F. J. Ryerson

Mineral-aqueous fluid partitioning of trace elements at 900°C and 2.0 GPa: Constraints on the trace element chemistry of mantle and deep crustal fluids

Abstract To constrain the trace element composition of aqueous fluids in the deep crust and upper mantle, mineral-aqueous fluid partition coefficients (Dmin/fluid) for U, Th, Pb, Nb, Ba, and Sr have been measured for clinopyroxene, garnet, amphibole, and olivine in experiments at 2.0 GPa and 900°C. Clinopyroxene-and garnet-fluid partition coefficients are similar for Nb (0.01-0.7) and Ba (∼10−4-10−5 ), whereas values of Dcpx/fluid for Sr (0.5−4), Th (0.6–9), and Pb (0.04–0.09) are ∼1Ox (Th, Pb) to ∼1000x (Sr) higher than Dgarnet/fluid . At the same f O2 (FMQ + 1), garnet-fluid partition coefficients for U are ∼10x higher than those for clinopyroxene. Amphibole-fluid partition coefficients are uniformly high (-I) for all elements studied, and, with the exception of Ba, interelement fractionations are similar to clinopyroxene. The olivine-fluid partition coefficient for Nb is similar to values measured for the other silicates, whereas Dolivine/fluid for U, Th, Pb, Sr, and Ba are significantly lower. Clinopyroxene and garnet partition coefficients follow Henrys Law up to ∼300 ppm of either Ba, Pb, or Sr in the fluid. Both the major-element chemistry of clinopyroxene and fluid have some influence on partitioning, with the magnitude of these effects varying according to element type. Although clinopyroxene concentrations of Pb, Ba, and Sr were found to be homogeneous, core-to-rim decreases in wt% Al2O3 were found to correlate with reductions in the concentrations of Nb, U and Th, and hence Dcpx/fluid. Both increases in solute content and the addition of NaCl to fluids lower the measured partition coefficients. A decrease in experiment f O2 reduces DTh/DU for clinopyroxene, which is consistent with the compatibility of U4+ relative to U6+ in the clinopyroxene structure. Comparison of mineral/ fluid partition coefficients with mineral /basaltic melt values from the literature reveal notable distinctions in partitioning behavior for fluids vs. melts. Mineral-melt and mineral-fluid partitioning for elements such as Ba, Pb, and Sr are similar, but in contrast, U, Th, and Nb are more strongly partitioned into silicate melts than aqueous fluids. Such differences may provide a means of discerning the products of melt- vs. fluid-mediated metasomatism. Bulk eclogite- and lherzolite-aqueous fluid partition coefficients, calculated from mineral/aqueous fluid values, are used to illustrate how partitioning data can constrain (1) the trace element composition of fluids that may be a product of dehydration of basaltic oceanic crust and (2) the effect of the subarc mantle on trace element fractionation processes. The silicate assemblage produced during basalt dehydration (garnet + cpx ± amphibole) does not selectively deplete the coexisting fluid in Nb relative to the other elements studied, nor is Nb preferentially withdrawn from the fluid by passage through an amphibole lherzolite mantle. Results, therefore, reaffirm the notion that residual rutile is necessary to selectively deplete slab-derived fluids in high field strength elements. Calculations also indicate that fluids with excess [238U ] relative to [230Th] may be produced during dehydration of basaltic oceanic crust, and such excesses are retained or enhanced during transit through the mantle wedge, provided that mildly oxidizing conditions prevail. Slab-derived fluids can therefore produce the requisite low ratios of high-field-strength/large-ion-lithophile elements (such as Nb/Th) and [238U]/[230Th] > 1 in the source regions of island arc basalts by metasomatism of the mantle wedge. In addition to constraints on the composition of the fluid liberated during slab dehydration, our data allow us to estimate the trace element composition of the material returned to the deep mantle during subduction. Calculations indicate that, following dehydration, the U/Pb ratio in basaltic crust is increased and Rb/Sr is likely to be dramatically reduced. Subduction and prolonged aging of this material produces an isotopic reservoir with the characteristics of the HIMU component sampled by some oceanic island basalts.

