Oxygen isotope composition of three types of unique nodules which consist of amorphous silica-apatite, cristobalite-apatite and tridymite-apatite associations interspersed amidst basaltic pyroclastics from the Har Peres volcano, Golan Heights, Israel is reported. Unusual isotopic temperature (75°C estimated from oxygen isotope fractionation between cristobalite (δ18O = +25.5‰)-apatite (δ18O = +12.9‰) pair suggests that the nodule was not formed by present-day pedogenesis as has been previously proposed, but was a xenolith incorporated probably from the underlying siliceous phosphorites at a higher temperature. An observed negative oxygen isotopic fractionation (δ18O = −5.1‰) between tridymite (δ18O = +9.9‰) and associated apatite (δ18O = +15.0‰) is indicative of the nodular formation under disequilibrium conditions. A plausible mechanism of formation of the apatite (and calcite) associated with tridymite is an epitaxial overgrowth on template tridymite of magmatic origin under the current weathering regime. Oxygen isotopic evidence indicates a complicated origin for the nodules.

Oxygen-isotope data were obtained for synthetic aluminum-hydroxide phases precipitated over 65–125 mo and have been compared to results from similar experiments conducted for 3–56 mo. The Al(OH)3 polymorphs, gibbsite, nordstrandite, and bayerite, were synthesized, but gibbsite was dominant in most samples, and commonly the only phase present. Using pure gibbsite samples, the following oxygen-isotope fractionation factors, ${\alpha }_{gibbsite-H_{2}O}$, were obtained: 1.0167 ± 0.0003 (9 ± 1°C), 1.0147 ± 0.0007 (24 ± 2°C), 1.0120 ± 0.0003 (51 ± 2°C). These values, and the associated equation for an oxygen-isotope geothermometer for the interval 0–60°C 103ln ${\alpha }_{gibbsite-H_{2}O}=2.04\times {10}^6/T^2-3.61\times {10}^3/T+3.65$ (T in K), are not significantly different from those obtained from experiments of much shorter duration. These results, and the good agreement with ${\alpha }_{gibbsite-H_{2}O}$ values obtained for well-constrained natural systems, suggest that the experimentally determined fractionation factors describe equilibrium conditions for gibbsite that has precipitated directly from solution.

As also proposed by others using a modified-increment calculation, our synthesis experiments suggest that ${\alpha }_{Al(OH{)}_3-H_{2}O}$ is polymorph-dependent at low temperatures and that a significant temperature-dependent trend exists in the values of ${\alpha }_{Al(OH{)}_3-H_{2}O}$. However, previously calculated fractionation factors obtained using the modified-increment method are higher than those obtained from the experiments, with this discrepancy becoming larger as temperature decreases.

Crystallization processes of the illite-smectite (I-S) mixed-layer mineral series during alteration of felsic vitric materials in volcaniclastic sediments through two drill holes (IT-2 and IT-8) near the Kakkonda active geothermal system, Japan, were examined by optical microscopy, scanning and transmission electron microscopy (SEM and TEM), electron microprobe analysis, X-ray diffraction (XRD), and oxygen isotope analysis. Temperatures measured through the drill holes increased nearly linearly with depth up to 317°C at the bottom (1700 m) of IT-8. Homogenization temperature measurements of fluid inclusions indicated that the alteration occurred at temperatures similar to the present temperatures. In selected volcaniclastic rocks, excluding andesitic rocks and black shales, clay minerals occurred as glass replacements and pore fillings as seen under SEM and optical microscopy, and exhibited predominantly euhedral hexagonal and elongated forms under TEM, implying that they precipitated in situ through hydrolytic reactions of glass and fluid. Based on XRD examination, I-S minerals showed a sigmoidal variation in illite layer percentage (%I) in the range of ∼150 to 220°C and R0 I-S minerals with intermediate %I between 20 and 40% rarely occurred (where R is the Reichweite parameter). The chemical composition also showed a specific variation with depth. Intermediate clays including smectite and I-S minerals are enriched in Al compared to those reported previously from hydrothermal alteration of almost equivalent parent rocks. The oxygen isotope data indicated that the reacting solution was percolating groundwater in the shallow levels and with fossil seawater in the deeper levels. Furthermore, calculating the fluid/rock (W/R) ratio from the isotope variations revealed that the alteration occurred at a nearly constant W/R ratio condition irrespective of %I. Consequently, the observed specific variations in structure and chemical composition of I-S minerals reflect the compositional variations of fluid participating in the crystallization at given temperatures under the conditions of a given original rock and constant W/R ratio. High pH and Na-rich solutions generated by progressive hydrolytic reactions between felsic glass and groundwater favored the precipitation of Al-rich smectite up to ∼150°C and was followed by precipitation of an aluminous R1 I-S mineral with few intermediate R0 I-S minerals at higher temperatures. The crystallization obeys Ostwald’s step rule behavior of smectite illitization processes under a high geothermal gradient.

