Pub Date : 2023-09-01DOI: 10.1093/petrology/egad070
{"title":"Correction to: Arc–Backarc Exchange Along the Tonga–Lau System: Constraints From Volatile Elements","authors":"","doi":"10.1093/petrology/egad070","DOIUrl":"https://doi.org/10.1093/petrology/egad070","url":null,"abstract":"","PeriodicalId":16751,"journal":{"name":"Journal of Petrology","volume":"1 1","pages":"0"},"PeriodicalIF":0.0,"publicationDate":"2023-09-01","publicationTypes":"Journal Article","fieldsOfStudy":null,"isOpenAccess":false,"openAccessPdf":"","citationCount":null,"resultStr":null,"platform":"Semanticscholar","paperid":"135428964","PeriodicalName":null,"FirstCategoryId":null,"ListUrlMain":null,"RegionNum":2,"RegionCategory":"地球科学","ArticlePicture":[],"TitleCN":null,"AbstractTextCN":null,"PMCID":"","EPubDate":null,"PubModel":null,"JCR":null,"JCRName":null,"Score":null,"Total":0}
ABSTRACT The mechanism behind the destabilization of the North China Craton (NCC) remains a contentious topic among researchers. Large-scale Cretaceous adakitic magmatism in the NCC offers insights into the decratonization process. This study focuses on the Huashan and Laoniushan plutons located in the Lesser Qinling on the southern margin of the NCC and compiles published data for coeval adakitic rocks to investigate the role of water in adakitic rock petrogenesis during the peak destruction of NCC. Both the Huashan and Laoniushan plutons exhibit adakitic signatures, including high Sr (193–1080 ppm), low Yb (<14.8 ppm) and Y (<1.24 ppm) concentrations, as well as high Sr/Y (18–100) and La/Yb (24–58) ratios. The zircon Hf–O isotope compositions suggest that the primary source for the Huashan and Laoniushan plutons is the mafic lower crust of NCC. Nevertheless, there are significant differences in trace element characteristics between the two plutons. Specifically, the Huashan pluton displays higher Na2O/K2O ratios, lower levels of Rb, Rb/Sr, Nb, Ta content, and a weak Eu anomaly in comparison to the Laoniushan pluton. These variations in geochemical attributes cannot be accounted for by mechanisms like mantle-derived magma mixing, crustal contamination, or fractional crystallization processes. Instead, these disparities are attributed to distinct modes of crustal anatexis, involving both water-fluxed and dehydration melting. Subsequently, we conducted thermodynamic simulations of the melting process of mafic lower crust under different pressure (0.5–1.5 GPa) and water content conditions (1–3 wt.%). The simulation results suggest that the Huashan pluton is most likely formed through water-fluxed melting in a scenario with normal crustal thickness (1 GPa). On the other hand, the Laoniushan pluton might have originated from dehydration melting under normal crustal thickness and pressure conditions. Notably, high pressure (>1.5 GPa) is not necessary for the formation of intracontinental adakitic rocks. The release of water from metasomatized lithospheric mantle and subsequent hydration of the lower continental crust triggers extensive adakitic magmatism in the NCC. These findings emphasize the significance of deep water cycling in understanding large-scale magmatic events and illuminate the decratonization mechanism.
