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1.
We determined erosion rates on timescales of 101–104 years for two catchments in the northeastern Rhenish Massif, in order to unravel the Quaternary landscape evolution in a Variscan mountain range typical of central Europe. Spatially averaged erosion rates derived from in situ produced 10Be concentrations in stream sediment of the Aabach and M?hne watersheds range from 47 ± 6 to 65 ± 14 mm/ka and integrate over the last 9–13 ka. These erosion rates are similar to local rates of river incision and rock uplift in the Quaternary and to average denudation rates since the Mesozoic derived from fission track data. This suggests that rock uplift is balanced by denudation, i.e., the landscape is in a steady state. Short-term erosion rates were derived from suspended and dissolved river loads subsequent to (1) correcting for atmospheric and anthropogenic inputs, (2) establishing calibration curves that relate the amount of suspended load to discharge, and (3) estimating the amount of bedload. The resulting solid mass fluxes (suspended and bedload) agree with those derived from the sediment volume trapped in three reservoirs. However, resulting geogenic short-term erosion rates range from 9 to 25 mm/ka and are only about one-third of the rates derived from 10Be. Model simulations in combination with published sediment yield data suggest that this discrepancy is caused by at least three factors: (1) phases with higher precipitation and/or lower evapotranspiration, (2) rare flood events not captured in the short-term records, and (3) prolonged periods of climatic deterioration with increased erosion and sediment transport on hillslopes.  相似文献   

2.
柳振江  王建平  郑德文  刘家军  刘俊  付超 《岩石学报》2010,26(12):3597-3611
矿床形成后会经历不同形式的变化,区域隆升与剥蚀是影响矿床变化保存最为关键的因素之一。构造-热年代学是目前广泛运用于研究区域隆升剥蚀的一种重要手段,本文以我国最大金矿集中区———胶东西北部金矿及赋矿围岩玲珑花岗岩为研究对象,尝试将构造-热年代学引用到矿床成矿后变化与保存研究。通过磷灰石裂变径迹热年代学测试获得玲珑花岗岩距今110Ma以来的隆升演化历史,结果显示岩体剥蚀速率很小,平均0.0303±0.0044mm.a-1,自金矿形成后区域热-构造运动趋于平静,这对矿床的保存非常有利。胶东金矿成矿深度范围集中于4~10km,根据剥蚀速度计算玲珑花岗岩剥蚀量仅为2.0~4.2km,远未达到金矿最大成矿深度。当前本区金矿勘探和开采深度普遍小于2km,深部金矿找矿潜力良好。  相似文献   

3.
Along the upper reaches of the Gediz River in western Turkey, in the eastern part of the Aegean extensional province, the land surface has uplifted by 400 m since the Middle Pliocene. This uplift is revealed by progressive gorge incision, and its rate can be established because river terraces are capped by basalt flows that have been K–Ar and Ar–Ar dated. At present, the local uplift rate is 0.2 mm a−1. Uplift at this rate began around the start of the Middle Pleistocene, following a span of time when the uplift was much slower. This was itself preceded by an earlier uplift phase, apparently in the late Late Pliocene and early Early Pleistocene, when the uplift rate was comparable to the present. The resulting regional uplift history resembles what is observed in other regions and is analogously interpreted as the isostatic response to changing rates of surface processes linked to global environmental change. We suggest that this present phase of surface uplift, amounting so far to 150 m, is being caused by the nonsteady-state thermal and isostatic response of the crust to erosion, following an increase in erosion rates in the late Early Pleistocene, most likely as a result of the first large northern-hemisphere glaciation during oxygen isotope stage 22 at 870 ka. We suggest that the earlier uplift phase, responsible for the initial 250 m of uplift, resulted from a similar increase in erosion rates caused by the deterioration in local climate at 3.1 Ma. This uplift thus has no direct relationship to the crustal extension occurring in western Turkey, the rate and sense of which are thought not to have changed significantly on this time scale. Our results thus suggest that the present, often deeply incised, landscape of western Turkey has largely developed from the Middle Pleistocene onwards, for reasons not directly related to the active normal faulting that is also occurring. The local isostatic consequences of this active faulting are instead superimposed onto this “background” of regional surface uplift. Modelling of this surface uplift indicates that the effective viscosity of the lower continental crust beneath this part of Turkey is of the order of 1019 Pa s, similar to a recent estimate for beneath central Greece. The lower uplift rates observed in western Turkey, compared with central Greece, result from the longer typical distances of fluvial sediment transport, which cause weaker coupling by lower-crustal flow between offshore depocentres and eroding onshore regions that provide the sediment source.  相似文献   

