Formation and accretion history of terrestrial planets from runaway growth through to late time: implications for orbital eccentricity

Remnant planetesimals might have played an important role in reducing the orbital eccentricities of the terrestrial planets after their formation via giant impacts. However, the population and the size distribution of remnant planetesimals during and after the giant impact stage are unknown, because simulations of planetary accretion in the runaway growth and giant impact stages have been conducted independently. Here we report results of direct N-body simulations of the formation of terrestrial planets beginning with a compact planetesimal disk. The initial planetesimal disk has a total mass and angular momentum as observed for the terrestrial planets, and we vary the width (0.3 and 0.5AU) and the number of planetesimals (1000-5000). This initial configuration generally gives rise to three final planets of similar size, and sometimes a fourth small planet forms near the location of Mars. Since a sufficient number of planetesimals remains, even after the giant impact phase, the final orbital eccentricities are as small as those of the Earth and Venus.Comment: 36 pages, 9 figures, 1 table, Accepted in Ap

The Melting of Carbonated Pelites from 70 to 700 km Depth

Phase assemblages, melting relations and melt compositions of a dry carbonated pelite (DG2) and a carbonated pelite with 1·1 wt % H2O (AM) have been experimentally investigated at 5·5-23·5 GPa and 1070-1550°C. The subsolidus mineralogies to 16 GPa contain garnet, clinopyroxene, coesite or stishovite, kyanite or corundum, phengite or potassium feldspar (≤8 GPa with and without H2O, respectively), and then K-hollandite, a Ti phase and ferroan dolomite/Mg-calcite or aragonite + ferroan magnesite at higher pressures. The breakdown of clinopyroxene at >16 GPa causes Na-rich Ca-carbonate containing up to 11 wt % Na2O to replace aragonite and leads to the formation of an Na-rich CO2 fluid. Further pressure increase leads to typical Transition Zone minerals such as the CAS phase and one or two perovskites, which completely substitute garnet at the highest investigated pressure (23·5 GPa). Melting at 5·5-23·5 GPa yields alkali-rich magnesio-dolomitic (DG2) to ferro-dolomitic (AM) carbonate melts at temperatures 200-350°C below the mantle geotherm, lower than for any other studied natural composition. Melting reactions are controlled by carbonates and alkali-hosting phases: to 16 GPa clinopyroxene remains residual, Na is compatible and the magnesio- to ferro-dolomitic carbonate melts have extremely high K2O/Na2O ratios. K2O/Na2O weight ratios decrease from 26-41 at 8 GPa to 1·2 at 16 GPa when K-hollandite expands its stability field with increasing pressure. At >16 GPa, Na is repartitioned between several phases, and again becomes incompatible as at <3 GPa, leading to Na-rich carbonate melts with K2O/Na2O ratios 1. This leaves the pressure interval of c. 4-15 GPa for ultrapotassic metasomatism. Comparison of the solidus with typical subducting slab-surface temperatures yields two distinct depths of probable carbonated pelite melting: at 6-9 GPa where the solidus has a negative Clapeyron slope between the intersection of the silicate and carbonate melting reactions at ∼5 GPa, and the phengite or potassium feldspar stability limit at ∼9 GPa. The second opportunity is related to possible slab deflection along the 660 km discontinuity, leading to thermal relaxation and partial melting of the fertile carbonated pelites, thus recycling sedimentary CO2, alkalis and other lithophile and strongly incompatible elements back into the mantl

The Biotite to Phengite Reaction and Mica-dominated Melting in Fluid + Carbonate-saturated Pelites at High Pressures

