化学变化规律,指出蛇绿岩、堆积杂岩具有层状镁铁超镁铁岩体的主要特点,两者在成因上是相似的。文中提出蛇绿岩、堆积杂岩是在岩浆房中经液态岩浆重力堆积形成的观点,并对液态堆积机制进行了解释。     

鲁西中生代高镁辉石闪长岩中超镁铁质捕虏体成因:氧化态和铂族元素证据

鲁西莱芜晚中生代高镁闪长岩中含丰富的超镁铁质捕虏体,主要为纯橄岩(>80%),少量方辉橄榄岩(<5%)和橄榄辉石岩(<15%)。测定了代表性捕虏体的亲铁、亲铜元素含量,根据其矿物化学成分计算了捕虏体的氧化-还原状态。方辉橄榄岩遭受了不同程度富Si熔体的交代,全岩富含Cr、Co、Ni和IPGE,高IPGE/PPGE值,是地幔深度部分熔融的残留。纯橄岩由铬铁矿和高Mg橄榄石组成,全岩富含Cr、Co、Ni,贫IPGE,低IPGE/PPGE值,可能为一种镁铁质熔体的堆晶岩。纯橄岩中橄榄石成分表现出一定的变化范围,局部镁橄榄石（Fo）成分高达94,可能是堆晶中橄榄石与足够的铬铁矿反应的结果,原始熔体可能为玻镁安山质岩浆。方辉橄榄岩和纯橄岩都显示高f_O2值,FMQ+1.4~+2.4,与研究区早古生代相对还原的陆下岩石圈地幔(f_O2低于C-CO₂缓冲反应)形成鲜明对比。数据表明,中生代扬子大陆和华北克拉通碰撞之前,特提斯大洋板块和扬子大陆边缘相继俯冲到华北克拉通东缘之下,导致当时岩石圈地幔的f_O2陡升。     

Mineral chemistry of ultramafic and mafic cumulates as an indicator of the arc-related origin of the Mersin ophiolite (southern Turkey)

The Mersin ophiolite, represented by approximately 6-km-thick oceanic lithospheric section on the southern flank of the Taurus calcareous axis, formed in the Mesozoic Neo-Tethyan ocean some time during Late Cretaceous in southern Turkey. The ultramafic and mafic cumulates having over 3 km thickness consist of dunite ± chromite, wehrlite, clinopyroxenite at the bottom and pass into gabbroic cumulates in which leucogabbro, olivine-gabbro and anorthosite are seen. Crystallization order is olivine (Fo₉₁₋₈₀) ± chromian spinel (Cr# 60-80), clinopyroxene (Mg#₉₅₋₇₇), plagioclase (An_95.6−91.6) and orthopyroxene (Mg#₆₈₋₇₇). Mineral chemistry of ultramafic and mafic cumulates suggest that highly magnesian olivines, clinopyroxenes and absence of plagioclase in the basal ultramafic cumulates are in good agreement with products of high-pressure crystal fractionation of primary basaltic melts beneath an island-arc environment. Major, trace element geochemistry of the cumulative rocks also indicate that Mersin ophiolite was formed in an arc environment. Coexisting Ca-rich plagioclase and Forich olivine in the gabbroic cumulates show arc cumulate gabbro characteristics. Field relations as well as the geochemical data support that Mersin ophiolite formed in a supra-subduction zone tectonic setting in the southern branch of the Neo-Tethys in southern Turkey.     