Rutile saturation in magmas: implications for TiNbTa depletion in island-arc basalts

Abstract The TiO 2 contents of rutile-saturated melts ranging from basalt to rhyodacite have been investigated at P = 8–30 kbar and T = 1000–1300°C under hydrous, CO 2 -saturated, and volatile-absent conditions. Dissolved TiO 2 is positively correlated with T and not strongly dependent on P total . For fixed P and T , TiO 2 content decreases markedly as the melts become more felsic. The distribution of TiO 2 between rutile and liquid, expressed as a wt.% concentration ratio, D (rut/liq), is given by: In D = −3.16 + (9373T) + 0.026P − 0.152FM where T is in Kelvins, P in kbar and FM is a melt composition parameter, FM = [Na+K+ 2(Ca+Fe+Mg)]/Al· 1/Si in which the chemical symbols represent cation fractions. The first term expresses the competition of aluminate and titanate anions for charge-compensating cations, and the second term expresses the inverse dependence of dissolved TiO 2 on SiO 2 content. There is no apparent dependence of rutile solubility on water content. For ranges of probable solidus conditions, rutile saturation in basaltic, andesitic, and dacitic liquids requires 7–9, 5–7, and 1–3 wt.% TiO 2 , respectively. These concentrations are well in excess of those found in the respective rock types, so depletion in Nb, Ta, and Ti and reduced Nb/U and Nb/Th ratios in volcanic rocks erupted at convergent plate margins cannot be attributed to residual rutile in their source regions. Thus, Nb, Ta and Ti depletion must be an inherent property of the source region. We suggest that the island-arc source region has been depleted in Nb and Ta by a previous episode of melt extraction (MORB), zoning refining, or equilibration with a percolating melt or fluid. Such a process markedly depletes the LILE and HFSE element concentrations in the residuum, but ratios such as Nb/U, Nb/Th and U/Th remain relatively constant due to similar solid-melt partition coefficients. The depletion of Nb relative to Th in the source regions of island-arc magmas occurs during hybridization of the source by rutile-saturated (Nb/Ta-depleted) melts or aqueous fluids. If the hybridizing agent is a melt, a relatively felsic composition, produced under low T (900°) hydrous conditions, is required.

Did the Indo-Asian collision alone create the Tibetan plateau?

It is widely believed that the Tibetan plateau is a late Cenozoic feature produced by the Indo-Asian collision. However, because Tibet was the locus of continental accretion and subduction throughout the Mesozoic, crustal thickening during that time may also have contributed to growth of the plateau. This portion of the geologic history was investigated in a traverse through the central Lhasa block, southern Tibet. Together with earlier studies, our mapping and geochronological results show that the Lhasa block underwent little north-south shortening during the Cenozoic. Rather, our mapping shows that ∼60% crustal shortening, perhaps due to the collision between the Lhasa and Qiangtang blocks, occurred during the Early Cretaceous. This observation implies that a significant portion of southern Tibet was raised to perhaps 3–4 km elevation prior to the Indo-Asian collision.