Subsolidus and melting experiments were performed at 2·0-3·7 GPa and 750-1300°C on a carbonated pelite in the model system K2O-CaO-MgO-Al2O3-SiO2-H2O-CO2 to define stabilities of potassic micas and fluid-present melting reactions. The biotite to phengite reaction occurs at pressures between 2· 4 and 2·6 GPa for temperatures of 750-850°C, and the amphibole to clinopyroxene reaction from 2·0 GPa, 875°C to 2·5 GPa, 740°C. Dolomite is the carbonate phase stable at subsolidus conditions. The biotite to phengite reaction preserves K2O, but is not H2O conservative, as a fluid is produced from the decomposition of zoisite. Phengite + quartz control fluid-saturated melting at a pressure (P) >2·6 GPa, whereas biotite + quartz dominate at P <2· 4 GPa. Incongruent melting occurs through the reactions phengite or biotite + zoisite + quartz/coesite + fluid = silicate melt + clinopyroxene + kyanite. Overstepping of the solidus, located at 850-950°C, results in 7-24 wt % metaluminous K-rich granitic melts. The experiments define the melting surface of the model system, projected from kyanite + quartz/coesite + fluid onto the K2O-CaO-MgO plane. The solidus melts in the studied system occur at a peritectic point consuming mica + zoisite and forming clinopyroxene. With increasing temperature (T), carbonated pelites then evolve along a peritectic curve along which further clinopyroxene is produced until zoisite is exhausted. This is then followed by a peritectic curve consuming clinopyroxene and producing garnet. A comparison of CO2-bearing with CO2-free experiments from the literature suggests that the main effect of adding calcite to a continental sediment is not the minor shift of typically 20-30°C of reactions involving fluid, but the change in bulk Ca/(Mg + Fe) ratio stabilizing calcic phases at the expense of ferromagnesian phases. The experiments suggest that in most subduction zones, CO2, H2O and K2O will be carried to depths in excess of 120-150 km through carbonates and K-micas, as partial melting occurs only at temperatures at the uppermost end of thermal models of subduction zones. Nevertheless, the release of fluid through P-induced decomposition of amphibole and zoisite provides some H2O for arc magma formation. Melting at higher temperatures (e.g. resulting from slower burial rates or from incorporation of subducted crust into the mantle) will produce potassic granitic melts and provide a substantial volatile and K source for the formation of arc magma

Trapped Liquid, Paleo-porosity and Formation Time Scale of a Chromitite-(Ortho)pyroxenite Cumulate Section, Bushveld, South Africa

To evaluate compaction and interstitial melt expulsion during cumulate formation, a 20 m cumulate section including the UG2 and UG3 chromitites from a 264 m drill core through the Upper Critical Zone of the Bushveld Complex (South Africa) has been studied. The cumulates in the studied section are as follows: 3 m plagioclase pyroxenite to pyroxenite, pegmatoid footwall pyroxenite at the lower contact to UG2, 0·7 m UG2 chromitite, 6·8 m pyroxenite, 0·24 m UG3 chromitite, 2·0 m plagioclase-rich pyroxenite changing locally to norite, the two 5 cm leader stringers UG3a and UG3b, and 7 m total of olivine pyroxenites grading into plagioclase pyroxenites. All pyroxenites are dominated by orthopyroxene (opx) and the cumulate sequence is topped by mottled anorthosite grading into norite. Stratigraphic concentrations of major and trace elements of 52 bulk-rock samples were determined. Bulk-rock Mg-numbers are 0·79-0·81 throughout the silicate cumulate units, and 0·40-0·46 in the chromitite layers. The stratigraphic distribution of six incompatible trace elements (K, Rb, Ba, Cs, Zr and Th) has been used to determine the amount of trapped liquid (FTL) or paleo-porosity in the cumulate rocks. Final porosities (volume fractions), based on averages from the six trace elements, are 0·06-0·33 in the pyroxenites. In chromitite layers, trapped melt fractions of 0·12-0·36 are calculated from incompatible trace element concentrations, but bulk SiO2 concentrations and X-ray tomography yield 0·04-0·17 higher porosities. Hence, the bulk silicate fraction in the chromitites may not necessarily correspond to the trapped liquid fraction, as poikilitic opx was crystallizing while the silicate melt still equilibrated. Using a previously derived experiment-based model for compaction time scales, gravitationally driven chemical compaction in the UG2-UG3-pyroxenite section is calculated to occur within 1-10 years. This time frame corresponds to the times necessary to cool a 20 m layer by 10-50°C, the temperature interval argued to encompass the liquidus and almost complete solidification. Compaction within a decade can in fact easily develop the paleo-porosities indirectly observed today and is probably stopped by crystallization of the interstitial liquid. Contrary to previous assertions, melt expulsion from the cumulate pile does not hinder compaction; calculated permeabilities would allow for the migration of an order of magnitude higher amount of melt than has to be expelled from the 20 m pile of cumulate. The pegmatoid zones in the chromitite footwalls enriched in incompatible trace elements are consistent with a collection of interstitial melts expelling from the underlying compacting pyroxenites. Their entrapment below the chromitite layers suggests that these act as permeability barriers. This is in part due to their finer grain size compared with the pyroxenites, but is mainly due to the crystallization of large poikilitic opx during compactio

