ALBERT

Description: To investigate how large volumes of silicic melts segregate to form granitic plutons, we conducted a case study of a zoned pluton, in which SiO 2 increases from intermediate (69 wt%) to highly silicic compositions (74 wt%) toward the contact with metasedimentary wallrock in the outer 25 m of the pluton. All other major, minor, and trace elements vary systematically with SiO 2 and indicate that outward increasing SiO 2 is due to a decrease in mafic elements and minerals. Whole-rock oxygen isotopes and elemental variation diagrams do not support mixing with wallrock as an explanation for the Si-rich boundary layer. Instead, mafic enclaves, which are common in the pluton, also decrease in abundance in the outer 25 m of the pluton, suggesting a mechanical origin for the Si-rich boundary layer. The coupling of mechanical and geochemical boundary layers, combined with geochemical modeling, indicate that the silica-rich, enclave-poor boundary layer formed by hindered settling or compaction of a crystal-rich (crystal fractions 〉60%) magmatic mush. Segregation of melts at high crystal fraction is known to be a slow process. However, petrography and Zr-based thermometry indicate that the residual Si-rich liquids were water-saturated. Water decreases melt viscosity, which helps expulsion, but equally importantly, water also delays much of the latent heat release to late in the thermal and crystallization history of a cooling magma. We show that the higher the water content, the longer the time interval over which a magma chamber resides at the stage when water-saturated, high-silica liquids form, allowing sufficient time for exfiltration of silicic liquids before the magma body freezes.

Description: First-row transition element (FRTE) concentrations in primitive mantle-derived melts have been used as direct indicators of mantle source mineralogy (e.g., Ti, Mn, Fe, Co, Ni, Zn) and as proxies to trace the oxidation state of the mantle (e.g., Sc, V, Cu, Zn). Ga and Ge, which share chemical similarities with FRTEs, may also have the ability to trace mineralogical heterogeneities in the source of mantle-derived melts. Although the partitioning behaviors of most FRTEs are well constrained during mantle melting, partition coefficients of Cu, Ga, and Ge between mantle minerals and melt are still uncertain. Here we report new measurements that constrain partition coefficients of Cu, Ga, and Ge between olivine (Ol), orthopyroxene (Opx), clinopyroxene (Cpx), and basaltic melt from graphite capsule experiments carried out at 1.5–2 GPa and 1290–1500 °C. We suggest that discrepancies between recent experimental studies on Cu partitioning reflect one or more of the following causes: compositional control on partitioning, the effect of oxygen fugacity, Cu loss, Fe loss, non-Henrian behavior, and/or lack of complete chemical equilibrium. The partitioning values obtained from this study are 0.13 (±0.06), 0.12 (±3), and 0.09 for D Cu Ol/melt , D Cu Opx/melt , and D Cu Cpx/melt , respectively. Using values from this study and from the literature, we show that melting of a sulfide-bearing peridotite source with an initial D Cu peridotite/melt ranging from 0.49 to 0.60 can explain the Cu content of primitive MORBs. Here, we also support the hypothesis that Ga partitioning between pyroxenes and melt strongly depends on the Al 2 O 3 content of pyroxenes. Using pyroxene compositions from experiments, and previous partition data from literature, we recommend D Ga Px/melt values for low- P (1.5 GPa) spinel peridotite melting ( D Ga Opx/melt = 0.23 and D Ga Cpx/melt = 0.28), intermediate- P (2.8 GPa) spinel peridotite melting ( D Ga Opx/melt = 0.42 and D Ga Cpx/melt = 0.40), high- P (3 GPa) garnet peridotite melting ( D Ga Opx/melt = 0.38 and D Ga Cpx/melt = 0.37), high- P (4 GPa) garnet peridotite melting ( D Ga Opx/melt = 0.26 and D Ga Cpx/melt = 0.30), and MORB-like eclogite melting at 2–3 GPa ( D Ga Cpx/melt = 0.78). Consistent with previous studies, we find that Ga is incompatible in olivine during low- P peridotite melting ( D Ga Ol/melt = 0.08). Using values from this study and from the literature, we support the hypothesis that the Ga, Ga/Sc, and Ti contents of most mantle-derived melts require garnet in their source, but that additional lithologies (e.g., metasomatic veins) may be necessary to explain the chemical variability of those melts. Here we also obtain Ge partition coefficients applicable to low- P peridotite melting of 0.67, 1.04, and 1.12 for D Ge Ol/melt , D Ge Opx/melt , and D Ge Cpx/melt , respectively. Last, to provide a comprehensive picture of FRTE, Ga, and Ge partitioning during mantle melting, we provide a complete set of recommended partitioning values, based on results from this study and from the literature, for all FRTEs, Ga, and Ge, relevant for partial melting of spinel and garnet peridotite, as well as for MORB-like eclogite.

