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Zircon grains from metasiltstone beds in the Precambrian Ashburn Formation were dated to constrain the age of the stromatolite Archaeozoon acadiense. The largest group of analyzed grains consists of mostly well-rounded detrital zircon no younger than 1060 Ma with significant peaks at about 1200-1100, 1500, 1800, 2050, and 2700 Ma, as well as a few Mesoarchean ages. A second population of apparently euhedral grains gives ages in the range 680-400 Ma. The distribution of detrital zircon ages along with field relationships suggest that the Ashburn and Martinon formations are proximal and distal parts of the Green Head Group, respectively, and hence a previously reported maximum depositional age of ca. 602 [+ or -] 8 Ma for the Martinon Formation also applies to stromatolite in the Ashburn Formation. Paleozoic zircon recovered from the Ashburn Formation cannot be detrital because the rocks were intruded and contact metamorphosed by the Golden Grove Plutonic Suite at ca. 550-525 Ma. The Paleozoic zircon grains were found in four independently processed samples and have also been seen in other studies in the region, showing that they are not due to laboratory contamination. Zircon formed during younger magmatic activity may have been transported in hydrothermal fluids and emplaced into these metasedimentary rocks in veins. A single euhedral zircon recovered from an atypically thick vein gave an age of 659 [+ or -] 33 Ma. Hydrothermally transported zircon might provide a window into younger igneous processes for which evidence is not otherwise preserved in this and other areas. Key words: U--Pb geochronology, zircon, hydrothermal transport, detrital ages, Ashburn Formation. Des grains de zircon provenant de lits de metasiltstone dans la Formation precambrienne d'Ashburn ont ete dates afin de circonscrire l'age du stromatolite Archaeozoon acadiense. Le plus grand groupe de grains analyses est principalement constitue de zircons detritiques arrondis d'un age minimum de 1060 Ma avec des pics significatifs a environ 1200-1100, 1500, 1800, 2050 et 2700 Ma, ainsi que quelques ages mesoarcheens. Une deuxieme population de grains en apparence idiomorphes donne des ages dans la fourchette de 680-400 Ma. La distribution des ages de zircons detritiques combinee aux relations de terrain donne a penser que les Formations dAshburn et de Martinon sont les parties proximale et distale du Groupe de Green Head, respectivement, et qu'un age de depot maximum deja publie d'environ 602 [+ or -] 8 Ma pour la Formation de Martinon s'applique egalement aux stromatolites dans la Formation dAshburn. Des zircons paleozo'iques provenant de la Formation dAshburn ne peuvent etre detritiques puisque les roches ont ete recoupees par la serie plutonique de Golden Grove vers 550-525 Ma et en ont subi un meta-morphisme de contact. Les grains de zircon paleozo'iques ont ete trouves dans quatre echantillons traites de maniere independante et ont egalement ete releves dans d'autres etudes dans la region, ce qui demontre qu'ils ne sont pas dus a une contamination en laboratoire. Des zircons formes durant des episodes magmatiques plus recents pourraient avoir ete transported dans des fluides hydrothermaux et mis en place dans des veines dans ces roches metasedimentaires. Un unique grain de zircon idiomorphe provenant d'une veine inhabituellement epaisse a donne un age de 659 [+ or -] 33 Ma. Des zircons transported par des fluides hydrothermaux pourraient ouvrir une fenetre sur des processus ignes plus jeunes dont d'autres preuves ne sont pas preservees dans cette region et ailleurs. [Traduit par la Redaction] Mots-cles: geochronologie U--Pb, zircon, transport hydrothermal, ages de zircons detritiques, Formation d'Ashburn., Introduction The Saint John area of southern New Brunswick, Canada (Fig. 1), has been a centre of geological investigation since the mid part of the 19th century, when the Steinhammer [...]

