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.     

Crystallization, melt inclusion, and redox history of a Martian meteorite: Olivine-phyric shergottite Larkman Nunatak 06319

The Larkman Nunatak (LAR) 06319 olivine-phyric shergottite is composed of zoned megacrysts of olivine (Fo_76-55 from core to rim), pyroxene (from core to rim En₇₀Fs₂₅Wo₅, En₅₀Fs₂₅Wo₂₅, and En₄₅Fs₄₅Wo₁₀), and Cr-rich spinel in a matrix of maskelynite (An₅₂Ab₄₅), pyroxene (En_30-40Fs_40-55Wo_10-25,), olivine (Fo₅₀), Fe-Ti oxides, sulfides, phosphates, Si-rich glass, and baddeleyite. LAR 06319 experienced equilibration shock pressures of 30-35 GPa based on the presence of localized shock melts, mechanical deformation of olivine and pyroxene, and complete transformation of plagioclase to maskelynite with no relict birefringence. The various phases and textures of this picritic basalt can be explained by closed system differentiation of a shergottitic melt. Recalculated parent melt compositions obtained from melt inclusions located in the core of the olivine megacrysts (Fo_>72) resemble those of other shergottite parent melts and whole-rock compositions, albeit with a lower Ca content. These compositions were used in the MELTS software to reproduce the crystallization sequence. Four types of spinel and two types of ilmenite reflect changes in oxygen fugacity during igneous differentiation. Detailed oxybarometry using olivine-pyroxene-spinel and ilmenite-titanomagnetite assemblages indicates initial crystallization of the megacrysts at 2 log units below the Fayalite-Magnetite-Quartz buffer (FMQ - 2), followed by crystallization of the groundmass over a range of FMQ - 1 to FMQ + 0.3. Variation is nearly continuous throughout the differentiation sequence.LAR 06319 is the first member of the enriched shergottite subgroup whose bulk composition, and that of melt inclusions in its most primitive olivines, approximates that of the parental melt. The study of this picritic basalt indicates that oxidation of more than two log units of FMQ can occur during magmatic fractional crystallization and ascent. Some part of the wide range of oxygen fugacities recorded in shergottites may consequently be due to this process. The relatively reduced conditions at the beginning of the crystallization sequence of LAR 06319 may imply that the enriched shergottite mantle reservoir is slightly more reduced than previously thought. As a result, the total range of Martian mantle oxygen fugacities is probably limited to FMQ − 4 to − 2. This narrow range could have been generated during the slow crystallization of a magma ocean, a process favored to explain the origin of shergottite mantle reservoirs.     

Petrogenesis of olivine-phyric shergottite Yamato 980459, revisited

Primitive magmas provide critical information on mantle sources, but most Martian meteorites crystallized from fractionated melts. An olivine-phyric shergottite, Yamato 980459 (Y-980459), has been interpreted to represent a primary melt, because its olivine megacrysts have magnesian cores (Fo84-86) that appear to be in equilibrium with the Y-980459 whole-rock composition based on Fe-Mg partitioning. However, crystal size distribution (CSD) plots for Y-980459 olivines show a size gap, suggesting a cumulus origin for some megacrysts. Because melting experiments using the Y-980459 whole-rock composition have been used to infer the thermal structure and volatile contents of the Martian mantle, the interpretation that this rock is primitive should be scrutinized.We report major, minor and trace element compositions of Y-980459 olivines and compare them with results from melting experiments (both hydrous and anhydrous) and thermodynamic calculations. Cores of the olivine megacrysts have major and minor element contents identical to those of the most magnesian olivines from the experiments, but they differ slightly from those of thermodynamic calculations. This is probably because the Y-980459 whole-rock composition lies near the limit of the range of liquids used to calibrate these models. The megacryst cores (Fo80-85) exhibit minor and trace element (Mn-Ni-Co-Cr-V) characteristics distinct from other olivines (megacryst rims and groundmass olivines, Fo < 80), implying that the megacryst cores crystallized under more reduced conditions (∼IW + 1).Y-980459 contains pyroxenes with orthopyroxene cores mantled by pigeonite and augite. We also found some reversely zoned pyroxenes that have augite cores (low-Mg#) mantled by orthopyroxenes (high-Mg#), although they are uncommon. These reversely zoned pyroxenes are interpreted to have grown initially as atoll-like crystals with later crystallization filling in the hollow centers, implying disequilibrium crystallization at a moderate cooling rate (3-7 °C/h). The calculated REE pattern of a melt in equilibrium with normally zoned pyroxene is parallel to those of glass and the Y-980459 whole-rock as well as other depleted olivine-phyric shergottites, suggesting that Y-980459 was derived from a depleted mantle reservoir.Considering the CSD patterns of Y-980459 olivines, we propose that the olivine megacrysts are cumulus crystals which probably formed in a feeder conduit by continuous melt replenishment, and the parent melt composition was indistinguishable from the Y-980459 whole-rock with 0-2 wt% of H₂O and 0-5 wt% of CO₂. The final magma pulse entrained these cumulus olivines and then crystallized groundmass olivines and pyroxenes. Although Y-980459 contains small amounts of cumulus olivine (<∼6 vol%), we conclude that the Y-980459 whole-rock composition closely approximates a Martian primary melt composition.     