Melting of Amphibole-bearing Wehrlites: an Experimental Study on the Origin of Ultra-calcic Nepheline-normative Melts

Olivine + clinopyroxene ± amphibole cumulates have been widely documented in island arc settings and may constitute a significant portion of the lowermost arc crust. Because of the low melting temperature of amphibole (∼1100°C), such cumulates could melt during intrusion of primary mantle magmas. We have experimentally (piston-cylinder, 0·5-1·0 GPa, 1200-1350°C, Pt-graphite capsules) investigated the melting behaviour of a model amphibole-olivine-clinopyroxene rock, to assess the possible role of such cumulates in island arc magma genesis. Initial melts are controlled by pargasitic amphibole breakdown, are strongly nepheline-normative and are Al2O3-rich. With increasing melt fraction (T > 1190°C at 1·0 GPa), the melts become ultra-calcic while remaining strongly nepheline-normative, and are saturated with olivine and clinopyroxene. The experimental melts have strong compositional similarities to natural nepheline-normative ultra-calcic melt inclusions and lavas exclusively found in arc settings. The experimentally derived phase relations show that such natural melt compositions originate by melting according to the reaction amphibole + clinopyroxene = melt + olivine in the arc crust. Pargasitic amphibole is the key phase in this process, as it lowers melting temperatures and imposes the nepheline-normative signature. Ultra-calcic nepheline-normative melt inclusions are tracers of magma-rock interaction (assimilative recycling) in the arc crus

Ultra-calcic Magmas Generated from Ca-depleted Mantle: an Experimental Study on the Origin of Ankaramites

Ultra-calcic ankaramitic magmas or melt inclusions are ubiquitous in arc, ocean-island and mid-ocean ridge settings. They are primitive in character (XMg > 0·65) and have high CaO contents (>14 wt %) and CaO/Al2O3 (>1·1). Experiments on an ankaramite from Epi, Vanuatu arc, demonstrate that its liquidus surface has only clinopyroxene at pressures of 15 and 20 kbar, with XCO2 in the volatile component from 0 to 0·86. The parental Epi ankaramite is thus not an unfractionated magma. However, forcing the ankaramite experimentally into saturation with olivine, orthopyroxene and spinel results in more magnesian, ultra-calcic melts with CaO/Al2O3 of 1·21-1·58. The experimental melts are not extremely Ca-rich but high in CaO/Al2O3 and in MgO (up to 18.5 wt %), and would evolve to high-CaO melts through olivine fractionation. Fractionation models show that the Epi parent magma can be derived from such ultra-calcic experimental melts through mainly olivine fractionation. We show that the experimental ultra-calcic melts could form through low-degree melting of somewhat refractory mantle. The latter would have been depleted by previous melt extraction, which increases the CaO/Al2O3 in the residue as long as some clinopyroxene remains residual. This finding corrects the common assumption that ultra-calcic magmas must come from a Ca-rich pyroxenite-type source. The temperatures necessary for the generation of ultra-calcic magmas are ≥1330°C, and their presence would suggest melting regimes that are at the upper temperature end of previous interpretations made on the basis of picritic magma

Fractional crystallization of high-K arc magmas: biotite- versus amphibole-dominated fractionation series in the Dariv Igneous Complex, Western Mongolia

