We have used a computer model (TRACES) to simulate low pressure differentiation of natural basalt... more We have used a computer model (TRACES) to simulate low pressure differentiation of natural basaltic magmas in an attempt to investigate the chemical dynamics of open system magmatic processes. Our results, in the form of simulated liquid lines of descent and the calculated equilibrium mineralogy, were determined for perfect fractional crystallization; fractionation paired with recharge and eruption (PRF); fractionation paired with assimilation (AFC); and fractionation paired with recharge, eruption, and assimilation (FEAR). These simulations were calculated in an attempt to assess the effects of combinations of petrogenetic processes on major and trace element evolution of natural systems and to test techniques that have been used to decipher the relative roles of these processes. If the results of PRF calculations are interpreted in terms of a mass balance based fractionation model (e.g., BRYAN et al., 1969), it is possible to generate low residuals even if one assumes that fractional crystallization was the only active process. In effect, the chemical consequences of recharge are invisible to mass balance models. Pearce element ratio analyses, however, can effectively discern the effects of PRF versus simple fractionation. The fractionating mineral proportions, and therefore, bulk distribution coefficients (6) of a differentiating system are dependent on the recharge or assimilation rate. Comparison of the results of simulations assuming constant d with the results calculated by TRACE5 show that the steady state liquid concentrations of some elements can differ by a factor of 2 to 5. If the PRF simulation is periodic, with episodes of mixing separated by intervals of fractionation, parallel liquidus mineral control lines are produced. Most of these control lines do not project back to the parental composition. This must be an important consideration when attempting to calculate a potential parental magma for any natural suite where magma chamber recharge has occurred. Most basaltic magmas cannot evolve to high silica compositions without magnetite fractionation. Small amounts of rhyolite assimilation (assimilation/fract.ionation < 0.1). however, can drive evolving bar&s to more silica rich compositions. If mass balance models are used to interpret these synthetic AFC data, low residuals are obtained if magnetite is added to the crystallizing assemblage. This approach works even for cases where magnetite was not a fractionating phase. Thus, the mass balance results are mathematically correct, but are geologically irrelevant.
An updated model for pyroxene-melt equilibria at 1 atm has been developed and calibrated using ne... more An updated model for pyroxene-melt equilibria at 1 atm has been developed and calibrated using new and existing experimental data in order to refine calculations of liquid lines of descent, which simulate the effect of igneous differentiation processes. We combine the Davidson and Lindsley (1985) model for activities of components in clinopyroxene and orthopyroxene solid solutions, al', where i represents a quadrilateral endmember, with the Nielsen and Drake (1979) expressions for component activities in the melt, a~ (two-lattice melt model). The chemical potential differences for pyroxene-melt equilibria are expressed in the form:
Plagioclase ultraphyric basalts (PUB) are a class of Mid-Ocean Ridge (MOR) lavas found in a varie... more Plagioclase ultraphyric basalts (PUB) are a class of Mid-Ocean Ridge (MOR) lavas found in a variety of ocean floor environments, are characterized by abundant (15-40 volume %) plagioclase megacrysts and a diverse trace element and isotopic signature. Paradoxically, we never see lavas erupted on the seafloor that are in equilibrium with these PUB megacrysts. Based on petrographic evidence, melt inclusion composition, and new data on depth of entrapment calculated from CO2 contents in plagioclase-hosted inclusions, many of the megacrysts formed at upper mantle pressures (~3-7 kbars). To constrain the composition of the parent magmas of the plagioclase megacrysts, we conducted a series of experiments at 5 and 10 kbars using MORB glasses as starting materials. The experimental results were consistent with the presence of a pseudoazeotrope in the anorthitic segment of the plagioclase + basalt pseudobinary. This has the effect of dropping the anorthitic end of the feldspar loop, lowering ...