Petrology and trace element geochemistry of Robert Massif 04261 and 04262 meteorites, the first examples of geochemically enriched lherzolitic shergottites

Shergottites sampled two distinct geochemical reservoirs on Mars. Basaltic and olivine-phyric shergottites individually sampled both geochemically enriched and depleted reservoirs, whereas lherzolitic shergottites are previously known only to exhibit a relatively limited intermediate geochemical signature that may have resulted from the mixing of the two geochemical end-member reservoirs. Here we show that recently discovered shergottites Robert Massif (RBT) 04261 and RBT 04262 are the first examples of lherzolitic shergottites originating from the enriched reservoir.RBT 04261 and RBT 04262, initially identified as olivine-phyric shergottites, are actually lherzolitic shergottites. Both meteorites exhibit nearly identical textures and mineral compositions, suggesting that they should be paired. Each consists of two distinct textures: poikilitic and non-poikilitic. The poikilitic areas are composed of pyroxene oikocrysts enclosing olivine grains; all pyroxene oikocrysts have pigeonite cores mantled by augite. The non-poikilitic areas are composed of olivine, pyroxene, maskelynite and minor amounts of merrillite, chromite and ilmenite. Olivine and pyroxene show the lowest Mg-number, and maskelynite has the lowest anorthite component among the lherzolitic shergottites. Moreover, the modal abundances of maskelynite in these two meteorites are distinctly higher than the other lherzolitic shergottites.The rare earth element (REE) budgets of RBT 04261 and RBT 04262 are dominated by merrillite. The slightly light rare earth element (LREE)-enriched pattern of this mineral is similar to that of merrillite in the geochemically enriched basaltic shergottites Shergotty and Zagami, and unlike the LREE-depleted pattern of merrillite in the other lherzolitic shergottites. The REE patterns of both high- and low-Ca pyroxenes are also similar to those in Shergotty and Zagami. The REE pattern of a melt calculated to be in equilibrium with the core of a pyroxene oikocryst is parallel to that of the RBT 04262 whole-rock as well as whole-rock compositions of other geochemically enriched basaltic shergottites. These observations imply that RBT 04262 sampled an enriched and oxidized reservoir similar to that sampled by some of the basaltic shergottites and are consistent with an oxidizing condition for the formation of RBT 04262 (log fO₂ = QFM-1.6).The petrographic and geochemical observations presented here suggest that RBT 04261 and RBT 04262 represent the most evolved magma among the lherzolitic shergottites and that this magma originated from a geochemically enriched reservoir on Mars. Based on an evaluation of the relationship between petrographic, geochemical and chronological signatures for shergottites including RBT 04261 and RBT 04262, we propose that both geochemically enriched and depleted shergottites were ejected from the same launch site on Mars.     

Petrology and chemistry of the Picritic Shergottite North West Africa 1068 (NWA 1068)

We report on the petrology and chemistry of North West Africa 1068 (NWA 1068), a shergottite recently recovered in Morocco. This meteorite has a total known mass of about 577 g and comprises 23 fragments. The largest fragment is a greenish-brown rock devoid of fusion crust. It displays a porphyritic texture consisting of a fine-grained groundmass and olivine grains. Excluding the impact melt pockets and the minor carbonate veins produced by terrestrial weathering, modal analyses indicate the following mineral proportions: 52 vol% pyroxenes, 22% maskelynite, 21% olivine, 2% phosphates (merrillite and chlorapatite), 2% opaque oxides (mainly ilmenite and chromite) and sulfides, and 1% K-rich mesostasis. Olivines with various habits occur as clusters often associated with chromite, or single crystals ranging in size from 50 μm to 2 millimeters (“megacrysts”). These crystals originate probably from disrupted cumulates with strong affinities with peridotitic shergottites.The bulk composition of NWA 1068 has been determined for 45 elements. It is an Al-poor ferroan basaltic rock, rich in MgO. Its major element composition is similar to those reported for other picritic shergottites, especially EETA79001A. Furthermore, key element ratios such as Fe/Mn (45), Al/Ti (6.6), Na/Ti (1.83), Ga/Al (4.4 × 10⁻⁴) and Na/Al (0.28) are typical of Martian meteorites. The trace elements demonstrate unambiguously that NWA 1068 is unpaired with any of the other hot desert finds: it is the first picritic shergottite with a REE pattern similar to those of Shergotty, Zagami, and Los Angeles.Incompatible element abundances indicate that NWA 1068 was not formed from a “primitive” shergottitic melt. It derived more likely from a basaltic shergottite, which has accumulated (and possibly partly digested) fragments of an olivine-rich lithology, in full agreement with major element abundances and petrographical interpretations.     