4.
The eastern Ecuadorian Andes appear as a fold-and-thrust belt adjacent to a continental foredeep represented by one of the world's largest tropical alluvial megafans, the Pastaza megafan, debouching into the Amazonian lowland. The apex of the Pliocene–Pleistocene megafan situated in the present-day wedge top (Subandean Zone) has been cut by an erosion surface, the western part of which has been uplifted of 500 m along the frontal thrust, forming a poorly dissected plateau, the Mera plateau. This erosion surface erased most of the previous fluvial landscape but preserved a large thrust-related anticlinal hinge deforming less erodible underlying strata, the Mirador fold and smaller-sized anticlines. This surface has been then incised by two antecedent major rivers, the Pastaza and the Napo, and few tributaries. The plateau edge is marked by a series of large scale gently sloping landslides clustered along a 70 km long concave eastward line associated with the frontal thrust fault. The newly formed immature rivers issued from the landslides or sourced within east-dipping remnants of the erosion surface downstream of the landslide line constitute the greatest part of the streams feeding the Ecuadorian Amazonian basin. At 70 to 100 km from the landslide line, the drainage abruptly changes from highly immature to mature with a well-defined hinge line representing the outer limit of landslide and tectonic control. The diversions of the Pastaza River indicate ongoing fold growth since at least the late Pleistocene in the Eastern Cordillera, and the early Holocene in the Mera plateau. The preserved terraces of the Pastaza valley are all degradational and are ascribed to periods of tectonic (seismic) activity alternating with periods of tectonic quiescence or decreased seismic activity rather than to climatic events. 14C dating of the plateau erosion surface and of the upper Pastaza terraces indicates that the minimum average incision rate since 18,000 years BP varies locally in the upper Pastaza valley from 0.5 to 0.67 cm year−1 , increasing from 18,000 years BP to now. A comparison of these incision rates with fold-and-thrust fault uplift rates indicates that incision in the upper Pastaza valley was a result of rapid uplift (up to 1 cm year−1) along the Mirador fold-and-thrust which caused a restoration of the local equilibrium profile of the upper reach, combined with smaller local fault uplift along the westernmost thrust faults. The uplift of the whole Mera plateau with respect to the upper Amazonian basin gives a minimum average uplift rate of 2.8 cm year−1 since 18,000 years BP. The overall uplift of the Mera plateau and the Eastern Cordillera is likely to have been caused by a regional-scale low angle thrust ramp emerging as the frontal thrust fault.  相似文献   