塔里木东北缘坡十Ni矿化侵入体中橄榄石和铬尖晶石对成岩成矿及源区特征的约束

坡十Ni矿化超镁铁侵入体的矿化岩相主要为第二侵入期次的（斜长）单辉橄榄岩、（斜长）二辉橄榄岩、 纯橄岩等岩相。坡十超镁铁岩的橄榄石成分变化范围较大, 橄榄石的Fo值在76.8～89.6之间, Ni含量为767×10^-6～4 580×10^-6。铬尖晶石的Mg^#值和Cr^#值变化范围分别为19.4～41.9和49.8～64.8, 原生铬尖晶石中Cr₂O₃和Al₂O₃表现为负相关, 蚀变改造的铬尖晶石则表现为正相关。橄榄石成分剖面显示坡十母岩浆处于一个动态的岩浆系统中, 成分稳定的新鲜岩浆的补给、 持续向上的动力及浅部橄榄石快速分离结晶,造成了不同深度橄榄石成分的不同变化。坡十侵入体母岩浆估算结果为MgO=14.49％, FeO=10.01％,模拟结果显示橄榄石中Ni含量的变化主要受橄榄石结晶分异和硫化物不混溶作用共同控制,其中橄榄石与硫化物熔体发生明显的Fe-Ni交换反应。坡十母岩浆中橄榄石分离结晶造成的硫饱和,是坡十硫化物熔离的重要因素。橄榄石高Fo值、母岩浆高MgO、超镁铁岩中斜长石发育、矿物高结晶温度和铬尖晶石成分的弧岩浆特征显示,塔里木东北缘坡十侵入体是俯冲交代的岩石圈地幔部分熔融形成的母岩浆的产物,表现出低压高温的演化特征,其中源区熔融机制可能与塔里木二叠纪地幔柱提供的热源或该区大规模拆沉作用造成的软流圈上涌有关。     

The age of Dar al Gani 476 and the differentiation history of the martian meteorites inferred from their radiogenic isotopic systematics

Samarium-neodymium isotopic analysis of the martian meteorite Dar al Gani 476 yields a crystallization age of 474 ± 11 Ma and an initial ε_Nd¹⁴³ value of +36.6 ± 0.8. Although the Rb-Sr isotopic system has been disturbed by terrestrial weathering, and therefore yields no age information, an initial ⁸⁷Sr/⁸⁶Sr ratio of 0.701249 ± 33 has been estimated using the Rb-Sr isotopic composition of the maskelynite mineral fraction and the Sm-Nd age. The Sr and Nd isotopic systematics of Dar al Gani 476, like those of the basaltic shergottite QUE94201, are consistent with derivation from a source region that was strongly depleted in incompatible elements early in the history of the solar system. Nevertheless, Dar al Gani 476 is derived from a source region that has a slightly greater incompatible enrichment than the QUE94201 source region. This is not consistent with the fact that the parental magma of Dar al Gani 476 is significantly more mafic than the parental magma of QUE94201, and underscores a decoupling between the major element and trace element-isotopic systematics observed in the martian meteorite suite.Combining the ε_Nd¹⁴²-ε_Nd¹⁴³ isotopic systematics of the martian meteorites yields a model age for planetary differentiation of 4.513_+0.033^−0.027 Ga. Using this age, the parent/daughter ratios of martian mantle sources are calculated assuming a two-stage evolutionary history. The calculated sources have very large ranges of parent/daughter ratios (⁸⁷Rb/⁸⁶Sr = 0.037-0.374; ¹⁴⁷Sm/¹⁴⁴Nd = 0.182-0.285; ¹⁷⁶Lu/¹⁷⁷Hf = 0.028-0.048). These ranges exceed the ranges estimated for terrestrial basalt source regions, but are very similar to those estimated for the sources of lunar mare basalts. In fact, the range of parent/daughter ratios calculated for the martian meteorite sources can be produced by mixing between end-members with compositions similar to lunar mare basalt sources. Two of the sources have compositions that are similar to olivine and pyroxene-rich mafic cumulates with variable proportions of a Rb-enriched phase, such as amphibole, whereas the third source has the composition of liquid trapped in the cumulate pile (i.e. similar to KREEP) after ∼99% crystallization. Correlation between the proportion of trapped liquid in the meteorite source regions and estimates of f_O2, suggest that the KREEP-like component may be hydrous. The success of these models in reproducing the martian meteorite source compositions suggests that the variations in trace element and isotopic compositions observed in the martian meteorites primarily reflect melting of the crystallization products of an ancient magma ocean, and that assimilation of evolved crust by mantle derived magmas is not required. Furthermore, the decoupling of major element and trace element-isotopic systematics in the martian meteorite suite may reflect the fact that trace element and isotopic systematics are inherited from the magma source regions, whereas the major element abundances are limited by eutectic melting processes at the time of magma formation. Differences in major element abundances of parental magma, therefore, result primarily from fractional crystallization after leaving their source regions.     