We present a database detailing the volumetric evolution of plagioclase-hosted melt inclusions du... more We present a database detailing the volumetric evolution of plagioclase-hosted melt inclusions during incremental heating on a heating stage. The heating experiments were designed to induce volumetric re-equilibration. At ~40°C above the temperature at which the daughter minerals melted, irreversible volume expansion lowered the pressure in the melt inclusions, and led to exsolution of CO2 into vapor bubbles. With each additional few degrees of heating, additional episodes of CO2 exsolution, bubble nucleation and expansion of the vapor bubbles occurred. Re-equilibration of melt inclusions in plagioclase occurred through a combination of ductile and brittle deformation of the host surrounding the nelt inclusions. The volumetric evolution of the vapor bubbles and melt inclusions were estimated from both microtomography (3D) and photographs (2D).
This dataset presents compiled experimental mineral/melt partitioning data for orthopyroxene from... more This dataset presents compiled experimental mineral/melt partitioning data for orthopyroxene from the literature (e.g. no unpublished collections). This dataset can help users to calculate differentiation scenarios (by sorting for compositions that match the system of interest), test models or create calibration datasets for models, and facilitate experimental design and data management plans. The format of the data is oriented vertically – each column represents the data from one published experiment – including experimental conditions, source publication, analytical techniques and the composition of the starting composition, liquid, mineral or fluid and the partition coefficient for the elements analyzed. This format is inclusive, and therefore complex, and care should be taken in processing the data from this raw form. Please see the "Readme" tab for further explanations and instructions. This data is presented in the traceDs template, a format for documenting experimen...
This dataset presents compiled experimental mineral/melt partitioning data for garnet from the li... more This dataset presents compiled experimental mineral/melt partitioning data for garnet from the literature (e.g. no unpublished collections). This dataset can help users to calculate differentiation scenarios (by sorting for compositions that match the system of interest), test models or create calibration datasets for models, and facilitate experimental design and data management plans. The format of the data is oriented vertically – each column represents the data from one published experiment – including experimental conditions, source publication, analytical techniques and the composition of the starting composition, liquid, mineral or fluid and the partition coefficient for the elements analyzed. This format is inclusive, and therefore complex, and care should be taken in processing the data from this raw form. Please see the "Readme" tab for further explanations and instructions. This data is presented in the traceDs template, a format for documenting experimental dat...
This dataset presents compiled experimental mineral/melt partitioning data for spinel from the li... more This dataset presents compiled experimental mineral/melt partitioning data for spinel from the literature (e.g. no unpublished collections). This dataset can help users to calculate differentiation scenarios (by sorting for compositions that match the system of interest), test models or create calibration datasets for models, and facilitate experimental design and data management plans. The format of the data is oriented vertically – each column represents the data from one published experiment – including experimental conditions, source publication, analytical techniques and the composition of the starting composition, liquid, mineral or fluid and the partition coefficient for the elements analyzed. This format is inclusive, and therefore complex, and care should be taken in processing the data from this raw form. Please see the "Readme" tab for further explanations and instructions. This data is presented in the traceDs template, a format for documenting experimental dat...
This dataset presents compiled experimental mineral/melt partitioning data for amphibole from the... more This dataset presents compiled experimental mineral/melt partitioning data for amphibole from the literature (e.g. no unpublished collections). This dataset can help users to calculate differentiation scenarios (by sorting for compositions that match the system of interest), test models or create calibration datasets for models, and facilitate experimental design and data management plans. The format of the data is oriented vertically – each column represents the data from one published experiment – including experimental conditions, source publication, analytical techniques and the composition of the starting composition, liquid, mineral or fluid and the partition coefficient for the elements analyzed. This format is inclusive, and therefore complex, and care should be taken in processing the data from this raw form. Please see the "Readme" tab for further explanations and instructions. This data is presented in the traceDs template, a format for documenting experimental ...
This dataset presents compiled experimental mineral/melt partitioning data for olivine from the l... more This dataset presents compiled experimental mineral/melt partitioning data for olivine from the literature (e.g. no unpublished collections). This dataset can help users to calculate differentiation scenarios (by sorting for compositions that match the system of interest), test models or create calibration datasets for models, and facilitate experimental design and data management plans. The format of the data is oriented vertically – each column represents the data from one published experiment – including experimental conditions, source publication, analytical techniques and the composition of the starting composition, liquid, mineral or fluid and the partition coefficient for the elements analyzed. This format is inclusive, and therefore complex, and care should be taken in processing the data from this raw form. Please see the "Readme" tab for further explanations and instructions. This data is presented in the traceDs template, a format for documenting experimental da...
