James S. Beard

Reactive bulk assimilation: A model for crust-mantle mixing in silicic magmas

Bulk assimilation of small (millimeters to ∼1 km) fragments of crust—driven and (ultimately) masked by reactions during xenolith melting and magma crystallization—is an important mechanism for crust-mantle mixing. Xenoliths containing mica or amphibole undergo dehydration melting when incorporated into a host magma, yielding mainly plagioclase, pyroxene, Fe-Ti oxides, and hydrous melt. The xenolith is physically compromised by partial melting and begins to disintegrate; xenolithic melt and crystals are mixed into the host magma. Xenocrystic zircon is liberated at this stage. The cryptic character of assimilation is greatly enhanced in any hydrous magma by hydration crystallization reactions (the reverse of dehydration melting). All pyroxenes and oxides (phenocrysts, xenocrysts, or crystals having a hybrid signature) will be subject to these reactions, producing feldspars, amphiboles, and micas that incorporate material from several sources, a particularly effective mixing mechanism. Implicit in the model is a reduced energy penalty for bulk assimilation—much of the assimilant remains in solid form—compared to melt-assimilation models. A large role for bulk assimilation supports stoping as a credible mechanism for the ascent of magmas. While the assimilation of low-density crust and concomitant fractionation provide the isostatic impetus for ascent, the wholesale incorporation and processing of crustal rocks in the magma chamber helps create the room for ascent.

Effect of water on the composition of partial melts of greenstone and amphibolite

Closed-system partial melts of hydrated, metamorphosed arc basalts and andesites (greenstones and amphibolites), where only water structurally bound in metamorphic minerals is available for melting (dehydration melting), are generally water-undersaturated, coexist with plagioclase-rich, anhydrous restites, and have compositions like island arc tonalites. In contrast, water-saturated melting at water pressures of 3 kilobars yields strongly peraluminous, low iron melts that coexist with an amphibolebearing, plagioclase-poor restite. These melt compositions are unlike those of most natural silicic rocks. Thus, dehydration melting over a range of pressures in the crust of island arcs is a plausible mechanism for the petrogenesis of islands arc tonalite, whereas water-saturated melting at pressure of 3 kilobars and above is not.

A fossil, serpentinization-related hydrothermal vent, Ocean Drilling Program Leg 173, Site 1068 (Iberia Abyssal Plain): some aspects of mineral and fluid chemistry

The basement at Site 1068, Ocean Drilling Program (ODP) Leg 173 (serpentinized peridotite in fault contact with overlying amphibolite-clast-dominated sedimentary and tectonic breccias) is host to a hydrothermal system rooted in serpentinization reactions occurring at depth. The serpentinite grades downward from cataclasites at the fault, through brecciated, recrystallized, tochilinite-bearing serpentinite, to awaruite-bearing massive, mesh-textured serpentinite. Andradite is common throughout and is a major sink for iron. The breccias are similarly zoned, from tectonized rocks near the fault upward into sedimentary breccias. Mg-silicate vein assemblages and rodingitized amphibolite clasts near the fault give way to calcite veins and nonpervasive albite-chlorite alteration upsection. Marcasite (± pyrrhotite at the fault) is the sulfide phase and occurs only in the tectonic breccias. Fe oxides are magnetite near the fault and hematite and ferric oxyhydroxides upsection. The zonation reflects mixing of seawater with a fluid whose composition (low fO2, fS2 Si, CO2, high Ca, Fe, Ca/Mg, pH) is controlled by serpentinization reactions. The deepest serpentinites have strongly reduced mineral assemblages that are unusual in a totally serpentinized peridotite. This probably reflects equilibration with a fluid derived from ongoing serpentinization at depth. The upper serpentinites, on through the mineral sequences seen in the breccias reflect increasing input from seawater upsection. Increased fO2 and fS2 stabilizes increasingly S- and O-rich assemblages. Calcite (and ferric oxide) precipitation decreases pH, stabilizing marcasite. Relative to mid-ocean ridge hydrothermal systems, fluids in serpentinite-hosted hydrothermal systems are poor in S and rich in Mg and are unlikely to host large sulfide ore deposits.

