Subsolidus Reduction Research Articles

Possibility of Lunar Crustal Magmatism Producing Strong Crustal Magnetism

AbstractThe Moon generated a long‐lived core dynamo magnetic field, with intensities at least episodically reaching ∼10–100 μT during the period prior to ∼3.56 Ga. While magnetic anomalies observed within impact basins are likely attributable to the presence of impactor‐added metal, other anomalies such as those associated with lunar swirls are not as conclusively linked to exogenic materials. This has led to the hypothesis that some anomalies may be related to magmatic features such as dikes, sills, and laccoliths. However, basalts returned from the Apollo missions are magnetized too weakly to produce the required magnetization intensities (>0.5 A/m). Here, we test the hypothesis that subsolidus reduction of ilmenite within or adjacent to slowly cooled mafic intrusive bodies could locally enhance metallic FeNi contents within the lunar crust. We find that reduction within hypabyssal dikes with high‐Ti or low‐Ti mare basalt compositions can produce sufficient FeNi grains to carry the minimum >0.5 A/m magnetization intensity inferred for swirls, especially if ambient fields are >10 μT or if fine‐grained Fe‐Ni metals in the pseudo‐single domain grain size range are formed. Therefore, there exists a possibility that certain magnetic anomalies exhibiting various shapes such as linear, swarms, and elliptical patterns may be magmatic in origin. Our study highlights that the domain state of the magnetic carriers is an under‐appreciated factor in controlling a rock's magnetization intensity. The results of this study will help guide interpretations of lunar crustal field data acquired by future rovers that will traverse lunar magnetic anomalies.

Oxide and sulphide isograds along a Late Archean, deep‐crustal profile in Tamil Nadu, south India

AbstractOxide–sulphide–Fe–Mg–silicate and titanite–ilmenite textures as well as their mineral compositions have been studied in felsic and intermediate orthogneisses across an amphibolite (north) to granulite facies (south) traverse of lower Archean crust, Tamil Nadu, south India. Titanite is limited to the amphibolite facies terrane where it rims ilmenite or occurs as independent grains. Pyrite is widespread throughout the traverse increasing in abundance with increasing metamorphic grade. Pyrrhotite is confined to the high‐grade granulites. Ilmenite is widespread throughout the traverse increasing in abundance with increasing metamorphic grade and occurring primarily as hemo‐ilmenite in the high‐grade granulite facies rocks. Magnetite is widespread throughout the traverse and is commonly associated with ilmenite. It decreases in abundance with increasing metamorphic grade. In the granulite facies zone, reaction rims of magnetite + quartz occur along Fe–Mg silicate grain boundaries. Magnetite also commonly rims or is associated with pyrite. Both types of reaction rims represent an oxidation effect resulting from the partial subsolidus reduction of the hematite component in ilmenite to magnetite. This is confirmed by the presence of composite three oxide grains consisting of hematite, magnetite and ilmenite. Magnetite and magnetite–pyrite micro‐veins along silicate grain boundaries formed over a wide range of post‐peak metamorphic temperatures and pressures ranging from high‐grade SO2 to low‐grade H2S‐dominated conditions. Oxygen fugacities estimated from the orthopyroxene–magnetite–quartz, orthopyroxene–hematite–quartz, and magnetite–hematite buffers average 2.5 log units above QFM. It is proposed that the trends in mineral assemblages, textures and composition are the result of an external, infiltrating concentrated brine containing an oxidizing component such as CaSO4 during high‐grade metamorphism later acted upon by prograde and retrograde mineral reactions that do not involve an externally derived fluid phase.

The most reduced rock from the moon, Apollo 14 basalt 14053: Its unique features and their origin

The Apollo 14 high-Al basalt, 14053, is the most reduced lunar rock examined to date. Both fayalite in the mesostasis and spinel minerals have been extensively reduced in the exterior of the rock, whereas the interior contains relatively limited reduction. It is shown that these products are the effects of solar-wind hydrogen that was implanted on the exterior of the “normal” 14053 basalt after it originally crystallized and was weathered to become part of the regolith. Subsequent reheating, probably in an impact-ejecta blanket, caused extreme subsolidus hydrogen reduction, particularly of the weathered exterior of this rock. The limited permeability of the rock prevented the entire rock from being subjected to the same degree of reduction. It is proposed that this extreme reduction, especially of the mesostasis, also affected the phosphate minerals, F-Cl apatite and merrillite, each of which vary greatly in REE contents. This effect on the phosphate minerals could relate to the upset Sm-Nd radiogenetic systematics, whereas the Rb-Sr system may have been largely immune to the subsolidus reduction.