One of the commonly used analytical approaches for measuring oxygen isotope ratios δ18O of solids (organic and inorganic) is to pyrolyze the samples to gaseous phases and then send the gas into an isotope ratio mass spectrometer system. Solid samples for δ18O measurements are usually stored in silver cups because of its low reactivity towards oxygen and other oxidants. Samples in silver cups can be dropped directly into the carbon column of the pyrolysis furnace. However, the silver cups can tarnish and then be oxidized over a prolonged storage period. We find that while a small amount of silver oxides does not affect measurements with appreciable sample sizes, it can skew isotope results of small samples. We thus recommend careful storage of samples in silver cups to minimize oxidation, such as under an air-isolated condition, and avoiding prolonged storage for accurate δ18O measurements.

Twenty vapour-phase garnets were studied in two samples of the Topopah Spring Tuff of the Paintbrush Group from Yucca Mountain, in southern Nevada. The Miocene-age Topopah Spring Tuff is a 350 m thick, devitrified, moderately to densely welded ash-flow tuff that is zoned compositionally from high-silica rhyolite to latite. During cooling of the tuff, escaping vapour produced lithophysae (former gas cavities) lined with an assemblage of tridymite (commonly inverted to cristobalite or quartz), sanidine and locally, hematite and/or garnet. Vapour-phase topaz and economic deposits associated commonly with topaz-bearing rhyolites (characteristically enriched in F) were not found in the Topopah Spring Tuff at Yucca Mountain. Based on their occurrence only in lithophysae, the garnets are not primary igneous phenocrysts, but rather crystals that grew from a F-poor magmaderived vapour trapped during and after emplacement of the tuff. The garnets are euhedral, vitreous, reddish brown, trapezohedral, as large as 2 mm in diameter and fractured. The garnets also contain inclusions of tridymite. Electron microprobe analyses of the garnets reveal that they are almandinespessartine (48.0 and 47.9 mol.%, respectively), have an average composition of (Fe1.46Mn1.45Mg0.03Ca0.10)(Al1.93Ti0.02)Si3.01O12 and are comparatively homogeneous in Fe and Mn concentrations from core to rim. Composited garnets from each sample site have δ18O values of 7.2 and 7.4%. The associated quartz (after tridymite) has δ18O values of 17.4 and 17.6%, values indicative of reaction with later, low-temperature water. Unaltered tridymite from higher in the stratigraphic section has a δ18O of 11.1% which, when coupled with the garnet δ18O values in a quartz-garnet fractionation equation, indicates isotopic equilibration (vapour-phase crystallization) at temperatures of ∼600°C. This high-temperature mineralization, formed during cooling of the tuffs, is distinct from the later and commonly recognized low-temperature stage (generally 50−70°C) of calcite, quartz and opal secondary mineralization, formed from downward-percolating meteoric water, that locally coats fracture footwalls and lithophysal floors.