Drilling constraints on lithospheric accretion and evolution at Atlantis Massif, Mid‐Atlantic Ridge 30°N

Expeditions 304 and 305 of the Integrated Ocean Drilling Program cored and logged a 1.4 km section of the domal core of Atlantis Massif. Postdrilling research results summarized here constrain the structure and lithology of the Central Dome of this oceanic core complex. The dominantly gabbroic sequence recovered contrasts with predrilling predictions; application of the ground truth in subsequent geophysical processing has produced self-consistent models for the Central Dome. The presence of many thin interfingered petrologic units indicates that the intrusions forming the domal core were emplaced over a minimum of 100-220 kyr, and not as a single magma pulse. Isotopic and mineralogical alteration is intense in the upper 100 m but decreases in intensity with depth. Below 800 m, alteration is restricted to narrow zones surrounding faults, veins, igneous contacts, and to an interval of locally intense serpentinization in olivine-rich troctolite. Hydration of the lithosphere occurred over the complete range of temperature conditions from granulite to zeolite facies, but was predominantly in the amphibolite and greenschist range. Deformation of the sequence was remarkably localized, despite paleomagnetic indications that the dome has undergone at least 45 degrees rotation, presumably during unroofing via detachment faulting. Both the deformation pattern and the lithology contrast with what is known from seafloor studies on the adjacent Southern Ridge of the massif. There, the detachment capping the domal core deformed a 100 m thick zone and serpentinized peridotite comprises similar to 70% of recovered samples. We develop a working model of the evolution of Atlantis Massif over the past 2 Myr, outlining several stages that could explain the observed similarities and differences between the Central Dome and the Southern Ridge.

Magnetite–melt HFSE partitioning

Abstract Results from doped, hydrous experiments on natural mafic-to intermediate-composition lavas at 2–5 kbar pressure were combined with existing 1 atm data to evaluate the effects of composition and temperature on the partitioning behavior of the high field strength elements (HFSE), Zr, Nb, Ta and Hf between magnetite and natural silicate melts. Magnetite composition was found to be the strongest controlling factor on partitioning behavior. The partition coefficients ( D ) for Zr, Nb, Hf, and Ta correlate with D Ti , Ti and Al content of the magnetite, temperature and pressure. The partition coefficients for the HFSE are similar to one another for any given magnetite–melt pair, but range from 2 in titanomagnetite. In addition, the relationship between Ti and the HFSE changes as a function of pressure and temperature, with the HFSE becoming more incompatible relative to Ti at lower temperatures and/or higher pressures. This change in the relationship between D Ti and D HFSE with temperature and pressure means that the expressions presented in Nielsen et al. (1994) [Nielsen, R.L., Forsythe, L.M., Gallaghan, W.E., Fisk, M.R., 1994. Major and trace element magnetite–melt partitioning. Chem. Geol. 117, 167–191.] are not valid for hydrous, aluminous systems. Expressions were derived to describe the relationship between D HFSE and temperature, pressure, Fe 2+ /Mg exchange, Ti/Al ratio of the magnetite, and D Ti . These expressions reproduce the input data within 35–50% (1 σ ) over a range extending from highly incompatible to compatible (

Partial melting of apatite-bearing charnockite, granulite, and diorite: Melt compositions, restite mineralogy, and petrologic implications

Melting experiments (P = 6.9 kbar, T = 850–950°C, NNO < ƒ02 < HM) were done on mafic to felsic charnockites, a dioritic gneiss, and a felsic garnet granulite, all common rock types in the Grenville basement of eastern North America. A graphite-bearing granulite gneiss did not melt. Water (H2O+ = 0.60 to 2.0 wt %) is bound in low-grade, retrograde metamorphic minerals and is consumed during the earliest stages of melting. Most melts are water-undersaturated. Melt compositions range from metaluminous, silicic granodiorite (diorite starting composition) to peraluminous or weakly metaluminous granites (all others). In general, liquids become more feldspathic, less silicic, and less peraluminous and are enriched in FeO, MgO, and TiO2 with increasing temperature. Residual feldspar mineralogy controls the CaO, K2O, and Na2O contents of the partial melts and the behavior of these elements can be used, particularly if the degree of source melting can be ascertained, to infer some aspects of the feldspar mineralogy of the source. K-feldspar, a common restite phase in the charnockite and granulite (but not the diorite) should control the behavior of Ba and, possibly, Eu in these systems and yield signatures of these elements that can distinguish source regions and, in some cases, bulk versus melt assimilation. Apatite, a common restite phase, is enriched in rare earth elements (REE), especially middle REE. Retention of apatite in the restite will result in steep, light REE-enriched patterns for melts derived from the diorite and charnockites.

