Plagioclase Crystal Size Distributions Research Articles

Unraveling the tectonic setting and crystallization history of the Equatorial Atlantic Magmatic Province

In NE Brazil, Early Cretaceous mafic dikes and sills intrude the Precambrian Borborema Province and the Paleozoic Parnaíba basin, where they form the Equatorial Atlantic Magmatic Province. The Rio Ceará-Mirim dike swarm (RCM) is a giant arcuate structure ca. 1100 km in length composed of sets of dikes ranging from E-W to NE-SW. There are two additional giant swarms with WNW and NE-SW orientations spanning 380 km and 355 km, respectively, in the NW and SE regions of the Borborema Province. We investigated the tectonic setting and crystallization history of the mafic magmatism through orientation, dimensions (960 dikes), and 32 representative samples of plagioclase crystal size distributions (CSD) to shed light on the connection between the dikes and sills. Plagioclase compositions in EQUAMP intrusions indicate normal crystallization trends with the magma becoming more evolved towards the final stages of crystallization. CSD data suggests that residence times are similar for dikes and sills (≤ 1.5 yrs), although some dikes display higher residence times (3.6–1.8 yrs) in agreement with a longer crystallization history. These results suggest a deeper crystallization setting for the dikes in agreement with a plumbing system in that dikes penetrate the “stiffer” basement units to feed upward the sills accommodated between sedimentary layers with contrasted rheologies. The regional distribution of the mafic magmatism resulted from a bulk NW-trending extensional deformation, where the brittle (re)activation and control of pre-existing structures allowed the nucleation of the arcuate giant dike system and other satellite swarms. We also examine connections between tholeiitic igneous events in NE South America and tectonomagmatic episodes affecting the Pangea and West Gondwana landscapes in the Mesozoic Era.

The Geochemical and Textural Transition between the Reef Package and its Hanging Wall, Stillwater Complex, Montana, USA

Abstract The highest grade Pd-Pt deposit on Earth, the J-M Reef, is hosted in coarse-grained to pegmatoidal cumulates called the Reef Package. Decades of mine development of the J-M Reef have revealed that a distinct discontinuity in rock fabric marks the top of the rock unit that hosts economic-grade sulfide mineralization. Mine geologists refer to this discontinuity as the hanging wall contact. This contact is the top of the Reef Package and is always locatable—either by the change in rock fabric or by distinctive hanging wall textures of silicate minerals—even when the reef sulfide mineralization is absent. This rather subtle textural feature is used reliably by mine geologists to follow the Reef during exploration and mine development. Although some high tenor sulfides (>1000 ppm Pd in 100% sulfide) are found sporadically in the hanging wall cumulates, these accumulations are too small to be economically viable. We present quantitative rock fabric data for four Reef Package and hanging wall intersections collected by electron back-scattered diffraction (EBSD). Plagioclase fabrics in the hanging wall are characterized by low variance in grain sizes and a strong point maximum concentration of (010) and a perpendicular girdle distribution of [100] consistent with an axial B-type fabric. These fabrics are indicative of either compaction of the crystal mush or crystal settling of nucleated crystals, the bulk magma in a chamber. Conversely, the fabrics of the Reef Package show higher variance grain in size distributions and weak to undeveloped preferred orientation of plagioclase crystals that did not undergo significant alignment or textural equilibration of plagioclase grains. The absence of foliation in the Reef Package stands in contrast both to hanging wall fabrics and to other reported EBSD datasets of plagioclase crystals orientations from the Bushveld Complex, the Skaergaard Intrusion, and the Rum Intrusion. Furthermore, plagioclase crystal size distributions for the Reef Package show flatter slopes and convex profiles with fewer crystals at small size fractions indicating the dissolution of small crystals during partial melting and textural coarsening (i.e. Ostwald ripening) and crystal growth. Crystal growth was favored over the nucleation of new crystals during prolonged interaction with a hot infiltrating melt into the resident mush resulting in the coarse-grained textures of the Reef Package cumulates. The hanging wall contact represents a boundary between partially remelted crystal mush of the Reef Package, where sulfide mineralization formed and accumulated, and an overlying essentially barren cumulate pile. The hanging wall cumulates formed following the cessation of footwall erosion and the resumption of crystal accumulation by normal magma chamber processes.