This dataset presents compiled experimental mineral/melt partitioning data for plagioclase from t... more This dataset presents compiled experimental mineral/melt partitioning data for plagioclase from the literature (e.g. no unpublished collections). This dataset can help users to calculate differentiation scenarios (by sorting for compositions that match the system of interest), test models or create calibration datasets for models, and facilitate experimental design and data management plans. The format of the data is oriented vertically – each column represents the data from one published experiment – including experimental conditions, source publication, analytical techniques and the composition of the starting composition, liquid, mineral or fluid and the partition coefficient for the elements analyzed. This format is inclusive, and therefore complex, and care should be taken in processing the data from this raw form. Please see the "Readme" tab for further explanations and instructions. This data is presented in the traceDs template, a format for documenting experimenta...
This dataset presents compiled experimental mineral/melt partitioning data for clinopyroxene from... more This dataset presents compiled experimental mineral/melt partitioning data for clinopyroxene from the literature (e.g. no unpublished collections). This dataset can help users to calculate differentiation scenarios (by sorting for compositions that match the system of interest), test models or create calibration datasets for models, and facilitate experimental design and data management plans. The format of the data is oriented vertically – each column represents the data from one published experiment – including experimental conditions, source publication, analytical techniques and the composition of the starting composition, liquid, mineral or fluid and the partition coefficient for the elements analyzed. This format is inclusive, and therefore complex, and care should be taken in processing the data from this raw form. Please see the "Readme" tab for further explanations and instructions. This data is presented in the traceDs template, a format for documenting experimen...
We present a database characterizing the composition and volatile contents of plagioclase-hosted ... more We present a database characterizing the composition and volatile contents of plagioclase-hosted melt inclusions from a unique sample from the Blanco Transform Fault (A91-1R). The melt inclusions were homogenized on a 1-atm vertical furnace at two temperatures from 30 min to 4 days to gain a better understanding of the post entrapment processes induced by the homogenization experiments. The composition and volatile contents of the melt inclusions indicate their mantle origins.
This dataset contains major and trace element geochemistry of MORB Plagioclases Ultraphyric Basal... more This dataset contains major and trace element geochemistry of MORB Plagioclases Ultraphyric Basalts (PUB) whole rock samples, plagioclase megacrysts and melt inclusions hosted within plagioclase.
ABSTRACT ABSTRACT At mid-ocean ridges, a variety of crustal processes overprint mantle-derived me... more ABSTRACT ABSTRACT At mid-ocean ridges, a variety of crustal processes overprint mantle-derived melts and can obscure original mantle compositions. To address the nature of this crustal fi lter, we report 87Sr/86Sr ratios from plagioclase phenocrysts and host glasses in mid-oceanic-ridge basalts from the Juan de Fuca Ridge, Blanco Transform Zone, and the Southwest Indian Ridge. Microdrilled isotopic analyses reveal signifi cant inter- and intracrystalline disequilibria within individual samples. These disequilibria suggest that a range of isotopically distinct melt components contribute to individual plagioclase crystals and to the magmas that transport them to the surface. Low Cl/K values both in the host glass and in plagioclase-hosted melt inclusions largely rule out incorporation of seawater-derived material as an explanation for differences in 87Sr/86Sr. Instead, the observed heterogeneity implies derivation of magmas from isotopically diverse mantle sources. Importantly, the range of Sr isotope values preserved in a single sample is similar to the range of compositions seen at the ridge segment scale. Unlike analyses of host glass compositions, which are the result of extensive crustal processing, isotopic analyses of phenocryst phases record fi ne-scale aggregation of these distinct mantlederived melts and are thus an important and underutilized tool in interpreting the nature of the mid-oceanic-ridge basalts.