Description: Thickening of arc lithosphere influences the extent of magmatic differentiation and is thereby important for the evolution of juvenile arcs into mature continental crust. Here, we use mantle xenoliths from the late Mesozoic Sierra Nevada continental arc in California (USA) to constrain the pressure, temperature, and compositional evolution of the deep lithosphere beneath a mature arc. These xenoliths consist of spinel peridotites and garnet-bearing spinel peridotites. The former are characterized by coarse-grained protogranular textures having bulk compositions indicative of high-degree melting. The latter are characterized by porphyroclastic textures, garnet coronas around spinels, garnet exsolution lamellae in pyroxenes, and pyroxenes with high-Al cores and low-Al rims. The garnet-bearing spinel peridotites range from depleted to fertile compositions, but the high Cr-numbers [molar Cr/(Cr + Al)] of spinel cores reflect high-degree melting. These observations suggest that the protoliths of the garnet-bearing spinel peridotites were melt-depleted spinel peridotites. Constraints from geothermobarometry and bulk compositions coupled with mantle melting models suggest that the protoliths underwent shallow melt depletion (1–2 GPa, 1300–1400°C), followed by compression, cooling, and final equilibration within the garnet stability field (~3 GPa, 〈800°C). The deepest equilibrated samples are the most refertilized, suggesting that refertilization occurred during compression. We interpret this P – T –composition path to reflect progressive thickening of the Sierran arc lithosphere perhaps as a result of magmatic inflation or tectonic thickening. We hypothesize that newly formed arc lithospheric mantle thickens enough to pinch out the asthenospheric wedge, juxtaposing Sierran arc lithosphere against the subducting oceanic plate. This could have terminated arc magmatism and initiated cooling of the deep Sierran lithosphere.

Description: Crustal xenoliths (pyroxenites and plagioclase + quartz + pyroxene lithologies) from the Quaternary Big Pine volcanic field on the eastern flank of the Sierra Nevada Batholith in California (USA) represent the products of metasomatic reaction between the margins of a Cretaceous granodioritic pluton and Paleozoic marbles, possibly at mid-crustal depths based on the equilibration temperatures recorded by Ti-in-quartz geothermometry. This interpretation is based on the presence of plagioclase showing relict plutonic textures, pyroxenite characterized by nearly pure diopside clinopyroxene, recrystallized plagioclase with anomalously high anorthite content, textures indicating replacement of plagioclase by clinopyroxene (and vice versa), ‘ghost’ plagioclase rare earth element signatures in some clinopyroxenes, and the presence of phlogopite endmember micas at the contact between clinopyroxene-rich and plagioclase-rich zones. These observations suggest that the xenoliths represent fragments of an ‘endoskarn’, the outer sheath of a pluton that chemically reacted with carbonate country-rock. Mass transfer between the carbonate country-rock and the pluton involved transfer of Ca and Mg from the carbonate into the pluton and transfer of Na, K, Al and Si from the pluton to the carbonate, the latter generating extensive endoskarns. The Ca metasomatism of the pluton converted alkali feldspar components into anorthite-rich plagioclase, releasing Na and K, which left the plutonic system. K, in particular, migrated towards the carbonate and precipitated phlogopite upon entering clinopyroxene-rich lithologies. Mass-balance calculations, based on theory and residual enrichments in immobile elements such as Ti, suggest that the pluton experienced net mass loss (〉15%) in the form of Si, Al, Na and K to the surrounding country-rock, but a net gain in Ca and Mg.