Tertiary ultrapotassic volcanic rocks from Serbia occasionally display low levels of K2O and K2O/ Na2O. In these rocks, analcime regularly appears as pseudomorphs after pre-existing leucite microphenocrysts. The process ofleucite transformation in Serbian ultrapotassic rocks is very thorough: fresh leucite survives only in ugandites from the Koritnik lava flows as well as in rare inclusions in Cpx. This paper focuses on the impact of ‘analcimization’ on the mineralogy and geochemistry ofthe Serbian ultrapotassic rocks, using the samples where leucite survived as a monitor for the process.

Analcimization has had a great impact on the geochemistry of the rocks, but affects only a restricted number of chemical parameters. These are the falsification of the original K2O/Na2O ratio, the decoupling oflarge-ion lithophile elements resulting in considerable depletion of Rb and K2O, but not ofBa, and sporadic, but extreme enrichment ofCs in some analcime-bearing samples (up to 900 ppm). Analcimization is also recognized by an increase in whole-rock δ18O values of ∼3% compared to fresh rocks, which correlates with the level of whole-rock hydration. Finally, the 87Sr/86Sr enrichment at nearly constant 143Nd/144Nd demonstrated by some rocks can also be explained by the analcimization ofleucite. For samples with variable 87Sr/86Sr from the same lava flow, 87Sr/86Sr values correlate with modal analcime abundance (ex-leucite), loss on ignition of whole-rock and whole-rock δ18O values. The extreme depletion in K and enrichment in Na, together with modification of other geochemical parameters, may have led to the misinterpretation of the origin and geodynamic affiliations of the Serbian ultrapotassic rocks, had the effects of analcimization not been taken into account.

The magma source and evolution of a zoned breccia pipe on the southern Beara Peninsula in southwest Ireland are investigated using the geochemistry of the host mineral assemblages. The clast-poor inner zone of the pipe has a magnesium-rich silicocarbonatite whole-rock composition (14.30 wt.% MgO; 31.80 wt.% SiO2). The silicocarbonatite has retained an ultimate mantle source 13C isotopic composition after metamorphism, consistent with the presence of mantle debris. The silicocarbonatite is Cr-, Ni- and Co-rich (847 ppm, 611 ppm and 60 ppm, respectively) but REE depleted compared with volcanic dolomite carbonatites worldwide. The mineral assemblage consists of Sr-rich (0.55 wt.% SrO) ferroan dolomite, magnesite and pseudomorphs of chlorite after phlogopite, consistent with derivation from a carbonated and hydrated mantle. However, chrome spinel crystals (≤4 40.14 wt.% Cr2O3) are compositionally indistinguishable from unmetasomatized spinel macrocrysts in kimberlites. The silicocarbonatite is inferred to represent a magma produced by partial melting of metasomatized mantle at physical conditions between those in which primary dolomite carbonatite and ultramafic magmas of high-pressure origin form. The primary silicocarbonatite magma ascended and sampled mantle material in a manner similar to kimberlite, and subsequently lost volatile components due to release of metasomatic fluids and later metamorphism.

Oxygen isotope measurements using SIMS and laser-fluorination methods confirm the presence of concentric and sector zoning in low-temperature (200°C to <400°C) hydrothermal quartz from Alpine veins. While concentric zoning is most readily explained by changes in the chemical composition of the fluid or temperature of crystallization, the reasons for sector zoning are more difficult to explain. Relative enrichment in 18O for crystallographically different sectors of quartz corresponds to m >r >z. Sector zoning is, however, largely limited to the exterior zones of crystals and/or to crystals with large Al (>1000 ppm) and trace element contents, probably formed at temperatures <250°C. Differences in δ18O between the prismatic (m) relative to the rhombohedral (r and z) growth sectors of up to 2% can be explained by a combination of a face-related crystallographic and/or a growth rate control. In contrast, isotopic sector zoning of up to about 1.5% amongst the different rhombohedral faces increases in parallel with the trace element content and is likely to represent disequilibrium growth. This is indicated by non-systematic, up to 2%, differences within single growth zones and the irregular, larger or smaller, δ18O values (of several permil) of the exterior compared to the inner zones of the same crystals. Disequilibrium growth may be related to the large trace element content incorporated into the growing quartz at lower temperatures (<250°C) and/or be related to fluid-vapour separation, allowing crystal growth from both a vapour as well as a liquid phase.