Plagioclase ultraphyric basalts (PUBs) have been sampled along most mid-ocean ridges with ultrasl... more Plagioclase ultraphyric basalts (PUBs) have been sampled along most mid-ocean ridges with ultraslow to intermediate spreading rates. Over the past 40 years, the prevalent models for their origin assume positive buoyancy of plagioclase in basaltic liquids resulting in selective concentration of plagioclase phenocrysts by floatation. However, when the global population of PUB lavas is examined, this hypothesis becomes less compelling. PUB host lavas demonstrate a large range of compositions and densities, similar to aphyric glasses from the same ridge segments. Most importantly, the majority of PUB host liquids are less dense than their phenocryst cargo, meaning that plagioclase floatation within a magma chamber cannot be the driving force for phenocryst enrichment. Furthermore, PUB lavas have never been sampled on axis at fast-spreading centers or from locations with noted contemporaneous axial magma chambers, where PUBs should be abundant if plagioclase is buoyant in mid-ocean ridge basalt (MORB). Instead, we argue that the high modal abundance of plagioclase results from interaction between magma and preexisting zones of crystal cumulates within the lower crust, possibly followed by loss of olivine during magma ascent. PUBs erupt when the magma maintains an ascent velocity greater than the settling rate of the plagioclase phenocrysts, which precludes long crustal residence times for these magmas. In addition to being a proxy for lower spreading rates, our findings also suggest that PUB eruption can also be used as a proxy for the absence of a magma chamber or transport through a conduit system that bypassed an axial chamber.
Our current understanding of trace element behavior is in great part dependent on the experimenta... more Our current understanding of trace element behavior is in great part dependent on the experimental database that defines the trace element constraints for geologically significant systems. The accuracy of that database is dependent on a number of factors, including analytical precision, kinetics, and the relevance of the experiments to the specific system of interest. One of the cross cutting issues in this field relates to the use of dopants (added components) in trace element experiments. In designing such experiments, we must balance the improved analytical precision gained by addition of elements of interest against the concern that those added components will affect the behavior of the system. One of the most important of those effects relates to the Henry's Law limit - a concentration level at which there is an inflection point in the activity-composition behavior. Many previous investigations have run experimental tests designed to define the Henry's Law limit in diff...
We have used a computer model (TRACES) to simulate low pressure differentiation of natural basalt... more We have used a computer model (TRACES) to simulate low pressure differentiation of natural basaltic magmas in an attempt to investigate the chemical dynamics of open system magmatic processes. Our results, in the form of simulated liquid lines of descent and the calculated equilibrium mineralogy, were determined for perfect fractional crystallization; fractionation paired with recharge and eruption (PRF); fractionation paired with assimilation (AFC); and fractionation paired with recharge, eruption, and assimilation (FEAR). These simulations were calculated in an attempt to assess the effects of combinations of petrogenetic processes on major and trace element evolution of natural systems and to test techniques that have been used to decipher the relative roles of these processes. If the results of PRF calculations are interpreted in terms of a mass balance based fractionation model (e.g., BRYAN et al., 1969), it is possible to generate low residuals even if one assumes that fractional crystallization was the only active process. In effect, the chemical consequences of recharge are invisible to mass balance models. Pearce element ratio analyses, however, can effectively discern the effects of PRF versus simple fractionation. The fractionating mineral proportions, and therefore, bulk distribution coefficients (6) of a differentiating system are dependent on the recharge or assimilation rate. Comparison of the results of simulations assuming constant d with the results calculated by TRACE5 show that the steady state liquid concentrations of some elements can differ by a factor of 2 to 5. If the PRF simulation is periodic, with episodes of mixing separated by intervals of fractionation, parallel liquidus mineral control lines are produced. Most of these control lines do not project back to the parental composition. This must be an important consideration when attempting to calculate a potential parental magma for any natural suite where magma chamber recharge has occurred. Most basaltic magmas cannot evolve to high silica compositions without magnetite fractionation. Small amounts of rhyolite assimilation (assimilation/fract.ionation < 0.1). however, can drive evolving bar&s to more silica rich compositions. If mass balance models are used to interpret these synthetic AFC data, low residuals are obtained if magnetite is added to the crystallizing assemblage. This approach works even for cases where magnetite was not a fractionating phase. Thus, the mass balance results are mathematically correct, but are geologically irrelevant.