5.
Late Quaternary landscape development along the Rancho Marino coastal range front in the central‐southern Pacific Coast Ranges of California has been documented using field mapping, surveying, sedimentary facies analysis and a luminescence age determination. Late Quaternary sediments along the base of the range front form a single composite marine terrace buried by alluvial fans. Marine terrace sediments overlie two palaeoshore platforms at 5 m and 0 m altitude. Correlation with the nearby Cayucos and San Simeon sites links platform and marine terrace development to the 125 ka and 105 ka sea‐level highstands. Uplift rate estimates based on the 125 ka shoreline angle are 0.01–0.09 m ka?1 (mean 0.04 m ka?1), and suggest an increase in regional uplift along the coast towards the NW where the San Simeon fault zone intersects the coastline. Furthermore, such low rates suggest that pre‐125 ka uplift was responsible for most of the relief generation at Rancho Marino. The coastal range front landscape development is, thus, primarily controlled by post 125 ka climatic and sea‐level changes. Post 125 ka sea‐level lowering expanded the range front piedmont area to a width of 7.5 km by the 18 ka Last Glacial Maximum lowstand. This sea‐level lowering created space for alluvial fan building along the range front. A 45 ± 3 ka optically stimulated luminescence (OSL) age provides a basal age for alluvial fan building or marks the time by which distal alluvial fan sedimentation has reached 300 m from the range front slope. Fan sedimentation is related to climatic change, with increased sediment supply to the range front occurring during (1) glacial period cold stage maxima and/or (2) the Late Pleistocene–Holocene transition, when respective increases in precipitation and/or storminess resulted in hillslope erosion. Sea‐level rise after the 18 ka lowstand resulted in range front erosion, with elevated localised erosion linked to the higher relief and steeper slopes in the SE. This study demonstrates that late Quaternary coastal range front landscape development is driven by interplay of tectonics, climatic and sea‐level change. In areas of low tectonic activity, climatic and sea‐level changes dominate coastal landscape development. When the sea‐level controlled shoreline is in close proximity to the coastal range front, localised patterns of sedimentation and erosion are passively influenced by the pre‐125 ka topography. Copyright © 2008 John Wiley & Sons, Ltd.  相似文献   

6.
Erosion-driven uplift of the modern Central Alps   总被引：2，自引：0，他引：2  
We present a compilation of data of modern tectono-geomorphic processes in the Central European Alps which suggest that observed rock uplift is a response to climate-driven denudation. This interpretation is predominantly based on the recent quantification of basin-averaged Late Holocene denudation rates that are so similar to the pattern and rates of rock uplift rates as determined by geodetic leveling. Furthermore, a GPS data-based synthesis of Adriatic microplate kinematics suggests that the Central Alps are currently not in a state of active convergence. Finally, we illustrate that the Central Alps have acted as a closed system for Holocene redistribution of sediment in which the peri-Alpine lakes have operated as a sink for the erosional products of the inner Central Alps.While various hypotheses have been put forward to explain Central Alpine rock uplift (e.g. lithospheric forcing by convergence, mantle processes, or ice melting) we show with an elastic model of lithospheric deformation, that the correlation between erosion and rock uplift rates reflects a positive feedback between denudation and the associated isostatic response to unloading. Thus, erosion does not passively respond to advection of crustal material as might be the case in actively converging orogens. Rather, we suggest that the geomorphic response of the Alpine topography to glacial and fluvial erosion and the resulting disequilibrium for modern channelized and associated hillslope processes explains much of the pattern of modern denudation and hence rock uplift. Therefore, in a non-convergent orogen such as the Central European Alps, the observed vertical rock uplift is primarily a consequence of passive unloading due to erosion.  相似文献   

7.
The Atlas Mountains have been uplifted by two mechanisms: Cenozoic thickening of the crust and thinning of the mantle lithosphere due to a buoyant thermal anomaly, previously inferred by indirect criteria to have started some 15 Ma. Because crustal shortening‐related uplift and mantle‐related uplift affect the topography at different spatial scales, we use scattered direct surface evidence to clarify the palaeoelevation dynamics. Uplifted Messinian shallow marine sediments in the southern margin of the Saïss Basin and in the northern Middle Atlas, tilted Pliocene lacustrine deposits in the Saïss Basin and in the piedmont of the southern High Atlas and drainage‐network reorganization in the Saïss Basin underscore the long‐wavelength rock uplift of the Atlas domain of mantle origin. The low erosion of the aforementioned deposits indicates that such uplift is a true surface uplift that occurred in post‐Miocene times at a minimum rate ranging from 0.17 to 0.22 mm yr?1.  相似文献   

