Silica and volatile-element metasomatism of Archean mantle: a xenolith-scale example from the Kaapvaal Craton

Textural evidence in a composite garnet harzburgite mantle xenolith from Kimberley, South Africa, suggests metasomatism of a severely melt-depleted substrate by a siliceous, volatile-rich fluid. The fluid reacted with olivine-rich garnet harzburgite, converting olivine to orthopyroxene, forming additional garnet and introducing phlogopite, and small quantities of sulfide and probable carbonate. Extensive reaction (>50%) forming orthopyroxenite resulted from channelized flow in a vein, with orthopyroxene growth in the surrounding matrix from a pervasive grain-boundary fluid. The mineralogy of the reaction assemblage and the bulk composition of the added component dominated by Si and Al, with lesser quantities of K, Na, H, C and S, are consistent with experimental studies of hybridization of siliceous melts or fluids with peridotite. However, low Na, Fe and Ca compared with melts of eclogite suggest a fluid phase that previously evolved by reaction with peridotitic mantle. Garnet and phlogopite trace element compositions indicate a fluid rich in large-ion lithophile (LIL) elements, but poor in high field-strength elements (HFSE), qualitatively consistent with subduction zone melts and fluids. An Os isotope (T_RD) model age of 2.97 ± 0.04 Ga and lack of compositional zonation in the xenolith indicate an ancient origin, consistent with proposed 2.9 Ga subduction and continental collision in the Kimberley region. The veined sample reflects the silicic end of a spectrum of compositions generated in the Kimberley mantle lithosphere by the metasomatizing effects of fluids derived from oceanic lithosphere. These results provide petrographic and chemical evidence for fluid-mediated Si-, volatile- and trace-element metasomatism of Archean mantle, and support models advocating large-scale modification of regions of Archean subcontinental mantle by subduction processes that occurred in the Archean.     

Phase equilibria of the Shergotty meteorite: Constraints on pre‐eruptive water contents of martian magmas and fractional crystallization under hydrous conditions

Abstract— An experimental investigation of the Shergotty meteorite was performed at 0.1 MPa under anhydrous conditions at the quartz‐fayalite‐magnetite buffer and at 100 and 200 MPa under H₂O‐saturated conditions at the nickel‐nickel oxide buffer. The results of these experiments are used to infer magmatic conditions recorded by co‐crystallization of augite and pigeonite phenocrysts found in Shergotty and to investigate the effect of H₂O on fractional crystallization paths followed by shergottite magmas. The phase relations and compositions of the homogeneous magnesian pyroxene cores in Shergotty are most closely approximated by crystallization under H₂O‐saturated conditions at 1120 °C (± 10 °C) and 56 MPa (± 18 MPa), corresponding to dissolved H₂O contents of 1.8 wt% (± 0.6 wt%) and a depth of 5 km (± 1.5 km) in the martian crust (uncertainties are 2s? values). The Shergotty magma then lost this water during ascent and eruption. Fractional crystallization of the Shergotty magma under anhydrous conditions produces liquids that follow a strong Fe‐enrichment trend at nearly constant SiO₂. Crystallization under H₂O‐saturated conditions generates derivative liquids, depleted in FeO and Al₂O₃ and enriched in SiO₂, that are compositionally similar to the Mars Pathfinder andesite rock composition. The presence of ～1.8 wt% water in Shergotty parental magmas could result from assimilation of hydrated crustal materials or from dehydration of hydrous phases in the mantle source region.     

Tax records from western Norway,as an index of Little Ice Age environmental and economic deterioration

Data from general tax commissions held in Sunnfjord Fogderi, Norway, reveal a substantial decline in rural prosperity between 1667 and 1723. Late seventeenth and eighteenth century incidence of serious physical damage to farmlands is documented in tax relief proceedings. Environmental deterioration characterised the early years of the Little Ice Age in western Norway.     

Experiments on liquid immiscibility along tholeiitic liquid lines of descent

Crystallization experiments have been conducted on compositions along tholeiitic liquid lines of descent to define the compositional space for the development of silicate liquid immiscibility. Starting materials have 46–56 wt% SiO₂, 11.7–17.7 wt% FeO_tot, and Mg-number between 0.29 and 0.36. These melts fall on the basaltic trends relevant for Mull, Iceland, Snake River Plain lavas and for the Sept Iles layered intrusion, where large-scale liquid immiscibility has been recognized. At one atmosphere under anhydrous conditions, immiscibility develops below 1,000–1,020°C in all of these compositionally diverse lavas. Extreme iron enrichment is not necessary; immiscibility also develops during iron depletion and silica enrichment. Variations in melt composition control the development of silicate liquid immiscibility along the tholeiitic trend. Elevation of Na₂O + K₂O + P₂O₅ + TiO₂ promotes the development of two immiscible liquids. Increasing melt CaO and Al₂O₃ stabilizes a single-liquid field. New data and published phase equilibria show that anhydrous, low-pressure fractional crystallization is the most favorable condition for unmixing during differentiation. Pressure inhibits immiscibility because it expands the stability field of high-Ca clinopyroxene, which reduces the proportion of plagioclase in the crystallizing assemblage, thus enhancing early iron depletion. Magma mixing between primitive basalt and Fe–Ti–P-rich ferrobasalts can serve to elevate phosphorous and alkali contents and thereby promote unmixing. Water might decrease the temperature and size of the two-liquid field, potentially shifting the binodal (solvus) below the liquidus, leading the system to evolve as a single-melt phase.     