An updated model for pyroxene-melt equilibria at 1 atm has been developed and calibrated using ne... more An updated model for pyroxene-melt equilibria at 1 atm has been developed and calibrated using new and existing experimental data in order to refine calculations of liquid lines of descent, which simulate the effect of igneous differentiation processes. We combine the Davidson and Lindsley (1985) model for activities of components in clinopyroxene and orthopyroxene solid solutions, al', where i represents a quadrilateral endmember, with the Nielsen and Drake (1979) expressions for component activities in the melt, a~ (two-lattice melt model). The chemical potential differences for pyroxene-melt equilibria are expressed in the form:
Plagioclase ultraphyric basalts (PUB) are a class of Mid-Ocean Ridge (MOR) lavas found in a varie... more Plagioclase ultraphyric basalts (PUB) are a class of Mid-Ocean Ridge (MOR) lavas found in a variety of ocean floor environments, are characterized by abundant (15-40 volume %) plagioclase megacrysts and a diverse trace element and isotopic signature. Paradoxically, we never see lavas erupted on the seafloor that are in equilibrium with these PUB megacrysts. Based on petrographic evidence, melt inclusion composition, and new data on depth of entrapment calculated from CO2 contents in plagioclase-hosted inclusions, many of the megacrysts formed at upper mantle pressures (~3-7 kbars). To constrain the composition of the parent magmas of the plagioclase megacrysts, we conducted a series of experiments at 5 and 10 kbars using MORB glasses as starting materials. The experimental results were consistent with the presence of a pseudoazeotrope in the anorthitic segment of the plagioclase + basalt pseudobinary. This has the effect of dropping the anorthitic end of the feldspar loop, lowering ...
We present a database detailing the volumetric evolution of plagioclase-hosted melt inclusions du... more We present a database detailing the volumetric evolution of plagioclase-hosted melt inclusions during incremental heating on a heating stage. The heating experiments were designed to induce volumetric re-equilibration. At ~40°C above the temperature at which the daughter minerals melted, irreversible volume expansion lowered the pressure in the melt inclusions, and led to exsolution of CO2 into vapor bubbles. With each additional few degrees of heating, additional episodes of CO2 exsolution, bubble nucleation and expansion of the vapor bubbles occurred. Re-equilibration of melt inclusions in plagioclase occurred through a combination of ductile and brittle deformation of the host surrounding the nelt inclusions. The volumetric evolution of the vapor bubbles and melt inclusions were estimated from both microtomography (3D) and photographs (2D).
This dataset presents compiled experimental mineral/melt partitioning data for orthopyroxene from... more This dataset presents compiled experimental mineral/melt partitioning data for orthopyroxene from the literature (e.g. no unpublished collections). This dataset can help users to calculate differentiation scenarios (by sorting for compositions that match the system of interest), test models or create calibration datasets for models, and facilitate experimental design and data management plans. The format of the data is oriented vertically – each column represents the data from one published experiment – including experimental conditions, source publication, analytical techniques and the composition of the starting composition, liquid, mineral or fluid and the partition coefficient for the elements analyzed. This format is inclusive, and therefore complex, and care should be taken in processing the data from this raw form. Please see the "Readme" tab for further explanations and instructions. This data is presented in the traceDs template, a format for documenting experimen...
This dataset presents compiled experimental mineral/melt partitioning data for garnet from the li... more This dataset presents compiled experimental mineral/melt partitioning data for garnet from the literature (e.g. no unpublished collections). This dataset can help users to calculate differentiation scenarios (by sorting for compositions that match the system of interest), test models or create calibration datasets for models, and facilitate experimental design and data management plans. The format of the data is oriented vertically – each column represents the data from one published experiment – including experimental conditions, source publication, analytical techniques and the composition of the starting composition, liquid, mineral or fluid and the partition coefficient for the elements analyzed. This format is inclusive, and therefore complex, and care should be taken in processing the data from this raw form. Please see the "Readme" tab for further explanations and instructions. This data is presented in the traceDs template, a format for documenting experimental dat...