Investigations in the Stillwater Complex: Part II. Petrology and Petrogenesis of the Ultramafic Series

The Ultramafic series of the Stillwater Complex has been dividedinto two major zones: a Peridotite zone formed of 20 macro-rhythmicunits of dunite-harzburgite-orthopyroxenite, and an overlyingOrthopyroxenite zone. The stratigraphic section has been determinedat Mountain View (2065 m) and at Chrome Mountain (840 m). TheMountain View section apparently formed in a subsiding basinwhereas the rocks at Chrome Mountain accumulated in a relativelystable, higher area of the chamber floor. In both sections,Mg/(Mg + Fe) in cumulus mafic minerals increases with stratigraphicheight in the lower 400 m, then remains relatively constantthrough the rest of the series. The base of the series is marked by the first appearance oflaterally extensive olivine-rich cumulates. The accretion ofthe cumulates and the growth of the chamber proceeded throughperiodic injections of olivine-saturated mafic magma. The lowercontact of the cycles represents a hiatus in crystallizationand a return to a more primitive magma composition. Althoughhotter, the primitive magma was more dense, so it entered thechamber at or near the floor and did not immediately mix withthe more differentiated orthopyroxene-saturated magma alreadypresent. As it cooled by transfer of heat across its upper surface,the primitive magma crystallized olivine and differentiatedin situ to form the lower dunite. With the accumulation of olivinenear the base, the crystal/liquid ratio, and thus the density,decreased at the top of the layer eventually resulting in mixingand the formation of harzburgite. After removal of olivine byresorption and settling from the hybrid magma, orthopyroxenealone crystallized forming an orthopyroxenite. Chromitite layersprobably formed by the mixing of primitive olivine± chromite-saturatedmagma and narrow layers of orthopyroxene-saturated magma trappedunderneath. The Mg-enrichment trend in the lower 400 m resulted from reactionof cumulus olivine and/or orthopyroxene with progressively decreasingvolumes of intercumulus liquid. As heat loss through the floordecreased, accumulation rate approached a steady state, thefraction of trapped liquid remained more or less constant andvariation in Mg/(Mg + Fe) was governed dominantly by cumulusprocesses. The constant NiO abundances in olivine throughoutthe section are consistent with the model for the formationof the macro-rhythmic units. Depletion of NiO was dampened byrepeated additions of parental magma, localized equilibriumcrystallization, mixing, and the effect of postcumulus equi-librationwith varied amounts of trapped liquid. Discordant dunite bodies, which are common at Chrome Mountain,formed by the replacement by olivine of earlier formed cumulates.The replacement involved the incongruent dissolution of ortho-pyroxeneat near-solidus temperatures by a late-stage, hydrous vaporprobably derived from the magma. The vapor phase migrated alongfractures formed by the readjustment of the cumulate pile.     