Amphibole stability in primitive arc magmas: effects of temperature,H<Subscript>2</Subscript>O content,and oxygen fugacity

The water-saturated phase relations have been determined for a primitive magnesian andesite (57 wt% SiO₂, 9 wt% MgO) from the Mt. Shasta, CA region over the pressure range 200–800 MPa, temperature range of 915–1,070 °C, and oxygen fugacities varying from the nickel–nickel oxide (NNO) buffer to three log units above NNO (NNO+3). The phase diagram of a primitive basaltic andesite (52 wt% SiO₂, 10.5 wt% MgO) also from the Mt. Shasta region (Grove et al. in Contrib Miner Petrol 145:515–533; 2003) has been supplemented with additional experimental data at 500 MPa. Hydrous phase relations for these compositions allow a comparison of the dramatic effects of dissolved H₂O on the crystallization sequence. Liquidus mineral phase stability and appearance temperatures vary sensitively in response to variation in pressure and H₂O content, and this information is used to calibrate magmatic barometers-hygrometers for primitive arc magmas. H₂O-saturated experiments on both compositions reveal the strong dependence of amphibole stability on the partial pressure of H₂O. A narrow stability field is identified where olivine and amphibole are coexisting phases in the primitive andesite composition above 500 MPa and at least until 800 MPa, between 975–1,025 °C. With increasing H₂O pressure (\({P}_{\text {H}_2{\rm O}}\)), the temperature difference between the liquidus and amphibole appearance decreases, causing a change in chemical composition of the first amphibole to crystallize. An empirical calibration is proposed for an amphibole first appearance barometer-hygrometer that uses Mg# of the amphibole and \(f_{\text {O}_2}\):

$$ P_{\text{H}_{2}{\rm O}}({\rm MPa})=\left[{\frac{{\rm Mg\#}}{52.7}}-0.014 * \Updelta {\rm NNO}\right]^{15.12} $$

This barometer gives a minimum \({P}_{\text{H}_{2}{\rm O}}\) recorded by the first appearance of amphibole in primitive arc basaltic andesite and andesite. We apply this barometer to amphibole antecrysts erupted in mixed andesite and dacite lavas from the Mt. Shasta, CA stratocone. Both high H₂O pressures (500–900 MPa) and high pre-eruptive magmatic H₂O contents (10–14 wt% H₂O) are indicated for the primitive end members of magma mixing that are preserved in the Shasta lavas. We also use these new experimental data to explore and evaluate the empirical hornblende barometer of Larocque and Canil (2010).

     

Magnesium isotopic composition of the Earth and chondrites

To constrain further the Mg isotopic composition of the Earth and chondrites, and investigate the behavior of Mg isotopes during planetary formation and magmatic processes, we report high-precision (±0.06‰ on δ²⁵Mg and ±0.07‰ on δ²⁶Mg, 2SD) analyses of Mg isotopes for (1) 47 mid-ocean ridge basalts covering global major ridge segments and spanning a broad range in latitudes, geochemical and radiogenic isotopic compositions; (2) 63 ocean island basalts from Hawaii (Kilauea, Koolau and Loihi) and French Polynesia (Society Island and Cook-Austral chain); (3) 29 peridotite xenoliths from Australia, China, France, Tanzania and USA; and (4) 38 carbonaceous, ordinary and enstatite chondrites including 9 chondrite groups (CI, CM, CO, CV, L, LL, H, EH and EL).Oceanic basalts and peridotite xenoliths have similar Mg isotopic compositions, with average values of δ²⁵Mg = −0.13 ± 0.05 (2SD) and δ²⁶Mg = −0.26 ± 0.07 (2SD) for global oceanic basalts (n = 110) and δ²⁵Mg = −0.13 ± 0.03 (2SD) and δ²⁶Mg = −0.25 ± 0.04 (2SD) for global peridotite xenoliths (n = 29). The identical Mg isotopic compositions in oceanic basalts and peridotites suggest that equilibrium Mg isotope fractionation during partial melting of peridotite mantle and magmatic differentiation of basaltic magma is negligible. Thirty-eight chondrites have indistinguishable Mg isotopic compositions, with δ²⁵Mg = −0.15 ± 0.04 (2SD) and δ²⁶Mg = −0.28 ± 0.06 (2SD). The constancy of Mg isotopic compositions in all major types of chondrites suggest that primary and secondary processes that affected the chemical and oxygen isotopic compositions of chondrites did not significantly fractionate Mg isotopes.Collectively, the Mg isotopic composition of the Earth’s mantle, based on oceanic basalts and peridotites, is estimated to be −0.13 ± 0.04 for δ²⁵Mg and −0.25 ± 0.07 for δ²⁶Mg (2SD, n = 139). The Mg isotopic composition of the Earth, as represented by the mantle, is similar to chondrites. The chondritic composition of the Earth implies that Mg isotopes were well mixed during accretion of the inner solar system.     