The Oligocene Avaj volcanic – plutonic complex of Central Iran: A record of magma evolution and mineral equilibria

• High-An plagioclase feldspar cores formed during early stage of evolution of the primary magma . • Mineral-melt equilibrium conditions are determined in clinopyroxenes and plagioclase cores. • Zoning in plagioclase and smooth CSD patterns invoke an unsteady-state magma system. • Parental mafic magmas were hydrous and contained <4 wt% H 2 O. • The residence time of plutonic and volcanic magmas was 117 and 13 years, respectively. The Avaj Oligocene volcanic – plutonic complex is part of extensive Cenozoic magmatic activity within the Urumieh-Dokhtar magmatic arc of Iran. We use whole rock geochemistry, mineral compositions and crystal size distributions (CSD) in a suite of co-genetic basalt, basaltic andesite and gabbro to determine their petrogenesis. Ca-rich cores in plagioclase (An 79-86 ) overlap empirically modelled compositions, indicating equilibrium crystallization from melts represented by the whole-rock compositions. Clinopyroxene compositions (Mg# 74–80) are compatible with mildly fractionated mantle-derived magmas in an arc setting. Mineral-melt equilibrium is inferred from high Al contents and close correspondence between the measured DiHd and predicted Kd Fe–Mg (0.23–0.32) in clinopyroxenes, and Kd (Plg/melt) (An–Ab) values of plagioclase cores (0.11–0.15). Clinopyroxene-melt thermometers indicate crystallization at 1119–1173 °C for volcanic and 1099–1134 °C for plutonic rocks. Plagioclase crystal core saturation temperatures range from 1088–1162 °C (volcanic) and 1121–1163 °C (plutonic); these values overlap calculated mineral-melt equilibrium temperatures. Plagioclase CSDs are nearly straight for both volcanic and plutonic samples, with higher nucleation density and steeper slopes for the plutonic samples. Major element variations suggest the Avaj rocks represent co-genetic magmas related by fractional crystallization of the observed mineral phases. We suggest minor crustal assimilation occurred during ascent from a deeper reservoir to a shallower one; CSD data indicate longer magma residence time for plagioclase in the plutonic samples (∼117 years) compared to the residence time of basaltic samples (∼13 years).

Multiple timescale constraints for high-flux magma chamber assembly prior to the Late Bronze Age eruption of Santorini (Greece)

The rhyodacitic magma discharged during the 30–80 km3 DRE (dense rock equivalent) Late Bronze Age (LBA; also called ‘Minoan’) eruption of Santorini caldera is known from previous studies to have had a complex history of polybaric ascent and storage prior to eruption. We refine the timescales of these processes by modelling Mg–Fe diffusion profiles in orthopyroxene and clinopyroxene crystals. The data are integrated with previously published information on the LBA eruption (phase equilibria studies, melt inclusion volatile barometry, Mg-in-plagioclase diffusion chronometry), as well as new plagioclase crystal size distributions and the established pre-LBA history of the volcano, to reconstruct the events that led up to the assembly and discharge of the LBA magma chamber. Orthopyroxene, clinopyroxene and plagioclase crystals in the rhyodacite have compositionally distinct rims, overgrowing relict, probably source-derived, more magnesian (or calcic) cores, and record one or more crystallization (plag ≫ opx > cpx) events during the few centuries to years prior to eruption. The crystallization event(s) can be explained by the rapid transfer of rhyodacitic melt from a dioritic/gabbroic region of the subcaldera pluton (mostly in the 8–12 km depth range), followed by injection, cooling and mixing in a large melt lens at 4–6 km depth (the pre-eruptive magma chamber). Since crystals from all eruptive phases yield similar timescales, the melt transfer event(s), the last of which took place less than 2 years before the eruption, must have involved most of the magma that subsequently erupted. The data are consistent with a model in which prolonged generation, storage and segregation of silicic melts were followed by gravitational instability in the subcaldera pluton, causing the rapid interconnection and amalgamation of melt-rich domains. The melts then drained to the top of the pluton, at fluxes of up to 0.1–1 km3 year− 1, where steep vertical gradients of density and rheology probably caused them to inject laterally, forming a short-lived holding chamber prior to eruption. This interpretation is consistent with growing evidence that some large silicic magma chambers are transient features on geological timescales. A similar process preceded at least one earlier caldera-forming eruption on Santorini, suggesting that it may be a general feature of this rift-hosted magmatic system.