Opaque mineralogy and magnetic properties of selected banded iron‐formations, Hamersley Basin, Western Australia

The oxide mineralogy and rock magnetic properties of unmineralised banded iron‐formations in selected portions of four drillholes in the Hamersley Basin, Western Australia are reviewed. In all four drillholes, petrographic studies indicate that primary euhedral to subhedral hematite is partially replaced by magnetite as a result of subsolidus reduction. All drillholes show partial recrystallisation of the secondary magnetite, suggesting that early subsolidus reduction was probably a regional event occurring during prograde metamorphism. Incomplete replacement of primary hematite by magnetite within and between sedimentary band structures indicates that equilibration in the magnetite stability field was not reached even at the mesoband scale. Subsequent subsolidus oxidation of magnetite and the formation of a second‐generation hematite are documented in only two of the drillholes. Goethite‐filled veins and thick selvages of goethite around some veins reflect movement of circulating oxidising fluids. The absence of goethite and second‐generation hematite in two of the drillholes indicates that subsolidus oxidation is not a regional event, but very much localised. Rapid changes in down‐hole susceptibility measurements correlate directly with detailed petrographic results as susceptibility readings change with the hematite/magnetite ratio on a mesoband scale. Acquisition of the main remanence correlates with the formation of hematite as the primary oxide phase followed by partial replacement by magnetite as a result of subsolidus reduction, supporting regional models requiring pre‐folding remanence. The strong orientation of the primary hematite parent parallel to band structures in the banded iron‐formations has influenced the direction of crystallisation remanent magnetisation during subsolidus reduction to the magnetite daughter. The strong planar alignment has also produced a planar magnetic fabric and marked anisotropy of magnetic susceptibility. A natural remanent magnetisation overprint and reduction in anisotropy of magnetic susceptibility are only recorded in samples that have undergone subsolidus oxidation and the recognition of localised post‐metamorphic oxidation overprinting can also explain ore deposit models requiring post‐folding remanence. The relative timing of and between oxidising fluid events is not known, but both petrographic and rock magnetic evidence to date suggests that there was at least one and probably two post‐folding oxidising events in the area of study.

Oxide Mineralogy and Magnetic Properties of the Koidu Kimberlite Complex, Sierra Leone, West Africa

SUMMARY A good correlation between opaque mineralogy and rock magnetic properties has been established in a study of a small number of oriented samples from four kimberlite dykes and one pipe in the Koidu complex of Sierra Leone. The pipe sample is mineralogically distinctive, in that it alone contains deuteric titanomaghaemite after titanomagnetite, and haematite after chlorite. All of the kimberlites contain xenocrystic, upper-mantle derived ilmenite that has undergone sub-solidus reduction to form Mg-Ti magnetite. Three of the dykes contain serpentinized olivine with microscopic to sub-microscopic magnetite and identifiable metal Ni-Fe alloys, and finely textured Mg titanomagnetite and magnetite as atolls surrounding complexly zoned Cr-Mg- A1 spinels. Consistent with these observations, Curie and blocking temperatures range from 520 to 590°C and high alternating field (AF) coercivities imply single domain or pseudo-single domain particles as the bulk carrier of natural remanent magnetization (NRM). In the pipe sample, however, the remanence persisting above 600°C is comparatively strong. Preliminary remanent vectors, isolated by either thermal or AF demagnetization, from four of the five samples are moderately consistent, and yield a mean (n = 4) of D = 323, Z = -17, k = 18, and ag5 = 22.

Macrocryst Fe - Ti oxides in olivine melilitites from Namaqualand-Bushmanland South Africa

Ilmenite macrocrysts in olivine melilitites from Namaqualand-Bushmanland, South Africa, have decomposed by subsolidus reduction to form oriented Mg-titanomagnetite along {0001} ilmenite planes. Residual ilmenite contains 10–11 wt% MgO, ∼1 wt% MnO, and ∼0.1 wt% Cr2O3. This macrocryst assemblage is mantled by an annulus of Mg-titanomagnetite, followed by an overgrowth of radiating magnetite + perovskite. Terminal compositions of these magnetites are similar to groundmass spinels, and to the outermost margins of magnetite macrocrysts that have very high Fe3+ core contents. The assemblages are remarkably similar to oxide intergrowths in kimberlites and an upper mantle derivation is proposed for ilmenite macrocrysts in these melilitites. Oxidation states in the source regions are also very similar, whether on-or off-craton, being slightly above FMQ (∼NNO), but reduced to FMQWM with the onset of decompression, volatile loss, and carbonate immiscibility. In the case of the melilitites, late stage, low pressure crystallization above NNO precipated abundant magnetite + perovskite. The oxide fO2 data are consistent with, and refine the fO2 estimates obtained previously for the behavior of Fe/Mg and Ni contents in olivine from the same suite of samples.