An Engineering Approach to Seismic Risk Management in Hardrock Mines

The problem of mining-induced seismicity in hardrock mines has become significant as underground mines from around the world are pushing production to deeper levels. At many mines, the risk associated with large seismic events and rockburst damage must be managed to ensure the safety of mine workers and minimise production losses. In this paper, an engineering approach to seismic risk management is described. It relies on accepted risk management techniques, which principally include the identification and understanding of hazards from which risk mitigation measures can be developed. This is achieved using simple but effective analysis techniques of high resolution microseismic data.     

Evidence of hydrous differentiation and crystal accumulation in the low-MgO,high-Al2O3 Lake Basalt from Medicine Lake volcano,California

The late Pleistocene Lake Basalt of Medicine Lake volcano, California is comprised of variably porphyritic basalt and basaltic andesite flows and scoria. These eruptives are similar in composition and phenocryst abundance to the low-MgO, high-Al²O³ mafic magmas common in convergent margin settings. The petrogenesis of the magmas that produced the Lake Basalt has been inferred from field relations, melting experiments and subsequent major and trace element modeling. Their formation involved both hydrous differentiation and plagioclase accumulation and thus the Lake Basalt can be used to constrain the relative contributions of these processes to the production of high-Al²O³ arc basalt. Phenocryst-poor lavas of the Lake Basalt formed by hydrous differentiation; their compositions and observed phenocrysts were reproduced in 1 kbar, H²O-saturated melting experiments. Anorthite-rich plagioclase compositions of the lavas of the Lake Basalt necessitate crystallization from melts with between 4 and 6 wt% dissolved H²O. Phenocryst-rich lavas of the Lake Basalt, with 18 modal% phenocrysts and greater, formed by plagioclase accumulation in magmas similar to the phenocryst-poor lavas. This interpretation is supported by the depleted incompatible element abundances and enriched Sr/Zr ratio of the more porphyritic lavas relative to the phenocryst-poor lavas. We model the formation of the Lake Basalt as a two-stage process that combines a differentiation model and a plagioclase accumulation model. Stage one involved hydrous fractionation, granitic assimilation and mixing with undifferentiated parent magma. This process generated lavas with up to 19.2 wt% A1²O³ and 7 modal% phenocrysts. In stage two, plagioclase accumulated in these liquids and produced more aluminous and porphyritic lavas with up to 21.8 wt% A1²O³ and 33 modal% phenocrysts.     

Straddling the tholeiitic/calc-alkaline transition: the effects of modest amounts of water on magmatic differentiation at Newberry Volcano,Oregon

Melting experiments have been performed at 1 bar (anhydrous) and 1- and 2-kbar H₂O-saturated conditions to study the effect of water on the differentiation of a basaltic andesite. The starting material was a mafic pumice from the compositionally zoned tuff deposited during the ~75 ka caldera-forming eruption of Newberry Volcano, a rear-arc volcanic center in the central Oregon Cascades. Pumices in the tuff of Newberry caldera (TNC) span a continuous silica range from 53 to 74 wt% and feature an unusually high-Na₂O content of 6.5 wt% at 67 wt% SiO₂. This wide range of magmatic compositions erupted in a single event makes the TNC an excellent natural laboratory in which to study the conditions of magmatic differentiation. Our experimental results and mineral–melt hygrometers/thermometers yield similar estimates of pre-eruptive H₂O contents and temperatures of the TNC liquids. The most primitive (mafic) basaltic andesites record a pre-eruptive H₂O content of 1.5 wt% and a liquidus temperature of 1,060–1,070 °C at upper crustal pressure. This modest H₂O content produces a distinctive fractionation trend that is much more enriched in Na, Fe, and Ti than the calc-alkaline trend typical of wetter arc magmas, but slightly less enriched in Fe and Ti than the tholeiitic trend of dry magmas. Modest H₂O contents might be expected at Newberry Volcano given its location in the Cascade rear arc, and the same fractionation trend is also observed in the rim andesites of the rear-arc Medicine Lake volcano in the southern Cascades. However, the Na–Fe–Ti enrichment characteristic of modest H₂O (1–2 wt%) is also observed to the west of Newberry in magmas erupted from the arc axis, such as the Shevlin Park Tuff and several lava flows from the Three Sisters. This shows that modest-H₂O magmas are being generated directly beneath the arc axis as well as in the rear arc. Because liquid lines of descent are particularly sensitive to water content in the range of 0–3 wt% H₂O, they provide a quantitative and reliable tool for precisely determining pre-eruptive H₂O content using major-element data from pumices or lava flows. Coupled enrichment in Na, Fe, and Ti relative to the calc-alkaline trend is a general feature of fractional crystallization in the presence of modest amounts of H₂O, which may be used to look for “damp” fractionation sequences elsewhere.