The origin of felsic microgranitoid enclaves: Insights from plagioclase crystal size distributions and thermodynamic models

Magma mixing is widely recognized in contemporary petrology as one of the primary igneous processes. Microgranitoid enclaves (MEs) are considered to be remnants of such mixing processes, and the term has a well-established genetic implication. However, microgranitoid enclaves span a wide range of compositions, and felsic varieties are also frequently reported.Nd–Sr isotope and textural data from felsic microgranitoid enclaves (FMEs), mafic microgranitoid enclaves (MMEs) and host granites from the Salto pluton, Itu Granitic Province, show that the cm-sized MMEs are dioritic, have medium-grained igneous textures and xenocrysts of alkali feldspar and quartz. The FMEs are cm- to meter-sized, have spheric shapes, show corrugated contacts with the host granites, and have resorbed feldspars and deformed quartz crystals interpreted as xenocrysts set in a fine-grained groundmass.Compared to the host granites, both MME and FME samples have increased FeO, MgO, TiO2, P2O5 and Zr contents, but their Sr and Nd isotope signatures are identical: FME 87Sr/86Sri=0.7088–0.7063, εNdi=−10.0 to −10.2; MME 87Sr/86Sri=0.7070, εNdi=−10.5; host granite 87Sr/86Sri 0.7056–0.7060, εNdi=−10.2 to −10.3. These indicate that the enclaves derive from a similar source, although the melts from which they formed were probably hotter and chemically more primitive than their host granites.Crystal size distributions (CSDs) of plagioclase in samples drilled from rinds and cores of three FMEs show that the rind samples are systematically finer-grained than the samples from the cores, which indicates that the FMEs cooled inwards and contradict interpretations that the FMEs are autoliths. Thermal modeling suggests that a slightly more primitive, hotter magma would be thermally equilibrated with an evolved resident melt within weeks after mixing/mingling. Upon thermal equilibrium, the FMEs would have an increased crystal cargo, and the resulting touching framework would impart a solid-like behavior to the FME-forming magma, which would lead to a contrast in rheology, fragmentation, dragging and preservation of felsic replenishment batches as distinct enclaves.

Crystal size distributions of plagioclase in lavas from the July–August 2001 Mount Etna eruption

During the 2001 eruption of Mount Etna, two independent vent systems simultaneously erupted two different lavas. The Upper Vents system (UV), opened between 3100 and 2650 m a.s.l., emitted products that are markedly porphyritic and rich in plagioclase, while the Lower Vents system (LV), opened at 2100 and 2550 m a.s.l., emitted products that are sparsely porphyritic with scarce plagioclase. In this study, the crystal size distributions (CSDs) of plagioclase were measured for a series of 14 samples collected from all the main flows of the 2001 eruption. The coefficient of R 2 determination was used to evaluate the goodness of fit of linear models to the CSDs, and the results are represented as a grid of R 2 values by using a numerical code developed ad hoc. R 2 diagrams suggest that the 2001 products can be separated into two main groups with slightly different characteristics: plagioclase CSDs from the UVs can be modeled by three straight lines with different slopes while the plagioclase CSDs from the LVs are largely concave. We have interpreted the CSDs of the UVs as representing three different populations of plagioclases: (i) the large phenocrysts (type I), which started to crystallize at lower cooling rate in a deep reservoir from 13 to 8 months before eruption onset; (ii) the phenocrysts (type II), which crystallized largely during continuous degassing in a shallow reservoir; and (iii) the microlites, which crystallized during magma ascent immediately prior to the eruption. The plagioclase CSD curves for the LVs lava are interpreted to reflect strong and rapid changes in undercooling induced by strong and sudden degassing.

