Gregory A. Snyder

A chemical model for generating the sources of mare basalts - Combined equilibrium and fractional crystallization of the lunar magmasphere

Abstract It is generally considered that mare basalts were generated by the melting of a cumulate mantle formed in an early Moon-wide magma ocean or magmasphere. However, the nature and chemistry of this cumulate mantle and the logistics of its origin have remained elusive. Extensive studies of terrestrial layered mafic intrusions over the past sixty years have emphasized the imperfection of fractional crystallization and attendant crystal-crystal and crystal-liquid separation in a convecting magma chamber. Crystal-liquid and crystal-crystal separations were similarly inefficient during evolution of the lunar magma ocean (LMO), allowing for the trapping of interstitial liquid and entrainment of a small proportion of less-dense plagioclase into the denser mafic cumulate mush. Indeed, petrography of lunar highlands samples demonstrates this for anorthosites (with 1–10% olivine). The residual liquid after 80–90% crystallization was very evolved (in fact KREEPy) and, even in small proportions (1–5%), would have a noticeable effect on the trace-element chemistry of melts generated from these cumulates. This trapped residual liquid would elevate total REE abundances in the cumulate pile, while synchronously deepening the already negative Eu anomaly. Essentially, this trapped liquid will make the cumulate more fertile for melting to generate both KREEP basalt and mare basalt magmas. Plagioclase entrained in the mafic cumulate pile adds an essential Al component to the high-Ti basalt source and will moderate the requisite negative Eu anomaly in the cumulate. Early in the evolution of the lunar mantle, when the LMO still was largely liquid, it is likely that vigorous convection was an important factor in crystallization. Such convection would allow crystals to remain suspended and in equilibrium with the LMO liquid for relatively long periods of time. This extended period of equilibrium crystallization would then have been followed by fractional crystallization once plagioclase became a liquidus phase and began to float to form the lunar highlands crust. Previous authors have proposed a three-component model for the evolution of high-Ti mare basalt source regions. This model includes KREEP, early (olivine-rich, high Mg#) cumulates, and late (ilmeniterich, low Mg#) cumulates in various proportions. However, we propose a model for high-Ti basalt parent magmas which is in accord with studies of terrestrial layered intrusions. This model for the high-Ti source includes trapped instantaneous residual liquid (TIRL; 1–3%) and entrainment of a small (2–5%) proportion of plagioclase into the late-stage cumulate pile in order to account for both the observed Al compositions and trace-element characteristics of high-Ti mare basalts. Melting of this relatively shallow, ilmenite- and clinopyroxene-bearing, late-stage cumulate can generate high-Ti mare basalt magmas. Furthermore, we are in agreement with other workers that only through a process of nonmodal melting will the high Ti values for the parent magmas be realized. Large-scale convective overturn of the cumulate pile and mixing of KREEP with early- and late-stage cumulates is not required. However, localized overturn of the upper tenth of the cumulate pile is likely and, in fact, required to achieve an appropriate major-element balance for the high-Ti mare basalt source region.

Archean mantle heterogeneity and the origin of diamondiferous eclogites, Siberia; evidence from stable isotopes and hydroxyl in garnet

Abstract We have determined the phase relationships of melting of synthetic granite (two ternary feldspars + quartz) in the presence of an H2O-CO2 fluid. Synthesis and reversed experiments were conducted in a piston-cylinder apparatus over the range 650-900 °C and 6-15 kbar. At XH₂O = 0.75, melting occurred between 670 and 680 °C (15 kbar), 700 and 710 °C (10 kbar), and 710 and 720 °C (7.4 kbar). At XH₂O= 0.5, melting occurred between 760 and 770 °C (15 kbar), 780 and 790 °C (10 kbar), and 800 and 820 °C (6-7.4 kbar). At XH₂O = 0.25, melting occurred between 830 and 840 °C (15 kbar), 830 and 840 °C (10 kbar), and 860 and 870 °C (6-7.4 kbar). These results provide important constraints on the maximum temperatures of regional metamorphism attainable in vapor-saturated metapelitic and quartzofeldspathic rocks that escaped widespread melting. At pressures below 10 kbar, a fluid phase of XH₂O= 0.75, 0.5, and 0.25 limits temperatures to below ~700-725, ~800-825, and ~850-875 °C, respectively. As a consequence, the formation of granulite does not require CO2 concentrations in a coexisting fluid to exceed an XCO₂ of 0.25-0.5, a range that may include dilution of the H2O component of the fluid through internal buffering by devolatilization reactions. Therefore, the formation of granulites by the influx of CO2 may be a less common phenomenon than previously thought.