The nakhlite meteorites: Augite-rich igneous rocks from Mars

The seven nakhlite meteorites are augite-rich igneous rocks that formed in flows or shallow intrusions of basaltic magma on Mars. They consist of euhedral to subhedral crystals of augite and olivine (to 1 cm long) in fine-grained mesostases. The augite crystals have homogeneous cores of Mg′=63% and rims that are normally zoned to iron enrichment. The core-rim zoning is cut by iron-enriched zones along fractures and is replaced locally by ferroan low-Ca pyroxene. The core compositions of the olivines vary inversely with the steepness of their rim zoning - sharp rim zoning goes with the most magnesian cores (Mg′=42%), homogeneous olivines are the most ferroan. The olivine and augite crystals contain multiphase inclusions representing trapped magma. Among the olivine and augite crystals is mesostasis, composed principally of plagioclase and/or glass, with euhedra of titanomagnetite and many minor minerals. Olivine and mesostasis glass are partially replaced by veinlets and patches of iddingsite, a mixture of smectite clays, iron oxy-hydroxides and carbonate minerals. In the mesostasis are rare patches of a salt alteration assemblage: halite, siderite, and anhydrite/gypsum. The nakhlites are little shocked, but have been affected chemically and biologically by their residence on Earth.Differences among the chemical compositions of the nakhlites can be ascribed mostly to different proportions of augite, olivine, and mesostasis. Compared to common basalts, they are rich in Ca, strongly depleted in Al, and enriched in magmaphile (incompatible) elements, including the LREE. Nakhlites contain little pre-terrestrial organic matter. Oxygen isotope ratios are not terrestrial, and are different in anhydrous silicates and in iddingsite. The alteration assemblages all have heavy oxygen and heavy carbon, while D/H values are extreme and scattered. Igneous sulfur had a solar-system isotopic ratio, but in most minerals was altered to higher and lower values. High precision analyses show mass-independent fractionations of S isotopes. Nitrogen and noble gases are complex and represent three components: two mantle sources (Chas-E and Chas-S), and fractionated Martian atmosphere.The nakhlites are igneous cumulate rocks, formed from basaltic magma at ∼1.3 Ga, containing excess crystals over what would form from pure magma. After accumulation of their augite and olivine crystals, they were affected (to various degrees) by crystallization of the magma, element diffusion among minerals and magma, chemical reactions among minerals and magma, magma movement among the crystals, and post-igneous chemical equilibration. The extent of these modifications varies, from least to greatest, in the order: MIL03346, NWA817, Y000593, Nakhla=Governador Valadares, Lafayette, and NWA998.Chemical, isotopic, and chronologic data confirm that the nakhlites formed on Mars, most likely in thick lava flows or shallow intrusions. Their crystallization ages, referenced to crater count chronologies for Mars, suggest that the nakhlites formed on the large volcanic constructs of Tharsis, Elysium, or Syrtis Major. The nakhlites were suffused with liquid water, probably at ∼620 ma. This water dissolved olivine and mesostasis glass, and deposited iddingsite and salt minerals in their places. The nakhlites were ejected from Mars at ∼10.75 Ma by an asteroid impact and fell to Earth within the last 10,000 years.Although the nakhlites are enriched in incompatible elements, their source mantle was strongly depleted. This depletion event was ancient, as the nakhlites’ source mantle was fractionated while short-lived radionuclides (e.g., ) were still active. This differentiation event may have been core formation coupled with a magma ocean, as is inferred for the moon.     

A Rb-Sr and Sm-Nd isotope geochronology and trace element study of lunar meteorite LaPaz Icefield 02205

Rubidium-strontium and samarium-neodymium isotopes of lunar meteorite LaPaz Icefield (LAP) 02205 are consistent with derivation of the parent magma from a source region similar to that which produced the Apollo 12 low-Ti olivine basalts followed by mixing of the magma with small amounts (1-2 wt%) of trace element-enriched material similar to lunar KREEP-rich sample SaU 169. The crystallization age of LAP 02205 is most precisely dated by an internal Rb-Sr isochron of 2991 ± 14 Ma, with an initial ⁸⁷Sr/⁸⁸Sr at the time of crystallization of 0.699836 ± 0.000010. Leachable REE-rich phosphate phases of LAP 02205 do not plot on a Sm-Nd mineral isochron, indicating contamination or open system behavior of the phosphates. Excluding anomalous phases from the calculation of a Sm-Nd isochron yields a crystallization age of 2992 ± 85 (initial ε¹⁴³Nd = +2.9 ± 0.8) that is within error of the Rb-Sr age, and in agreement with other independent age determinations for LAP 02205 from Ar-Ar and U-Pb methods. The calculated ¹⁴⁷Sm/¹⁴⁴Nd source ratios for LAP 02205, various Apollo 12 and 15 basalts, and samples with strong affinities to KREEP (SaU 169, NWA 773, 15386) are uncorrelated with their crystallization ages. This finding does not support the involvement of a common KREEP component as a heat source for lunar melting events that occurred after crystallization of the lunar magma ocean.     