Petrography and geochemistry of the chassignite Northwest Africa 2737 (NWA 2737)

We report on the petrology and geochemistry of the Northwest Africa 2737 (NWA 2737) meteorite that was recovered from the Morrocan Sahara in 2000. It is the second member of the chassignite subclass of the SNC (Shergotitte-Nakhlite-Chassignite) group of meteorites that are thought to have originated on Mars. It consists of black olivine- and spinel-cumulate crystals (89.7 and 4.6 wt%, respectively), with intercumulus pyroxenes (augite 3.1 wt% and pigeonite-orthopyroxene 1.0 wt%), analbite glass (1.6 wt%) and apatite (0.2 wt%). Unlike Chassigny, plagioclase has not been observed in NWA 2737. Olivine crystals are rich in Mg, and highly equilibrated (Fo = 78.7 ± 0.5 mol%). The black color of olivine grains may be related to the strong shock experienced by the meteorite as revealed by the deformation features observed on the macroscopic to the atomic scale. Chromite is zoned from core to rim from Cr_83.4Uv_3.6Sp_13.0 to Cr_72.0Uv_6.9Sp_21.1. Pyroxene compositional trends are similar to those described for Chassigny except that they are richer in Mg. Compositions range from En_78.5Wo_2.7Fs_18.8 to En_76.6Wo_3.2Fs_20.2 for the orthopyroxene, from En_73.5Wo_8.0Fs_18.5 to En_64.0Wo_22.1Fs_13.9for pigeonite, and from En_54.6Wo_32.8Fs_12.6 to En_46.7Wo_44.1Fs_9.2 for augite. Bulk rock oxygen isotope compositions confirm that NWA 2737 is a new member of the martian meteorite clan (Δ¹⁷O = 0.305 ± 0.02‰, n = 2). REE abundances measured in NWA 2737 mineral phases are similar to those in Chassigny and suggest a genetic relationship between these two rocks. However, the parent melt of NWA 2737 was less evolved and had a lower Al abundance.     

Comparative petrology, geochemistry, and petrogenesis of evolved, low-Ti lunar mare basalt meteorites from the LaPaz Icefield, Antarctica

New data is presented for five evolved, low-Ti lunar mare basalt meteorites from the LaPaz Icefield, Antarctica, LAP 02205, LAP 02224, LAP 02226, LAP 02436, and LAP 03632. These basalts have nearly identical mineralogies, textures, and geochemical compositions, and are therefore considered to be paired. The LaPaz basalts contain olivine (Fo_64-2) and pyroxene (Fs₃₂Wo₈En₆₀ to Fs_84-86Wo₁₅En_2-0) crystals that record extreme chemical fractionation to Fe-enrichment at the rims, and evidence for silicate liquid immiscibility and incompatible element enrichment in the mesostasis. The basalts also contain FeNi metals with unusually high Co and Ni contents, similar to some Apollo 12 basalts, and a single-phase network of melt veins and fusion crusts. The fusion crust has similar chemical characteristics to the whole rock for the LaPaz basalts, whereas the melt veins represent localized melting of the basalt and have an endogenous origin. The crystallization conditions and evolved nature of the LaPaz basalts are consistent with fractionation of olivine and chromite from a parental liquid similar in composition to some olivine-phyric Apollo 12 and Apollo 15 basalts or lunar low-Ti pyroclastic glasses. However, the young reported ages for the LaPaz mare basalts (∼2.9 Ga) and their relative incompatible element enrichment compared to Apollo mare basalts and pyroclastic glasses indicate they cannot be directly related. Instead, the LaPaz mare basalts may represent fractionated melts from a magmatic system fed by similar degrees of partial melting of a mantle source similar to that of the low-Ti Apollo mare basalts or pyroclastic glasses, but which possessed greater incompatible element enrichment. Despite textural differences, the LaPaz basalts and mare basalt meteorite NWA 032 have similar ages and compositions and may originate from the same magmatic system on the Moon.     

Mineralogy and petrology of the diabasic rocks in a differentiated olivine diabase sill complex,Sierra Ancha,Arizona

The Precambrian Sierra Ancha sill complex, more than 700 feet thick, is a multiple intrusion with a central layer of feldspathic olivine-rich diabase, and upper and lower layers of olivine diabase derived from a high-alumina basalt magma. Minor rock types include albite diabase and albite-diabase pegmatite. Deuteric alteration was extensive. Principal primary minerals are plagioclase (An₇₂ to An₁₆), augite (Wo₄₃En₄₄Fs₁₃ to Wo₄₀En₃₈Fs₂₂), olivine (Fo₇₄ to Fo₅₄), orthopyroxene (En₇₇ to En₄₄), magnetite (Mgt₆₆Usp₃₄ to Mgt₈₉Usp₁₁), and ilmenite (Ilm₈₆Hem₁₄ to Ilm₉₆Hem₄). Ilmenite formed by reaction-exsolution from magnetite_ss is consistently different in compositon from primary ilmenite. Primary ilmenite became enriched in Mn and depleted in Mg as crystallization proceded. A systematic Fe-Mg partition between contacting olivine and orthopyroxene suggests that equilibrium prevailed on an extremely local scale during crystallization. Albite-diabase pegmatite contains a mineral assemblage including augite, ferrosalite (Wo₄₉En₂₈Fs₂₃ to Wo₄₉En₁₄Fs₃₇), albite (An₂ to An₀), and iron-rich chlorite. Altered diabase and albite diabase also have unusually calcium-rich pyroxenes. The calcium-rich pyroxenes, which occur in assemblages like those characterizing some spilites, are richer in calcium and lower in aluminum and titanium than basaltic augite.Contribution No. 1712 of the Division of Geological Sciences, California Institute of Technology, Pasadena, California.     

