We characterize the textural and geochemical features of ocean crustal zircon recovered from plagiogranite, evolved gabbro, and metamorphosed ultramafic host-rocks collected along present-day slow ...and ultraslow spreading mid-ocean ridges (MORs). The geochemistry of 267 zircon grains was measured by sensitive high-resolution ion microprobe-reverse geometry at the USGS-Stanford Ion Microprobe facility. Three types of zircon are recognized based on texture and geochemistry. Most ocean crustal zircons resemble young magmatic zircon from other crustal settings, occurring as pristine, colorless euhedral (Type 1) or subhedral to anhedral (Type 2) grains. In these grains, Hf and most trace elements vary systematically with Ti, typically becoming enriched with falling Ti-in-zircon temperature. Ti-in-zircon temperatures range from 1,040 to 660°C (corrected for
a
TiO2
≈ 0.7,
a
SiO2
≈ 1.0, pressure ≈ 2 kbar); intra-sample variation is typically ~60–150°C. Decreasing Ti correlates with enrichment in Hf to ~2 wt%, while additional Hf-enrichment occurs at relatively constant temperature. Trends between Ti and U, Y, REE, and Eu/Eu* exhibit a similar inflection, which may denote the onset of eutectic crystallization; the inflection is well-defined by zircons from plagiogranite and implies solidus temperatures of ~680–740°C. A third type of zircon is defined as being porous and colored with chaotic CL zoning, and occurs in ~25% of rock samples studied. These features, along with high measured La, Cl, S, Ca, and Fe, and low (Sm/La)
N
ratios are suggestive of interaction with aqueous fluids. Non-porous, luminescent CL overgrowth rims on porous grains record uniform temperatures averaging 615 ± 26°C (2SD,
n
= 7), implying zircon formation below the wet-granite solidus and under water-saturated conditions. Zircon geochemistry reflects, in part, source region; elevated HREE coupled with low U concentrations allow effective discrimination of ~80% of zircon formed at modern MORs from zircon in continental crust. The geochemistry and textural observations reported here serve as an important database for comparison with detrital, xenocrystic, and metamorphosed mafic rock-hosted zircon populations to evaluate provenance.
Ocean Drilling Program Hole 735B at Atlantis Bank on the Southwest Indian Ridge sampled 1508 m of plutonic oceanic crust, hosted in the footwall of an oceanic detachment fault. We present new ...high-precision isotope dilution-thermal ionization mass spectrometry (ID-TIMS) U–Pb zircon dates from samples spanning the length of Hole 735B, and from the shallower adjacent Hole 1105A (158 m). The new dates provide the most complete and precise record of both the spatial and temporal distribution of magmatism during accretion of the lower oceanic crust to date. Whole rock and mineral geochemistry from Hole 735B define three main igneous series. Weighted mean 206Pb/238U dates suggest each igneous series intruded beneath the preceding series. Weighted mean 206Pb/238U dates range from 12.175 to 11.986 Ma in Series 1; 11.974 to 11.926 Ma in Series 2; and 11.936 to 11.902 Ma in Series 3 (±0.015 to 0.069 Ma). Weighted mean 206Pb/238U dates from Hole 1105A range from 11.9745 to 11.9573 Ma (±0.0082 to 0.0086 Ma). The Hole 1105A dates are coeval with Series 2 in Hole 735B, consistent with previous correlations of Fe-Ti oxide-rich layers between the two holes, suggesting individual magmatic series formed sheet-like bodies that were ≥250 m thick and extended ≥1.1 km parallel to the ridge axis (E–W) and ≥0.48 km in the spreading direction (N–S). The data suggest a total duration of magmatism in Hole 735B of ≥0.214±0.032 Ma, corresponding to accretion over a horizontal distance of ≥2.6±0.4 km. The crust at Atlantis Bank was formed during active detachment faulting, and the successive underplating of each magmatic unit may have been favored in this environment. The combined U–Pb dates, and reported Ti-in-zircon temperatures, are consistent with magmatic cooling rates of 103–104 °C/Ma over the temperature interval of 900–700 °C.