Fe-rich Dunite Xenoliths from South African Kimberlites: Cumulates from Karoo Flood Basalts

Fe-rich dunite xenoliths within the Kimberley kimberlites compriseolivine neoblasts with minor elongated, parallel-oriented ilmenite,and rarely olivine porphyroclasts and spinel. Compared withtypical mantle peridotites, olivines in the Fe-rich duniteshave lower forsterite (Fo_87–89) and NiO contents (1300–2800ppm), which precludes a restitic origin for the dunites. Chrome-richspinels are remnants of a metasomatic reaction that producedilmenite and phlogopite. Trace element compositions differ betweenporphyroclastic and neoblastic olivine, the latter having higherTi, V, Cr and Ni and lower Zn, Zr and Nb contents, documentingtheir different origins. The dunites have high ¹⁸⁷Os/ ¹⁸⁸Osratios (0·11–0·15) that result in youngmodel ages for most samples, whereas three samples show isotopicmixtures between Phanerozoic neoblasts and ancient porphyroclasticmaterial. Most Fe-rich dunite xenoliths are interpreted to berecrystallized cumulates related to fractional crystallizationof Jurassic Karoo flood basalt magmatism, whereas the porphyroclastsare interpreted to be remnants from a much earlier (probablyArchaean Ventersdorp) magmatic episode. The calculated parentalmagma for the most primitive olivine neoblasts in the Fe-richdunites is similar to low-Ti Karoo basalts. Modelling the crystalfractionation of the inferred parental magma with pMELTS yieldselement fractionation trends that mirror the element variationof primitive low-Ti Karoo basalts. KEY WORDS: dunite xenoliths; fractional crystallization; Karoo; large igneous province; pMELTS; Re–Os; trace elements     

Peridotitic and gabbroic rocks associated with the shield-forming lavas of réunion

The peridotitic and gabbroic rocks described occur a) as a tectonically emplaced layered body in Piton des Neiges volcano, b) as blocks in basaltic agglomerate, Piton des Neiges, and c) as nodular inclusions in lavas of both Piton des Neiges and Piton de la Fournaise volcanoes. All are associated with the olivine basalts of the early shield-forming growth stages and not later alkaline lavas, thereby contrasting with the Hawaiian situation. Rock-types include dunite, clinopyroxenite, wehrlite, feldspathic wehrlite, olivine eucrite, allivalite, (bytownite) anorthosite and gabbro. The peridotites and most of the gabbroic rocks are inferred to be cumulates formed in floored magma chambers occurring at depths from 30 km upwards. The inclusion suite is probably derived from repetitive layered units consisting predominantly of ol + sp cumulates with sporadic development of ol + cpx±sp and ol + cpx + plag cumulate horizons.     

Olivine: a monitor of magma evolutionary paths in kimberlites and olivine melilitites

Petrographic and chemical criteria indicate that the overwhelming majority of olivines in kimberlites are probably cognate phenocrysts. The implied low volume of xenocryst olivines requires that primitive kimberlite magmas are highly ultrabasic liquids. Two chemically distinctive olivine populations are present in all of the kimberlites studied. The dominant olivine population, which includes large rounded olivines and smaller euhedral crystals, is Mg-rich relative to late-stage rim compositions. It is characterized by a range in 100 Mg/(Mg + Fe) and uniform Ni concentration, reflecting Rayleigh-type crystallization during magma evolution. The most Mg-rich of these olivines are considered to be similiar to those in the mantle source rocks. The second compositional population, generally very subordinate, though markedly more abundant in the megacrystrich Monastery kimberlite, is Fe-rich relative to rim compositions. This group of olivines crystallized from evolved liquids in equilibrium with iron-rich megacrysts, both entrained by the kimberlite magma during ascent. Differences between the chemical fields of Fe-rich olivines in Group I and Group II kimberlites point to relatively deeper derivation of the latter suite. Olivine chemistry can be used to characterize kimberlite magma sub-types, and may prove to be a useful tool for evaluating the diamond potential of kimberlites.     

Coupled Ce-Nd isotope systematics and rare earth elements differentiation of the moon

¹³⁸Ce/¹⁴²Ce and ¹⁴³Nd/¹⁴⁴Nd isotope ratios of lunar samples are determined to constrain the petrogenetic differentiation and evolution of the moon. High-precision Ce-Nd isotope data, well-defined Rb-Sr isochrons, and rare earth elements (REE) abundances of lunar samples show that unexpectedly low La/Ce ratios of evolved lunar highland samples are preserved from at least 3.9 Ga. Precise analysis of REE abundances indicates that the low La/Ce ratio results from a depletion of La relative to other REE. This depletion can be seen in pristine KREEP basalts and Mg-suite rocks from 3.85 to 4.46 Ga. As REE abundances of all these samples are controlled by the presence of a KREEP component, the depletion was probably inherited from a late crystallization sequence of the lunar magma ocean related to the production of the original KREEP component.     