Petrogenesis of the Elephant Moraine A79001 meteorite: Multiple magma pulses on the shergottite parent body

The EETA 79001 achondrite consists of two distinct igneous lithologies joined along a planar, non-brecciated contact. Both are basaltic rocks composed primarily of pigeonite, augite, and maskelynite, but one contains zoned megacrysts of olivine, orthopyroxene, and chromite that represent disaggregated xenoliths of harzburzite. Both lithologies probably formed from successive volcanic flows or multiple injections of magma into a small, shallow chamber. Many similarities between the two virtually synchronous magmas suggest that they are related. Possible mechanisms to explain their differences involve varying degrees of assimilation, fractionation from similar parental magmas, or partial melting of a similar source peridotite; of these, assimilation of the observed megacryst assemblage seems most plausible. However, some isotopic contamination may be required in any of these petrogenetic models. The meteorite has suffered extensive shock metamorphism and localized melting during a large impact event that probably excavated and liberated it from its parent body. Both basaltic lithologies and the inferred ultramafic protolith of the megacryst assemblage are petrologically similar to other members of the shergottite group, and all may have been derived from a volcanic-plutonic complex on a planetary body.     

Northwest Africa 773: lunar mare breccia with a shallow-formed olivine-cumulate component, inferred very-low-Ti (VLT) heritage, and a KREEP connection

Lunar meteorite Northwest Africa 773 (herein referred to as NWA773) is a breccia composed predominantly of mafic volcanic components, including a prominent igneous clast lithology. The clast lithology is an olivine-gabbro cumulate, which, on the basis of mineral and bulk compositions, is a hypabyssal igneous rock related compositionally to volcanic components in the meteorite. The olivine-gabbro lithology exhibits cumulus textures and, in our largest section of it, includes some 48% olivine (Fo₆₄ to Fo₇₀, average Fo₆₇), 27% pigeonite (En₆₀Fs₂₄Wo₁₆ to En₆₇Fs₂₇Wo₆), 11% augite (En₅₀Fs₁₇Wo₃₃ to En₄₇Fs₁₃Wo₄₀), 2% orthopyroxene (En₇₀Fs₂₆Wo₄), 11% plagioclase (An₈₀ to An₉₄), and trace barian K-feldspar, ilmenite, Cr-spinel, RE-merrillite, troilite, and Fe-Ni metal. The Mg/Fe ratios of the mafic silicates indicate equilibration of Fe and Mg; however, the silicates retain compositional variations in minor and trace elements that are consistent with intercumulus crystallization. Accessory mineralogy reflects crystallization of late-stage residual melt. Both lithologies (breccia and olivine cumulate) of the meteorite have very-low-Ti (VLT) major-element compositions, but with an unusual trace-element signature compared to most lunar VLT volcanic compositions, i.e., relative enrichment in light REE and large-ion-lithophile elements, and greater depletion in Eu than almost all other known lunar volcanic rocks. The calculated composition of the melt that was in equilibrium with pyroxene and plagioclase of the cumulate lithology exhibits a KREEP-like REE pattern, but at lower concentrations. Melt of a composition calculated to have been in equilibrium with the cumulate assemblage, plus excess olivine, yields a major-element composition that is similar to known green volcanic glasses. One volcanic glass type from Apollo 14 in particular, green glass B, type 1, has a very low Ti concentration and REE characteristics, including extremely low Eu concentration, that make it a candidate parent melt for the olivine-gabbro cumulate. We infer an origin for the parent melt of NWA773 volcanic components by assimilation of a trace-element-rich partial or residual melt by a magnesian, VLT magma deep in the lunar crust or in the mantle prior to transportation to the near-surface, accumulation of olivine and pyroxene in a shallow chamber, eruption onto a volcanic surface, and incorporation of components into local, predominantly volcanic regolith, prior to impact mixing of the volcanic terrain and related hypabyssal setting, and ejection from the surface of the Moon. Volcanic components such as these probably occur in the Oceanus Procellarum region near the site of origin of the green volcanic glasses found in the Apollo 14 regolith.     