The oxygen isotope composition of phosphate from tooth enamel of rodents (δ18Op) constitutes a valuable proxy to reconstruct past air temperatures in continental environments. This method has been applied to rodent dental remains from three genera, Arvicola sp., Microtus sp. and Dicrostonyx sp., coming from Taillis-des-Coteaux, Vienne, France. This archaeological site contains an exceptionally preserved sedimentary sequence spanning almost the whole Upper Palaeolithic, including seven stratigraphic layers dated from 35 to 17 cal ka BP. The abundant presence of rodent remains offers the opportunity to quantify the climatic fluctuations coeval of the various stages of human occupation of the site. Differences between δ18Op values of Arvicola sp. and Microtus sp. teeth are interpreted as the result of heterochrony in tooth formation as well as differences in ecology. Mean δ18Op values of Microtus sp. are preferentially used to reconstruct summer air temperatures, which range from 16.0 ± 3.7 to 19.1 ± 3.1°C throughout the sedimentary sequence; however, the highest variability is observed during the last glacial maximum.

We present a new oxygen isotope (δ18O) record from carbonate-rich lake sediments from central Vermont. The record from Twin Ponds spans from 13.5 cal ka BP (1950 AD) to present, but contains a 6 ka long hiatus starting shortly after 7.5 cal ka BP. We compare the record for ca. 13.5–7.5 cal ka BP with published δ18O data from the region after using a Bayesian approach to produce many possible chronologies for each site. Principal component analysis then identified chronologically-robust, multi-site oxygen isotope signals, including negative values during the Younger Dryas, but no significant deviations from the early Holocene mean of the regional records. However, differences among sites indicate significant trends that likely relate to interacting changes in the regional gradients of seasonal temperatures and precipitation as well as moisture sources, moisture pathways, and aridity that were controlled by large-scale climatic controls such as insolation, the progressive decline of the Laurentide Ice Sheet, and changes in oceanic circulation. Centennial shifts punctuate these trends at ca. 9.3 and 8.2 cal ka BP, and reveal that the local character of these short-lived features requires a detailed understanding of lake hydrology and regional isotopic gradients to yield reliable information for regional climate reconstructions.

Oxygen isotope compositions of biogenic phosphates from mammals are widely used as proxies of the isotopic compositions of meteoric waters that are roughly linearly related to the air temperature at high- and mid-latitudes. An oxygen isotope fractionation equation was determined by using present-day European arvicoline (rodents) tooth phosphate: δ18Op = 20.98(±0.59) + 0.572(±0.065) δ18Ow. This fractionation equation was applied to the Late Pleistocene karstic sequence of Gigny, French Jura. Comparison between the oxygen isotope compositions of arvicoline tooth phosphate and Greenland ice core records suggests to reconsider the previously established hypothetical chronology of the sequence. According to the δ18O value of meteoric water–mean air temperature relationships, the δ18O value of arvicoline teeth records variations in mean air temperatures that range from 0° to 15°C.

