Model Endmember Research Articles

Lutetium incorporation in magmas at depth: Changes in melt local environment and the influence on partitioning behaviour

The structure of two Lu doped (4000 ppm) model end member silicate liquids, a highly polymerised haplogranite (Si–Al–Na–K–O) and a less polymerised anorthite–diopside (Si–Al–Mg–Ca–O), have been studied up to 8 GPa using in situ x-ray diffraction techniques. The results are the first to identify trace rare Earth element incorporation in silicate melts at high pressure. At pressures below 5 GPa, the bonding environment of Lu–O was found to be dependent on composition with coordination number CNLu–O=8 and bond distance rLu–O=2.36 Å in the haplogranite melt, decreasing to CNLu–O=6 and rLu–O=2.29 Å in the anorthite–diopside melt. This compositional variance in coordination number at low pressure is consistent with observations made for Y–O in glasses at ambient conditions and is coincident with a dramatic increase in the partition coefficients previously observed for rare Earth elements with increasing melt polymerisation. With increasing pressure we find that CNLu–O and rLu–O remain constant in the haplogranite melt. However, an abrupt change in both Lu–O coordination and bond distance is observed at 5 GPa in the anorthite–diopside melt, with CNLu–O increasing from 6 to 8-fold and rLu–O from 2.29 to 2.39 Å. This occurs over a similar pressure range where a change in the P-dependence in the reported rare Earth element partition coefficients is observed for garnet–, clinopyroxene–, and olivine–melt systems. This work shows that standard models for predicting trace elements at depth must incorporate the effect of pressure-induced structural transformations in the melt in order to realistically predict partitioning behaviour.

Nearshore along-strike variability: Is the concept of the systems tract unhinged?

Sequence stratigraphic models assume that nearshore strata have relatively consistent and laterally persistent stacking at the systems tract scale and therefore may not fully describe the three-dimensional stratigraphic architecture in areas displaying marked nearshore along-strike variability. A stratigraphic model of nearshore deposits is presented that corrects for this assumption by explaining variations in along-strike stratal geometries in terms of a systematic change in the orientation of a shoreline trend or clinoform rollover, a scenario comparable to deflection around a hinge. The model defines hinge zones that are both fixed and moving with respect to time, and was created from outcrop, well-log, and seismic reflection data. Model end members predict contemporaneous progradational, aggradational, and retrogradational stacking bounded by surfaces displaying significant along-strike changes in architecture, implying that sequence stratigraphic surfaces can be diachronous. We advocate examining the impact of stratigraphic variations parallel to depositional strike by testing for the presence of uniform versus differential progradation, phenomena responsible for creating unhinged and hinged nearshore depositional systems, respectively. Understanding these differences will improve subsurface predictions and provide a more complete understanding of the stratigraphic evolution of sedimentary basins.

Recharge mixing in a complex distributary spring system in the Missouri Ozarks, USA

Toronto Springs is a complex distributary karst spring system with 11 perennial springs in the Missouri Ozarks, USA. Carroll Cave (CC) and Wet Glaize Creek (WG) were previously identified as principal recharge sources. This study (1) characterized physical and chemical properties of springs and recharge sources; (2) developed end-member mixing models to estimate contributing proportions of CC and WG; and (3) created a conceptual model for the system. Samples analyzed for major ions and specific conductivity, in conjunction with a rotating continuous monitoring program to identify statistically comparable baseflow conditions, were used to assess differences among the sites. Monitoring data showed that the springs differed depending upon recharge proportions. Cluster analysis of average ion concentrations supported the choice of CC and WG as mixing model end members. Results showed a range in the proportions of the recharge sources, from surface-water to groundwater dominated. A conceptual model suggests that a system of distinct conduits beneath the WG flood plain transmits water to the individual springs. These conduits controlled the end-member recharge contributions and water chemistry of the springs. Interpretation of relative proportions of recharge contributions extends existing knowledge of karst hydrologic geometry beyond that of point-to-point connections to revealing complex surface-water/groundwater mixing in heterogeneous distributary spring systems.