Description: Continents are long-term storage sites for sedimentary carbonates. Global flare-ups in continental arc volcanism, when arc magmas intersect and interact with stored carbonates, thus have the potential for elevating the global baseline of deep Earth carbon inputs into the atmosphere, leading to long-lived greenhouse conditions. Decarbonation residues, known as skarns, are ubiquitously associated with the eroded remnants of ancient batholiths, attesting to the potential link between continental arc magmatism and enhanced global CO 2 inputs to the atmosphere.

Description: The two most important magmatic differentiation series on Earth are the Fe-enriching tholeiitic series, which dominates the oceanic crust and island arcs, and the Fe-depleting calc-alkaline series, which dominates the continental crust and continental arcs. It is well known that calc-alkaline magmas are more oxidized when they erupt and are preferentially found in regions of thick crust, but why these quantities should be related remains unexplained. We use the redox-sensitive behavior of europium (Eu) in deep-seated, plagioclase-free arc cumulates to directly constrain the redox evolution of arc magmas at depth. Primitive arc cumulates have negative Eu anomalies, which, in the absence of plagioclase, can only be explained by Eu being partly reduced. We show that primitive arc magmas begin with low oxygen fugacities, similar to that of mid-ocean ridge basalts, but increase in oxygen fugacity by over two orders of magnitude during magmatic differentiation. This intracrustal oxidation is attended by Fe depletion coupled with fractionation of Fe-rich garnet. We conclude that garnet fractionation, owing to its preference for ferrous over ferric iron, results in simultaneous oxidation and Fe depletion of the magma. Favored at high pressure and water content, garnet fractionation explains the correlation between crustal thickness, oxygen fugacity, and the calc-alkaline character of arc magmas.

Description: The Cretaceous to early Paleogene (ca. 140–50 Ma) was characterized by a greenhouse baseline climate, driven by elevated concentrations of atmospheric CO 2 . Hypotheses for the elevated CO 2 concentrations invoke an increase in volcanic CO 2 production due to higher oceanic crust production rates, higher frequency of large igneous provinces, or increases in pelagic carbonate deposition, the last leading to enhanced carbonate subduction into the mantle source regions of arc volcanoes. However, these are not the only volcanic sources of CO 2 during this time interval. We show here that ocean-continent subduction zones, manifested as a global chain of continental arc volcanoes, were as much as 200% longer in the Cretaceous and early Paleogene than in the late Paleogene to present, when a cooler climate prevailed. In particular, many of these continental arcs, unlike island arcs, intersected ancient continental platform carbonates stored on the continental upper plate. We show that the greater length of Cretaceous–Paleogene continental arcs, specifically carbonate-intersecting arcs, could have increased global production of CO 2 by at least 3.7–5.5 times that of the present day. This magmatically driven crustal decarbonation flux of CO 2 through continental arcs exceeds that delivered by Cretaceous oceanic crust production, and was sufficient to drive Cretaceous–Paleogene greenhouse conditions. Thus, carbonate-intersecting continental arc volcanoes likely played an important role in driving greenhouse conditions in the Cretaceous–Paleogene. If so, the waning of North American and Eurasian continental arcs in the Late Cretaceous to early Paleogene, followed by a fundamental shift in western Pacific subduction zones ca. 52 Ma to an island arc–dominated regime, would have been manifested as a decline in global volcanic CO 2 production, prompting a return to an icehouse baseline in the Neogene. Our analysis leads us to speculate that long-term (〉50 m.y.) greenhouse-icehouse oscillations may be linked to fluctuations between continental- and island arc–dominated states. These tectonic fluctuations may result from large-scale changes in the nature of subduction zones, changes we speculate may be tied to the assembly and dispersal of continents. Specifically, dispersal of continents may drive the leading edge of continents to override subduction zones, resulting in continental arc volcanism, whereas assembly of continents or closing of large ocean basins may be manifested as large-scale slab rollback, resulting in the development of intraoceanic volcanic arcs. We suggest that greenhouse-icehouse oscillations are a natural consequence of plate tectonics operating in the presence of continental masses, serving as a large capacitor of carbonates that can be episodically purged during global flare-ups in continental arcs. Importantly, if the global crustal carbonate reservoir has grown with time, as might be expected because platform carbonates on continents do not generally subduct, the greenhouse-driving potential of continental arcs would have been small during the Archean, but would have increased in the Neoproterozoic and Phanerozoic after a significant reservoir of crustal carbonates had formed in response to the evolution of life and the growth of continents.