Many studies have documented hydrous fractionation of calc-alkaline basalts producing tonalitic, granodioritic, and granitic melts, but the origin of more alkaline arc sequences dominated by high-K monzonitic suites has not been thoroughly investigated. This study presents results from a combined field, petrologic, and whole-rock geochemical study of a paleo-arc alkaline fractionation sequence from the Dariv Range of the Mongolian Altaids. The Dariv Igneous Complex of Western Mongolia is composed of a complete, moderately hydrous, alkaline fractionation sequence ranging from phlogopite-bearing ultramafic and mafic cumulates to quartz–monzonites to late-stage felsic (63–75 wt% SiO2) dikes. A volumetrically subordinate more hydrous, amphibole-dominated fractionation sequence is also present and comprises amphibole (±phlogopite) clinopyroxenites, gabbros, and diorites. We present 168 whole-rock analyses for the biotite- and amphibole-dominated series. First, we constrain the liquid line of descent (LLD) of a primitive, alkaline arc melt characterized by biotite as the dominant hydrous phase through a fractionation model that incorporates the stepwise subtraction of cumulates of a fixed composition. The modeled LLD reproduces the geochemical trends observed in the “liquid-like” intrusives of the biotite series (quartz–monzonites and felsic dikes) and follows the water-undersaturated albite–orthoclase cotectic (at 0.2–0.5 GPa). Second, as distinct biotite- and amphibole-dominated fractionation series are observed, we investigate the controls on high-temperature biotite versus amphibole crystallization from hydrous arc melts. Analysis of a compilation of hydrous experimental starting materials and high-Mg basalts saturated in biotite and/or amphibole suggests that the degree of K enrichment controls whether biotite will crystallize as an early high-T phase, whereas the degree of water saturation is the dominant control of amphibole crystallization. Therefore, if a melt has the appropriate major-element composition for early biotite and amphibole crystallization, as is true of the high-Mg basalts from the Dariv Igneous Complex, the relative proximity of these two phases to the liquidus depends on the H2O concentration in the melt. Third, we compare the modeled high-K LLD and whole-rock geochemistry of the Dariv Igneous Complex to the more common calc-alkaline trend. Biotite and K-feldspar fractionation in the alkaline arc series results in the moderation of K2O/Na2O values and LILE concentrations with increasing SiO2 as compared to the more common calc-alkaline series characterized by amphibole and plagioclase crystallization and strong increases in K2O/Na2O values. Lastly, we suggest that common calc-alkaline parental melts involve addition of a moderate pressure, sodic, fluid-dominated slab component while more alkaline primitive melts characterized by early biotite saturation involve the addition of a high-pressure potassic sediment melt

Petrogenesis of Pyroxenites and Melt Infiltrations in the Ultramafic Complex of Beni Bousera, Northern Morocco

The origin of pyroxenites and their relation to melt migration in the mantle have been investigated in two pyroxenite-rich zones in the Beni Bousera massif. Based on combined field, microtextural, mineralogical and geochemical observations, the pyroxenites were separated into four types. Type-I Cr-diopside websterites contain bright green diopside and have primitive bulk Ni, Cr and Mg-number. Their trace element systematics are characterized by slight light rare earth element (LREE) enrichment compared with the middle (MREE) and heavy (H)REE, and negative high field strength element (HFSE) anomalies in bulk-rock and mineral compositions suggesting that they result from melting of metasomatized mantle. Trace element concentrations of melts calculated to be in equilibrium with Type-I cpx have a subduction-like signature and show a close similarity to certain lavas erupted in the Alboran Basin. Calculated mineral equilibration temperatures of ∼1200 to 1350°C are close to the basalt liquidus and higher than for other pyroxenite types in Beni Bousera, which generally yield 45 to 20-30 kbar. Type-III pyroxenites display strong variations of LREE and HFSE depletion and strong bulk Nb/Ta fractionation. Calculated melts in equilibrium with augitic cpx are variably enriched in incompatible trace elements similar to intraplate basalts. Type-IV pyroxenites are composed of green diopside, opx, garnet and plagioclase and/or spinel. Whole-rocks have high Na2O, CaO and Al2O3 concentrations and high Mg-number, are HREE depleted, and have positive Eu and Sr anomalies. Garnets are characterized by low HREE/MREE and positive Eu anomalies. The absence of bulk-rock HREE enrichment indicates a metamorphic origin for this garnet, which is corroborated by the presence of Al-rich metamorphic spinels. Relict magmatic plagioclase indicates a shallower (<10 kbar) crustal origin for these pyroxenites. Their metamorphic assemblage yields temperatures and pressures of 800-980°C and 14 kbar, indicating a pressure increase during the metamorphic overprint. The whole-rock geochemistry of Type-IV pyroxenites is comparable with that of rocks from the lower crustal section of the Kohistan (northern Pakistan) paleo-arc, indicating a possible origin of these rocks as cumulates in the deeper arc crust and subsequent delamination into the underlying mantl