Late Cenozoic tectonic evolution of the southern Chinese Tian Shan

Structural, sedimentological, magnetostratigraphic, and 40Ar/39Ar thermochronological investigations were conducted in the southern Chinese Tian Shan. On the basis of our own mapping and earlier investigations in the area, the Late Cenozoic southern Tian Shan thrust belt may be divided into four segments based on their style of deformation. From west to east, they are (1) Kashi-Aksu imbricate thrust system, (2) the Baicheng-Kuche fold and thrust system, (3) the Korla right-slip transfer system, and (4) the Lop-Nor thrust system. The westernmost Kashi-Aksu system is characterized by the occurrence of evenly spaced (12–15 km) imbricate thrusts. The Baicheng-Kuche and Korla systems are expressed by a major north dipping thrust (the Kuche thrust) that changes its strike eastward to become a NW striking oblique thrust ramp (the Korla transfer zone). The Lop Nor system in the eastern-most part of the southern Chinese Tian Shan consists of widely spaced thrusts, all involved with basement rocks. Geologic mapping and cross-section construction suggest that at least 20–40 km of crustal shortening with a horizontal shortening strain of 20–30% has occurred in the southern Chinese Tian Shan during the late Cenozoic. These estimates are minimum because of both conservative extrapolation of the thrust geometries and partial coverage of the thrust belt by the cross sections. The timing of initial thrusting is best constrained in the Kuche basin where crustal shortening may have occurred at 21–24 Ma, the time of a major facies transition between lacustrine and braided-fluvial sequences constrained in general by biostratigraphy and in detail by magnetostratigraphy. This estimate represents only a minimum age, as development of thrusts in the southern Chinese Tian Shan may have propagated southward toward the foreland. Thus the sedimentary record only represents the southernmost and therefore youngest phase of thrusting. If our estimate of timing for the thrust initiation (21–24 Ma) is correct, using the estimated magnitude of shortening (20–40 km) and shortening strain (20–30%), the averaged rates of late Cenozoic horizontal slip and shortening strain are 1–1.9 mm yr−1 and 2.9–4.5 × 10−16 s−1, respectively. Our reconnaissance 40Ar/39Ar thermochronological analysis in conjunction with earlier published results of apatite fission track analysis by other workers in the Chinese Tian Shan suggests that the magnitude of Cenozoic denudation is no more than 10 km, most likely less than 5 km. We demonstrate via a simple Airy-isostasy model that when the thermal effect on changes in surface elevation is negligible, determination of the spatial distribution and temporal variation of both horizontal shortening strain and denudation becomes a key to reconstructing the elevation history of the Tian Shan. Using this simple model, the loosely constrained magnitude of crustal-shortening strain and denudation in the southern Chinese Tian Shan implies that it may have been elevated 1.0–2.0 km since the onset of Cenozoic thrusting.

Rutile-aqueous fluid partitioning of Nb, Ta, Hf, Zr, U and Th: implications for high field strength element depletions in island-arc basalts

To assess the possible role of residual rutile in the retention of high field strength elements (HFSEs) during dehydration of a subducting slab we have measured rutile/aqueous fluid partition coefficients (Drutfl) for Nb, Ta, Hf, Zr, U and Th at 1–2 GPa and 900–1100°C. Partition coefficients for Nb, Ta, Hf and Zr are all in excess of 100 at 900°C and 1.0 GPa and values become larger with increasing pressure or decreasing temperature. Partition coefficients for U6+ and Th are lower ( ∼ 2 and 2000 times, respectively) than those for the HFSEs, and trends in Drutfl) with ionic radius indicate that cations with a large radius (i.e., > 0.8 A), including U4+, will have partition coefficients < 1. Results indicate that rutile will therefore selectively deplete coexisting fluids in HFSEs relative to large ion lithophile elements (LILEs). Calculations using these partition coefficients show that only small amounts of residual rutile ( ∼ 0.2wt%) are required to prevent HFSE enrichment of the mantle wedge by fluids derived from either pelagic sediments or the basaltic portion of the subducting slab. In addition, the measured HFSE concentrations of rutiles from eclogite-facies oceanic gabbros from the Rocciavre Massif (Western Alps) indicate that fluids that may have equilibrated with such rocks are strongly depleted in HFSEs and therefore would have no capacity to alter the HFSE content of the subarc mantle. We conclude that fluids derived from slab dehydration can be sufficiently depleted in HFSEs that subsequent enrichment of these elements in the mantle wedge does not occur. Uncertainty still remains, however, with regard to the capacity of such fluids to achieve the necessary enrichments in LILEs inferred for the subarc mantle.