Crystallization from a melt and crystallization at subsolidus conditions: comparison from crystal size distribution study on Gennargentu Rocks (Sardinia, Italy)

Crystal size distributions (CSDs) has been constrained in plagioclase and cordierite in quartz-diorite and migmatites. Euhedral plagioclase crystals in a plutonic body preserve the crystallisation processes that occurred during the cooling history of the body. A crystallisation from a liquid (i.e. plagioclase crystals in quartz diorite and in migmatite leucosomes) has been compared with a crystallisation at subsolidus conditions (i.e. plagioclase and cordierite crystals in contact metamorphic rocks). At contact with the metamorphic basement the texture is migmatitic. Plagioclase CSDs have been investigated in a plutonic body (i.e. quartz diorite), in the leucosome of migmatites and in the melanosome of un-melted contact metamorphic rocks from Gennargentu Complex (Sardinia, Italy). Cordierite CSDs have been investigated only in the hybrid rocks of the same complex. CSDs indicate that, initially, plagioclase crystals in the quartz diorite nucleated and grew in a cooling system at a constant cooling rate, producing a straight-line crystal size distributions. The plagioclase crystallisation continued until the latent heat was available and the temperature was high enough to allow the plagioclase growing. When the temperature was low enough to reach the solidus temperature of plagioclase crystals, the nucleation was inhibited and the conditions were suitable for a textural coarsening (Ostwald ripening process). The small crystals, due to their high surface energy per unit volume dissolved and “fed” the growth of larger crystals. Moreover, the maintenance of temperature near the plagioclase liquidus may also inhibit the nucleation and growth of other phases. Cordierite crystals from contact metamorphic rocks show a similar behaviour, while the plagioclases from the migmatites show an accumulation process. From CSDs measurements we were able to calculate the different cooling ages for the different sample types as following: dark quartz diorite samples crystallise in an average of time of 5000 years; the light quartz diorite shows a slow cooling (average of 7300 years).

The formation of columnar joints produced by cooling in basalt at Staffa, Scotland

Columnar jointing in basaltic lava flows on the island of Staffa, NW Scotland, was studied using a combination of field mapping and measurement of column dimensions, sample petrology and measurements of plagioclase crystal size distributions (CSDs) interpreted using theoretical models of cooling. Four different lava flow units were measured, and column ordering was assessed using the hexagonality index and relative standard deviations of column side length, top area and internal angle. Upper and lower colonnades consist of dominantly 5-, 6- and 7-sided columns, with a hexagonality index value very similar to that of Giant’s Causeway and other basaltic columnar jointed localities. CSDs from samples at different heights within one colonnade were used to infer the propagation of the solidus isotherm, which was consistent with a convective cooling mechanism within the colonnade interior. Sample petrology and CSD measurements suggest that entablature can form both by the interaction of propagating joint sets and flooding of the flow surface by water, and the most widely exposed unit on Staffa shows evidence of both mechanisms operating on the same flow. Crystal size distribution measurements can provide a useful tool for field interpretation of lava flow cooling mechanisms.

Origin of internal flow structures in columnar-jointed basalt from Hrepphólar, Iceland: I. Textural and geochemical characterization

Basalt columns from Hreppholar (Iceland) show distinct internal structures produced by alternating brighter and darker bands through the column, locally exhibiting viscous fingering features. Here, we present geochemical and petrographic data retrieved from analyses of major and trace elements and mineral chemistry from a cross section of a single basaltic column. This is combined with petrographic descriptions and data on crystal size distributions of plagioclase. We use our data from Hreppholar to test four existing models that have been proposed to explain banded structures inside columns: (1) deuteric alteration, (2) double-diffusive convection, (3) constitutional supercooling, and (4) crystallization-induced melt migration. We find that the internal structures at Hreppholar represent primary magmatic features, because approximately 20 % of the observed structures crosscut the column-bounding fracture for each meter along the main axis of the column. These features must thus have been formed before the column-delimiting crack advanced. Major and trace element analyses show small but significant variations across the column and strong correlation between oxides like FeO and TiO2, as well as K2O and P2O5. The geochemical variations correlate with the presence of darker/brighter bands visible on a polished surface and can be explained by a variation in the modal proportions of the main phenocryst phases (specifically variable plagioclase and titanomagnetite content). This banding enhances the internal structures apparent in the polished cross section from columnar joints at Hreppholar. The measured variations in major and trace element geochemistry, as well as mineral chemistry, are too small to distinguish between the proposed band-forming models. Plagioclase crystal size distributions, however, display a systematic change across the column that is consistent with late-stage migration of melt inside the column (i.e., the crystallization induced melt migration hypothesis). The central part of the columns have plagioclases indicative of slow cooling and these are also more steeply oriented (i.e., subparallel to the column axis) compared with plagioclases present in the more rapidly cooled edges. This redistribution of melt within individual columns may significantly affect the cooling rate of columnar-jointed lava flows and intrusions.

