KREEP
214228966
Mare basalt meteorites, magnesian suite rocks and KREEP reveal loss of zinc during and after lunar formation
Abstract Isotopic compositions of reservoirs in the Moon can be constrained from analysis of rocks generated during lunar magmatic differentiation. Mare basalts sample the largest lunar mantle volume, from olivine and pyroxene rich cumulates, whereas ferroan anorthosites and magnesian suite rocks represent early crustal materials. Incompatible element enriched rocks, known as 'KREEP, probably preserve evidence for the last highly differentiated melts. Here we show that mare basalts, including Apollo samples and meteorites, have remarkably consistent d 66 Zn values 1.4 0.2 %0 and Zn abundances (1.5 0.4 ppm) Analyses of magnesian suite rocks show them to be characterized by even heavier d 66 Zn values (2.5 to 9.3%0) and low Zn concentrations. KREEP rich impact melt breccia Sayh al Uhaymir 169 has a nearly identical Zn composition to mare basalts d 66 Zn 1.3 %0 and a low Zn abundance (0.5 ppm) Much of this variation can be explained through progressive depletion of Zn and preferential loss of the light isotopes in response to evaporative fractionation processes during a lunar magma ocean. Samples with isotopically light Zn can be explained by either direct condensation or mixing and contamination processes at the lunar surface. The d 66 Zn of Sayh al Uhaymir 169 is probably compromised by mixing processes of KREEP with mafic components. Correlations of Zn with Cl isotopes suggest that the urKREEP reservoir should be isotopically heavy with respect to Zn, like magnesian suite rocks. Current models to explain how and when Zn and other volatile elements were lost from the Moon include nebular processes, prior to lunar formation, and planetary processes, either during giant impact, or magmatic differentiation. Our results provide unambiguous evidence for the latter process. Notwithstanding, with the currently available volatile stable isotope datasets, it is difficult to discount if the Moon lost its volatiles relative to Earth either during giant impact or exclusively from later magmatic differentiation. If the Moon did begin initially volatile depleted, then the mare basalt d 66 Zn value likely preserves the signature, and the Moon lost 96% of its Zn inventory relative to Earth and was also characterized by isotopically heavy Cl d 37 Cl 8 %0 Alternative loss mechanisms, including erosive impact removing a steam atmosphere need to be examined in detail, but nebular processes of volatile loss do not appear necessary to explain lunar and terrestrial volatile inventories.
James M D Day Elishevah van Kooten Beda A Hofmann Frederic Moynier
219061552
Rheology of a KREEP analog magma: Experimental results applied to dike ascent through the lunar crust
Abstract Among the Apollo returned samples were basaltic rocks belonging to the KREEP suite, which are thought to have been erupted ~4.1 3.9 Ga via dike propagation from the residual magmatic layer residing below the lunar crust as the last part of the solidifying lunar magma ocean. The emplacement dynamics of KREEP magma reaching the lunar surface are generally poorly understood, in part because no rheological data are available to model the conditions of KREEP ascent in detail, even ~50 years after KREEP basalt discovery. This study provides the first rheological data for KREEP magmas, as a function of temperature and crystal content, from the liquidus temperature ~1205 degC) to ~16% crystals by volume at ~1177 degC. The lava's rheological behavior becomes non Newtonian at just a few degrees of cooling below the liquidus and is well represented by a shear thinning power law equation. We applied these data to a buoyancy driven dike ascent model, suggesting that if KREEP magma ascended through dikes comparable in size to those that fed the lunar mare, it most likely erupted below its liquidus in the range of 1180 degC to 1160 degC, with effective viscosities in the range 103 107 Pas and traversing the lunar crust on timescales of minutes to hours.