Celtis sp. (commonly known as “hackberry”) fruits were collected from 101 North American sites located in 13 states and one Canadian province between the years of 1979–1994. The biomineralized carbonate endocarp of the hackberry, which is a common botanical fossil found throughout the Quaternary sediments of the Great Plains, was analyzed for its δ18O value and plotted against the δ18O value of site environmental water to demonstrate the potential of the hackberry as a paleoclimate indicator. This correlation was reinforced by intensive studies on extracted tissue-water δ18O value and hackberry endocarp carbonate δ18O value from three trees in Sterling, Colorado. The observed correlation in the large data set between hackberry endocarp carbonate δ18O value and environmental water is [endocarp δ18O=38.56+0.69×environmental water δ18O] (R=0.88; R2=0.78; p value<0.0001). The relation of the hackberry carbonate to temperature in the Great Plains was the following: (average daily-maximum growing season temperature [°C])=6.33+0.67 (δ18O of endocarp carbonate) (R=0.73; R2=0.54; p value=0.0133). The δ18O value of early Holocene fossil hackberry carbonate in the Pintwater Cave, southern Nevada, suggested precipitation δ18O values more positive than today (∼−4‰ early Holocene vs ∼−9 to −10‰ today). This shift, combined with paleobotanical data, suggests an influx of summer monsoonal moisture to this region in the early Holocene. Alternatively, the more positive δ18O values could be viewed as suggestive of warmer temperatures, although the direct use of Great Plains hackberry/temperature relationships to the Great Basin is of debatable value.

The middle Paleolithic stratigraphic sequence of Les Pradelles (Charente, France) spans from the end of Marine Isotope Stage (MIS) 4 until the middle of MIS 3. Micromammal remains are present in all the stratigraphic levels, offering a rare opportunity to address the questions of both environmental and climatic fluctuations throughout this period. Climate modes were studied through the taphonomy, biodiversity and oxygen isotope compositions of phosphate (δ18O p ) from 66 samples of rodent tooth enamel. The δ18O p values from the lower sedimentary levels provide summer mean air temperatures of 19 ± 2°C (level 2/1) and of 16 ± 2°C (levels 2A, 2B and 4A). Within the middle of sequence (level 4B), a paleobiodiversity change can be identified with an increase of Dicrostonyx torquatus, which is associated with the largest amplitude in δ18O p values and the highest maximal δ18O p values. At the top of the sequence (level 5-2), a biodiversity change is observed with the increase of Microtus arvalis, but without any change in δ18O p values. The association of cold rodent species with unexpected high and large amplitudes in the δ18O p values of their teeth, possibly indicative of aridity, suggests their deposition during a Heinrich event.

Climate change during the so-called Little Ice Age (LIA) of the 15th to 19th centuries was once thought to be limited to the high northern latitudes, but increasing evidence reflects significant climate change in the tropics. One of the hypothesized features of LIA climate in the low latitudes is a more southerly mean annual position of the Intertropical Convergence Zone (ITCZ), which produced more arid conditions through much of the northern tropics. High-resolution stable oxygen isotope data and other sedimentary evidence from Laguna de Felipe, located on the Caribbean slope of the Cordillera Central of the Dominican Republic, support the hypothesis that the mean annual position of the ITCZ was displaced significantly southward during much of the LIA. Placed within the context of regional paleoclimate and paleoceanographic records, and reconstructions of global LIA climate, this shift in mean annual ITCZ position appears to have been induced by lower solar insolation and internal dynamical responses of the global climate system. Our results from Hispaniola further emphasize the global nature of LIA climate change and the sensitivity of circum-Caribbean climate conditions to what are hypothesized to be relatively small variations in global energy budgets.

The 18O/16O profile of a 554-m long ice core through Taylor Dome, Antarctica, shows the climate variability of the last glacial–interglacial cycle in detail and extends at least another full cycle. Taylor Dome shares the main features of the Vostok record, including the early climatic optimum with later cool phase of the last interglacial period in Antarctica. Taylor Dome δ18O fluctuations are more abrupt and larger than those at Vostok and Byrd Station, although still less pronounced than those of the Greenland GISP2 and GRIP records. The influence of the Atlantic thermohaline circulation on regional ocean heat transport explains the partly “North Atlantic” character of this Antarctic record. Under full glacial climate (marine isotope stage 4, late stage 3, and stage 2), this marine influence diminished and Taylor Dome became more like Vostok. Varying degrees of marine influence produce climate heterogeneity within Antarctica, which may account for conflicting evidence regarding the relative phasing of Northern and Southern Hemisphere climate change.