The Source Region and Melting Mineralogy of High-Titanium and Low-Titanium Lunar Basalts Deduced from Lu-Hf Isotope Data

Five high-Ti basalts from the Apollo 11 and 17 landing sites have been analyzed for their hafnium isotope composition. These data serve to better constrain the hafnium isotope variation of the Moon’s mantle. Variations in initial ϵHf and ϵNd values of low- and high-Ti basalts imply that the source region mineral assemblages of these lunar magma types are distinct. Low-Ti basalts have higher initial ϵHf values, at a given ϵNd value, than high-Ti basalts. The differences in the hafnium and neodymium isotopic composition of low- and high-Ti basalts reflect the fact that the source of low-Ti basalts had a [Lu/Hf]n ratio approximately four times greater than its [Sm/Nd]n ratio. In contrast, the high-Ti source region had subequal [Lu/Hf]n and [Sm/Nd]n ratios. If it is assumed that mare basalts are partial melts of the Moon’s cumulate mantle, the differences between low- and high-Ti basalts can only be explained by these mare magma types being generated from melting sources with distinctly different mineral assemblages. The large Lu/Hf fractionations, relative to Sm/Nd fractionations, of low-Ti basalts can best be produced by an assemblage of olivine and orthopyroxene with trace amount of clinopyroxene that crystallized early in the history of the Lunar Magma Ocean (LMO). The subequal [Lu/Hf]n and [Sm/Nd]n fractionations of high-Ti basalts can be produced from a variety of ilmenite-bearing mineral assemblages. Low- and high-Ti basalts have similar Lu/Hf ratios, approximately 0.6 times chondrite. The low Lu/Hf ratios measured for these mare magmas contrast sharply with the high Lu/Hf ratios (greater than chondritic) calculated for their sources from initial ϵHf values and an assumed chondritic bulk moon initial ϵHf value. The difference between the measured Lu/Hf of a lava, vs. the calculated Lu/Hf of its source, implies that during partial melting, Lu was preferentially retained in the residual source, relative to Hf. In order to explain the extreme fractionation of measured Lu/Hf ratios we suggest mare basalts can best be explained using a polybaric melting model. Initial melting of a garnet bearing source followed by continued melting in the spinel stability field can produce the required Lu/Hf fractionations and produce a liquid that last equilibrated with a residuum of olivine and orthopyroxene.

Diamonds and Their Mineral Inclusions, and What They Tell Us: A Detailed “Pull-Apart” of a Diamondiferous Eclogite

For the first time, three-dimensional, high-resolution X-ray computed tomography (HRXCT) of an eclogite xenolith from Yakutia has successfully imaged diamonds and their textural relationships with coexisting minerals. Thirty (30) macrodiamonds (≥1 mm), with a total weight of just over 3 carats, for an ore grade of some 27,000 ct/ton, were found in a small (4 × 5 × 6 cm) eclogite, U51/3, from Udachnaya. Based upon 3-D imaging, the diamonds appear to be associated with zones of secondary alteration of clinopyroxene (Cpx) in the xenolith. The presence of diamonds with secondary minerals strongly suggests that the diamonds formed after the eclogite, in conjunction with meta-somatic input(s) of carbon-rich fluids. Metasomatic processes are also indicated by the non-systematic variations in Cpx inclusion chemistry in the several diamonds. The inclusions in the diamonds vary considerably in major- and trace-element chemistry within and between diamonds, and do not correspond to the minerals of the host eclogite, whose compositions are extremely homogeneous. Some Cpx inclusions possess +Eu anomalies, probably inherited from their crustal source rocks. The only consistent feature for the Cpx crystals in the inclusions is that they have higher K2O than the Cpx grains in the host. The δ13C compositions are relatively constant at −5% both within and between diamonds, whereas δ15N values vary from −2.8% to −15.8%. Within a diamond, the total N varies considerably from 15 to 285 ppm in one diamond to 103 to 1250 ppm in another. Cathodoluminescent imaging reveals extremely contorted zonations and complex growth histories in the diamonds, indicating large variations in growth environments for each diamond. This study directly bears on the concept of diamond inclusions as time capsules for investigating the mantle of the Earth. If diamonds and their inclusions can vary so much within this one small xenolith, the significance of their compositions is a serious question that must be addressed in all diamond-inclusion endeavors.