Oxygen isotopes are a valuable tool for determining the role of supracrustal rocks in the genesis of magmas and help constrain the source regions of plutonism in the northern Appalachian mountains of North America. Zircons from 35 igneous or metaigneous rocks mainly from southern New England, USA, were analyzed for [sup.18]O/[sup.16]O to locate and investigate the origin of anomalous [δ.sup.18]O magmas and to evaluate the regional meteoric water-rock interaction. [δ.sup.18]O values of the zircons obtained using the laser fluorination technique vary between 2.3 [per thousand] and 11.0‰, relative to Vienna Standard Mean Ocean Water (VSMOW). Zircons from two plutons in the Avalon terrane reveal the presence of anomalously low- [δ.sup.18]O ( Les isotopes de l'oxygene sontun outil important pour determiner le role des roches supracrustales dans lagenese des magmas et ils aident a delimiter les regions sources du plutonisme dans le nord des montagnes appalachiennes de l'Amerique du Nord. Des zircons provenant de 35 roches ignees ou meta-ignees, principalement du sud de la NouvelleAngleterre, ont ete analyses pour [sup.18]O/[sup.16]O afin de localiser et d'etudier l'origine des magmas a anomalie [δ.sup.18]O et d'evaluer les interactions regionales entre l'eau meteorique et les roches. Les valeurs [δ.sup.18]O des zircons, obtenues au moyen de la technique de fluoration au laser, varient entre 2,3 ‰ et 11,0 ‰ VSMOW (eau oceanique moyenne normalisee de Vienne). Les zircons provenant de deux plutons du terrane d'Avalon revelent la presence de zircons a teneur [δ.sup.18]O anormalement basse (> (Ordovicien tardif a Silurien precoce) du nord du Connecticut (zircon [δ.sup.18]O en vrac = 7,56 ‰) indiquent l'incorporation de quantites importantes de composantes sedimentaires a teneur elevee [δ.sup.18]Oa partir des regions sources plus anciennes dans ces magmas. Les terranes d'Avalon et de Gander du sud-est de la Nouvelle-Angleterre et la bordure est de Laurentia ont subi de longs historiques d'enfouissement ou de subduction, de fusion et d'assimilation de roches provenant de la surface au cours du Neoproterozoique et du milieu du Paleozoique. [Traduit par lar Redaction], Introduction Compared with other rock-forming minerals such as quartz and feldspar, zircon is highly resistant to oxygen isotopic exchange during post-crystallization hydrothermal alteration (e.g., Valley 2003) and thus can be [...]

[1] Zircon U-Pb geochronological data on over 900 zircon grains for Cambrian to Silurian sandstone samples from the South China Block constrain the pre-Devonian tectonic setting of, and the interrelationships between, the constituent Cathaysia and Yangtze blocks. Zircons range in age from 3335 to 465 Ma. Analyses from the Cathaysia sandstone samples yield major age clusters at ~2560, ~1850, ~1000, and 890-760 Ma. Zircons from the eastern and central Yangtze sandstone samples show a similar age distribution with clusters at -2550, ~1860, ~1100, and ~860-780 Ma. A minor peak at around 1450 Ma is also observed in the Cathaysia and central Yangtze age spectra, and a peak at ~490 Ma represents magmatic zircons from Middle Ordovician sandstone in the eastern Yangtze and Cathaysia blocks. The Cambrian and Ordovician strata show a transition from a carbonate-dominated succession in the central Yangtze Block, to an interstratified carbonate-siliciclastic succession in the eastern Yangtze Block, to a neritic siliciclastic succession in the Cathaysia Block. Paleocurrent data across this succession consistently indicate directions toward the W-NNW, from the Cathaysia Block to the Yangtze Block. Our data, together with other geological constraints, suggest that the Cathaysia Block constitutes a fragment on the northern margin of east Gondwana and both Cathaysia and east Gondwana constituted the source for the analyzed early Paleozoic samples. The similar age spectra for the Cambrian to Silurian sandstone samples from the Yangtze and Cathaysia blocks argue against the independent development and spatial separation of these blocks in the early Paleozoic but rather suggest that the sandstone units accumulated in an intracontinental basin that spanned both blocks. Subsequent basin inversion and Kwangsian orogenesis possibly at 400-430 Ma also occurred in an intracontinental setting probably in response to the interaction of the South China Block with the AustralianIndian margin of east Gondwana. Citation: Wang, Y., F. Zhang, W. Fan, G. Zhang, S. Chen, P. A. Cawood, and A. Zhang (2010), Tectonic setting of the South China Block in the early Paleozoic: Resolving intracontinental and ocean closure models from detrital zircon U-Pb geochronology, Tectonics, 29, TC6020, doi: 10.1029/2010TC002750.