Tertiary structural evolution of the Gangdese Thrust System, southeastern Tibet

For providing a square end on a dye spring centre of the type comprising a helical main spring having a wire lacing the turns of which extend between the adjacent helices of the main spring, the invention provides an end adapter defining an annular channel which fits on the terminal helix of the dye spring centre and in which are protrusions of different heights to engage the terminal helix and hold the adapter square.

A Late Miocene-Pliocene origin for the Central Himalayan inverted metamorphism

Abstract Perhaps the best known occurrence of an inverted metamorphic sequence is that found immediately beneath the Himalayan Main Central Thrust (MCT), generally thought to have been active during the Early Miocene. However, in situ 208 Pb/ 232 Th dating of monazite inclusions in garnet indicates that peak metamorphic recrystallization of the MCT footwall occurred in this portion of the central Himalaya at only ca. 6 Ma. The apparent inverted metamorphism appears to have resulted from activation of a broad shear zone beneath the MCT which tectonically telescoped the young metamorphic sequence. This explanation may resolve some outstanding problems in Himalayan tectonics, such why the MCT and not the more recently initiated thrusts marks the break in slope of the present day mountain range. It also renders unnecessary the need for exceptional physical conditions (e.g., high shear stress) to explain the generation of the Himalayan leucogranites.

Geochronologic and thermobarometric constraints on the evolution of the Main Central Thrust, central Nepal Himalaya

The Main Central Thrust (MCT) juxtaposes the high-grade Greater Himalayan Crystallines over the lower-grade Lesser Himalaya Formation; an apparent inverted metamorphic sequence characterizes the shear zone that underlies the thrust. Garnet-bearing assemblages sampled along the Marysandi River and Darondi Khola in the Annapurna region of central Nepal show striking differences in garnet zoning of Mn, Ca, Mg, and Fe above and below the MCT. Thermobarometry of MCT footwall rocks yields apparent inverted temperature and pressure gradients of ∼18°C km−1 and ∼0.06 km MPa−1, respectively. Pressure-temperature (P-T) paths calculated for upper Lesser Himalaya samples that preserve prograde compositions show evidence of decompression during heating, whereas garnets from the structurally lower sequences grew during an increase in both pressure and temperature. In situ (i.e., analyzed in thin section) ion microprobe ages of monazites from rocks immediately beneath the Greater Himalayan Crystallines yield ages from 18 to 22 Ma, whereas late Miocene and Pliocene monazite ages characterize rocks within the apparent inverted metamorphic sequence. A Lesser Himalayan sample collected near the garnet isograd along the Marysandi River transect contains 3.3±0.1 Ma monazite ages (P ≈ 0.72 GPa, T ≈ 535°C). This remarkably young age suggests that this portion of the MCT shear zone accommodated a minimum of ∼30 km of slip over the last 3 Ma (i.e., a slip rate of >10 mm yr−1) and thus could account for nearly half of the convergence across the Himalaya in this period. The distribution of ages and P-T histories reported here are consistent with a thermokinematic model in which the inverted metamorphic sequences underlying the MCT formed by the transposition of right-way-up metamorphic sequences during late Miocene-Pliocene shearing.

Seasonal stable isotope evidence for a strong Asian monsoon throughout the past 10.7 m.y