Quantitative petrological evidence for the origin of K-feldspar megacrysts in dacites from Taapaca volcano, Chile

K-feldspar megacrysts are common in granitoids, but relatively rare in chemically equivalent volcanic rocks. Dacites from Taapaca volcano have euhedral sanidine megacrysts up to 5 cm long. Small crystals, where present, are rounded. Growth of the megacrysts engulfed plagioclase and amphibole crystals. Crystal size distributions (CSD) of sanidine megacrysts are hump shaped. All these data show that megacrysts developed from the host magma by coarsening: this was enabled by the cycling of magma temperature around the sanidine liquidus temperature in response to injections of more mafic magma and subsequent magmatic overturns. Plagioclase crystals enclosed in the megacrysts are small and have short, steep, straight CSDs, which contrasts with the CSDs of plagioclase in the groundmass which are shallower and extend to larger sizes. This shows that plagioclase was also coarsened approximately synchronously with sanidine, in response to the same temperature conditions.

Igneous Layering, Fractional Crystallization and Growth of Granitic Plutons: the Dolbel Batholith in SW Niger

This study reassesses the development of compositional layering during the growth of granitic plutons, with emphasis on fractional crystallization and its interaction with both injection and inflationrelated deformation. The Dolbel batholith (SW Niger) consists of 14, kilometre-sized plutons emplaced by pulsed magma inputs. Each pluton has a coarse-grained core and a peripheral layered series. Rocks consist of albite (An 11), K-feldspar (Or96^99, Ab1^4), quartz, edenite (XMg1⁄4 0 37^0 55), augite (XMg1⁄4 0 65^0 72) and accessories (apatite, titanite and Fe^Ti-oxides). Whole-rock compositions are metaluminous, sodic (K2O/Na2O1⁄4 0 49^0 62) and iron-rich [FeOtot/(FeOtotþMgO)1⁄4 0 65^0 82]. The layering is present as size-graded and modally graded, sub-vertical, rhythmic units. Each unit is composed of three layers, which are, towards the interior: edenite plagioclase (Ca/p), edeniteþ plagioclaseþ augiteþ quartz (Cq), and edeniteþ plagioclaseþ augiteþ quartzþ K-feldspar (Ck). All phases except quartz show zoned microstructures consisting of external intercumulus overgrowths, a central section showing oscillatory zoning and, in the case of amphibole and titanite, complexly zoned cores. Ba and Sr contents of feldspars decrease towards the rims. Plagioclase crystal size distributions are similar in all units, suggesting that each unit experienced a similar thermal history. Edenite, characteristic of the basal Ca/p layer, is the earliest phase to crystallize. Microtextures and phase diagrams suggest that edenite cores may have been brought up with magma batches at the site of emplacement and mechanically segregated along the crystallized wall, whereas outer zones of the same crystals formed in situ.The subsequent Cq layers correspond to cotectic compositions in the Qz^Ab^Or phase diagram at PH2O1⁄4 5 kbar. Each rhythmic unit may therefore correspond to a magma batch and their repetition to crystallization of recurrent magma recharges. Microtextures and chemical variations in major phases allow four main crystallization stages to be distinguished: (1) open-system crystallization in a stirred magma during magma emplacement, involving dissolution and overgrowth (core of edenite and titanite crystals); (2) in situ fractional crystallization in boundary layers (Ca/p and Cq layers); (3) equilibrium ‘en masse’eutectic crystallization (Ck layers); (4) compaction and crystallization of the interstitial liquid in a highly crystallized mush (e.g. feldspar intercumulus overgrowths). It is concluded that the formation of the layered series in the Dolbel plutons corresponds principally to in situ differentiation of successive magma batches. The variable thickness of the Ck layers and the microtextures show that crystallization of a rhythmic unit stops and it is compacted when a new magma batch is injected into the chamber. Therefore, assembly of pulsed magma injections and fractional crystallization are independent, but complementary, processes during pluton construction.