[1] We combine detrital zircons (DZ) and epsilon neodymium ([[epsilon].sub.Nd]) signatures with field mapping in the Paro Formation in western Bhutan. DZ age spectra are strongly variable and display signatures that have been used to uniquely identify both Greater Himalayan (GH) and Lesser Himalayan (LH) strata. DZ age peaks from six quartzite samples require sources for ~0.5, 0.8, 1.2, 1.4, 1.7, 1.8, and 2.5 Ga zircons in the Paro Formation. The youngest (~0.5 Ga) zircons argue for a Cambrian maximum deposition age. Two samples have a youngest 1.8 Ga peak typically attributed to Paleoproterozoic LH rocks. A ~450 Ma crystallization age from two granite samples constrains the minimum deposition age as Ordovician. New [[epsilon].sub.Nd] signatures from six detrital samples from the Paro Formation show significant variation with lithology. Schists have [[epsilon].sub.Nd] (0) values between -12.0 and -16.9, while quartzite values vary between -18.8 and -24.5. These data imply that the Paro Formation was derived from both young and old sources, with DZ and [[epsilon].sub.Nd] values obtained from the same quartzite samples requiring old detritus while the end values obtained from interbedded schist require younger detritus. Using published isotopic and chronologic definitions of Himalayan strata, schist-rich layers would be considered GH, while the interbedded quartzite would be LH. Thus, the Paro Formation refutes the generally accepted notion that different Himalayan tectonostratigraphic zones have unique DZ and end signatures. Our data recommend caution in the use of DZ and [[epsilon].sub.Nd] signatures for tectonic interpretation, especially when making correlations with studies that extend 1000s of km along strike. Citation: Tobgay, T., S. Long, N. McQuarrie, M. N. Ducea, and G. Gehrels (2010), Using isotopic and chronologic data to fingerprint strata: Challenges and benefits of variable sources to tectonic interpretations, the Paro Formation, Bhutan Himalaya, Tectonics, 29, TC6023, doi: 10.1029/2009TC002637.

Detrital zircon provenance investigations of mid-Paleozoic sandstone from the western Ellesmerian clastic wedge and Cordilleran miogeocline in northern Yukon and Northwest Territories, northwestern Canada, provide critical new data on the source of foreland basin sedimentation attributed to terrane accretion and plate convergence along the ancestral Arctic margin of North America. Late Devonian and early Mississippian clastic wedge strata yield 'exotic' ca. 360-390, 430-460, 530-680, and 1500-1600 Ma detrital zircon populations that are consistent with source rocks that originated near the Caledonian and Timanian orogenic belts. Specifically, the Pearya and Arctic Alaska-Chukotka terranes, the landmass of Crockerland, and Caledonian rocks in eastern Greenland are the inferred sources for exotic detrital zircons in clastic wedge strata. Progressive recycling of Ellesmerian foreland basin sediments into the continental margin environment along northwestern Laurentia is indicated by the presence of ca. 360-430 Ma and 1500-1600 Ma detritai zircons in post-tectonic, middle to late Mississippian miogeoclinal strata in Yukon. Provenance data from these Mississippian samples record a dramatic shift in the source of the Cordilleran miogeocline, since Caledonian and Baltican (Timanide) detritai zircon signatures are not recognized in preLate Devonian sedimentary rocks in western Canada. Devonian strata of the Alexander terrane and Yreka subterrane (eastern Klamath terrane) have Caledonian and Baltican detrital zircon age signatures similar to Ellesmerian clastic wedge sandstones, implying that several Cordilleran terranes originated in the paleo-Arctic realm. Speculative correlations suggest that the Arctic Alaska--Chukotka terrane was located to the west of Crockerland and the Canadian Arctic Islands in pre-Cretaceous time, prior to opening of the Amerasian basin. Rifting models for the western Arctic Ocean featuring counterclockwise rotation of the Arctic Alaska--Chukotka terrane away from the Canadian Arctic Islands may need reevaluation. doi:10.1130/B30120.1

Detrital zircon populations from Silurian--Devonian clastic rocks of NW Svalbard were analysed by U--Pb laser ablation inductively coupled plasma mass spectrometry to investigate the pre-Caledonian provenance of Svalbard's Northwestern Terrane. Changes in the resulting age spectra suggest a major shift in sources from the Laurentian--Avalonian suture in the latest Silurian to the local metasedimentary basement of the Northwestern Terrane in the Late Silurian--Early Devonian, and in the Lochkovian to Grenvillian-Sveeonorwegian sources. These data, together with structural, additional geochronological and metamorphic data from Svalbard, East Greenland and Avalonia, support the amalgamation of Svalbard as the result of long-distance transport along sinistral strike-slip faults. A unifying model for the final amalgamation of Svalbard, consistent with the stratigraphical and tectonothermal history of Svalbard, involves fragments from the Grampian orogen and Avalonian ernst originally accreted to the Laurentian margin being subsequently transported northward along sinistral strike-slip faults during Scandian deformation. Supplementary material: The sample preparation procedures, laser ablation ICP-MS analytical methods, and data table are available at http://www.geolsoc.org.uk/SUP18421. doi: 10.1144/0016-76492010-062.