•New TIMS U–Pb zircon dates constrain the magmatic history of ODP Holes 735B/1105A.•Hole 735B crust was formed over ≥0.214±0.032 Ma.•Dated samples become progressively younger with increased depth in the core.•The age-depth profile indicates each igneous series underplated the prior series.•Magmatic cooling rates were 103–104 °C/Ma from 900–700 °C.
The depth extent, strength, and composition of oceanic detachment faults remain poorly understood because the grade of deformation‐related fabrics varies widely among sampled oceanic core complexes ...(OCCs). We address this issue by analyzing fault rocks collected from the Kane oceanic core complex at 23°30′N on the Mid‐Atlantic Ridge. A portion of the sample suite was collected from a younger fault scarp that cuts the detachment surface and exposes the interior of the most prominent dome. The style of deformation was assessed as a function of proximity to the detachment surface, revealing a ∼450 m thick zone of high‐temperature mylonitization overprinted by a ∼200 m thick zone of brittle deformation. Geothermometry of deformed gabbros demonstrates that crystal‐plastic deformation occurred at temperatures >700°C. Analysis of the morphology of the complex in conjunction with recent thermochronology suggests that deformation initiated at depths of ∼7 km. Thus we suggest the detachment system extended into or below the brittle‐plastic transition (BPT). Microstructural evidence suggests that gabbros and peridotites with high‐temperature fabrics were dominantly deforming by dislocation‐accommodated processes and diffusion creep. Recrystallized grain size piezometry yields differential stresses consistent with those predicted by dry‐plagioclase flow laws. The temperature and stress at the BPT determined from laboratory‐derived constitutive models agree well with the lowest temperatures and highest stresses estimated from gabbro mylonites. We suggest that the variation in abundance of mylonites among oceanic core complexes can be explained by variation in the depth of the BPT, which depends to a first order on the thermal structure and water content of newly forming oceanic lithosphere.
Key Points
The detachment fault is defined by a 450 m zone of mylonitic deformation
The fault deformed viscously and rooted at depths >7 km and temperatures >700oC
Variation among oceanic faults is due to water content and/or thermal structure
Three-quarters of the oceanic crust formed at fast-spreading ridges is composed of plutonic rocks whose mineral assemblages, textures and compositions record the history of melt transport and ...crystallization between the mantle and the sea floor. Despite the importance of these rocks, sampling them in situ is extremely challenging owing to the overlying dykes and lavas. This means that models for understanding the formation of the lower crust are based largely on geophysical studies and ancient analogues (ophiolites) that did not form at typical mid-ocean ridges. Here we describe cored intervals of primitive, modally layered gabbroic rocks from the lower plutonic crust formed at a fast-spreading ridge, sampled by the Integrated Ocean Drilling Program at the Hess Deep rift. Centimetre-scale, modally layered rocks, some of which have a strong layering-parallel foliation, confirm a long-held belief that such rocks are a key constituent of the lower oceanic crust formed at fast-spreading ridges. Geochemical analysis of these primitive lower plutonic rocks--in combination with previous geochemical data for shallow-level plutonic rocks, sheeted dykes and lavas--provides the most completely constrained estimate of the bulk composition of fast-spreading oceanic crust so far. Simple crystallization models using this bulk crustal composition as the parental melt accurately predict the bulk composition of both the lavas and the plutonic rocks. However, the recovered plutonic rocks show early crystallization of orthopyroxene, which is not predicted by current models of melt extraction from the mantle and mid-ocean-ridge basalt differentiation. The simplest explanation of this observation is that compositionally diverse melts are extracted from the mantle and partly crystallize before mixing to produce the more homogeneous magmas that erupt.