{"title":"Large-scale Cretaceous adakitic magmatism induced by water-fluxed melting of continental crust during the North China Craton destruction","authors":"Shuo Xue, Wanzhu Zhang, Ming-Xing Ling, Weidong Sun, Xing Ding","doi":"10.1093/petrology/egad066","DOIUrl":"https://doi.org/10.1093/petrology/egad066","url":null,"abstract":"ABSTRACT The mechanism behind the destabilization of the North China Craton (NCC) remains a contentious topic among researchers. Large-scale Cretaceous adakitic magmatism in the NCC offers insights into the decratonization process. This study focuses on the Huashan and Laoniushan plutons located in the Lesser Qinling on the southern margin of the NCC and compiles published data for coeval adakitic rocks to investigate the role of water in adakitic rock petrogenesis during the peak destruction of NCC. Both the Huashan and Laoniushan plutons exhibit adakitic signatures, including high Sr (193–1080 ppm), low Yb (&lt;14.8 ppm) and Y (&lt;1.24 ppm) concentrations, as well as high Sr/Y (18–100) and La/Yb (24–58) ratios. The zircon Hf–O isotope compositions suggest that the primary source for the Huashan and Laoniushan plutons is the mafic lower crust of NCC. Nevertheless, there are significant differences in trace element characteristics between the two plutons. Specifically, the Huashan pluton displays higher Na2O/K2O ratios, lower levels of Rb, Rb/Sr, Nb, Ta content, and a weak Eu anomaly in comparison to the Laoniushan pluton. These variations in geochemical attributes cannot be accounted for by mechanisms like mantle-derived magma mixing, crustal contamination, or fractional crystallization processes. Instead, these disparities are attributed to distinct modes of crustal anatexis, involving both water-fluxed and dehydration melting. Subsequently, we conducted thermodynamic simulations of the melting process of mafic lower crust under different pressure (0.5–1.5 GPa) and water content conditions (1–3 wt.%). The simulation results suggest that the Huashan pluton is most likely formed through water-fluxed melting in a scenario with normal crustal thickness (1 GPa). On the other hand, the Laoniushan pluton might have originated from dehydration melting under normal crustal thickness and pressure conditions. Notably, high pressure (&gt;1.5 GPa) is not necessary for the formation of intracontinental adakitic rocks. The release of water from metasomatized lithospheric mantle and subsequent hydration of the lower continental crust triggers extensive adakitic magmatism in the NCC. These findings emphasize the significance of deep water cycling in understanding large-scale magmatic events and illuminate the decratonization mechanism.","PeriodicalId":16751,"journal":{"name":"Journal of Petrology","volume":"35 1","pages":"0"},"PeriodicalIF":0.0,"publicationDate":"2023-08-29","publicationTypes":"Journal Article","fieldsOfStudy":null,"isOpenAccess":false,"openAccessPdf":"","citationCount":null,"resultStr":null,"platform":"Semanticscholar","paperid":"136284052","PeriodicalName":null,"FirstCategoryId":null,"ListUrlMain":null,"RegionNum":2,"RegionCategory":"地球科学","ArticlePicture":[],"TitleCN":null,"AbstractTextCN":null,"PMCID":"","EPubDate":null,"PubModel":null,"JCR":null,"JCRName":null,"Score":null,"Total":0}
Pub Date : 2023-08-29DOI: 10.1093/petrology/egad064
Xiaofei Pu, Rebecca A Lange, Gordon M Moore
Abstract A collection of Quaternary, high-MgO (≤13.4 wt%) basanite and minette cinder and lava cones, with an enhanced arc geochemical signature, are located along the northern margin of the N-S Colima rift in western Mexico. The Colima rift overlies the lithospheric suture between the Jalisco block and Guerrero terrane, as well as the tear between the Rivera and Cocos subducting oceanic plates. From the literature, volatile analyses of olivine-hosted melt inclusions in the Colima cone samples document notably high concentrations of dissolved H2O in the melt (≤ 7.0 wt%) as well as degassing-induced phenocryst growth over a range of depths ≤ 25 km. In this study, it is shown that the high-MgO character of the Colima suite reflects liquid compositions, consistent with evidence for their rapid transit to the surface, without stalling in a crustal magma chamber. The most Mg-rich olivine analyzed in each sample matches the equilibrium composition at the liquidus based on olivine-melt Mn-Mg and Fe2+-Mg exchange coefficients. Application of both a Mg- and Ni-based