ECLOGITIC INCLUSIONS IN DIAMONDS: EVIDENCE OF COMPLEX MANTLE PROCESSES OVER TIME

The first ion-probe trace element analyses of clinopyroxene-garnet pairs both included within diamonds and from the eclogite host xenoliths are reported; these diamondiferous eclogites are from the Udachnaya and Mir kimberlite pipes, Yakutia, Russia. The major and trace element analyses of these diamond-inclusion and host-rock pairs are compared in order to determine the relative ages of the diamonds, confirm or deny genetic relationships between the diamonds and the eclogites, evaluate models of eclogite petrogenesis, and model igneous processes in the mantle before, during, and after diamond formation. The most striking aspect of the chemical compositions of the diamond inclusions is the diversity of relationships with their eclogite hosts. No single distinct pattern of variation from diamond inclusion minerals to host minerals is found for all four samples. Garnet and clinopyroxene inclusions in the diamonds from two samples (U-65/3 and U-66/3) have lower Mg#s, lower Mg, and higher Fe contents, and lower LREE than those in the host eclogite. We interpret such variations as due to metasomatism of the host eclogite after diamond formation. One sample, U-41/3 shows enrichment in diamond-inclusion MREE enrichment relative to the eclogite host and may indicate a metasomatic event prior to, or during, diamond formation. Bulanova [2] found striking differences between inclusions taken from within different portions of the very same diamond. Clinopyroxene inclusions taken from the central (early) portions of Yakutian diamonds were lower in Mg# and Mg contents (by up to 25%) than those later inclusions at the rims of diamonds. These trends are parallel to those between diamond inclusions and host eclogites determined for four of the five samples from the present study and may merely represent changing magmatic and/or P-T conditions in the mantle. Garnet trace element compositions are similar in relative proportions, but variable in abundances, between diamond inclusions and host eclogites. This is probably due to the rapid diffusion of trace elements in garnet under mantle temperatures and consequent alteration of the garnet, and not due to juvenile diamonds ‘locking in’ source heterogeneities (c.f., [3]). Trace element compositions of clinopyroxenes included in diamonds are generally similar to those in the host eclogite. However, one host clinopyroxene does show enrichment in the LREE compared to that in the inclusion and may be attributed to mantle metasomatism, not related to kimberlite transport. In another eclogite, M-46, the host clinopyroxene is depleted in the LREE and Fe, and enriched in the HREE and Mg, relative to the inclusion and is consistent with partial melting of the eclogite subsequent to diamond formation. SmNd ratios in clinopyroxenes appear to be little affected by these processes for most samples, allowing SmNd isotopic studies to yield important information about ancient protoliths. Eclogitic mineral inclusions in Yakutian diamonds appear consanguineous with the diamonds, a contention supported by the observations of Bulanova [2]. Therefore, ReOs whole-rock and SmNd clinopyroxene age determinations of the Udachnaya eclogites also yield the time of diamond formation, approximately 2.9 Ga [32,33].

Precise Mossbauer milliprobe determination of ferric iron in rock-forming minerals and limitations of electron microprobe analysis

Abstract For estimations of P - T conditions of igneous and metamorphic rocks, Fe3+ in coexisting minerals is either assumed to be zero or is calculated from electron microprobe analyses (EMPA) based upon stoichiometry and charge balance. Geothermobarometers that involve Fe2+ - Mg2+ exchange can be significantly affected by either neglecting Fe3+ or using incorrect values. Ratios of Fe3+/ΣFe in garnet and clinopyroxene measured by a Mössbauer milliprobe were compared to those calculated from EMPA of garnet and clinopyroxene from eclogite xenoliths from the Udachnaya kimberlite in Yakutia. The effects of Fe3+ contents in garnet and clinopyroxene on temperature estimations were evaluated. The following Fe3+/ΣFe (in at%) values were obtained (EMPA/Mössbauer): Gt = 9.4/6.0; 11.5/7.0; 19.4/16.0; and 24.7/15.0; Cpx = 22.0/22.9; 34.2/22.0. The effects of Fe3+ in clinopyroxenes on calculated temperatures are illustrated by taking eclogitic clinopyroxene compositions and changing contents of certain elements within the range of standard deviations for EMPA of those particular elements. Increasing Na2O contents from 5.67 to 5.74 wt% (< 2.0% relative error) would lead to increasing Fe3+/ΣFe from 31.6 to 47.1%, thereby decreasing the calculated temperature from 1026 to 941 °C. Various Fe3+/ΣFe values for garnet and clinopyroxene were also tested for their effects on calculated temperatures: for clinopyroxene, T decreases with increasing Fe3+/ΣFe whereas for garnet, T increases with increasing Fe3+/ΣFe. This compensation effect between garnet and clinopyroxene moderates the variation in temperature estimations of eclogites based on Fe3+ corrected vs. uncorrected microprobe analyses. Little correlation exists between EMPA-calculated and Mössbauer-measured Fe3+/ΣFe values for these mantle-derived garnets and clinopyroxenes. Even a small relative error in Fe3+ may significantly change calculated temperatures of equilibration, seriously affecting petrologic interpretations. In particular, uncertainty in Fe3+ calculated from EMPA of silicate minerals leads to serious questions with regard to KD values obtained from natural assemblages.