Questions remain as to the nature of climatic change through the last glacial–interglacial transition in the eastern Mediterranean region, particularly the relative contribution of evaporation and precipitation to regional water balance. Here changes in oxygen isotope values through this time period from Eski Acıgöl, a crater lake in central Turkey, are investigated using hydrological and isotope mass balance models. These allow changes in evaporation and precipitation to be quantified and their relative importance evaluated. We show that it is the volumetric flux rate of water passing through the lake system and not the precipitation-to-evaporation ratio per se which controlled the stable isotope record in Eski Acıgöl. Early Holocene precipitation is shown to be much greater than that during both the latter part of the last glaciation and the present day. We test these calculated values against other records in the same region, firstly with other lake records in Anatolia, the Konya basin and Lake Van, and secondly with isotope-inferred palaeo-precipitation data from Soreq cave in Israel. This reveals a contrast between pre- and post-LGM precipitation values in Turkey (wetter and drier, respectively) and also suggests that during the last glacial–interglacial transition there was a more marked precipitation gradient than at present between northern/interior and southern/coastal parts of the eastern Mediterranean region.

Oxygen isotope compositions of phosphate in tooth enamel from large mammals (i.e. horse and red deer) were measured to quantify past mean annual air temperatures and seasonal variations between 145 ka and 33 ka in eastern France. The method is based on interdependent relationships between the δ18O of apatite phosphate, environmental waters and air temperatures. Horse (Equus caballus germanicus) and red deer (Cervus elaphus) remains have δ18O values that range from 14.2‰ to 17.2‰, indicating mean air temperatures between 7°C and 13°C. Oxygen isotope time series obtained from two of the six horse teeth show a sinusoidal-like signal that could have been forced by temperature variations of seasonal origin. Intra-tooth oxygen isotope variations reveal that at 145 ka, winters were colder (− 7 ± 2°C) than at present (3 ± 1°C) while summer temperatures were similar. Winter temperatures mark a well-developed West–East thermal gradient in France of about − 9°C, much stronger than the −4°C difference recorded presently. Negative winter temperatures were likely responsible for the extent and duration of the snow cover, thus limiting the food resources available for large ungulates with repercussions for Neanderthal predators.

We investigate seasonal variations in the diet and drinking water of four Great Lakes mastodon (Mammut americanum) specimens using stable isotope analysis of serially sampled inner-enamel bioapatite structural carbonate (δ13Csc, δ18Osc), and previously published bulk analyses. Isotopic analyses and thin section measurements showed that mastodon tooth enamel extension rates (~ 12–4 mm/yr, decreasing toward the cervix) were lower than those of mammoths or modern elephants. Mastodons had distinct and highly regular seasonal variations in δ13Csc and δ18Osc, which we interpret in the context of local glacial history and vegetation changes. Seasonal variations in δ18O were large but variations in δ13C were small, and may have been obscured if coarser sampling methods than our inner-enamel sampling approach were used. Thus, our approach may be particularly useful for understanding relatively small seasonal changes in δ13C within C3 environments. The seasonal patterns, though not entirely conclusive, suggest that the Ontario mastodons did not migrate over very long distances. Rather, the climate and seasonal dietary patterns of mastodons within the region changed over time, from ~ 12,400 to 10,400 14C yr BP (~ 15,000 – 12,000 cal yr BP). Insights gained using these methods can contribute to a better understanding of megafaunal extinctions and Paleoamerican lifeways.