Alexander Sehlke Alan G Whittington
225390379
The chlorine isotopic composition of lunar KREEP from magnesian suite troctolite 76535
Abstract We conducted in situ Cl isotopic measurements of apatite within intercumulus regions and within a holocrystalline olivine hosted melt inclusion in magnesian suite troctolite 76535 from Apollo 17. These data were collected to place constraints on the Cl isotopic composition of the last liquid to crystallize from the lunar magma ocean (i.e. urKREEP, named after its enrichments in incompatible lithophile trace elements like potassium, rare earth elements, and phosphorus) The apatite in the olivine hosted melt inclusion and within the intercumulus regions of the sample yielded Cl isotopic compositions of 28.3 0.9%0 (2s) and 30.3 1.1%0 (2s) respectively. The concordance of these values from both textural regimes we analyzed indicates that the Cl isotopic composition of apatites in 76535 likely represents the Cl isotopic composition of the KREEP rich magnesian suite magmas. Based on the age of 76535, these results imply that the KREEP reservoir attained a Cl isotopic composition of 28 30%0 by at least 4.31 Ga, consistent with the onset of Cl isotopic fractionation at the time of lunar magma ocean crystallization or shortly thereafter. Moreover, lunar samples that yield Cl isotopic compositions higher than the value for KREEP are likely affected by secondary processes such as impacts and/or magmatic degassing. The presence of KREEP rich olivine hosted melt inclusions within one of the most pristine and ancient KREEP rich rocks from the Moon provides a new opportunity to characterize the geochemistry of KREEP. In particular, a broader analysis of stable isotopic compositions of highly and moderately volatile elements could provide an unprecedented advancement in our characterization of the geochemical composition of the KREEP reservoir and of volatile depletion processes during magma ocean crystallization, more broadly.
Francis M McCubbin Jessica J Barnes
211129324
Magmatic Evolution Model for the Host Magma of Rocks in Procellarum KREEP Terrane
Introduction: Mg suite rocks and KREEP (K Rare Earth Element and P rich) basalts of the Procellarum KREEP Terrane (PKT) paradoxically contain both Mg rich components and an incompatible element enriched component [e.g. 1 3] I previously estimated the concentrations of trace elements in the host magmas of plutonic rocks including Mg suite rocks of the PKT region (i.e. the PKT host magma) based on the trace element compositions of plagioclase in Apollo (Ap) 14 samples determined by secondary ion mass spectrometry, and found that the PKT host magmas have high Ti and Ba concentrations (Fig. 1; [4] Here, I explore the formation of the PKT host magma and KREEP basalts from the bulk silicate moon (BSM) by applying two BSM evolution models: one from the lunar primitive upper mantle (LPUM [5] and one from a crustal component enriched BSM with a sub chondritic Ti/Ba ratio (cBSM) as previously proposed based on ferroan anorthosites (FANs [6] Polybaric multi step BSM evolution model: I modeled the polybaric multi step evolution of the BSM using MELTS software [7, 8] by extending the twostep model [6, 9] (Fig. 2) Ba and Sr concentrations were calculated based on the partition coefficients from model 2 of [10] In the first step, an equilibrium melt of LPUM or cBSM composition and accounting for 40 wt% of the BSM was generated at high pressure (0.8 GPa) leaving early cumulates (S1) In the second step, the magma ascended to a shallow depth (0.3 GPa) where
Shigeko Togashi
211141230
An Impact Origin of the Lunar Procellarum KREEP Terrane
thew_jones@brown.edu) Alexander J. Evans1, Brandon C. Johnson2, Matthew B. Weller1, James T. Keane3, and Sonia M. Tikoo4, 1Department of Earth, Environmental and Planetary Sciences, Brown University, Providence, RI 02906, 2Department of Earth, Atmospheric, and Planetary Sciences, Purdue University, 3Division of Geological and Planetary Sciences, California Institute of Technology, 4School of Earth, Energy and Environmental Sciences, Stanford University.