The melt-filled pore structure in the final stages of solidification of cumulates must lie somewhere between the two end-members of impingement (in which pore topology is controlled entirely by the ...juxtaposition of growth faces of adjacent grains) and textural equilibrium (in which pore topology is controlled by the minimization of internal energies). The exact position between these two end-members is controlled by the relative rates of crystal growth and textural equilibration. For samples in which growth has stopped, or is very slow, textural equilibrium will prevail. A close examination of dihedral angles in natural examples demonstrates that these two end-member textures can be distinguished. The impingement end-member results in a population of apparent solid–melt dihedral angles with a median of ∼60° and a standard deviation of ∼25–30°, whereas the texturally equilibrated end-member population has a median of ∼28° and a standard deviation of ∼14°. For the specific case of cumulates in the Rum Layered Intrusion, residual porosity in troctolitic cumulates was close to the impingement end-member, whereas that in peridotites was close to melt-bearing textural equilibrium. Suites of glass-bearing samples from small, or frequently disturbed, magma systems show modification of initial impingement textures. These modifications may be a consequence of textural equilibration or of diffusion-limited growth during quenching. Distinction can be made between these two processes by a consideration of grain shape. The geometry of interstitial phases in suites of fully solidified cumulates from the Rum Layered Intrusion shows variable approach to sub-solidus textural equilibrium from an initial state inherited by pseudmorphing of the last melt. Textural equilibration at pore corners occurs as a continuous process, with a gradual movement of the entire dihedral angle population towards the equilibrium final state. If the initial, pseudomorphed state is one of disequilibrium (i.e. a melt-present impingement texture) this change is accompanied by a reduction in the spread of the population. If it is one of equilibrium, the change is accompanied by an initial increase in the spread of the population, followed by a decrease. These observations demonstrate that previously published models of dihedral angle change involving the instantaneous establishment of the equilibrium angle in the immediate vicinity of the pore corner are incorrect.
Nineteen uranium-lead zircon ages of lower crustal gabbros from Atlantis Bank, Southwest Indian Ridge, constrain the growth and construction of oceanic crust at this slow-spreading midocean ridge. ...Approximately 75% of the gabbros accreted within error of the predicted seafloor magnetic age, whereas approximately25% are significantly older. These anomalously old samples suggest either spatially varying stochastic intrusion at the ridge axis or, more likely, crystallization of older gabbros at depths of approximately5 to 18 kilometers below the base of crust in the cold, axial lithosphere, which were uplifted and intruded by shallow-level magmas during the creation of Atlantis Bank.
The rates of slip on oceanic detachment faults and how those rates compare to sea-floor spreading rates constitute fundamental data required to constrain how oceanic core-complexes form and their ...role during crustal accretion. We combine sea-surface magnetic data, with the magnetic polarity of shallow-core samples and Pb/U SHRIMP ages of igneous zircon to determine the time-averaged half-spreading rate during oceanic detachment faulting at Atlantis Bank, 100 km south of the ultraslow-spreading Southwest Indian Ridge (SWIR). The Pb/U zircon ages correlate well with the magnetic ages and so highlight that magmatic accretion and faulting were coeval for over 2 Myr, creating and exposing a >1.5-km-thick layer of gabbro for >35 km parallel-to-spreading. We use bivariate linear regression of distance–age data and forward modeling of magnetic anomaly data to calculate a half-spreading rate during detachment faulting of 14.1+1.8/−1.5 km/Myr (95% confidence limits). When integrated with regional constraints on spreading history, we note that detachment faulting coincided with a short-lived regional increase in the full-spreading rate along the SWIR and, for the ridge segment containing Atlantis Bank, spreading was highly asymmetric with ~80% of plate-motion accommodated by detachment faulting. Consequently, the detachment fault effectively formed the plate-boundary at the surface in this spreading segment. Highly asymmetric spreading was confined to the spreading segment containing Atlantis Bank and to the duration of detachment faulting. So the ridge segment containing Atlantis Bank migrated northward relative to its symmetrically spreading eastern neighbour, such that the intervening non-transform discontinuity shortened. We suggest that the highly asymmetric spreading may be a characteristic feature of oceanic detachment faulting, an inference supported by more poorly constrained half-spreading rates determined at several other oceanic core-complexes.
Most phenocryst populations in volcanic rocks, and those preserved in shallow-level igneous intrusions, are clustered (variously referred to as clots, clumps or glomerocrysts). These clusters of ...crystals are the building blocks that accumulate to form the high-porosity, touching crystal frameworks from which igneous cumulates form. Examination of touching crystal frameworks in olivine- (komatiite cumulates and experimental charges) and plagioclase-dominant crystal populations (Holyoke flood basalt, Connecticut, USA) reveal complex, high-porosity, clustered crystal arrangements. Olivine touching frameworks in komatiite flows are interpreted to form in hundreds of days. Plagioclase frameworks are calculated to have formed in less than 17 years for a crystal growth rate of 1 × 10−10 mm/s to less than 3 years for a growth rate of 5 × 10−10 mm/s based on crystal size distributions. The origin of crystal clusters is likely to involve either (or a combination of) heterogeneous nucleation, remobilization of cumulate mushes or crystals sticking together during settling and/or flow. The spatial distribution pattern of clustered crystal frameworks from both natural and experimental examples constrains fields on spatial packing diagrams that allow the identification of touching and non-touching crystal populations, and further improve our understanding of crystal packing arrangements and cluster size distributions.
