Abstract Quantifying the time scales of magmatic differentiation is critical for understanding the rate at which silicic plutonic and volcanic rocks form. Directly dating this process is difficult because locations with both clear evidence for fractional crystallization and the accessory phases necessary for radiometric dating are rare. Early zircon saturation, however, appears to be characteristic of many high-K, arc-related melts due to their generally elevated initial Zr concentrations. Thus, high-K plutonic series are ideal candidates to study the time scales of magmatic differentiation using zircon U-Pb geochronology. This study focuses on the Dariv Igneous Complex in western Mongolia where early saturation of zircon in a suite of cogenetic, upper crustal (<0.5 GPa) igneous rocks ranging from ultramafic cumulates to evolved granitoids allows us to date magmatic differentiation. Crystallization ages from six samples across the sequence indicate that magmatic fractionation from a basalt to high-silica (>65 wt% SiO2) melt occurred in ≤590 ± 350 k.y. This estimate is greater than modeled time scales of conductive cooling of a single intrusion and physical segregation of minerals from a melt, suggesting that continued influx of heat through magmatic activity in the complex may have prolonged cooling and thus time scales associated with the production of silica-enriched melts.
Abstract During the differentiation of arc magmas, fractionating liquids follow a series of cotectics, where the co‐crystallization of multiple minerals control the melt compositional trajectories, commonly referred to as liquid lines of descent (LLD). These cotectics are sensitive to intensive properties, including fractionation pressure and melt H 2 O concentration, and changes in these variables produce systematic differences in the LLDs of arc lavas. Based on a compilation of experimental studies, we develop two major element proxies that exploit differences in LLDs to constrain the fractionation conditions of arc magmas. Near‐primary fractionating magmas evolve along the olivine‐clinopyroxene cotectic, which is pressure‐sensitive. We use this sensitivity to develop a proxy for early fractionation pressure based on the normative mineral compositions of melts with 8 ± 1 wt.% MgO. Fractionation in more evolved magmas is controlled by the clinopyroxene‐plagioclase cotectic, which is strongly sensitive to magmatic H 2 O contents. We use this relationship to develop an H 2 O proxy that is calibrated to the normative mineral components of melts with 2–4 wt.% MgO. These two proxies provide new tools for estimating the variations in pressure and temperature between magmatic systems. We applied these proxies to compiled major element data and phenocryst assemblages from modern volcanic arcs and show that in island arcs early fractionation is relatively shallow and magmas are dominantly H 2 O‐poor, while continental arcs are characterized by more hydrous and deeper early fractionation. These differences likely reflect variations in the relative contributions of decompression and flux melting in combination with distinct upper plate controls on arc melt generation.
Abstract Eleven isobaric experimental series simulate the fractional crystallization of 1,150 km deep lunar magma ocean. Crystallization begins at 1,850 ° C with olivine (to 32 per cent solidified, pcs), followed at 1,600 ° C by olivine + opx ± Cr‐spinel (to 62 pcs), at 1,210 ° C cpx + plagioclase ± olivine ± Ti‐spinel (to 97 pcs) and at 1,060 ° C quartz + cpx + plagioclase + Ti‐spinel, leaving 1.8 wt% residual magma that crystallizes minor K‐feldspar and apatite in addition. Melt compositions remain near 45 wt% SiO 2 , while FeO increases from 11 to 26 wt%, TiO 2 peaks at 4 wt% at Ti‐spinel saturation. The available experimental liquid lines of descent yield an overall fractional crystallization sequence of olivine→opx→cpx + plagioclase→quartz→FeTi‐oxide. Plagioclase appears concomitantly with cpx, a result of the low magma ocean floor pressures (≤1 GPa) after 66%–76% of olivine + opx‐fractionation. A few wt% of FeTi‐oxides form mostly once the quartz + plagioclase + cpx‐cotectic is reached, cumulate densities remain ≤3,740 kg/m 3 . Scaled to a full magma ocean, plagioclase appears at 210–120 km depth, mainly as a function of bulk Al 2 O 3 . As buoyancy driven plagioclase‐cpx separation is likely limited, these depths may correspond to the primordial lunar crustal thickness. Allowing for complete plagioclase flotation to the quartz + plagioclase + cpx + FeTi‐oxide ± olivine cotectic yields 95–70 km primordial crust of anorthosite and quartz‐gabbro, far in excess of the 35–50 km observed. This supports an overturn of primordial layers, remelting of dense gabbroic cumulates in the harzburgitic cumulate mantle leading to further mixing and differentiation. We posit that such complex density induced convection led to a lunar marble cake mantle with primitive and fairly evolved reprocessed cumulates next to each other.