8.
Coupling between tectonics and surface processes is usually ill‐quantified as other factors such as climate and lithology affect the later. We provide catchment‐wide 10Be denudation rates of the Mand catchment in the Zagros Fold Belt (Iran) to infer correlations between these rates and ongoing tectonic shortening in the region. Denudation rates are generally low (~0.05–0.1 mm/a) but increase to ~1 mm/a near the Halikan anticline, where changes in precipitation, lithology or hillslope gradient are insignificant. The denudation rates upstream and downstream of the Halikan anticline are consistent with the GPS convergence rates in these areas. The sharp increase in denudation rates over the Halikan anticline denotes its growth as previously detected from terrace incision. It also reveals small wavelength coupling between crustal deformation and erosion. Denudation rates are therefore a useful and sensitive tool that helps constraining non‐brittle active tectonics such as folding of a sedimentary cover.  相似文献   

9.
Functional relationships between erosion rates and topography are central to understanding controls on global sediment flux and interactions among tectonics, climate, and erosion in shaping topography. Based on such relations digital elevation models (DEMs) allow predicting landscape-scale erosion rates to the degree that process models can be calibrated and to the extent that such processes reflect elevation, drainage area, and aspect, or their derivatives such as slope and curvature. Digital elevation models allow investigating the influence of erosional processes on landscape form and evolution through generalized quantitative expressions often referred to as ‘erosion laws’. The analytical forms of such expressions are derived from physical principles, but only limited data are available to guide calibration to particular landscapes. In addition, few studies have addressed how different transport laws interact to set landscape-scale erosion rates in different environments. Conventionally, landscape-scale sediment flux is considered to be linearly related to slope or relief, but recent analyses point toward non-linear relations for steep terrain in which changes in the frequency of landsliding accommodate increased rates of rock uplift. In such situations, landscape-scale erosion rates are more closely tied to erosion potential predicted by models of bedrock river incision. Consequently, I propose that using DEMs to predict absolute or relative erosion rates at the landscape-scale counter-intuitively involves the rate of fluvial processes as governing the sediment flux from steep landscapes, and rates of hillslope processes as governing sediment flux from low-gradient landscapes. To cite this article: D.R. Montgomery, C. R. Geoscience 335 (2003).  相似文献   

10.
Ambiguous landscape histories can arise from equivocal or incomplete geomorphological, sedimentological or geochronological evidence. In this study, we apply quantitative analyses to robustly assess the origin and age of a field of rounded mounds, known as ‘The Hillocks’. Using clast analysis, the sediment is shown to be consistent with a landslide origin but inconsistent with other glacial sediments in the region. Cosmogenic 10Be exposure age dating suggests The Hillocks formed ~8 ka. Ground‐penetrating radar reveals that the deposit rests upon deltaic foreset beds; combined with topographical data, we calculate a deposit volume of ~15–27 M m3, consistent with the estimated volume of the proposed source area. Overall, our data support a rock avalanche origin, indicating that by 8 ka the valley was ice‐free at The Hillocks’ location, and the level of Lake Wakatipu was lower than 340 m asl by this time. The Dart River delta shoreline was situated somewhere between The Hillocks and the present day shoreline at that time, and has prograded at a maximum average rate of 1 m a?1 since ~8 ka. These findings are significant given the lack of landforms by which to constrain glacial or post‐glacial landscape histories in this region of New Zealand. Copyright © 2019 John Wiley & Sons, Ltd.  相似文献   

11.
A study of erosion rates by in-situ 10Be concentrations in granites of Miocene high-elevation paleosurfaces in Corsica indicates maximum erosion rates between 8 and 24 mm/kyear. The regional distribution of measured erosion rates indicates that the local climatic conditions, namely precipitation, the petrographic composition of granites, and the degree of brittle deformation govern erosion rates. Chemical erosion dominates even at elevations around 2,000 m in presently subalpine climate conditions. Field evidence indicates that erosion operates by continuous dissolution and/or disintegration to grains (grusification). The erosion rates are relatively high with respect to the preservation of inferred Early Miocene landscapes. We infer temporal burial in the Middle Miocene and significantly lower erosion rates in the Neogene until ∼3 Ma to explain the preservation of paleosurfaces, in line with fission track data. Valley incision rates that are a magnitude higher than erosion rates on summit surfaces result in relief enhancement and long-term isostatic surface uplift. On the other hand, widening and deepening of valleys by cyclic glaciation progressively destroys the summit surface relics.
Wolfgang FrischEmail:
  相似文献   