ABSTRACT
Inherited factors account for around one third of all colorectal cancers (CRCs) and include rare high penetrance mutations in APC, MSH2, MSH6, and POLE. Here, we sought novel ...tumor‐suppressor genes that predispose to CRC by exome resequencing 50 sporadic patients with advanced CRC (18 diagnosed ≤35 years of age) at a mean coverage of 30×. To help identify potentially pathogenic alleles, we initially sought rare or novel germline truncating mutations in 1,138 genes that were likely to play a role in colorectal tumorigenesis. In total, 32 such mutations were identified and confirmed, and included an insertion in APC and a deletion in POLE, thereby validating our approach for identifying disease alleles. We sought somatic mutations in the corresponding genes in the CRCs of the patients harboring the germline lesions and found biallelic inactivation of FANCM, LAMB4, PTCHD3, LAMC3, and TREX2, potentially implicating these genes as tumor suppressors. We also identified a patient who carried a germline truncating mutation in NOTCH3, part of the Notch signaling cascade that maintains intestinal homeostasis. Our whole exome analyses provided further gene lists to facilitate the identification of potential predisposition alleles.
We exomeresequenced 50 sporadic patients with advanced CRC. To identify predisposition alleles, we sought rare/novel germline truncating mutations in 1138 genes considered likely to play a role in colorectal tumorigenesis. Thirty‐two such mutations were identified, including an insertion in APC, thereby validating our approach. We sought somatic mutations in the corresponding genes in the CRCs of the patients harbouring the germline lesions and found biallelic inactivation of FANCM, LAMB4, PTCHD3, LAMC3 and TREX2, implicating their potential role in CRC‐predisposition.
Zircons recovered from oceanic gabbro exposed on Atlantis Bank, Southwest Indian Ridge, typically display oscillatory and sector zoning consistent with igneous crystallization from mafic magmas. In ...one rock (of twenty investigated), weak-oscillatory-zonation patterns are overprinted by secondary textural features characterized by mottled, convoluted and wavy internal zonation patterns that are frequently associated with secondary micron- to submicron-scale micro-porosity. These zircons are hosted in a felsic vein that intruded an oxide gabbro, both of which are cross-cut by monomineralic amphibole- and quartz-rich veinlets. Zircons with weak-oscillatory-zonation patterns record a weighted-average
206Pb/
238U age of 12.76
±
0.20
Ma (mswd
=
1.5), and have high trace element concentrations e.g., ΣREEs (∼
0.4–2.2
wt.%), Y (∼
0.6–2.8
wt.%), P (∼
0.4–0.9
wt.%), and Th/U (0.1–0.5). These zircons are anomalously old (≥1
Myr) relative to the magnetic age for this portion of oceanic crust (11.75
Ma). In contrast, zircons with non-igneous, secondary textures have a younger weighted-average
206Pb/
238U age of 12.00
±
0.16
Ma (mswd
=
1.7), and have lower trace element concentrations e.g., ΣREEs (∼
0.2–0.8
wt.%), Y (∼
0.3–1.0
wt.%), P (∼
0.1–0.3
wt.%), and slightly lower Th/U (0.1–0.3). The weighted-average age of these zircons is similar to the magnetic anomaly age, and other
206Pb/
238U ages of nearby rocks. We do not observe a correlation between crystallographic misorientation, internal texture, or trace element chemistry. We suggest that the decrease in trace element concentrations associated with the development of non-igneous alteration textures is attributed to the purging of non-essential structural constituent cations from the zircon crystal lattice at amphibolite-facies conditions. The mechanism of alteration/re-equilibration was likely an interface-coupled dissolution–reprecipitation processes that affected pre-existing, anomalously old zircons during shallow-level magmatic construction of Atlantis Bank at ∼
12.0
Ma.
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