Anorthosite formation by plagioclase flotation in ferrobasalt and implications for the lunar crust

The Sept Iles layered intrusion (Quebec, Canada) is dominated by a basal Layered Series made up of troctolites and gabbros, and by anorthosites occurring (1) at the roof of the magma chamber (100-500 m-thick) and (2) as cm- to m-size blocks in gabbros of the Layered Series. Anorthosite rocks are made up of plagioclase, with minor clinopyroxene, olivine and Fe-Ti oxide minerals. Plagioclase displays a very restricted range of compositions for major elements (An₆₈-An₆₀), trace elements (Sr: 1023-1071 ppm; Ba: 132-172 ppm) and Sr isotopic ratios (⁸⁷Sr/⁸⁶Sr_i: 0.70356-0.70379). This compositional range is identical to that observed in troctolites, the most primitive cumulates of the Layered Series, whereas plagioclase in layered gabbros is more evolved (An₆₀-An₃₈). The origin of Sept Iles anorthosites has been investigated by calculating the density of plagioclase and that of the evolving melts. The density of the FeO-rich tholeiitic basalt parent magma first increased from 2.70 to 2.75 g/cm³ during early fractionation of troctolites and then decreased continuously to 2.16 g/cm³ with fractionation of Fe-Ti oxide-bearing gabbros. Plagioclase (An₆₉-An₆₀) was initially positively buoyant and partly accumulated at the top of the magma chamber to form the roof anorthosite. With further differentiation, plagioclase (<An₆₀) became negatively buoyant and anorthosite stopped forming. Blocks of anorthosite (autoliths) even fell downward to the basal cumulate pile. The presence of positively buoyant plagioclase in basal troctolites is explained by the low efficiency of plagioclase flotation due to crystallization at the floor and/or minor plagioclase nucleation within the main magma body. Dense mafic minerals of the roof anorthosite are shown to have crystallized from the interstitial liquid.The processes related to floating and sinking of plagioclase in a large and shallow layered intrusion serve as a proxy to refine the crystallization model of the lunar magma ocean and explain the vertically stratified structure of the lunar crust, with (gabbro-)noritic rocks at the base and anorthositic rocks at the top. We propose that the lunar crust mainly crystallized bottom-up. This basal crystallization formed a mafic lower crust that might have a geochemical signature similar to the magnesian-suite without KREEP contamination, while flotation of some plagioclase grains produced ferroan anorthosites in the upper crust.     

Evidence for heterogeneous enriched shergottite mantle sources in Mars from olivine-hosted melt inclusions in Larkman Nunatak 06319