Concordant Rb-Sr, Sm-Nd, and Ar-Ar ages for Northwest Africa 1460: A 346 Ma old basaltic shergottite related to “lherzolitic” shergottites

Multiple lines of evidence show that the Rb-Sr, Sm-Nd, and Ar-Ar isotopic systems individually give robust crystallization ages for basaltic (or diabasic) shergottite Northwest Africa (NWA) 1460. In contrast to other shergottites, NWA 1460 exhibits minimal evidence of excess ⁴⁰Ar, thus allowing an unambiguous determination of its Ar-Ar age. The concordant Rb-Sr, Sm-Nd, and Ar-Ar results for NWA 1460 define its crystallization age to be 346 ± 17 Ma (2σ). In combination with petrographic and trace element data for this specimen and paired meteorite NWA 480, these results strongly refute the suggestion by others that the shergottites are ∼4.1 Ga old. Current crystallization and cosmic-ray exposure (CRE) age data permit identification of a maximum of nine ejection events for Martian meteorites (numbering more than 50 unpaired specimens as of 2008) and plausibly as few as five such events. Although recent high resolution imaging of the Martian surface has identified limited areas of sparsely cratered terrains, the meteorite data suggest that either these areas are representative of larger areas from which the meteorites might come, or that the cratering chronology needs recalibration. Time-averaged ⁸⁷Rb/⁸⁶Sr = 0.16 for the mantle source of the parent magma of NWA 1460/480 over the ∼4.56 Ga age of the planet is consistent with previously estimated values for bulk silicate Mars in the range 0.13-0.16, and similar to values of ∼0.18 for the “lherzolitic” shergottites. Initial ε_Nd for NWA 1460/480 at 350 ± 16 Ma ago was +10.6 ± 0.5, which implies a time-averaged ¹⁴⁷Sm/¹⁴⁴Nd of 0.217 in the Martian mantle prior to mafic melt extraction, similar to values of 0.211-0.216 for the “lherzolitic” shergottites. These time-averaged values do not imply a simple two-stage mantle/melt evolution, but must result from multiple episodes of melt extractions from the source regions. Much higher “late-stage” ε_Nd values for the depleted shergottites imply similar processes carried to a greater degree. Thus, NWA 1460/480, the “lherzolitic” shergottites and perhaps EET 79001 give the best (albeit imperfect) estimate of the Sr- and Nd-isotopic characteristics of bulk silicate Mars.     

Phase relations in transitional and alkali basaltic glasses from Iceland

Basaltic glasses from the three alkalic areas of Iceland (Snaefellsnes Volcanic Zone, Sudurland Volcanic Zone and Vestmannaeyjar Volcanic Area) contain plagioclase, olivine, clinopyroxene, chromian spinel and titanomagnetite as phenocryst phases. The glasses are hypersthene to nepheline normative alkali basaltic with FeO/ MgO ratios between 1.4–4.7. Olivine ranges in composition from Fo₉₀ to Fo₅₅, plagioclase from An₉₀ to An₅₀ and clinopyroxene from En₄₅Fs₁₀Wo₄₅ to En₄₀Fs₁₇Wo₄₃. Clinopyroxene reveals systematic Ti:Al metastable crystallization trends related to the composition of the enclosing glass. Two types of phenocryst are present in most glasses and show a bimodality in size and composition. Microphenocryst phases are those most likely to have crystallized from the enclosing glass, while macrophenocrysts may have crystallized from a liquid of slightly less evolved composition. The glasses show complex phenocryst-glass relations which can be related to a polybaric effect. The normative glass compositions are related to 2-phase cotectic surfaces in the basalt tetrahedron and define the position of the 3-phase cotectic line. In general with increasing FeO/MgO in the glass the phenocryst assemblages vary from clinopyroxene, olivine and plagioclase along a clinopyroxene-olivine surface to olivine and plagioclase along an olivine-plagioclase surface. The normative glass compositions show a deflection from clinopyroxene-bearing to clinopyroxene-free glasses. The appearance of plagioclase together with clinopyroxene and olivine can be explained in the light of experimental investigations of the effect of pressure on phase relations. The major element variation of the glasses is interpreted as representing mantle derived magma batches of primary liquids, modified to some degree by high (6 kbar) and intermediate to low pressure (below 3 kbar) crystal fractionation towards equilibrium phase relations during ascent and residence in crustal magma chambers. The observed deflection in normative compositions of the glasses marks the position of the high pressure 3-phase cotectic line. The bimodality in size and composition of plagioclase and olivine phenocrysts can be related to high pressure crystal fractionation in the melt. The Fe-Ti basalt glasses from Sudurland are believed to be quenched high pressure compositions.     