Zircon crystals from lateritic soils at Nsimi, Cameroon, were investigated using electron microprobe analysis (EMPA), transmission electron microscopy (TEM), scanning electron microscopy (SEM), and Raman spectroscopy to determine the extent of radiation damage from alpha-decay events. The soils belong to a small watershed developed on granitic rocks of the Congo craton (2.9 Ga). Interactions with fluids are evidenced by significant CaO (up to 1.5 wt%), [Al.sub.2][O.sub.3] (up to 2.9 wt%), and [Fe.sub.2][O.sub.3] (up to 2.9 wt%) concentrations in U[O.sub.2] rich regions (0.05 to 1 wt%) of the zircon. Regional heating up to 500 [degrees]C, related to the Pan-African orogeny about 0.6 Ga ago, has lead to the recrystallization of the radiation-damaged grains and the formation of a nanoporous microstructure. The correlation observed between the presence of dissolution features and the actual damage state of zircon shows that zircon dissolution occurs under tropical weathering conditions and with preferential dissolution of the highly radiation-damaged regions. Congruent dissolution of zircon and the limited mobility of Zr are supported by the absence of zirconium oxide precipitates in the fractures of weathered grains of zircon. Keywords: Zircon, Nsimi, laterite, nanopore, zirconium, metamict, radiation damage, TEM

U--Pb detrital zircon ages in variably metamorphosed, dominantly fine-grained clastic successions are used in northeastern Australia to identify two major successions along the East Gondwana margin. The older succession is of probable Late Neoproterozoic age and is considered part of a passive margin associated with rifting at c. 600 Ma. Most detrital zircons have ages in the range 1000-1300 Ma and were probably derived from an extension of a Late Mesoproterozoic (1050-1200Ma) orogenic belt from the central Australian Musgrave Complex located 1500 km to the west. No evidence has been found for 600-800 Ma rifting of a Rodinian supercontinent and therefore it is suggested that breakup must have occurred well outboard of the present Early Palaeozoic East Gondwana margin. The younger succession is of Early Palaeozoic age and contains the distinctive 500-600 Ma detrital zircon signature that is widespread in East Gondwana in addition to some samples with ages in the range 460-510 Ma consistent with local igneous sources. The younger succession is related to the active margin of Gondwana that developed on the former passive margin in a back-arc setting, and the source of 510-600 Ma zircons is considered to be a composite of rift-related and back-arc volcanic sources.

Using detrital zircon geochronology, turbidite deposystems fed from distinct sediment sources can be distinguished within the Songpan-Ganzi complex, a collapsed Middle to Late Triassic turbidite basin of central China. A southern Songpan-Ganzi deposystem initially was sourced solely by erosion of the Qinling-Dabie orogen during early Late Triassic time, then by Qinling-Dabie orogen, North China block, and South China block sources during middle to late Late Triassic time. A northern Songpan-Ganzi system was sourced by erosion of the Qinling-Dabie orogen and the North China block throughout its deposition. These separate deposystems were later tectonically amalgamated to form one complex and then uplifted as the eastern Tibet Plateau. Keywords: turbidite, collisional orogen, unroofing, detrital zircon, China.