The late Quaternary glacial history of the Amery Oasis, and Prince Charles Mountains is of significant interest because about 10% of the total modern Antarctic ice outflow is discharged via the adjacent Lambert Glacier system. A glacial thrust moraine sequence deposited along the northern shoreline of Radok Lake between 20–10 ka bp, overlies a layer of thin, aragonite crusts which provide important constraints on the glacial history of the Amery Oasis. The modern Radok Lake is fed by the terminal meltwaters of the alpine Battye Glacier. The aragonite crusts were deposited in shallow water of ancestral Radok Lake 53 ka bp, during the A3 warm event in Isotope Stage 3. Oxygen isotope (δ18O) analysis of the last glacial-age aragonite crusts indicates that they precipitated from freshwater with a δ18OSMOW composition of -36%, which is 8% more depleted than the present water (-28%) in Radok Lake. A regional oxygen isotope (δ18O) and elevation relationship for snow is used to determine the source of meltwater and glacial ice in Radok Lake during the A3 warm event. This relationship indicates that Radok Lake received meltwater from the confluence of both Battye Glacier ice and an expansion of grounded western Lambert Glacier ice in the Amery embayment.

Oxygen isotope data are very useful in determining the source rocks of granitic magmas, particularly when used in combination with Sr, Pb, and Nd isotope studies. For example, unusually high δ18O values in magmas (δ18O> +8) require the involvement of some precursor parent material that at some time in the past resided on or near the Earth's surface, either as sedimentary rocks or as weathered or hydrothermally altered rocks. The isotopic systematics which are preserved in the Mesozoic and Cenozoic batholiths of western North America can be explained by grand-scale mixing of three broadly defined end-members: (1) oceanic island-arc magmas derived from a “depleted” (MORB-type?) source in the upper mantle (δ18O c. +6 and 87Sr/86Sr c. 0·703); (2) a high-18O (c. +13 to +17) source with a very uniform 87Sr/86Sr (c. 0·708 to 0·712), derived mainly from eugeosynclinal volcanogenic sediments and (or) hydrothermally altered basalts; and (3) a much more heterogeneous source (87Sr/86Sr c. 0·706 to 0·750, or higher) with a high δ18O (c. +9 to +15) where derived from supracrustal metasedimentary rocks and a much lower δ18O (c. +7 to +9) where derived from the lower continental crust of the craton. These end-members were successively dominant from W to E, respectively, within three elongate N–S geographic zones that can be mapped from Mexico all the way N to Idaho.

18O/16O studies (together with D/H analyses) can, however, play a more important and certainly a unique role in determining the origins of the aqueous fluids involved in the formation of granitic and rhyolitic magmas. Fluid-rock interaction effects are most clear-cut when low-18O, low-D meteoric waters are involved in the isotopic exchange and melting processes, but the effects of other waters such as seawater (with a relatively high δD c. 0) can also be recognised. Because of these hydrothermal processes, rocks that ultimately undergo partial melting may exhibit isotopic signatures considerably different from those that they started with. We discuss three broad classes of potential source materials of such “hydrothermal-anatectic” granitic magmas, based mainly on water/rock (w/r), temperature (T), and the length of time (t) that fluid-rock interaction proceeds: (Type 1) epizonal systems with a wide variation in whole-rock δ18O and extreme 18O/16O disequilibrium among coexisting minerals (e.g. quartz and feldspar); (Type 2) deeper-seated and (or) longer-lived systems, also with a wide spectrum of whole-rock δ18O, but with equilibrated 18O/16O ratios among coexisting minerals; (Type 3) thoroughly homogenised and equilibrated systems with relatively uniform δ18O in all lithologies. Low-18O magmas formed by melting of rocks altered in a Type 2 or a Type 3 meteoric-hydrothermal system are the only kinds of “hydrothermal-anatectic” granitic magmas that are readily recognisable in the geological record. Analogous effects produced by other kinds of aqueous fluids may, however, be quite common, particularly in areas of extensional tectonics and large-scale rifting. The greatly enhanced permeabilities in such fractured terranes make possible the deep convective circulation of ground waters and sedimentary pore fluids. The nature and origin of low-18O magmas in the Yellowstone volcanic field and the Seychelles Islands are briefly reviewed in light of these concepts, as is the development of high-D, peraluminous magmas in the Hercynian of the Pyrenees.