Pyroxene-selective impact smelting in ureilites

A characteristic feature of ureilite meteorites is reduction of FeO. But the reduction is usually confined to the rims of olivine. In the LAR 04315, LAP 03587 and Almahata Sitta ureilites, pyroxene was extensively reduced by impact smelting. In LAR 04315, the impact caused nearly all of the original pigeonite to melt or otherwise become sufficiently structurally compromised to allow smelting, and yet a minor proportion of the pyroxene escaped smelting and survived with its original composition (En_74.1Wo_10.2). Olivine mosaicism confirms that LAR 04315 experienced a major shock event. The smelted pyroxenes also show a distinctive patchiness in their interference colors (although each grain’s basic optical continuity, often including twinning, is still discernible). They also have reduced compositions, are ubiquitously porous (∼15%), and contain sprinklings of Fe-metal and felsic glass. For the most part the olivine underwent only very slight reduction. Much of the (small) pyroxene component of LAP 03587 shows the same oddly porous texture. LAR 04315 also contains large traces of silica and felsic glass (with a typical composition of, in wt%, 61 SiO₂, 23 Al₂O₃, 11 CaO, 3.7 Na₂O) glass; these two phases together form selvages that line the walls of many of the largest voids in the rock. Silica is a by-product of pyroxene smelting. The felsic glass probably derives largely from interstitial basaltic melt that predated the impact. However, the comparatively stiff surrounding/included silica may have promoted unusually high melt retention within LAR 04315 through the smelting episode (one aspect of which was a major stream-out, through the same large voids, of CO_x gas). The impact-smelted pyroxene of LAP 03587 is enigmatic because this ureilite also features little-shocked euhedral graphite laths and no olivine mosaicism. The fine-grained ureilitic component of Almahata Sitta appears to have likewise formed by impact smelting, but with more extensive melting of pyroxene (especially a Ca-rich pyroxene component), more pulverization and melting of olivine, and more displacement of both. However, in places the original coarse-equant ureilite texture is still discernible in relict form. Ordinarily, an impact shock melts olivine before, or at least no later than, pyroxene. But in the case of LAR 04315 and LAP 03587, the great shock event evidently occurred when the material was already anatectic or very nearly so; and thus the difference in melting temperature between pyroxene and olivine, ∼300 degrees lower for pyroxene, was decisive. If literature inferences of extremely fast cooling rates, implying shallow burial depths, are accurate, the proportion of CO_x gas generated by ureilite smelting exceeded by a very large factor (of order 10³ but possibly much greater) the volume represented as porosity in the final ureilites. The outflow of so much gas may have, by near-surface explosive expansion and jetting, enhanced the thoroughness of the impact-triggered catastrophic impact disruption of the parent asteroid.     

Chrome-diopside Megacryst-bearing Lamprophyre from the Late Cretaceous Mundwara Alkaline Complex,NW India: Petrological and Geodynamic Implications

The occurrence of a rare mantle-derived chrome-diopside megacryst (～8 mm), containing inclusions of olivine, in a lamprophyre dyke from the late Cretaceous polychronous (～100 – 68 Ma) Mundwara alkaline complex of NW India is reported. The olivine inclusions are forsteritic (Fo: 85.23) in composition, and their NiO (0.09 wt%) and CaO (0.13 wt%) contents imply derivation from a peridotitic mantle source. The composition of the chrome diopside (Cr₂O₃: 0.93 wt ) (Wo_45.27 En_48.47 Fs_5.07 and Ac_1.18) megacryst is comparable to that occurring in the garnet peridotite xenoliths found in diamondiferous kimberlites from Archaean cratons. Single pyroxene thermobarometry revealed that this chrome diopside megacryst was derived from a depth range of ～100 km, which is relatively much deeper than that of the chrome-diopside megacrysts (～40–50 km) reported in spinellherzolite xenoliths from the alkali basalts of Deccan age (ca. 66–67 Ma) from the Kutch, NW India. This study highlights that pre- Deccan lithosphere, below the Mundwara alkaline complex, was at least ～100 km thick and, likely, similar in composition to that of the cratonic lithosphere.     

Complex zoning between super-calcic pigeonite and augite from the Graveyard Point sill, Oregon: a record of the interplay between bulk and interstitial liquid fractionation

The 150 m thick late Miocene Graveyard Point sill (GPS) is situated at the Idaho-Oregon border near the southwestern edge of the western Snake River Plain. It records from bottom to top continuous fractional crystallization of a tholeiitic parent magma (lower chilled border, FeO/(FeO+MgO) = 0.59, Ni = 90 ppm) towards granophyres (late pods and dikes, FeO/(FeO+MgO) = 0.98, 78 wt% SiO₂ 3.5 wt% K₂O, <4 ppm Ni) showing a typical trend of Fe and P enrichment. Fractionating minerals are olivine (Fo₇₉-Fo₂), augite (X _Fe = 0.18−0.95), feldspars (An₈₀Or₁-An₁Or₆₂), Fe-Ti oxides (Ti-rich magnetite and ilmenite), apatite and in two samples super-calcic pigeonite (Wo_18–28 Fs_41–54). The granophyres may bear some quartz. Compositionally zoned minerals record a large interval of the fractionation process in every single sample, but this interval changes with stratigraphic height. In super-calcic pigeonite-bearing samples, olivine is scarce or lacking and because super-calcic pigeonite occurs as characteristic overgrowths on augite, its formation is interpreted to be related to the schematic reaction: augite + olivine (component in melt) + SiO₂ (in melt) = pigeonite, that defines the cotectic between augite and pigeonite in olivine-saturated basaltic systems. Line measurements with the electron microprobe reveal that the transition from augite to super-calcic pigeonite is continuous. However, some crystals show an abrupt “reversal” towards augite after super-calcic pigeonite growth. Two processes compete with each other in the GPS: fractional crystallization of the bulk liquid (the bulk melt separates from solids and interstitial liquids in the solidification front) and fractional crystallization of interstitial melt in the solidification front itself. Interplay between those two processes is proposed to account for the observed variations in mineral chemistry and mineral textures. Received: 25 November 1998 / Accepted: 14 June 1999     