This dataset presents compiled experimental mineral/melt partitioning data for spinel from the li... more This dataset presents compiled experimental mineral/melt partitioning data for spinel from the literature (e.g. no unpublished collections). This dataset can help users to calculate differentiation scenarios (by sorting for compositions that match the system of interest), test models or create calibration datasets for models, and facilitate experimental design and data management plans. The format of the data is oriented vertically – each column represents the data from one published experiment – including experimental conditions, source publication, analytical techniques and the composition of the starting composition, liquid, mineral or fluid and the partition coefficient for the elements analyzed. This format is inclusive, and therefore complex, and care should be taken in processing the data from this raw form. Please see the "Readme" tab for further explanations and instructions. This data is presented in the traceDs template, a format for documenting experimental dat...
This dataset presents compiled experimental mineral/melt partitioning data for amphibole from the... more This dataset presents compiled experimental mineral/melt partitioning data for amphibole from the literature (e.g. no unpublished collections). This dataset can help users to calculate differentiation scenarios (by sorting for compositions that match the system of interest), test models or create calibration datasets for models, and facilitate experimental design and data management plans. The format of the data is oriented vertically – each column represents the data from one published experiment – including experimental conditions, source publication, analytical techniques and the composition of the starting composition, liquid, mineral or fluid and the partition coefficient for the elements analyzed. This format is inclusive, and therefore complex, and care should be taken in processing the data from this raw form. Please see the "Readme" tab for further explanations and instructions. This data is presented in the traceDs template, a format for documenting experimental ...
This dataset presents compiled experimental mineral/melt partitioning data for olivine from the l... more This dataset presents compiled experimental mineral/melt partitioning data for olivine from the literature (e.g. no unpublished collections). This dataset can help users to calculate differentiation scenarios (by sorting for compositions that match the system of interest), test models or create calibration datasets for models, and facilitate experimental design and data management plans. The format of the data is oriented vertically – each column represents the data from one published experiment – including experimental conditions, source publication, analytical techniques and the composition of the starting composition, liquid, mineral or fluid and the partition coefficient for the elements analyzed. This format is inclusive, and therefore complex, and care should be taken in processing the data from this raw form. Please see the "Readme" tab for further explanations and instructions. This data is presented in the traceDs template, a format for documenting experimental da...
This dataset presents compiled experimental mineral/melt partitioning data for plagioclase from t... more This dataset presents compiled experimental mineral/melt partitioning data for plagioclase from the literature (e.g. no unpublished collections). This dataset can help users to calculate differentiation scenarios (by sorting for compositions that match the system of interest), test models or create calibration datasets for models, and facilitate experimental design and data management plans. The format of the data is oriented vertically – each column represents the data from one published experiment – including experimental conditions, source publication, analytical techniques and the composition of the starting composition, liquid, mineral or fluid and the partition coefficient for the elements analyzed. This format is inclusive, and therefore complex, and care should be taken in processing the data from this raw form. Please see the "Readme" tab for further explanations and instructions. This data is presented in the traceDs template, a format for documenting experimenta...
This dataset presents compiled experimental mineral/melt partitioning data for clinopyroxene from... more This dataset presents compiled experimental mineral/melt partitioning data for clinopyroxene from the literature (e.g. no unpublished collections). This dataset can help users to calculate differentiation scenarios (by sorting for compositions that match the system of interest), test models or create calibration datasets for models, and facilitate experimental design and data management plans. The format of the data is oriented vertically – each column represents the data from one published experiment – including experimental conditions, source publication, analytical techniques and the composition of the starting composition, liquid, mineral or fluid and the partition coefficient for the elements analyzed. This format is inclusive, and therefore complex, and care should be taken in processing the data from this raw form. Please see the "Readme" tab for further explanations and instructions. This data is presented in the traceDs template, a format for documenting experimen...
We present a database characterizing the composition and volatile contents of plagioclase-hosted ... more We present a database characterizing the composition and volatile contents of plagioclase-hosted melt inclusions from a unique sample from the Blanco Transform Fault (A91-1R). The melt inclusions were homogenized on a 1-atm vertical furnace at two temperatures from 30 min to 4 days to gain a better understanding of the post entrapment processes induced by the homogenization experiments. The composition and volatile contents of the melt inclusions indicate their mantle origins.