Solid-state reduction of iron in olivine-planetary and meteoritic evolution.

Iron-nickel metallic particles have been reported in meteorites1 and lunar2-5 and terrestrial6,7 rocks. The origin of these metallic particles is not unique as they may be formed by (1) condensation from a primordial solar nebula8; (2) crystallization from a melt; and (3) subsolidus reduction reactions under low oxygen or sulphur fugacity. We report here an electron microscopy study of the solid-state microstructural development in olivine single crystals (Fo92) in which half of the iron has been reduced to the metallic state by a gas-solid interaction in the temperature range 950-1,500 °C. The reaction, Fo92→Fo96+metallic Fe(Ni in solid solution)+pyroxene, begins with a homogeneous transformation involving fine-scale metallic precipitates resembling Guinier-Preston zones9. The microstructure develops by the growth of the first-formed precipitates during an Ostwald ripening process9 in which the precipitates located in the dislocation sub-boundaries develop in preference to precipitates in the subgrains. On the other hand, pyroxene is first observed to nucleate heterogeneously at pre-existing dislocations and its coarsening rate is more than an order-of-magnitude faster than that of the metallic phase. Besides the textural similarity of the observed microstructures with that reported for some of the lunar materials2, these results have important implications for the physical models of accretion of terrestrial planets, planetesimals and meteorites10, especially with respect to the distribution of siderophile elements. The rate of reaction observed here places constraints on models for the formation of the Earth's core by segregation of a metallic phase with or without reduction.

The redox state of planetary basalts

A well established data base exists for the oxidation‐reduction (redox) states of basalts on the Earth and on the Moon; the former equilibrate along the fayalite‐magnetite‐quartz buffer curve, and the latter crystallize in the field of metallic iron stability, below the iron wustite buffer. Preferred accumulation of volatiles and the disproportionation of water into hydrogen and oxygen in thick terrestrial lavas and in ponded lava lakes results in high states of oxidation equivalent to that of hematite stability. At the other extreme, lunar basalts exhibit the effects of subsolidus reduction and estimates yield T = 700‐1000°C and fO2 = 10−16 to 10−23 atms. Based on a number of cosmogenic properties, models for planetary interiors, and the sequence of condensation with heliocentric distance from the protosun, estimates for the redox states of inner solar system planetary basalts yield the following results: Basalts on Mercury and the Moon crystallize below the iron‐wustite buffer curve; Venusian basalts are more oxidized than those on Mercury, less oxidized than those on Earth, and crystallization within the field of wustite stability is suggested; basalts on Earth are dominantly in the field of magnetite stability in close proximity to the fayalite‐magnetite‐quartz buffer curve; Martian basalts are estimated to crystallize in the upper regions of magnetite stability and well into the hematite field of stability expressed in terms of temperature and oxygen fugacity.

Oxide minerals in lithic fragments from Luna 20 fines

One hundred and seventy-six oxide mineral grains in the Luna 20 samples were analyzed by electron microprobe. Spinel is the most abundant oxide, occurring in troctolite fragments. Next most abundant is ilmenite, which occurs in all rock types except those containing spinel. Chromite also occurs in all rock types except those containing spinel. Minor amounts of ulvöspinel, armalcolite, zirkelite, baddeleyite and an unidentified TiO 2-rich phase were also found. Spinel grains are predominantly spinel-hercynite solid solutions, commonly with very minor chromite. The Fe (Fe + Mg) ratio is generally lower than in spinel from Apollo 14 rocks. Chromites in non-mare rocks are similar to those from mare rocks. Ilmenite of mare origin is Mg-poor and Zr-rich compared to non-mare ilmenite; these elements may therefore be useful in determining the origin of ilmenite grains. Phase equilibria considerations suggest that spinel troctolite crystallized from a melt high in alumina; a likely candidate is the high-alumina basalt of Prinz et al. (1973a). Sub-micron wide rods of metallic Fe occur in plagioclase grains and may have formed by sub-solidus reduction processes.