Landscape-level nitrogen import and export in an ecosystem with complex terrain, Colorado Front Range

Knowledge of import, export, and transport of nitrogen (N) in headwater catchments is essential for understanding ecosystem function and water quality in mountain ecosystems, especially as these ecosystems experience increased anthropogenic N deposition. In this study, we link spatially explicit soil and stream data at the landscape scale to investigate import, export and transport of N in a 0.89 km2 site at the alpine-subalpine ecotone in the Front Range of the Rocky Mountains, Colorado, U.S.A. For two of the major N inputs to our site, N deposition in the snowpack and N fixation, a complementary relationship was found across the study site, with greater abundance of N-fixing plants in areas with less snow and substantial snow inputs in areas with low N fixer abundance. During the initial phases of snowmelt, mixing model end members for oxygen isotopes in nitrate (NO3 −) indicated that a substantial quantity of NO3 − is transported downhill into the forested subalpine without being assimilated by soil microbes. After this initial pulse, much less NO3 − entered the stream and most but not all of it was microbial in origin. Rising δ15N in stream NO3 − indicated greater influence of fractionating processes such as denitrification later in the season. NO3 − from both atmospheric and microbial sources was not exported from our site because it was consumed within the first several hundred meters of the stream; ultimately, N exports were in the form of dissolved organic nitrogen (DON) and particulate N (PN). The results of this study suggest that the highest elevation dry alpine meadows rely more heavily on N fixation as an N source and experience less of the effects of anthropogenic N deposition than mid and lower elevation areas that have more snow. Our data also suggest that mid-elevation krummholz, moist meadows, and talus slopes are exporting N as NO3 − shortly after the onset of snowmelt, but that this NO3 − is rapidly consumed as the stream flows through the subalpine forest. This consumption by assimilation and/or denitrification currently provides a buffer against increased inorganic N availability downstream.

High-Ti and low-Ti mafic potassic magmas: Key to plume-lithosphere interactions and continental flood-basalt genesis

The role of mantle plumes in the genesis of continental flood-basalts (CFB) remains controversial, primarily due to our limited knowledge of the composition of the subcontinental lithospheric mantle (SCLM). In this study we use the widespread Cretaceous mafic potassic magmatic rocks, emplaced around the margins of the Paranásedimentary basin, to probe large-scale compositional variations of the SCLM beneath southern Brazil and Paraguay. On the basis of Ti contents, together with major-, trace-element and isotopic ratios, these mafic potassic rocks may be subdivided into high-Ti and low-Ti groups. The former have relatively high average TiO 2 (4.64), CaO/Al 2O 3 (1.74) and 143Nd/ 144Nd i (0.51232), together with low La/Nb (1.1) and 87Sr/ 86Sr i (0.7050). The latter are characterised by much lower average TiO 2 (1.77), CaO/Al 2O 3 (0.72) and 143Nd/ 144Nd i (0.51182), together with higher La/Nb (2.01) and 87Sr 86Sr i (0.7068). These high-Ti and low-Ti groups are spatially separate and their distribution correlates with tectonic setting; the low-Ti magmas are associated with cratonic regions, whereas the high-Ti magmas are in Proterozoic mobile belts. The distribution of the subgroups of mafic potassic magmatic rocks correlates closely with the geochemical provinciality of the Early Cretaceous high-Ti and low-Ti Paranáflood-basalts. This is the first reported occurrence of extensive low-Ti mafic potassic magmatism associated both spatially and temporally with the low-Ti region of a major CFB province. Our study further reveals similar relationships between tectonic setting and the geochemical provinciality of mafic potassic magmas and continental flood-basalts across Gondwana. We use the bulk-rock compositions and radiogenic isotopic ratios of both the high-Ti and low-Ti mafic potassic magmatic rocks as end members in models of CFB genesis. Mixing calculations involving Sr and Nd isotopic ratios indicate that the flood-basalts may contain up to 50% of mafic potassic lithosphere-derived melts. Overall, the results of our geochemical modelling agree with geophysical arguments that the convecting asthenosphere is the predominant source of CFB magmas.