This dataset contains major and trace element geochemistry of MORB Plagioclases Ultraphyric Basal... more This dataset contains major and trace element geochemistry of MORB Plagioclases Ultraphyric Basalts (PUB) whole rock samples, plagioclase megacrysts and melt inclusions hosted within plagioclase.
ABSTRACT ABSTRACT At mid-ocean ridges, a variety of crustal processes overprint mantle-derived me... more ABSTRACT ABSTRACT At mid-ocean ridges, a variety of crustal processes overprint mantle-derived melts and can obscure original mantle compositions. To address the nature of this crustal fi lter, we report 87Sr/86Sr ratios from plagioclase phenocrysts and host glasses in mid-oceanic-ridge basalts from the Juan de Fuca Ridge, Blanco Transform Zone, and the Southwest Indian Ridge. Microdrilled isotopic analyses reveal signifi cant inter- and intracrystalline disequilibria within individual samples. These disequilibria suggest that a range of isotopically distinct melt components contribute to individual plagioclase crystals and to the magmas that transport them to the surface. Low Cl/K values both in the host glass and in plagioclase-hosted melt inclusions largely rule out incorporation of seawater-derived material as an explanation for differences in 87Sr/86Sr. Instead, the observed heterogeneity implies derivation of magmas from isotopically diverse mantle sources. Importantly, the range of Sr isotope values preserved in a single sample is similar to the range of compositions seen at the ridge segment scale. Unlike analyses of host glass compositions, which are the result of extensive crustal processing, isotopic analyses of phenocryst phases record fi ne-scale aggregation of these distinct mantlederived melts and are thus an important and underutilized tool in interpreting the nature of the mid-oceanic-ridge basalts.
Plagioclase ultraphyric basalts (PUBs) have been sampled along most mid-ocean ridges with ultrasl... more Plagioclase ultraphyric basalts (PUBs) have been sampled along most mid-ocean ridges with ultraslow to intermediate spreading rates. Over the past 40 years, the prevalent models for their origin assume positive buoyancy of plagioclase in basaltic liquids resulting in selective concentration of plagioclase phenocrysts by floatation. However, when the global population of PUB lavas is examined, this hypothesis becomes less compelling. PUB host lavas demonstrate a large range of compositions and densities, similar to aphyric glasses from the same ridge segments. Most importantly, the majority of PUB host liquids are less dense than their phenocryst cargo, meaning that plagioclase floatation within a magma chamber cannot be the driving force for phenocryst enrichment. Furthermore, PUB lavas have never been sampled on axis at fast-spreading centers or from locations with noted contemporaneous axial magma chambers, where PUBs should be abundant if plagioclase is buoyant in mid-ocean ridge basalt (MORB). Instead, we argue that the high modal abundance of plagioclase results from interaction between magma and preexisting zones of crystal cumulates within the lower crust, possibly followed by loss of olivine during magma ascent. PUBs erupt when the magma maintains an ascent velocity greater than the settling rate of the plagioclase phenocrysts, which precludes long crustal residence times for these magmas. In addition to being a proxy for lower spreading rates, our findings also suggest that PUB eruption can also be used as a proxy for the absence of a magma chamber or transport through a conduit system that bypassed an axial chamber.
Our current understanding of trace element behavior is in great part dependent on the experimenta... more Our current understanding of trace element behavior is in great part dependent on the experimental database that defines the trace element constraints for geologically significant systems. The accuracy of that database is dependent on a number of factors, including analytical precision, kinetics, and the relevance of the experiments to the specific system of interest. One of the cross cutting issues in this field relates to the use of dopants (added components) in trace element experiments. In designing such experiments, we must balance the improved analytical precision gained by addition of elements of interest against the concern that those added components will affect the behavior of the system. One of the most important of those effects relates to the Henry's Law limit - a concentration level at which there is an inflection point in the activity-composition behavior. Many previous investigations have run experimental tests designed to define the Henry's Law limit in diff...
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