12.
Geomorphological and glacial geological surveys and multiple cosmogenic nuclide analyses (10Be, 26Al, and 21Ne) allowed us to reconstruct the chronology of variations prior to the last glacial maximum of the East Antarctic Ice Sheet (EAIS) and valley glaciers in the Terra Nova Bay region. Glacially scoured coastal piedmonts with round-topped mountains occur below the highest local erosional trimline. They represent relict landscape features eroded by extensive ice overriding the whole coastal area before at least 6 Ma (pre-dating the build-up of the Mt. Melbourne volcanic field). Since then, summit surfaces were continuously exposed and well preserved under polar condition with negligible erosion rates on the order of 17 cm/Ma. Complex older drifts rest on deglaciated areas above the younger late-Pleistocene glacial drift and below the previously overridden summits. The combination of stable and radionuclide isotopes documents complex exposure histories with substantial periods of burial combined with minimal erosion. The areas below rounded summits were repeatedly exposed and buried by ice from local and outlet glaciers. The exposure ages of the older drift(s) indicate multiple Pleistocene glacial cycles, which did not significantly modify the pre-existing landscape.  相似文献   

13.
Boulders of the Younger Dryas Salpausselkä I (Ss I) formation west of Lahti, southern Finland, were sampled for surface exposure dating. The 10Be concentrations, determined by accelerator mass spectrometry, yield minimum exposure ages of 11 930 ± 950, 11 220 ± 890, 11 050 ± 910 and 11 540 ± 990 years, using recently published production rates scaled for latitude and elevation. This includes a correction to the production rate resulting from postglacial uplift of the Fennoscandian lithosphere (i.e. changing elevation) during the time of exposure. The error‐weighted mean exposure age of 11 420 ± 470 years of the analysed boulders agrees with previous varve dates of Ss I, which range from 11 680 to 11 430 calendar years BP. However, erosion has to be taken into account as a process affecting rock surfaces and therefore influencing exposure ages. Available information suggests an erosion rate of 5 mm/kyr, which increases the error‐weighted mean exposure age to a value of 11 610 ± 470 years. Within the errors, the formation of Ss I in the Vesala area west of Lahti falls into the Younger Dryas time bracket, as defined by the GRIP and GISP 2 ice core (Greenland).  相似文献   

14.
东海是月球上最年轻的多环撞击盆地,关于其形成机制的研究很多,但成果大都基于正撞击的机制提出的,虽然有部分学者提出东海是斜撞击的,但缺乏具体撞击参数。本文通过多源数据融合,综合分析LRO影像数据、LOLA地形数据、M~3高光谱数据和IIM高光谱数据,对东海地区的地貌特征、物质成分进行了较为系统的解译,发现在东海中央熔融区存在一条与东海撞击方向垂直的中央隆起区域(中央隆起线),其也是中央熔融区粗糙部分与光滑部分的分界线,结合撞击坑成坑理论,认为其可能是撞击过程冲击波作用引起的堆叠作用形成的。同时利用GRAIL数据及对该地区的重力异常的成因进行了分析,认为异常是由于压强、温度及岩石粘度的改变引起局部莫霍面抬升和中央熔融物的形成而出现的,进而估算出熔融物占盆地内物质的25%,约为1.1×10~6km~3。同时,对GRAIL数据的剖面分析结果也支持了本文的斜撞击理论。最后,综合多方面的信息和撞击理论获取东海盆地构造分布图,并根据中央隆起线、溅射物及线性构造的分布特征等,提出东海盆地理论上是由一直径在50~100km的撞击体以10~30km/s的速度自东偏北约20°~30°方向以20°~30°的角度斜撞击月表而形成的。这可为研究更早期的月球撞击坑提供理论参考。  相似文献   