This study reports U-Pb ages for zircons from the Los Ranchos Formation, which is part of the primitive island are sequence, the oldest volcanic rocks in the Greater Antilles. Zircons were analyzed from three samples: quartz porphyry (quartz keratophyre) from the lower part of the formarion (Quita Sueno Member), fragmental quartz porphyry from the upper part of the formation (Pueblo Viejo Member), and the Cotui quartz diorite stock that intrudes the lower part of the formation. The lower part of the Los Ranchos Formation, represented by the Quita Sueno sample, formed at either 113.9 [+ or -] 0.8 or 118.6 [+ or -] 0.5 Ma depending on interpretation of the data, whereas the upper part, represented by the Pueblo Viejo sample, formed ca. 110.9 [+ or -] 0.8 Ma. The Cotui stock (RD-73-601) was emplaced ca. 111.8 [+ or -] 0.6 Ma or 112.9 + 0.9 Ma depending on whether eight grains reflect subtle inheritance. These results show that the Los Ranchos Formation was emplaced during the Aptian-Albian transition, that the Cotui stock could have supplied magma to the volcanic sequence and fluids to the Pueblo Viejo gold-silver deposit in the upper part of the formation, and that carbonaceous sediments in the upper part of the formation formed at the same time as ocean anoxic event 1b (Paquier). Caribbean plate tectonic models involving invasion of anomalous Pacific crust into the Caribbean region appear to agree best with this age because they provide a mechanism for the change from primitive island arc (PIA) to calc-alkaline magmatism and account for the restricted ocean circulation necessary to generate regional anoxic conditions. Keywords: Los Ranchos Formation, U-Pb, island-arc tholeiite, Hispaniola, Caribbean, plate tectonics.

Precise dates for mafic igneous events are essential for tectonic reconstructions and understanding mantle dynamics, mass extinctions, and paleoclimate. Zirconolite (CaZr[Ti.sub.2][O.sub.7) is a uranium-bearing accessory mineral, found in a wide range of terrestrial and lunar rocks, that has been largely overlooked as a chronometer. In situ U-Pb geochronology of zirconolite from three dolerite intrusions in Western Australia demonstrates that it yields emplacement ages that are more precise than those obtained from coexisting zircon and baddeleyite. Dikes in the Stirling Range Formation give a zirconolite [sup.207]pb/[sup.206]Pb age of 1218 [+ or -] 3 Ma, indistinguishable from the less precise dates obtained from zircon (1215 [+ or -] 10 Ma) and baddeleyite (1217 [+ or -] 39 Ma) and coincident with dike emplacement in the adjacent craton margin and peak metamorphism in the Albany-Fraser orogen. Zirconolite from the 755 Ma Mundine Well dike swarm yields a [sup.207]Pb/[sup.206]Pb age of 754 [+ or -] 5 Ma. Sills intruding the Proterozoic Manganese Group contain zirconolite crystals that give a [sup.207]Pb/[sup.206]Pb age of 523 [+ or -] 14 Ma. Despite high U contents (550 ppm to 14,000 ppm) and greenschist facies metamorphism, zirconolite in these samples is apparently unaffected by loss of radiogenic Pb. Because of its remarkable properties for U-Pb geochronology, it may soon become the dominant tool for dating mafic igneous rocks as young as 500 Ma by ion microprobe, and thus will prove especially valuable in reconstructing Precambrian geologic history. Keywords: zirconolite, dolerite, SHRIMR geochronology, mafic rocks, zircon, baddeleyite.

Clastic sedimentary and low-grade metasedimentary rocks preserve populations of detrital zircons because of the unreactive nature of this mineral. However, evidence of new zircon growth has been found within highly heterogeneous populations of zircon from several greenschist facies slates from the Scottish Highlands. Small ( Keywords: zircon, slate, metamorphism, detrital, overgrowths.

New research on suggests that Earth's early years were in a temperature cool enough for water to exist. Until now, geologists have thought that the planet's first 500 million years were too hot for liquid water to condense on its surface.

U-Pb geochronology of detrital zircons from upper Paleozoic loessite (western United States) provides data bearing on atmospheric circulation within western equatorial Pangea. Zircon age spectra of four loessites from three localities representing middle Pennsylvanian (Desmoinesian) and Early Permian (Wolfcampian) time vary significantly, reflecting changing provenances attributable to temporal and spatial shifts in winds. Zircons from two Desmoinesian samples (from Arizona and Utah) show a dominant mode between 1800 and 1600 Ma, reflecting the Yavapai-Mazatzal terranes that cored the Ancestral Rockies uplifts and suggesting northeasterly winds. Both samples also contain a secondary cluster of Grenvillian grains (1300-1000 Ma), reflecting a south-southeasterly source. Ages for Wolfcampian samples (from New Mexico and Utah) differ from one another; the New Mexico loessite contains a large mode at 1700 Ma, missing in the Utah sample, reflecting their locations on opposing sides of the Ancestral Rocky Mountains, within a westerly wind regime. Inferred easterly winds for middle Pennsylvanian time match model predictions, and the presence of both northerly and southerly directions might reflect time-averaged fluctuation of the Inter-Tropical Convergence Zone. In contrast, monsoonal circulation and attendant westerly winds appear to have been well established by earliest Permian time. Keywords: U-Pb geochronology, loessite, detrital zircons, Pangea.