A Complex Petrogenesis for an Arc Magmatic Suite, St Kitts, Lesser Antilles

St Kitts forms one of the northern group of volcanic islands in the Lesser Antilles arc. Eruptive products from the Mt Liamuiga centre are predominantly olivine + hypersthene-normative, low-K basalts through basaltic andesites to quartz-normative, low-K andesites. Higher-Al and lower-Al groups can be distinguished in the suite. Mineral assemblages include olivine, clinopyroxene, orthopyroxene, plagioclase and titanomagnetite with rarer amphibole, ilmenite and apatite. Eruptive temperatures of the andesites are estimated as 963–950°C at fO2 NNO + 1 (where NNO is the nickel–nickel oxide buffer). Field and mineral chemical data provide evidence for magma mixing. Glass (melt) inclusions in the phenocrysts range in composition from andesite to high-silica rhyolite. Compositional variations are broadly consistent with the evolution of more evolved magmas by crystal fractionation of basaltic parental magmas. The absence of any covariation between 87Sr/86Sr or 143Nd/144Nd and SiO2 rules out assimilation of older silicic crust. However, positive correlations between Ba/La, La/Sm and 208Pb/204Pb and between 208Pb/204Pb and SiO2 are consistent with assimilation of small amounts (<10%) of biogenic sediments. Trace element and Sr–Nd–Pb isotope data suggest derivation from a normal mid-ocean ridge basalt (N-MORB)-type mantle source metasomatized by subducted sediment or sediment melt and fluid. The eruptive rocks are characterized by 238U excesses that indicate that fluid addition of U occurred <350 kyr ago; U–Th isotope data for mineral separates are dominated by melt inclusions but would allow crystallization ages of 13–68 ka. However, plagioclase is consistently displaced above these ‘isochrons’, with apparent ages of 39–236 ka, and plagioclase crystal size distributions are concave-upwards. These observations suggest that mixing processes are important. The presence of 226Ra excesses in two samples indicates some fluid addition <8 kyr ago and that the magma residence times must also have been less than 8 kyr.

A numerical model of hydrothermal cooling and crustal accretion at a fast spreading mid‐ocean ridge

There are two competing models for lower crustal accretion at fast spreading ridges. In the gabbro glacier model, all the lower crust solidifies in a magma lens at the dike/gabbro boundary and then flows to its final level. In the sheeted sill model, the lower crust is formed by sills emplaced at a variety of depths. We present a steady state numerical representation of the sill model that explicitly includes hydrothermal circulation. The crust is accreted uniformly at the ridge axis at all depths and hydrothermal circulation occurs in crustal rocks that have cooled below a threshold temperature. The results show that when the permeability exceeds a threshold value (∼4 × 10−14 m2 for a cracking temperature of 800°C and a uniform permeability), hydrothermal circulation can cool the entire crust near the ridge axis, producing vertical isotherms in the lower crust. At lower permeabilities, the properties of seawater lead to bimodal cooling with the lowermost crust cooling more slowly than shallower depths. These results are reasonably consistent with studies of the seismic structure of the East Pacific Rise and of plagioclase crystal size distributions in gabbros from Oman. We infer that the sheeted sill model cannot be discarded on thermal grounds.

Case studies of plagioclase growth and residence times in island arc lavas from Tonga and the Lesser Antilles, and a model to reconcile discordant age information