O of wet-season rainfall was significantly morenegative (29.5‰ SMOW) prior to 7.5 Ma than after ( 26.5‰SMOW). If this change is attributable to a lessening of the amounteffect in rainfall, this agrees with floral and soil geochemical datathat indicate increasing aridity beginning at 7.5 Ma.Keywords: Tibetan Plateau, monsoon, stable isotopes, paleohydrology,seasonality.INTRODUCTIONThe Tibetan Plateau is the engine that drives the modern Asianmonsoon by generating a high-altitude region of low pressure in thesummer as the plateau heats, and a region of high pressure in the winteras the plateau cools (Hastenrath, 1991). During the summer, warm airrises from the plateau, pulling moist air off the ocean, across the Indiansubcontinent, and into the highlands; this results in heavy summer rain-fall on the subcontinent. The opposite occurs in the winter, resultingin cold dry air spilling off the plateau and effectively excluding rainfrom the subcontinent. Thus, the presence of a strong wet-season–dry-season alternation implies the presence of a plateau broad and highenough to drive the monsoon.The timing of the uplift of the plateau remains a matter of con-siderable debate because there are few direct indicators of paleotopog-raphy in the geologic record. Consequently, past workers in Tibet, re-lying on indirect indicators of uplift, have proposed dates ranging from40 to 3.4 Ma, on the basis of initiation of potassic volcanism (Chunget al., 1998; Turner et al., 1993) or extension on the plateau (Harrisonet al., 1995; Coleman and Hodges, 1995), changes in marine sedimen-tation rates (Burbank et al., 1993), sediment types (Rea et al., 1998),or biota (Nigrini and Caulet, 1992; Kroon et al., 1991), and changesin stable carbon isotope and palynological patterns on the Indian sub-continent (Quade et al., 1989; Chen, 1981). Although different areasof the plateau may have risen at different times, many workers haveinferred rapid simultaneous uplift of large areas of the plateau at 7–8Ma by a process such as lithospheric delamination (Molnar et al.,1993). This inference was based on the following approximately coevalphenomena: a major change in plant communities of the Indian sub-continent (Quade et al., 1989), and shifts in marine upwelling patternsthat are linked to an intense monsoon (Kroon et al., 1991). Althoughthere is strong evidence for significant climate change at 7–8 Ma, it isunclear whether this is the onset of the monsoon. The floral transitionseems to have been a global rather than local phenomenon (Cerling etal., 1997) and monsoonally driven upwelling may have already beenpresent by 10–12 Ma (Nigrini and Caulet, 1992; Kroon et al., 1991).Because there is an intimate association between the intense sea-sonality of the modern monsoon and a high Tibetan Plateau and be-cause evaporation can be unambiguously recognized in the d

Tertiary deformation history of southeastern and southwestern Tibet during the Indo-Asian collision

Geologic mapping and geochronological analysis in southwest (Kailas area) and southeast (Zedong area) Tibet reveal two major episodes of Tertiary crustal shortening along the classic Indus-Tsangpo suture in the Yalu River valley. The older event occurred between ca. 30 and 24 Ma during movement along the north-dipping Gangdese thrust. The development of this thrust caused extensive denudation of the Gangdese batholith in its hanging wall and underthrusting of the Xigaze forearc strata in its footwall. Examination of timing of major tectonic events in central Asia suggests that the initiation of the Gangdese thrust was approximately coeval with the late Oligocene initiation and development of north-south shortening in the eastern Kunlun Shan of northern Tibet, the Nan Shan at the northeastern end of the Altyn Tagh fault, the western Kunlun Shan at the southwestern end of the Altyn Tagh fault, and finally the Tian Shan (north of the Tarim basin). Such regionally synchronous initiation of crustal shortening in and around the plateau may have been related to changes in convergence rate and direction between the Eurasian plate and the Indian and Pacific plates. The younger thrusting event along the Yalu River valley occurred between 19 and 10 Ma along the south-dipping Great Counter thrust system, equivalent to the locally named Renbu-Zedong thrust in southeastern Tibet, the Backthrust system in south- central Tibet, and the South Kailas thrust in southwest Tibet. The coeval development of the Great Counter thrust and the North Himalayan granite-gneiss dome belt is consistent with their development being related to thermal weakening of the north Himalayan and south Tibetan crust, due perhaps to thermal relaxation of an already thickened crust created by the early phase of collision between India and Asia or frictional heating along major thrusts, such as the Main Central thrust, beneath the Himalaya.