Abstract Crustal geochemical signatures in carbonatites may arise from carbon recycling through the mantle or from fluid-mediated interaction with the continental crust. To distinguish igneous from fluid-mediated processes, we experimentally determined rare earth element (REE) partitioning between calcite/melt and apatite/melt at subvolcanic emplacement conditions (1–2 kbar, 750–1000 °C). Our data allow modeling of calcite-apatite (Cc/Ap) partition coefficients (D), representing a new tool to bypass the previously required but largely unknown carbonatite melt composition. Experimentally determined magmatic calcite/apatite REE patterns are flat, as is ~0.75, and they show a slight U-shape that becomes more pronounced with temperature decreasing from 1000 to 750 °C. Application to texturally well-equilibrated natural Ca-carbonatites and calcite-bearing nephelinites shows that some calcite-apatite pairs follow this pattern and, hence, confirm the magmatic nature of the carbonates. values of other mineral pairs range from 10−2 to 10−3, which, together with a substantial light REE depletion in the calcite, is interpreted as fluid-mediated light REE removal during secondary calcite recrystallization. Calcite/apatite REE distributions are well suited to evaluate whether a carbonatite mineralogy is primary and magmatic or has been affected by secondary recrystallization. In this sense, our tool provides information about the sample's primary or secondary nature, which is essential when assigning isotopic crustal signatures (in Ca, C, or Sr) or REE patterns to related geologic processes.
We have developed a novel analytical technique for diamond-trap experiments to directly analyze high-pressure, high-temperature fluid and melt compositions in equilibrium with mantle material. Experiments were conducted at a pressure of 6 GPa and temperatures between 900-1200 °C in a multi-anvil apparatus with a synthetic K-free eclogite doped with 860 ppm Cs, ~20 wt% H2O, and a layer of diamond aggregates serving as a fluid/melt trap. Experiments at identical conditions were analyzed with two different methods. In the new, "freezing," approach, the capsule was frozen prior to opening and kept frozen during laser-ablation ICP-MS analysis, thus ablating the quenched fluid (precipitates together with water that unmixed upon quenching) in a solid state. Cesium, fractionating completely into the fluid or melt phase, was used as an internal standard for calculating the fluid compositions. Calculated uncertainties on H2O content in the fluid composition are 0.7-2.5%. In the conventional "evaporation" approach, water from the unmixed fluid was first evaporated from the capsule, then the remaining fluid precipitates were analyzed by LA-ICP-MS. The compositions of the residual eclogitic minerals were measured by electron microprobe, and the fluid composition was then determined by mass-balance. Uncertainties in mineral compositions lead to poor precision in fluid composition in this latter approach. Results of the two methods of fluid analysis were found to be in good agreement. Because the "freezing" approach analyzes the entire fluid directly and does not rely on mass balance for calculating fluid compositions, our new method provides a superior means for determining the composition of fluids. Secondly, it avoids loss of cations that remain soluble in water (e.g., Cs, K) after quenching the experiment.
Natural moissanite (SiC) is reported from dozens of localities, most commonly from ultramafic rocks where it may be associated with diamond and iron silicides. Yet, formation conditions of moissanite remain in the realm of speculation. The key property of SiC is its extremely reduced nature. We have experimentally equilibrated SiC with olivine and orthopyroxene at 1300-1700°C, 2 and 10 GPa, to determine the oxygen fugacity of the C + orthopyroxene = SiC + olivine + O2 buffer (MOOC) and the equilibrium X Mg of coexisting mantle silicates. The experiments resulted in olivine and orthopyroxene with X Mg of 0.993-0.998 in equilibrium with SiC and iron silicides. Calculated oxygen fugacities are 5-6.5 log units below the iron-wustite (IW) buffer at 2-10 GPa. The experimental results concur with calculated phase relations for harzburgitic mantle under reducing conditions that include metal alloys, carbides and silicides. The extremely reducing character of SiC precludes coexistence with silicates with appreciable Fe2+, and hence excludes equilibrium with mantle phases with typical X Mg 's of ~0.9. Calculated Fe-Mg diffusion lengths reveal that SiC grains of 1 mm would react with the Fe-component of olivine to iron carbide or metal and orthopyroxene within <1 Ma at temperatures above 800°C. We thus conclude that SiC forms through a low-temperature process (<700-800°C) where equilibrium is only reached at the grain scale. The most plausible formation mechanism is a strong fractionation of a C-O-H fluid from metamorphosed sediments originally rich in organic material. Such a fluid is initially saturated with graphite or diamond and is slightly more reduced than the H2O-C join. Fluid percolation in the mantle leads to H2O-sequestration by crystallizing hydrous phases (most likely serpentine, brucite or phase A), and hence O2-removal from the fluid causing its reduction and continuous graphite or diamond precipitation. A small, highly fractionated fluid fraction may then reach CH4 and H2 concentrations that allow SiC formation on grain boundaries without equilibration with the bulk rock on a larger scale. Such a mechanism is corroborated by the strongly negative δ13C of moissanites (-20 to -37), consistent with reduced fluids originating from metamorphosed organic carbon.
'/%3e%3cuse xlink:href='%23a' transform='rotate(23.036 .093 25.536)'/%3e%3cuse xlink:href='%23a' transform='rotate(45.87 1.273 16.18)'/%3e%3cuse xlink:href='%23a' transform='rotate(69.945 .996 12.078)'/%3e%3cuse xlink:href='%23a' transform='rotate(20.66 -19.689 31.932)'/%3e%3c/svg%3e)