The magnetic properties and morphology of metallic iron produced by subsolidus reduction of synthetic Apollo 11 composition glasses

Metallic iron has been precipitated from a synthetic high-iron, high-titanium Apollo 11 composition glass powder in a furnace with the oxygen fugacity controlled by gas mixing techniques. Measurable quantities of iron, as determined with a vibrating sample magnetometer capable of detecting 0.01 wt% iron in the absence of ferromagnetic minerals, were produced in experiments at temperatures between 700°C and 1045°C, with run times between 3 hr and 95 hr, and oxygen fugacities between 1 and 2 orders of magnitude below the iron-wustite (IW) buffer curve. Such conditions of ƒ O 2 and T are probably not greatly different from those occurring in a large lunar ejecta blanket. The oxygen fugacity determines the amount of iron produced for a given time and temperature, with about 1% produced if log ƒ O 2 is 1.4 units below the IW buffer curve and about 3.5% produced if log ƒ O 2 is 1.7 units below at 990°C. Above 950°C essentially all the iron is multidomain (>300Å) while below 950°C as much as 15% is single domain (150Å–300Å) and an appreciable quantity remains even smaller. Compaction of the sample slows the rate of reduction but does not influence the grain size of metal. The quantities and size distribution of the reduced iron in a number of the experimental runs are strongly analogous to certain lunar soils and breccias and indicate that reduction in an ejected blanket could partly account for the excess iron of lunar soils and breccias relative to the igneous rocks.

Stability relations of ilmenite and ulvöspinel in the Fe-Ti-O system and application of these data to lunar mineral assemblages

The fO 2 stability relations of ilmenite and ulvöspinel were determined using C-O H-N gas-flow apparatus with fO 2 measured by a solid ceramic (calcia-zirconia) oxygen electrolyte cell. For Fe+TiO 2 + 1/2 O 2 =FeTiO 3 (from 850°–1050°C), 1/2 log fO 2=(−11,250/T) + 0.98 and for Fe+FeTiO 3 + 1/2 O 2 =Fe 2TiO 4 (from 850°–1210°C), 1/2 logfO 2 = (−12,170/T) + 1.93 . These curves lie at significantly higher values of ƒO 2 than determined by previous investigators (i.e., 3/4 and 1/4 order of magnitude for ilmenite and ulvöspinel, respectively). In addition, for Fe+ 2TiO 2 + 1/2 O 2 =FeTi 2O 5 (1210°C) , ΔG r 0=−45.8 ± 0.6 kcal. The QFI curve crosses the ulvöspinel reduction curve at ∼950°C and is at lower values of fO 2 below this temperature. The occurrences of fayalite reduction to SiO 2 + Fe in lunar rock 14053, as well as a new finding of this assemblage in 14072, are evidence for extreme sub-solidus reduction, whereas ulvöspinel breakdown alone occurs under less reducing conditions. The ‘complete’ reduction of ulvöspinel to TiO 2 + Fe occurs in 2 steps: first, to ilmenite + Fe and then, however more slowly, to rutile + Fe. Thus, the presence of ulvöspinel but lack of ilmenite reduction in lunar rocks cannot be used as evidence that the fO 2 was between the associated curves — only upper limits of fO 2 can be inferred.

Subsolidus Reduction of Lunar Spinels

IT is now generally accepted that lunar basalts crystallized at extremely low oxygen fugacities. This contention is supported by the ubiquitous occurrence of metallic iron, metallic Ni–Fe alloys and the absence of Fe+3 in bulk rock analyses. Electrolytic cell measurements on the Apollo 12 basalts1 indicate crystallization oxygen fugacities (f02) at 1,200° C between 10−10.8 and 10−12.8 atm. The evidence presented in this communication shows that some basalts must have exceeded the lower limit by several orders of magnitude in fO2 and is based primarily on the decomposition of Cr-Al-ulvospinel; it is further supported in one case by the decomposition of olivine. Optical and analytical data on the opaque phase mineralogy of ulvospinel, chromite and ilmenite are presented in this communication and thermodynamic calculations relating to the decomposition of Fe+2-bearing spinels are considered. These data suggest that some rocks have undergone intense non-equilibrium subsolidus reduction. The reduction phenomenon is widespread and is considered to have developed either during initial deuteric cooling or as a result of a post-crystallization reduction event.

Subsolidus Reduction of Lunar Spinels

Subsolidus Reduction of Lunar Spinels