Chronology and petrogenesis of the lunar highlands alkali suite: Cumulates from KREEP basalt crystallization

Alkalic rocks from the highlands of the Moon, though relatively minor in volume, yield important information in understanding the later development of the lunar crust. However, until recently little information has been available on the crystallization ages of samples from this diverse suite of rocks. Previous workers suggested a link between pristine KREEP basalts and lunar quartz monzodiorites and granites. Through mineral and chemical modelling of all known pristine highlands alkali suite (HAS) rocks, and radiogenic isotopic analyses of rather large HAS clasts from the Apollo 14 landing site, we explore further this potential link. Two clasts of alkalic affinity from the western highlands of the Moon have been analyzed for their neodymium and strontium isotopic compositions. An alkali anorthosite (14304,267) yields a SmNd mineral isochron of 4108 ± 53 Ma (MSWD = 0.06) and an eNd(T) of −1.0 ± 0.2. Two splits of the same alkali norite clast (14304,270 and 14304,272) yield similar eNd values when calculated for this age. RbSr systematics for alkali anorthosite 14304,267 scatter about a line which yields an “age” of 4336 ± 81 Ma. However, the large mean square of weighted deviates (MSWD) for this line (12.3) indicates that the age is suspect and that RbSr sytematics may have been disturbed. The SmNd age for 14304,267, in conjunction with UPb zircon ages for two other highlands alkali suite (HAS) rocks from the Apollo 14 landing site and one from the Apollo 16 landing site indicate production of HAS rocks over an extended time period spanning at least 300 million years from 4.34 to 4.02 Ga. Since the last dregs of the lunar magma ocean (LMO) likely crystallized prior to 4.3 Ma, all rocks from this alkalic period cannot represent direct remnants of the late LMO and may include material resulting from the remelting of evolved portions of the Moon by upward-moving basaltic melts from the deep lunar interior. These “contaminated” melts may be represented by pristine KREEP basalts from the Apollo 15 landing site. These KREEP basalt melts could have crystallized within the crust to form cumulate gabbros, norites, anorthosites, monzodiorites, and possibly granites, with the proportion of trapped KREEPy residual liquid determining the large-ion lithophile element enrichment of the rock. Generally, the proportion of trapped KREEPy liquid is small (2–15%). The broad age range for the lunar HAS indicates that parental KREEP basalt magmatism was not a unique event, but was an important process possibly repeated several times throughout the first 600 to 700 million years of lunar history.

Comparative geochemistry of basalts from the moon, earth, HED asteroid, and Mars: implications for the origin of the moon

Abstract Most hypotheses for the origin of the Moon (rotational fission, co-accretion, and collisional ejection from the Earth, including “giant impact”) call for the formation of the Moon in a geocentric environment. However, key geochemical data for basaltic rocks from the Moon, Earth, the howardite-eucrite-diogenite (HED) meteorite parent body (probably asteroid 4-Vesta), and the shergottite-nakhlite-chassignite (SNC) meteorite parent body (likely Mars), provide no evidence that the Moon was derived from the Earth, and suggest that some objects with lunar-like compositions were produced without involvement of the Earth. The source region compositions of basalts produced in the Moon (mare basalts) were similar to those produced in the HED asteroid (eucrites) with regard to volatile-lithophile elements (Na, K, Rb, Cs, and Tl), siderophile elements (Ni, Co, Ga, Ge, Re, and Ir), and ferromagnesian elements (Mg, Fe, Cr, and V), and less similar to those in the Earth or Mars. Mare and eucrite basalts differ in their Mn abundances, Fe/Mn values, and isotopic composition, suggesting that the Moon and HED asteroid formed in different nebular locations. However, previous claims that the Moon and HED parent body differ significantly in the abundances of some elements, such as Ni, Co, Cr, and V, are not supported by the data. Instead, Cr-Mg-Fe-Ni-Co abundance systematics suggest a close similarity between the source region compositions and conditions involved in producing mare and eucrite basalts, and a significant difference from those of terrestrial basalts. The data imply that the Moon and HED asteroid experienced similar volatile-element depletion and similar fractionation of metallic and mafic phases. Among hypotheses of lunar origin, rotational fission, and small-impact collisional ejection seem less tenable than co-accretion, capture, or a variant of giant-impact collisional ejection in which the Moon inherits the composition of the impactor. Both the Moon and HED asteroid may have been derived from a class of objects that were common in the early solar system. “The most plausible model for the origin of the Moon in line with geochemical and cosmochemical constraints is an impact-induced “fission” of the proto-Earth.” — Wanke and Dreibus (1986 ) “Clearly, the Moon and eucrite parent body resemble each other to a high degree. Nature produced such a composition not once but at least twice. This calls into question an entire class of models that invoke ad hoc processes to explain the Moon by a unique chance event.” — Anders (1977 ) “The similarity between eucrites and lunar mare basalts are remarkable. Were it not for the differences in age and oxygen-isotope signature, it might be difficult to distinguish them on petrological or geochemical grounds.” — Taylor (1986 )