Ion microprobe U-Pb isotope analyses of zircons from fallout and intercalated ash-flow deposits of the early, most chemically evolved Bishop Tuff (east-central California) yield a weighted mean age of 823 +/- 11 ka (mean square of weighted deviates, MSWD = 0.8; n = 22). A similar mean age of 839 +/- 36 ka (MSWD = 0.6, n = 15) obtained on unpolished zircon rims suggests that the difference between these ages and that of 760 ka for the Bishop Tuff obtained by recent Ar isotope determinations may be largely analytical in origin. Considerations based only on the effect of (super 230)Th-(super 234)U disequilibria on the U-Pb ages yield an apparent maximum mean crystal residence time for zircon of approximately 100 k.y. The zircon residence times are significantly shorter than those inferred from previous estimates for the timing of phenocryst growth and suggest that, once evolved, the early Bishop Tuff rhyolite was not stored for long before erupting. Keywords: Long Valley caldera, residence time, zircon, (super 238)U-(super 206)pb, ion probe data, rhyolites, magma chamber.

Coesite (the dense, high-pressure polymorph of quartz) occurs as inclusions in mechanically strong minerals in deeply subducted, metamorphosed crustal rocks in a number of Eurasian collisional orogens. It is the primary indicator mineral of ultrahigh-pressure (P) metamorphism. Whereas some coesite inclusions are untransformed, most exhibit partial transformation to palisade quartz and a concomitant increase in volume (resulting in rupture and radial fracturing of the host grain). Coesite can be identified by its diagnostic Raman spectrum; the strongest band (at atmospheric pressure, room temperature) is at 521 cm(super -l). Laser Raman microspectroscopic analyses of coesite inclusions within garnet and zircon in ultrahigh-P metamorphic rocks from Kazakhstan, Indonesia, and China reveal consistent differences in the Raman spectra of (1) partially transformed coesite + quartz (main Raman band at 521 cm(super -1)) and (2) untransformed monomineralic coesite grains (main band at 525-526 cm(super -1)). Applying the room-temperature calibration of pressure dependence of the coesite Raman spectrum, we conclude that the latter coesite inclusions are subject to a remarkable pressure differential of 19-23 kbar with the host grains, and are still undergoing pressure-temperature conditions on or close to the quartz-coesite equilibrium boundary.

Advances in analysing techniques have made it possible to undertake U-Pb age determination in zircon fragments weighing under one microgram. A new study investigates the internal structures of zircon from the late Archean Darling Range Batholith, Australia, to determine their origins and their geological significance. The presence of secondary decomposition structures in zircons suggests that the initial closed system reconstitution of zircons is caused by slow cooling at high temperatures.

Zircon growth in high-grade metamorphic rocks may be triggered by net transfer reactions involving the breakdown of a phase bearing zirconium (Zr). We have measured the concentration of Zr in the major minerals in granulite-facies rocks of differing bulk composition. Both garnet and hornblende contain tens of parts per million Zr, and no other major phase contains significant Zr. Simple calculations show that reaction of either garnet or hornblende to form non-Zr-bearing phases will release sufficient Zr to account for at least some new zircon growth. U-Pb ages from new zircon, grown as a result of either hornblende or garnet breakdown, are not expected to record the time of peak metamorphism, but rather will record the time of particular metamorphic reactions, allowing direct correlation of zircon ages with petrologically derived pressure-temperature-time paths. This approach offers the potential for more rigorous interpretation of the metamorphic significance of zircon ages than has previously been possible.

Most of Earth's present-day crust formed at mid-ocean ridges. High-precision uranium-lead dating of zircons in gabbros from the Vema Fracture Zone on the Mid-Atlantic Ridge reveals that the crust there grew in a highly regular pattern characterized by shallow melt delivery. Combined with results from previous dating studies, this finding suggests that two distinct modes of crustal accretion occur along slow-spreading ridges. Individual samples record a zircon date range of 90,000 to 235,000 years, which is interpreted to reflect the time scale of zircon crystallization in oceanic plutonic rocks.