15.
The influence of the morphological setting on the denudation of carbonate landscapes and the respective contributions of mechanical and chemical weathering processes are still debated. We have addressed these questions by measuring 36Cl concentrations in 40 samples from the Luberon mountain, SE France, to constrain the denudation of various landscape elements. We observe a clear contrast between the local denudation rates from the flat summit surface, clustered around 30 mm/ka, and the basin‐average denudation rates across the flanks, ranging from 100 to 200 mm/ka. This difference highlights the transient evolution of the range, whose topography is still adjusting to previous uplift events. Such a pattern also suggests that carbonate dissolution is not the only driver of denudation in this setting, which appears to be significantly controlled by slope‐dependent processes.  相似文献   

16.
The Cretaceous and Cenozoic fill of the continental margins of southern Africa (South‐East Atlantic and Agulhas Margins) contains a continuous record of sediment supplied from the South African Plateau (SAP) for the past 134 million years. Estimates of solid sediment volumes deposited offshore were calculated from isopach maps and extrapolated vertical cross‐sections derived from a large amount of industrial geophysical data. Solid phase volumes and accumulation rates were calculated for six epochs: Lower Cretaceous (134–113 Ma), Mid Cretaceous (113–93.5 Ma), Upper Cretaceous (93.5–81 and 81–66 Ma), Palaeogene (66–25 Ma), Neogene (25–0 Ma). Our new compilation demonstrates the existence of two periods of elevated flux. The most important one occurs in the late Cretaceous (93.5–66 Ma) and was synchronous with an acceleration of onshore denudation as shown by thermochronometric data. After a period of extremely low accumulation rate, the second phase of elevated flux started in the Oligocene (~30–25 Ma) until present‐day. From these observations we suggest that the main phase of uplift of the SAP took place during the Upper Cretaceous. Two mechanisms, namely uplift caused by lithospheric delamination or by dynamic topography caused by the continent moving over the African Superplume, are viable explanations for our observations. The more recent and lower amplitude episode of enhanced accumulation rates is likely to correspond to a second period of uplift, potentially associated with the onset of uplift and extension along the East African Rift System.  相似文献   

17.
Slightly inclined Holocene marine terraces cover parts of two circular salt diapirs (Hormoz and Namakdan) in the Persian Gulf. Their relative altitude above present sea level results from a combination of general marine transgression/regression affecting the whole area, and of local uplift related to salt diapirism. Differential uplift rate of the studied diapirs in centre‐to‐rim profiles was calculated from results based on: (i) radiocarbon ages of skeletal remains of benthic faunas (19 samples), which originally grew close to sea level; (ii) original altitude of samples, estimated from general sea‐level oscillation curves for the last 10 kyr, and (iii) present sample altitude measured in the field. Calculated uplift rates increase from rim to centre on both diapirs in the range from: 2 mm yr?1 at the rim to 5–6 mm yr?1 at the interior of Hormoz, and 1–3 mm yr?1 at the rim to 3–5 mm yr?1 at the interior of Namakdan. Such uplift rate distributions fit into the parabolic profile of Newtonian fluid rather than to profiles typical for pseudoplastic fluids. The increase in uplift rate with distance from rim to centre of diapirs is gradual as demonstrated also by generally smooth surface of marine terraces. No tectonic dissections were found. The depositional history on both salt diapirs is similar although they are situated more than 100 km apart. Marine sedimentation started at about 9.6k cal. yr BP on Hormoz and at 8.6k cal. yr BP on Namakdan. Owing to rapid transgression, the sea partially truncated both salt diapirs and rapidly deepened, and carbonate mud was deposited on the peripheries of both salt diapirs. Between 7 and 5k cal. yr BP beach deposition replaced carbonate mud. Soon after 5k cal. yr BP, the sea retreated from most of the marine terraces on both salt diapirs. Copyright © 2006 John Wiley & Sons, Ltd.  相似文献   