Element Partitioning between Immiscible Carbonatite and Silicate Melts for Dry and H2O-bearing Systems at 1-3 GPa

Carbonatite and silicate rocks occurring within a single magmatic complex may originate through liquid immiscibility. We thus experimentally determined carbonatite/silicate melt partition coefficients (Dcarbonate melt/silicate melt, hereafter D) for 45 elements to understand their systematics as a function of melt composition and to provide a tool for identifying the possible conjugate nature of silicate and carbonatite magmas. Static and, when necessary, centrifuging piston cylinder experiments were performed at 1-3 GPa, 1150-1260°C such that two well-separated melts resulted. Bulk compositions had Na K, Na ∼ K, and Na K; for the latter we also varied bulk H2O (0-4 wt %) and SiO2 contents. Oxygen fugacities were between iron-wüstite and slightly below hematite-magnetite and were not found to exert significant control on partitioning. Under dry conditions alkali and alkaline earth elements partition into the carbonatite melt, as did Mo and P (DMo >8, DP= 1·6-3·3). High field strength elements (HFSE) prefer the silicate melt, most strongly Hf (DHf = 0·04). The REE have partition coefficients around unity with DLa/Lu = 1·6-2·3. Transition metals have D < 1 except for Cu and V (DCu ∼ 1·3, DV = 0·95-2). The small variability of the partition coefficients in all dry experiments can be explained by a comparable width of the miscibility gap, which appears to be flat-topped in our dry bulk compositions. For all carbonatite and silicate melts, Nb/Ta and Zr/Hf fractionate by factors of 1·3-3·0, in most cases much more strongly than in silicate-oxide systems. With the exception of the alkalis, partition coefficients for the H2O-bearing systems are similar to those for the anhydrous ones, but are shifted in favour of the carbonatite melt by up to an order of magnitude. An increase of bulk silica and thus SiO2 in the silicate melt (from 35 to 69 wt %) has a similar effect. Two types of trace element partitioning with changing melt composition can be observed. The magnitude of the partition coefficients increases for the alkalis and alkaline earths with the width of the miscibility gap, whereas partition coefficients for the REE shift by almost two orders of magnitude from partitioning into the silicate melt (DLa = 0·47) to strongly partitioning into the carbonatite melt (DLa = 38), whereas DLa/DLu varies by only a factor of three. The partitioning behavior can be rationalized as a function of ionic potential (Z/r). Alkali and alkaline earth elements follow a trend, the slope of which depends on the K/Na ratio and H2O content. Contrasting the sodic and potassic systems, alkalis have a positive correlation in D vs Z/r space in the potassic case and Cs to K partition into the silicate melt in the presence of H2O. For the divalent third row transition metals on the one hand and for the tri- and tetravalent REE and HFSE on the other, two trends of negative correlation of D vs Z/r can be defined. Nevertheless, the highest ionic strength network-modifying cations (V, Nb, Ta, Ti and Mo) do not follow any trend; understanding their behavior would require knowledge of their bonding environment in the carbonatite melt. Strong partitioning of REE into the carbonatite melt (DREE = 5·8-38·0) occurs only in H2O-rich compositions for which carbonatites unmix from evolved alkaline melts with the conjugate silicate melt being siliceous. We thus speculate that upon hydrous carbonatite crystallization, the consequent saturation in fluids may lead to hydrothermal systems concentrating REE in secondary deposit