Petrogenesis of olivine-phyric shergottites Sayh al Uhaymir 005 and Elephant Moraine A79001 lithology A

Martian meteorites Sayh al Uhaymir (SaU) 005 and lithology A of EETA79001 (EET-A) belong to a newly emerging group of olivine-phyric shergottites. Previous models for the origin of such shergottites have focused on mixing between basaltic shergottite-like magmas and lherzolitic shergottite-like material. Results of this work, however, suggest that SaU 005 and EET-A formed from olivine-saturated magmas that may have been parental to basaltic shergottites.SaU 005 and EET-A have porphyritic textures of large (up to ∼3 mm) olivine crystals (∼25% in SaU 005; ∼13% in EET-A) in finer-grained groundmasses consisting principally of pigeonite (∼50% in SaU 005; ∼60% in EET-A), plagioclase (maskelynite) and < 7% augite. Low-Ti chromite occurs as inclusions in the more magnesian olivine, and with chromian ulvöspinel rims in the more ferroan olivine and the groundmass. Crystallization histories for both rocks were determined from petrographic features (textures, crystal shapes and size distributions, phase associations, and modal abundances), mineral compositions, and melt compositions reconstructed from magmatic inclusions in olivine and chromite. The following observations indicate that the chromite and most magnesian olivine (Fo 74-70 in SaU 005; Fo 81-77 in EET-A) and pyroxenes (low-Ca pyroxene [Wo 4-6] of mg 77-74 and augite of mg 78 in SaU 005; orthopyroxene [Wo 3-5] of mg 84-80 in EET-A) in these rocks are xenocrystic. (1) Olivine crystal size distribution (CSD) functions show excesses of the largest crystals (whose cores comprise the most magnesian compositions), indicating addition of phenocrysts or xenocrysts. (2) The most magnesian low-Ca pyroxenes show near-vertical trends of mg vs. Al₂O₃ and Cr₂O₃, which suggest reaction with a magma. (3) In SaU 005, there is a gap in augite composition between mg 78 and 73. (4) Chromite cores of composite spinel grains are riddled with cracks, indicating that they experienced some physical stress before being overgrown with ulvöspinel. (5) Magmatic inclusions are absent in the most magnesian olivine, but abundant in the more ferroan, indicating slower growth rates for the former. (6) The predicted early crystallization sequence of the melt trapped in chromite (the earliest phase) in each rock produces its most magnesian olivine-pyroxene assemblage. However, in neither case is the total crystallization sequence of this melt consistent with the overall crystallization history of the rock or its bulk modal mineralogy.Further, the following observations indicate that in both SaU 005 and EET-A the fraction of solid xenocrystic or xenolithic material is small (in contrast to previous models for EET-A), and most of the material in the rock formed by continuous crystallization of a single magma (possibly mixed). (1) CSD functions and correlations of crystal size with composition show that most of the olivine (Fo 69-62 in SaU 005; Fo 76-53 in EET-A) formed by continuous nucleation and growth. (2) Groundmass pigeonites are in equilibrium with this olivine, and show continuous compositional trends that are typical for basalts. (3) The CSD function for groundmass pigeonite in EET-A indicates continuous nucleation and growth (Lentz and McSween, 2000). (4) The melt trapped in olivine of Fo 76 to 67 in EET-A has a predicted crystallization sequence similar to that inferred for most of the rock and produces an assemblage similar to its modal mineralogy. (5) Melt trapped in late olivine (Fo ∼ 64) in SaU 005 has a composition consistent with the inferred late crystallization history of the rock.The conclusion that only a small fraction of either SaU 005 or EET-A is xenocrystic or xenolithic implies that both rocks lost fractionated liquids in the late stages of crystallization. This is supported by: (1) high pigeonite/plagioclase ratios; (2) low augite contents; and (3) olivine CSD functions, which show a drop in nucleation rate at high degrees of crystallization, consistent with loss of liquid. For EET-A, this fractionated liquid may be represented by EET-B.     

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.     