Temporal variation of mineralogy and petrology in cognate gabbroic enclaves at Arenal volcano, Costa Rica

Gabbroic enclaves ejected during the current eruption phase (A-1) and during the latest prehistoric eruption phase (A-2) of Arenal Volcano show systematic variations in texture, mineralogy and composition as a function of host rock chemistry and timing of eruption. The most differentiated enclaves occur in the more differentiated A-2 lavas. Enclaves in the A-1 volcanics are consistently less evolved. Within the current A-1 eruption, the most mafic enclaves are amphibole-bearing rocks that were erupted during the first 2–3 years of activity (1968–1970). These enclaves occur in the most differentiated A-1 volcanics and are not in equilibrium with their host rocks. They crystallized from a hydrous melt that was slightly more mafic than anything erupted during the current cycle. We interpret the enclaves as sidewall crystallization products of a melt, possibly a high-alumina basalt, that was immediately parental to the A-1 lavas. Enclaves that occur in A-1 rocks erupted after 1970 and all of the A-2 enclaves are amphibole-free and less mafic than the early A-1 enclaves. Their chemistry suggests that they formed during the early to intermediate crystallization of their host lavas. None of the enclaves contain minerals that might have equilibrated with a primary, mantle-derived melt. Geothermometry is consistent with geochemistry, with amphibole-bearing A-1 enclaves yielding the highest pyroxene temperatures (ave. 1090° C) and A-2 enclaves the lowest (ave. 1030° C). Geobarometry suggests mid- to upper crustal depths for the crystallization of all enclaves. The enclaves are cognate and reflect pre-eruptive crystallization of Arenal magmas. They record evolution from a hydrous, basaltic magma to the drier basaltic andesites that characterize the current eruption. Volatiles appear to have been lost due to depressurization during the slow ascent of the magmas through the upper levels of the crust following the initial explosive eruption. Volatile loss and depressurization resulted in the destabilization and the progressive resorption of amphibole. The A-2 lavas may represent the long-term fractionation products of basaltic andesite magmas similar in composition to the A-1 lavas. Anorthitic plagioclase, commonly thought of as a phase stabilized by high Ca/Na and high water pressure, continued to crystallize in a system with relatively low Ca/Na and which had dehydrated and/or depressurized to the point at which amphibole was no longer stable. This suggests that compositional characteristics other than high Ca/Na or high water content may have stabilized the anorthite in the basaltic and basaltic andesite melts at Arenal. We speculate that the high-alumina content of the Arenal magmas may be the stabilizing factor.

Experimental, geological, and geochemical constraints on the origins of low-K silicic magmas in oceanic arcs

The results of recent experimental and geochemical studies demonstrate that typical, low-K, island arc dacites (IAD) and tonalites have liquidus water contents below 6 wt %, while many arc basalts, including evolved, low-Mg high-alumina basalt (HAB; H2O ≥ 4 wt%) and magnesian arc tholeiites (H2O ≥ 2 wt %) are water-rich. If these water contents are typical of arc basalts, fractionation at pressures of 200 MPa or more would produce water-rich magmas with major element chemical characteristics unlike >99% of observed IAD. In light of this, plausible mechanisms for IAD genesis include (1) dehydration melting of amphibolitized arc crust and (2) low-pressure (less than 200 MPa) fractionation or assimilation/fractional crystallization (AFC) (accompanied by devolatilization) of hydrous arc basalts. In general, a partial melting origin is favored for tonalitic plutons emplaced at pressures ≥200 MPa, for bimodal suites where geochemistry rules out a genetic relationship between the mafic and silicic end-members, and for IAD with isotopic characteristics distinct from potential basaltic or andesitic parents. Evidence for partial melting of amphibolite yielding IAD melts has been documented in several ancient island arc complexes. A fractionation origin is favored where there is isotopic homogeneity in a basalt-dacite system and for dacites having very low concentrations of incompatible trace elements. Bimodal magmatism in general and high concentrations of incompatible elements in silicic magmas appear to favor a partial (batch) melting origin over Raleigh fractionation but can also result from convectively driven batch fractionation processes (Brophy, 1991). Although both fractionation/AFC and partial melting may be legitimately invoked to explain dacitic magmatism in a given situation, a general model must also account for the absence of high-pressure fractionates of hydrous basalts. This observation seems to favor an important role for amphibolite melting in IAD genesis. It is also possible, however, that physical factors (e.g., neutral buoyancy) promote the formation of basaltic magma chambers in the upper crust or that convective processes, enhanced by large temperature gradients in the upper crust, may favor upper crustal over lower crustal fractionation.

Petrology and Tectonic Significance of Gabbros, Tonalites, Shoshonites, and Anorthosites in a Late Paleozoic Arc-Root Complex in the Wrangellia Terrane, Southern Alaska