olivine-melt thermometer, calibrated on the same experimental data set, to the Colima cone suite provides the temperature and dissolved H2O content at the liquidus. Because the Ni-thermometer is insensitive to dissolved H2O in the melt, it gives the actual temperature at the onset of olivine phenocryst growth. For the nine Colima samples that range from 13.4-9.2 wt% MgO, resulting temperatures range from 1221-1056 (± 6-11) °C. In contrast, the Mg-thermometer is sensitive to dissolved H2O in the melt, and its application (without a correction of H2O) gives the temperature of olivine crystallization under anhydrous conditions. When the Mg- and Ni-based temperatures are paired, the depression of the liquidus (∆T = TMg-TNi) due to dissolved H2O can be obtained. For the high-MgO (>9 wt%) Colima samples, ∆T values range from 188-109 °C. Corrections for the effect of pressure (i.e., evidence that phenocryst growth began at ~700 MPa), increase ∆T by ~21°C. An updated model calibration (on experiments from the literature) that relates ∆T with dissolved H2O in the melt shows that the best fit (R2= 0.95) is linear, wt% H2O = 0.047*∆T, with a standard error of ± 0.5 wt%. Although the experimental data set spans a wide range of melt composition (e.g., 47-58 wt% SiO2, 4.4-10.2 wt% MgO, 1.3-4.9 wt% Na2O, 0.1-5.0 wt% K2O, 0.3-5.3 wt% H2O), no dependence on anhydrous melt composition is resolved. Application of this updated model to the Colima suite gives H2O contents of 5.1-8.8 wt% H2O, consistent with those analyzed in olivine-hosted MIs from the literature. When the thermometry and hygrometry results for the Colima cone suite are compared to those for the adjacent calc-alkaline basalts from the Tancítaro Volcanic Field (TVF) in Michoacán, two distinct linear trends in a plot of wt% H2O vs. temperature are found, indicative of different mantle sources. It is proposed that the high-MgO (>11 wt%)
{"title":"Application of an Improved Olivine-Melt Thermometer/Hygrometer to the Colima Cone Basanites and Minettes of Western Mexico: Implications for the Mantle Source of Unusually High-MgO Melts","authors":"Xiaofei Pu, Rebecca A Lange, Gordon M Moore","doi":"10.1093/petrology/egad064","DOIUrl":"https://doi.org/10.1093/petrology/egad064","url":null,"abstract":"Abstract A collection of Quaternary, high-MgO (≤13.4 wt%) basanite and minette cinder and lava cones, with an enhanced arc geochemical signature, are located along the northern margin of the N-S Colima rift in western Mexico. The Colima rift overlies the lithospheric suture between the Jalisco block and Guerrero terrane, as well as the tear between the Rivera and Cocos subducting oceanic plates. From the literature, volatile analyses of olivine-hosted melt inclusions in the Colima cone samples document notably high concentrations of dissolved H2O in the melt (≤ 7.0 wt%) as well as degassing-induced phenocryst growth over a range of depths ≤ 25 km. In this study, it is shown that the high-MgO character of the Colima suite reflects liquid compositions, consistent with evidence for their rapid transit to the surface, without stalling in a crustal magma chamber. The most Mg-rich olivine analyzed in each sample matches the equilibrium composition at the liquidus based on olivine-melt Mn-Mg and Fe2+-Mg exchange coefficients. Application of both a Mg- and Ni-based olivine-melt thermometer, calibrated on the same experimental data set, to the Colima cone suite provides the temperature and dissolved H2O content at the liquidus. Because the Ni-thermometer is insensitive to dissolved H2O in the melt, it gives the actual temperature at the onset of olivine phenocryst growth. For the nine Colima samples that range from 13.4-9.2 wt% MgO, resulting temperatures range from 1221-1056 (± 6-11) °C. In contrast, the Mg-thermometer is sensitive to dissolved H2O in the melt, and its application (without a correction of H2O) gives the temperature of olivine crystallization under anhydrous conditions. When the Mg- and Ni-based temperatures are paired, the depression of the liquidus (∆T = TMg-TNi) due to dissolved H2O can be obtained. For the high-MgO (&gt;9 wt%) Colima samples, ∆T values range from 188-109 °C. Corrections for the effect of pressure (i.e., evidence that phenocryst growth began at ~700 MPa), increase ∆T by ~21°C. An updated model calibration (on experiments from the literature) that relates ∆T with dissolved H2O in the melt shows that the best