Petrology and origin of the shergottite meteorites

Shergottites contain cumulus pigeonite and augite, probably without cumulus plagioclase and crystallized under relatively oxidizing conditions. Shergotty and Zagami may differ in the relative proportions of cumulus pyroxenes and crystallized intercumulus liquid, but the compositions of pyroxenes and liquid are similar in both meteorites. Absence of olivine in melting experiments suggests that the shergottites crystallized from fractionated derivatives of primary liquids. Low-Ca pyroxene and augite apparently began to crystallize from these primary liquids prior to plagioclase. Shergottites can be readily distinguished from other achondrite groups by their mineralogies, crystallization sequences and inferred source region compositions. However, the source regions of the shergottites may be related to those of other achondrite types by addition or loss of volatile components.The bulk composition of the Earth's upper mantle overlaps that of permissible shergottite source regions. Shergottites and terrestrial basalts display similarities in oxidation state and concentrations of trace and minor elements with a wide range of cosmochemical and geochemical affinities. Accretion of similar materials to produce the terrestrial upper mantle and the shergottite parent body or accretion of the Earth's upper mantle from planetesimals similar to the shergottite parent body may account for many of their similarities. Models of the origin of the Earth's upper mantle which attribute its oxidation state, its siderophile element abundances and its volatile element abundances to uniquely terrestrial processes or conditions, or to factors unique to the origin and differentiation of large bodies, are unattractive in light of the similarities between shergottites and terrestrial basalts.     

Post-Collisional Miocene Alkaline Volcanism in the Oglakçi Region,Turkey: Petrology and Geochemistry

Miocene volcanism of the Oglakci region (Sivrihisar, Eskisehir) in northwestern Central Anatolia, Turkey, is represented by basaltic and trachytic groups of rocks. Samples of both groups have been investigated using mineral-chemical data together with whole-rock major-, trace-element, and radiogenic Sr-Nd isotopic data. The basaltic volcanic rocks consist of mugearites and shoshonites, whereas the trachytic rocks include trachytes, latite, and rhyolite. Both groups are of alkaline character. The basaltic rocks contain plagioclase (An_29-63), alkali feldspar (Or_12-74), olivine, orthopyroxene (En_64-67), clinopyroxene (Wo_43-48), biotite (Mg#_82-88), and Fe-Ti oxide phenocrysts, whereas the trachytic rocks contain plagioclase (An_21-64), alkali feldspar (Or_10-53), clinopyroxene (Wo_41-49), amphibole (Mg#_64-83), biotite (Mg#_79-85), Fe-Ti oxide, titanite, apatite, and quartz phenocrysts. The measured ⁸⁷Sr/⁸⁶Sr ratios of basaltic samples range from 0.7045 to 0.7048, and those of trachytic samples from 0.7054 to 0.7056. The basaltic samples have ¹⁴³Nd/¹⁴⁴Nd ratios ranging from 0.512753 to 0.512737, and those of trachytic samples are 0.512713 to 0.512674. Isotopic, major-, and trace-element data suggest that the Oglakci volcanic rocks are products of postcollisional magmatism and originated from a complex interplay of crustal assimilation, magma mixing, and fractional crystallization processes following the demise of Neotethys. Trace-element characters also are consistent with an OIB-like mantle source. These volcanic rocks probably were associated with extensional tectonics, which occurred within the Anatolian plate as a result of collision of the Eurasian and Afro-Arabian plates during the neotectonic evolution of Turkey.     

Ultramafic inclusions and high pressure megacrysts from a nephelinite sill,Nandewar Mountains,north-eastern New South Wales,and their bearing on the origin of certain ultramafic inclusions in alkaline volcanic rocks

Ultramafic inclusions and megacrysts are unusually abundant in a nephelinite sill in the Nandewar Mountains in north-eastern New South Wales. The inclusions are divisible into a Cr-diopside group and a Ti-augite group, the former being dominated by Cr-spinel Iherzolites of restricted modal composition, the latter by olivine and titaniferous Al-rich clinopyroxene assemblages which vary widely in their modal proportions. The principal megacryst species are olivine and black, titaniferous Al-rich clinopyroxene; additional but comparatively rare megacrysts include titanphlogopite, kaersutitic amphibole, and deep green, relatively Fe-rich clinopyroxene. The Cr-spinel Iherzolites conform closely in mineralogy and chemistry with the spinel lherzolites which dominate upper mantle xenolith assemblages in alkaline mafic volcanic rocks from other provinces. Megacrysts and Ti-augite inclusion mineral assemblages are consistently more Fe-rich than analogous phases in the Cr-diopside xenoliths and also display more extensive cryptic variation. The available experimental data on the high pressure liquidus or near-liquidus phases in olivine nephelinite and related compositions indicate that the olivine and black clinopyroxene megacrysts were precipitated at pressures in the vicinity of 15–20 kb. The similarity in the nature and compositions of the principal megacryst species to analogous phases in the Tiaugite group of inclusions indicates that the latter also represent cognate cumulates derived from the olivine nephelinite at broadly comparable pressures. High pressure fractionation of the host olivine nephelinite liquid, controlled mainly by the separation of olivine and aluminous clinopyroxene, produced only comparatively minor compositional changes in the derivative liquid. The hiatus in olivine compositions at approximately Fo_86–88, apparently characteristic of the olivines in coexisting Cr-diopside and Ti-augite inclusions, is assessed in terms of the compositions of olivine in equilibrium with alkali basaltic liquids at high pressures.