18.
At the largest time and space scales, the pace of erosion and chemical weathering is determined by tectonic uplift rates. Deviations from equilibrium arise from the transient response of landscape denudation to climatic and tectonic perturbations. We posit that the constraint of mass balance, however, makes it unlikely that such disequilibrium persists at the global scale over millions of years, as has been proposed for late Cenozoic erosion. We synthesize weathering fluxes, global sedimentation rates, sediment yields and tectonic motions to show a remarkable constancy in the pace of Earth‐surface evolution over the last 10 Ma and support the null hypothesis – that global rates of landscape change have remained constant over this time period, despite global climate change and mountain building events. This work undermines the hypothesis that increased weathering due to mountain building or climate change was the primary agent for a decrease in global temperatures.  相似文献   

19.
Understanding the evolution of geochemical and geomorphic systems requires measurements of long-term rates of physical erosion and chemical weathering. Erosion and weathering rates have traditionally been estimated from measurements of sediment and solute fluxes in streams. However, modern sediment and solute fluxes are often decoupled from long-term rates of erosion and weathering, due to storage or re-mobilization of sediment and solutes upstream from the sampling point. Recently, cosmogenic nuclides such as 10Be and 26Al have become important new tools for measuring long-term rates of physical erosion and chemical weathering. Cosmogenic nuclides can be used to infer the total denudation flux (the sum of the rates of physical erosion and chemical weathering) in actively eroding terrain. Here we review recent work showing how this total denudation flux can be partitioned into its physical and chemical components, using the enrichment of insoluble tracers (such as Zr) in regolith relative to parent rock. By combining cosmogenic nuclide measurements with the bulk elemental composition of rock and soil, geochemists can measure rates of physical erosion and chemical weathering over 1000- to 10,000-year time scales.  相似文献   

20.
Numerical simulations of diffusion‐controlled nucleation and growth of garnet porphyroblasts in regionally metamorphosed rocks constrain interfacial energy and rates of nucleation and Al intergranular diffusion. The 13 rocks analysed in this study were collected from seven localities exhibiting a diverse range of crystallization conditions. Kinetic parameters governing nucleation and intergranular diffusion were adjusted iteratively to achieve fits between simulated and natural porphyroblastic textures. Model fits were assessed primarily from textural characteristics precisely measured by high‐resolution X‐ray computed tomography. Interfacial energy for heterogeneous nucleation ranges from 0.007 to 0.255 J m?2 for the sample suite, assuming shape factors in the range 0.01–1.0. Nucleation rates change through space and time due to growth and impingement of Al depletion zones surrounding porphyroblasts. In some models, the overall rock‐wide nucleation rate rises steeply, achieves a steady state, and then falls rapidly as reactants are consumed; in others, the steady state is not achieved, but instead the rate simply peaks before falling. Maximum rock‐wide nucleation rates range from 10?14.7 to 10?10.7 nuclei cm?3 s?1, and maximum local rates range from 10?13.7 to 10?9.7 nuclei?cm?3 s?1 depending on Al supersaturation. Diffusive fluxes of Al are well constrained by the simulated textures, but rates of intergranular diffusion are subject to uncertainties in Al solubility and interconnected porosity. Best estimates of Al diffusivities at 600 °C span 10?12.3 to 10?10.5 m2 s?1 for the sample suite, a narrow range considering natural variability and the uncertainties in solubility and porosity. Eliminating some models suspected of higher uncertainty for these quantities yields diffusivities at 600 °C near 10?11.0 m2 s?1, with dispersion of less than half an order of magnitude. These simulations, which are among the first attempted for regionally metamorphosed rocks, emphasize that: (i) nucleation rates vary markedly in time and space during crystallization; (ii) nucleation extends well beyond equilibrium conditions; (iii) Al diffusivity likely varies over only a narrow range across common metamorphic circumstances; and (iv) better determinations of both Al solubility and interconnected porosity are needed to constrain rates of Al intergranular diffusion more precisely.  相似文献   

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