The time scales of crystal growth and residence in magmas have received much attention because such information can constrain the physical processes involved. However, crystals and liquids can have different ages, leading to apparently contradictory information from different dating methods. Two case studies are presented from island arc lavas that highlight such dichotomies. Plagioclase crystal size distributions and U–Th–Ra isotope data are presented for lavas from Late and Metis Shoal in Tonga and from Soufriere (St. Vincent) in the Lesser Antilles. In all three lavas, mineral data yield apparent U–Th ages of 30–50 kyr yet 226Ra-excesses are preserved in both the minerals and the whole rocks. The crystal size distributions suggest crystal growth over 100s of years but also frequently indicate the presence of mixed crystal populations. Furthermore, a cumulate from Soufriere yields the same U–Th age as its host lava. On 226Ra/Ba versus time evolution diagrams, curves for melts in equilibrium with plagioclase provide apparent ages of 1700–3300 yr for Tonga but for Soufriere they are displaced below that of the whole rock. Our interpretation is that the phenocrysts from all three lavas are zoned in both composition and age. The apparent U–Th ages reflect mixtures of old cores and young rims and indicate a volume-integrated age of 10s of kyr for the time for initial crystal growth, crystal–liquid segregation, the time of storage as a cumulate plus the time spent during entrainment and growth in the new magma prior to eruption. The 226Ra-excesses were imparted to overgrowths on the old cumulate cores during entrainment by subsequent injections of new magma. Because crystal growth, segregation and entrainment can all appear to occur in 100s to a few thousand years at most, the bulk of the U–Th time scale is inferred to record storage in cumulates which are then incorporated into new magma batches. Plagioclase growth rates of 3–9×10 −13 cm s −1 would reconcile the combined Ra–Th and crystal size distribution data. Magma differentiation time scales and eruptive periodicity are one to three orders of magnitude shorter than the inferred cumulate storage times, indicating complex phenocryst–liquid behaviour beneath island arc volcanoes.

Petrology and geochemistry of the lower ocean crust formed at the East Pacific Rise and exposed at Hess Deep: A synthesis and new results

The geochemistry and petrology of the lower oceanic crust record information about the compositions of melts extracted from the mantle, how these melts mix and crystallize, and the role of hydrothermal circulation in this portion of the crust. Unfortunately, lower oceanic crust formed at fast spreading ridges is rarely exposed at the seafloor making it difficult to study these processes. At Hess Deep, crust formed at the East Pacific Rise (EPR) is exposed due to the propagation of the Cocos‐Nazca spreading center westward. Here we review our state of knowledge of the petrology of lower crustal material from Hess Deep and document new mineral major and trace element compositions, amphibole‐plagioclase thermometry, and plagioclase crystal size distributions. Samples from the deeper parts of the gabbroic sequence contain clinopyroxene that is close to being in trace element equilibrium with erupted basalts but which can contain primitive (moderate Cr, high Mg#) orthopyroxene and very calcic plagioclase. Because primitive mid‐ocean ridge basalts (MORBs) are not saturated with orthopyroxene or very calcic plagioclase this suggests that melts added to the crust have variable compositions and that some may be in major but not trace element equilibrium with shallow depleted mantle. These apparently conflicting data are most readily explained if some of the melt extracted from the mantle is fully aggregated within the mantle but reacts with the shallow mantle during melt extraction. The occurrence of cumulates with these characteristics suggests that melts added to the crust do not all get mixed with normal MORB in the axial magma chamber (AMC), but rather that some melts partially crystallize in isolation within the lower crust. However, evidence that primitive melts fed the AMC, along with steep fabrics in shallow gabbros (from near the base of the dyke complex), provides support for models in which crystallization within the AMC followed by crystal subsidence is also an important process in lower crustal accretion. More evolved bulk compositions of gabbros from the upper than lower parts of the plutonic section are due to greater amounts of reaction with interstitial melt and not because their parental melt had become highly fractionated through the formation of large volumes of cumulates deeper in the crust. Amphibole‐plagioclase thermometry confirms previous reports that the initial ingress of fluid occurs at high‐temperatures in the shallow gabbros (Tave 713°C) and show that the temperature of amphibole formation was similar in deeper gabbros (Tave 722°C). This thermometry also suggests that fracture and grain boundary permeability for seawater‐derived fluids was open over the same temperature interval.