Larkman Nunatak (LAR) 06319 is an olivine-phyric shergottite whose olivine crystals contain abundant crystallized melt inclusions. In this study, three types of melt inclusion were distinguished, based on their occurrence and the composition of their olivine host: Type-I inclusions occur in phenocryst cores (Fo_77-73); Type-II inclusions occur in phenocryst mantles (Fo_71-66); Type-III inclusions occur in phenocryst rims (Fo_61-51) and within groundmass olivine. The sizes of the melt inclusions decrease significantly from Type-I (∼150-250 μm diameter) to Type-II (∼100 μm diameter) to Type-III (∼25-75 μm diameter). Present bulk compositions (PBC) of the crystallized melt inclusions were calculated for each of the three melt inclusion types based on average modal abundances and analyzed compositions of constituent phases. Primary trapped liquid compositions were then reconstructed by addition of olivine and adjustment of the Fe/Mg ratio to equilibrium with the host olivine (to account for crystallization of wall olivine and the effects of Fe/Mg re-equilibration). The present bulk composition of Type-I inclusions (PBC1) plots on a tie-line that passes through olivine and the LAR 06319 whole-rock composition. The parent magma composition can be reconstructed by addition of 29 mol% olivine to PBC1, and adjustment of Fe/Mg for equilibrium with olivine of Fo₇₇ composition. The resulting parent magma composition has a predicted crystallization sequence that is consistent with that determined from petrographic observations, and differs significantly from the whole-rock only in an accumulated olivine component (∼10 wt%). This is consistent with a calculation indicating that ∼10 wt% magnesian (Fo_77-73) olivine must be subtracted from the whole-rock to yield a melt in equilibrium with Fo₇₇. Thus, two independent estimates indicate that LAR 06319 contains ∼10 wt% cumulate olivine.The rare earth element (REE) patterns of Type-I melt inclusions are similar to that of the LAR 06319 whole-rock. The REE patterns of Type-II and Type-III melt inclusions are also broadly parallel to that of the whole-rock, but at higher absolute abundances. These results are consistent with an LAR 06319 parent magma that crystallized as a closed-system, with its incompatible-element enrichment being inherited from its mantle source region. However, fractional crystallization of the reconstructed LAR 06319 parent magma cannot reproduce the major and trace element characteristics of all enriched basaltic shergottites, indicating local-to-large scale major- and trace-element variations in the mantle source of enriched shergottites. Therefore, LAR 06319 cannot be parental to the enriched basaltic shergottites.     

Evolution of major mineral compositions and trace element abundances during fractional crystallization of a model lunar composition

The evolution of major mineral compositions and trace element abundances during fractional crystallization of a model lunar magma ocean have been calculated. A lunar bulk composition consistent with petrological constraints has been selected. Major mineral compositions have been calculated using published studies of olivine-melt, plagioclase-melt, and pyroxene-olivine equilibria. Trace element abundances have been calculated using experimentally-determined partition coefficients where possible. In the absence of experimental determinations, published partition coefficients obtained by analyzing phase separates from porphyritic volcanic rocks have been used. Trace elements studied are La, Sm, Eu, Lu, Rb, Sr( Eu²⁺), Ni, Co, and Cr.The first mineral to crystallize is olivine, which varies in composition from Fo₉₈ at the liquidus to Fo₉₅ at 50% solidification. Orthopyroxene crystallizes from 50 to 60% solidification with a restricted composition range of En₉₅-En₉₃. Plagioclase and Ca-rich clinopyroxene (X_Wo arbitrarily set equal to 0.5) co-crystallize during the final 40% solidification. Plagioclase changes in composition from An₉₇ to approximately An₉₃, while clinopyroxene evolves from En₄₆ to approximately En₄₀. The concomitant evolution of major element abundances in the melt is also discussed.The concentration of Ni in the melt decreases rapidly because solid-melt partition coefficients are significantly greater than unity at all stages of crystallization. The concentration of Cr in the melt increases slowly during olivine crystallization, then drops precipitously during the crystallization of orthopyroxene and clinopyroxene. The concentration of Co in the melt decreases slowly during olivine and orthopyroxene crystallization, after which it returns slowly to its initial concentration. Rubidium and Sr are not fractionated relative to one another until the onset of plagioclase crystallization. Ratios of Rb/Sr, normalized to their initial concentrations in the magma, do not rise above 10 until 95% of the magma has solidified. The ratios of Eu/Sm and La/Lu, normalized to their initial concentrations in the magma, remain essentially unfractionated until the onset of crystallization of clinopyroxene plus plagioclase, at which point the normalized La/Lu ratio increases to approximately 1.3 at 100% solidification and the normalized Eu/Sm ratio decreases to approximately 0.2 at 100% solidification.The model calculations are used to place approximate constraints on the bulk composition of the primitive Moon. Consideration of the effect on plagioclase composition of the activities of NaO_0.5 and SiO₂ in the melt suggests that the primitive Moon contained less than 0.4 wt % NaO_0.5 and approximately 42–43 wt % SiO₂. Concentrations of the REE in model lunar anorthosites are consistent with the returned samples. Concentrations of the REE in several model ‘highland basalts’ (considered to be representative of the average lunar terrae) are too low when compared with returned samples. Several possible explanations of this discrepancy are considered. The possible role of spinel in a twostage geochemical evolution of mare basalt liquids is discussed.