Nd and Sr isotopes from diamondiferous eclogites, Udachnaya Kimberlite Pipe, Yakutia, Siberia: Evidence of differentiation in the early Earth?

Nd and Sr isotopic data from diamond-bearing eclogites found in the Udachnaya kimberlite, Yakutia, Siberia, are interpreted as indicating an early (>1 4 Ga) differentiation event, whereby the mantle split into complementary depleted and enriched reservoirs. Reconstructed whole-rock 87Sr/S6Sr ratios (present-day) range from 0.70151 to 0.70315 and are consistent with a mantle origin for these rocks. The Nd isotopic evolution lines of four samples (U-5, U-37, U-41 and U-79) converge at 2.2-2.7 Ga. Sample U-5 is unique in exhibiting the most enriched signature of any of the samples yet analyzed (present-day end of --20), and this sample points unequivocally to an old, enriched component. A complementary depleted mantle component is suggested by two of the eclogite samples, U-86 and U-25, which yield ENd values (at 2.2 Ga) of + 13 and + 7, respectively. The two mantle reservoirs possibly formed prior to 4 Ga and evolved separately until 2.2-2.7 Ga. At that time, the reservoirs were melted forming eclogites both as residues (from the enriched reservoir) and as partial melts of peridotite (from the depleted reservoir), resulting in demonstrably different histories for eclogites from the same locality.

Processes involved in the formation of magnesian‐suite plutonic rocks from the highlands of the Earth's Moon

The earliest evolution of the Moon likely included the formation of a magma ocean and the subsequent development of anorthositic flotation cumulates. This primary anorthositic crust was then intruded by mafic magmas which crystallized to form the lunar highlands magnesian suite. The present study is a compilation of petrologic, mineral-chemical, and geochemical information on all pristine magnesian-suite plutonic rocks and the interpretation of this data in light of 18 “new” samples. Of these 18 clasts taken from Apollo 14 breccias, 12 are probably pristine and include four dunites, two norites, four troctolites, and two anorthosites. Radiogenic isotopic whole rock data also are reported for one of the “probably pristine” anorthositic troctolites, sample 14303,347. The relatively low Rb content and high Sm and Nd abundances of 14303,347 suggest that this cumulate rock was derived from a parental magma which had these chemical characteristics. Trace element, isotopic, and mineral-chemical data are used to interpret the total highlands magnesian suite as crustal precipitates of a primitive KREEP (possessing a K-, rare earth element (REE)-, and P-enriched chemical signature) basalt magma. This KREEP basalt was created by the mixing of ascending ultramafic melts from the lunar interior with urKREEP (the late, K-, REE-, and P-enriched residuum of the lunar magma ocean). The trace and major element compositions of nearly all magnesian-suite cumulates can be generated by 0–55% fractional crystallization of a primitive KREEP basalt combined with the trapping of varied proportions (generally ≤20%) of instantaneous, residual, KREEP-basalt liquid. A few samples of the magnesian suite with extremely elevated large-ion lithophile elements (5–10× other magnesian-suite rocks) cannot be explained by this model or any other model of autometasomatism, equilibrium crystallization, or “local melt-pocket equilibrium” without recourse to an extremely large-ion lithophile element-enriched parent liquid. It is difficult to generate parental liquids which are 2–4× higher in the REE than average lunar KREEP, unless the liquids are the basic complement of a liquid-liquid pair, i.e., the so-called “REEP-fraction,” from the silicate liquid immiscibility of urKREEP. Scarce age information on lunar rocks suggests that magnesian-suite magmatism was initiated at progressively more recent time from the northeast to the southwest on the lunar nearside from 4.45 to 4.25 Ga. This magmatic “event” could be due to melting of the lunar mantle beneath these regions and could have been generated either by latent heat during crystallization of the final, KREEP-rich (and, thus, Th- and U-rich), residual, lunar magma ocean liquid or heating due to radioactive decay of K, Th, and U.