A textural and geochemical investigation of high level gabbros from the Oman ophiolite: implications for the role of the axial magma chamber at fast-spreading ridges

Geophysical studies at fast-spreading ridges have identified a magma sill at, or slightly below, the base of the sheeted dyke complex. The role of this sill, the so-called axial magma chamber (AMC), in mid-ocean ridge basalt (MORB) differentiation and lower crustal accretion at fast-spreading ridges is unclear. Using the Oman ophiolite as an analogue for crust formed at a fast-spreading ridge, we investigate the composition of magmas delivered to the AMC using the compositions and textures of samples from the uppermost plutonics immediately below the sheeted dyke complex. The cores of clinopyroxene crystals are commonly as primitive as those found in deeper parts of the crust. This is true for major (Mg #), compatible trace (Cr) and incompatible trace (REEs) elements. Plagioclase crystal size distributions are ln-linear at all but the smallest crystal sizes suggesting a single crystal population. This indicates that the primitive crystal compositions are not carried to the AMC as xenocrysts formed deeper in the crust. Instead, these data demonstrate that primitive magmas that have previously undergone little or no differentiation can be delivered to the AMC. This supports models in which crystal subsidence from this body plays an important role in the accretion of the lower crust.

Variation of cooling rate with depth in lower crust formed at an oceanic spreading ridge: Plagioclase crystal size distributions in gabbros from the Oman ophiolite

Analysis of crystal size distributions (CSD) of plagioclase in gabbros from the Oman ophiolite indicates that cooling rates in the lower crust beneath a medium‐ to fast‐spreading oceanic ridge did not vary smoothly with depth. Cooling rates in the upper half of the gabbro section were approximately 1.5–2 times faster than in the lower half of the gabbro section during igneous crystallization and/or high temperature grain growth processes. Our results are inconsistent with thermal models for oceanic spreading ridges that approximate the effect of hydrothermal convection by using an enhanced thermal conductivity together with a conductive heat transfer mechanism. Instead, the Oman CSD data are consistent with recent seismic and modeling studies that suggest that hydrothermal convection leads to closely spaced, near‐vertical isotherms in the upper half of the plutonic section within a few kilometers of the ridge axis.

Steady-state volcanism, paleoeffusion rates, and magma system volume inferred from plagioclase crystal size distributions in mafic lavas: Dome Mountain, Nevada

The Dome Mountain ( DM) lavas, located in the moat of the Timber Mountain caldera of the SW Nevada volcanic field, cap a thick sequence of welded tuffs, massive and bedded tuffs, and mafic and rhyolitic lava flows. These 10.9-m.y.-old flows comprise a pile of some twenty flows with a total thickness of about 300 m. Individual flows range in thickness from 3 to 20 m and are characterized by a basal vesicular zone, a massive central region and an upper vesicular region containing, in most cases, cooling fractures. The flows dip gently away from the summit of DM, a prominent topographic feature of the region with an elevation of 2065 m. Silica contents range from 48.8 wt.% at the base of the pile to 59.4 wt.% at the top, although most of the flows are andesitic. Crystal size distributions (CSDs) for plagioclase in these comagmatic lavas have been determined to provide information on magma storage times. These times, when coupled with phase equilibria, can be used to infer cooling rates, which are indicative of the relative dimensions of magmatic bodies and the vigor of various magmatic cooling processes. Most of the CSDs show no sign of crystal fractionation or accumulation. Assuming a growth rate of 10−10 cm/s, calculated mean crystal residence times for most lavas cluster in the range of 1.5 to 4 years. Two of the lower lavas have residence times in the range of 4 to 9 years. The residence times correlate inversely with nucleation density and broadly correlate inversely with silica content: high-residence-time, low-silica lavas occur at the base of the pile, whereas low-residence-time, high-silica lavas occur at the top of the pile. One interpretation of the residence time data is that the lavas are from an open system of more or less constant residence time but varying spatially in system locations; e.g., a magma chamber in which steady-state recharge and eruption may have been reached. Alternatively, these lavas may represent small samples, closely spaced in time, of a slowly cooling, large batch or reservoir of magma. The greater residence times associated with the lower flows may indicate the increased amount of time necessary to form the conduit of the magma-volcano system by the oldest magma or simply that part of the system with the highest liquidus temperature. A minimum volume of 1.4 to 2.5 km3 for the magma chamber is estimated for the DM system using the estimated residence times in a simplified cooling model of a sheet of aspect ratio R. The actual system volume depends on R, which is strictly unknown, and thus the actual system volume may be much larger (e.g., ~ 200 km3). A mean paleoeffusion rate of 0.05 km3/yr is obtained for the lavas by assuming that the residence time of each lava corresponds to the amount of time elapsed since the eruption of the underlying flow.