Plutonic rocks intrusive into the late Paleozoic Tetelna Formation of southern Alaska are the underpinnings of the late Paleozoic Skolai arc of the Wrangellia Terrane. There are four groups of intrusive rocks within the Skolai arc: (1) Gabbro-diorite plutons that contain gabbroic to anorthositic cumulates along with a differentiated series of gabbros and diorites of basaltic to andesitic composition; (2) Silicic intrusions including tonalite, granodiorite, and granite; (3) Monzonitic to syenitic plutonic rocks of the Ahtell complex and related dikes and sills; (4) Fault-bounded bytownite anorthosite of uncertain age and association. These anorthosites may be related to post-Skolai, Nikolai Greenstone magmatism. The silicic rocks yield discordant U-Pb zircon ages of 290-320 Ma (early to late Pennsylvanian). Relative age relations suggest that the oldest intrusive rocks are the gabbro-diorite plutons, the youngest are the monzonitic rocks, and that the silicic rocks span this range. The gabbro-diorite plutons are similar to gabbroic plutonic rocks in modern and other ancient island arc complexes. They record the differentiation of calc-alkaline basalt to andesite by the fractionation of plagioclase, pyroxene, olivine, and Fe-Ti oxides. The silicic rocks do not appear to be related to either the gabbros or the monzonites. They may represent partial melts of Skolai arc crust. The monzonitic rocks of the Ahtell complex have shoshonitic chemistry. Similar shoshonitic rocks are widespread in both the Wrangellia terrane and the neighboring Alexander terrane and intrude the contact between the two. In modern oceanic arcs, shoshonitic rocks are typically associated with tectonic instability occurring during the initial stages of subduction or just prior to or during termination or flip of an established subduction zone. The nature of any tectonic instability which may have led to the cessation of subduction in the Skolai arc is unclear. Possibilities include collision of the arc with a ridge, an oceanic plateau, another arc, or a continental fragment. One possibility is that the shoshonitic magmatism marks the late Paleozoic amalgamation of Wrangellia and the Alexander terrane. The scarcity of arc rocks predating the shoshonites in the Alexander terrane supports this possibility, but structural corroboration is lacking.

Experimental melting of crustal xenoliths from Kilbourne Hole, New Mexico and implications for the contamination and genesis of magmas

Experiments (P=6.9 kb; T=900–1000°C) on four crustal xenoliths from Kilbourne Hole demonstrate the varying melting behavior of relatively dry crustal lithologies in the region. Granodioritic gneisses (samples KH-8 and KH-11) yield little melt (<5–25%) by 925°C, but undergo extensive (30–50%) melting between 950 and 1000°C. A dioritic charnockite (KH-9) begins to melt, with the consumption of all modal K-feldspar, by 900°C. It is as fertile a melt source as the granodiorites at lower temperatures, but is outstripped in melt production by the granodiorite gneisses at high temperature, yielding only 26% melt by 1000°C. A pelitic granulite (KH-12) proved to be refractory (confirming earlier predictions based on geochemistry) and did not yield significant melt even at 1000°C. All melts have the composition of metaluminous to slightly peraluminous granites and are unlikely to be individually recognizable as magma contaminants on the basis of major element chemistry. However, the relative stability of K-feldspar during partial melting will produce recognizable signatures in Ba, Eu, K/Ba, and Ba/Rb. Melts of KH-11, which retains substantial K-feldspar throughout the melting interval, are generally low in Ba (<500–800 ppm), have high K/Ba and low Ba/Rb (est.) (62–124 and 1–3, respectively). Melts of KH-9, in which all K-feldspar disappears with the onset of melting, are Ba-rich [2000–2600 ppm, K/Ba=16–22; Ba/Rb (est.) =25–47]. Melts of KH-8 have variable Ba contents; <500 ppm Ba at low temperature but >900 ppm Ba in high-temperature melts coexisting with a K-feldspar-free restite. Although REE were not measured in either feldspar or melt, the high Kspar/melt Kds for Eu suggests that the melts coexisting with K-feldspar will have strong negative Eu anomalies. Isotopic and trace element models for magma contamination need to take into account the melting behavior of isotopic reservoirs. For example, the most radiogenic (and incompatible element-rich) sample examined here (the pelitic granulite,87Sr/86Sr=0.757) is refractory, while samples with far less radiogenic Sr (87Sr/86Sr=0.708-0.732) produced substantial melt. This suggests that, in this area, the isotopic signature of contamination may be more subtle than expected. The experimental results can be used to model the petrogenesis of Oligocene volcanic rocks exposed 150 km to the NW of Kilbourne Hole, in the Black Range in the Mogollon-Datil volcanic field. The experimental results suggest that a crustal melting origin for the Kneeling Nun and Caballo Blanco Tuffs is unlikely, even though such an interpretation is permitted by Sr isotopes. Curstal contamination of a mantle-derived magma best explains the chemical and isotopic characteristics of these tuffs. Both experimental and geochemical data suggest that the rhyolites of Moccasin John Canyon and Diamond Creek could represent direct melts of granodiorite basement similar, but not identical, to the Kilbourne Hole granodiorites, perhaps slightly modified by crystal fractionation. The absence of volcanic rocks having87Sr/86Sr>0.74 in the region is consistent with the refractory character of the pelitic granulite.