Matt J Jones Alexander Joseph Evans Brandon Johnson Matthew B Weller James Tuttle Keane Sonia M Tikoo
224853883
Corrigendum to "Mare basalt meteorites, magnesian suite rocks and KREEP reveal loss of zinc during and after lunar formation" [Earth Planet. Sci. Lett. 531 (2020) 115998]
James M D Day Elishevah van Kooten Beda A Hofmann Frederic Moynier
214451406
KREEP Basalt 15382: Not as Pristine as Originally Thought
Introduction: KREEP (potassium [K] rare earth elements [REE] phosphorus [P] and other incompatible trace elements [ITE] is a chemical component, that has been identified at each sample return site from the Moon. It is thought to be derived from the last dregs of the lunar magma ocean, or urKREEP [1] KREEP basalts returned from the Moon represent the purest form of KREEP currently available for study. They identified from the Apollo 14, 15 and 17 samples. KREEP basalts are distinct from mare basalts as they possess a lower Ca/Al ratio reflecting a paucity of high Ca pyroxene [2] They are the result of endogenous melting of the lunar interior (e.g. [3] or by the mixture of KREEP and surrounding lithologies by impact melting (e.g. [4] Traditionally, highly siderophile element (HSE) abundances were used to differentiate between these two hypotheses (e.g. [5] but this has not been without issue [6] Recently, quantitative petrographic methods have been used to determine between the endogenous or impact origins for KREEP basalts and other lunar samples [7,8]
K A Cronberger Clive R Neal
150383609
Magmatic Evolution Estimated from Trace Elements in Plagioclase from Procellarum KREEP Terrane Breccias
Introduction: Typical plutonic rocks in the Procellarum KREEP (K, Rare Earth Elements, and P) Terrane (PKT) are of the Mg suite, i.e. cumulates containing calcic plagioclase (An98 84) coexisting with Mg rich mafic silicates (Mg# 95 60) (e.g. [1, 2] While Mgsuite plutonic rocks outside of the PKT do not necessarily show KREEP signature, those within the PKT paradoxically have both Mg rich components (primary melt or cumulate) and a KREEP component (incompatible element enriched) [e.g. 3 5] The trace element abundance of PKT Mg suite rocks may be keys to unveil "urKREEP" a late KREEP enriched residuum of the lunar magma ocean that was proposed as a common reservoir of KREEPy rocks [6] Although urKREEP has not been found in its original form [6, 7] its signature might be recorded in cumulate minerals [8, 9] Here we estimate the concentrations of Ba, Sr, and Ti in the host magmas of the plagioclase in breccias from the PKT by using secondary ion mass spectrometry (SIMS) analyses. We then discuss the formation of a highly evolved urKREEP magma from bulk silicate moon (BSM) by applying three models: lunar primitive upper mantle (LPUM [10] Taylor whole Moon (TWM [11] and crustal component enriched BSM with sub chondritic Ti/Ba ratio (cBSM [12] which we previously proposed from the study of ferroan anorthosites (FANs [12] New trace element analyses of PKT Mg suite rocks further test above BSM models with respects to chemical characters of urKREEP and the formation of KREEPy rocks. Methods and Results: We analyzed plagioclase trace element contents in breccias from the Apollo (Ap) 12 and 14 landing sites in the PKT using the same SIMS analytical method in [12] For the samples that show minimal trace element evidence of postmagmatic processes (Group a1 [12] the plagioclase contained 84 1,550 ppm Ba, 220 700 ppm Sr, and 0.022 0.086 wt% TiO2 (Fig. 1) We estimated the concentrations of Ba, Sr, and Ti of the plagioclase host magmas from SIMS analyses using plagioclase melt trace element partition coefficients (DElement Pl) We adopted values of 0.14 0.23 for the DBa Pl, 1.5 1.8 for the DSr Pl and 0.009 0.030 for the DTi Pl, depending on XAn (0.97 0.77) The host magmas for the Group a1 samples contained 570 6,800 ppm Ba, 148 390 ppm Sr, and 0.8 6.2 wt% TiO2, with sub chondritic Ti/Ba ratios (Fig. 1) Fig. 1. TiO2 Ba variations of plagioclase in FANs [12] and PKT breccias (this study) and those estimated for their host magmas. Groups (a1, a2, and b) are grades indicating post magmatic processes [12] where Group a1 is least affected. C1 (dotted line) represents the chondritic ratio.
Shigeko Togashi Noriko T Kita Akihiko Tomiya Yuichi Morishita
148567404
THE LUNAR GEOCHEMICAL ASYMMETRY IMPLICATIONS FOR KREEP AND MAGMA OCEAN CRYSTALLIZATION
Introduction: Our fundamental understanding of lunar evolution is grounded in the theory that the Moon underwent an early, extensive magma ocean near the end of its primary accretion phase [1 3] This theory explains well the compositions and ages associated with the lunar crust [2] however, the last stages of magma ocean crystallization have yet to be reconciled with the observed surface geochemical distribution, particularly that of lunar KREEP [4 6] KREEP a geochemical component with a high abundance of incompatible elements such as K, Rare Earth Elements, and P is the residuum from the magma ocean that crystallized beneath the less dense, feldspathic crust [3] As a result of the high enrichment of radioactive elements such as potassium, thorium, and uranium within KREEP material, the primary distribution and subsequent evolution of lunar KREEP has significant implications for the thermal and chemical evolution of the Moon [7]
Alexander Joseph Evans
204266927
Late crystallization of the lunar magma ocean and the origin of KREEP
F Welsch