fit (R2= 0.95) is linear, wt% H2O = 0.047*∆T, with a standard error of ± 0.5 wt%. Although the experimental data set spans a wide range of melt composition (e.g., 47-58 wt% SiO2, 4.4-10.2 wt% MgO, 1.3-4.9 wt% Na2O, 0.1-5.0 wt% K2O, 0.3-5.3 wt% H2O), no dependence on anhydrous melt composition is resolved. Application of this updated model to the Colima suite gives H2O contents of 5.1-8.8 wt% H2O, consistent with those analyzed in olivine-hosted MIs from the literature. When the thermometry and hygrometry results for the Colima cone suite are compared to those for the adjacent calc-alkaline basalts from the Tancítaro Volcanic Field (TVF) in Michoacán, two distinct linear trends in a plot of wt% H2O vs. temperature are found, indicative of different mantle sources. It is proposed that the high-MgO (&gt;11 wt%)","PeriodicalId":16751,"journal":{"name":"Journal of Petrology","volume":"31 1","pages":"0"},"PeriodicalIF":0.0,"publicationDate":"2023-08-29","publicationTypes":"Journal Article","fieldsOfStudy":null,"isOpenAccess":false,"openAccessPdf":"","citationCount":null,"resultStr":null,"platform":"Semanticscholar","paperid":"136281098","PeriodicalName":null,"FirstCategoryId":null,"ListUrlMain":null,"RegionNum":2,"RegionCategory":"地球科学","ArticlePicture":[],"TitleCN":null,"AbstractTextCN":null,"PMCID":"","EPubDate":null,"PubModel":null,"JCR":null,"JCRName":null,"Score":null,"Total":0}
Pub Date : 2023-08-23DOI: 10.1093/petrology/egad063
Joshua T. Munro, C. Harris
The 2060 ± 2 Ma Phalaborwa Complex is a pipe-like, ultramafic to carbonatite intrusion formed from multiple magma pulses. The complex is made up of a main pipe consisting of clinopyroxenites, ultramafic pegmatoids, carbonatites and foskorite (olivine-apatite-magnetite-calcite assemblage), surrounded by many smaller syenite plugs. The range in mineral δ18O values for all rock types and minerals analysed in the Phalaborwa Complex is 2.24 to 18.3‰. However, 24 analyses of the most abundant and robust mineral, diopside, all have δ18O values between 6.2 and 7.7‰. The δ18O values of baddeleyite, olivine, diopside, magnetite, apatite and aegirine are thought to be magmatic. Most mineral pairs have differences in δ18O value that are consistent with magmatic equilibrium at high temperatures down to closure temperature. Alkali feldspar and phlogopite have more variable δ18O values, and both minerals may have undergone subsolidus O-exchange. The δD values for petrographically fresh Phalaborwa Complex phlogopite range from -77 to -48‰ with a mean of -64 ± 9‰ (1σ, n=19). The phlogopite δD values are consistent with subduction-related magmatic water. Despite petrographic evidence for fluid-rock interaction in the carbonatite-foskorite rocks, the carbonatite δ13C and δ18O range (δ18O, 8.13 to 12.00‰; δ13C, -3.19 to -5.60‰) overlaps with the unaltered, primary igneous carbonatite field. Magma δ18O values estimated from silicate and oxide minerals are mostly higher than normal mantle magmas (pyroxenites, ~7.6‰; foskorite, 7.2‰). The δ18O value of syenite magma estimated from aegirine is 7.8 ± 0.9‰ (1σ, n=8), in equilibrium with whole-rock syenite δ18O values (8.7 ± 0.4‰, 1σ, n=5). Local basement rocks have average bulk δ18O values of 8.6‰, and realistic proportions of assimilation by a mantle-derived magma (δ18O, 5.7‰) could not have produced the δ18O values in the pyroxenites or foskorites. Instead, it is proposed that the high-δ18O values of Phalaborwa Complex magmas reflect that of the mantle source. High δ18O values are also a feature of the Rustenburg Layered Suite of the Bushveld Complex (2060 to 2055 Ma), which may have had a similar high-δ18O mantle source. The higher δ18O values of the satellite syenites are consistent with an origin by partial melting of metasomatised country rock.
2060±2 Ma Phalabowa杂岩是由多个岩浆脉冲形成的管状超镁铁质到碳酸岩侵入体。该杂岩由斜辉石、超镁铁质伟晶岩、碳酸盐岩和磷钾岩（橄榄石-磷灰石-磁铁矿-方解石组合）组成的主管组成，周围有许多较小的正长岩塞。Phalabowa杂岩中分析的所有岩石类型和矿物的δ18O值范围为2.24至18.3‰。然而，对最丰富和最坚固的矿物透辉石的24次分析的δ18奥值均在6.2至7.7‰之间。巴德雷质岩、橄榄石、透辉石、磁铁矿、磷灰石和赤铁矿的δ18O值被认为是岩浆的。大多数矿物对的δ18O值存在差异，这与高温至闭合温度下的岩浆平衡一致。碱长石和金云母的δ18O值变化较大，两种矿物可能都经历了亚固体O交换。岩相新鲜Phalabowa杂岩金云母的δD值范围为-77至-48‰，平均值为-64±9‰（1σ，n=19）。金云母的δD值与俯冲相关的岩浆水一致。尽管有岩相学证据表明碳酸盐岩-磷钾岩中存在流体-岩石相互作用，但碳酸盐岩δ13C和δ18O范围（δ18O，8.13至12.00‰；δ13C，-3.19至-5.60‰）与未改变的原生火成碳酸岩场重叠。由硅酸盐和氧化物矿物估算的岩浆δ18O值大多高于正常地幔岩浆（辉石岩，~7.6‰；磷钾石岩，7.2‰）。由赤铁矿估算的正长岩岩浆的δ18O价值为7.8±0.9‰（1σ，n=8），与全岩正长岩δ18O数值（8.7±0.4‰，1σ，n=5）平衡。局部基岩的平均体积δ18O值为8.6‰，地幔岩浆的实际同化比例（δ18O，5.7‰）不可能在辉石岩或磷钾石岩中产生δ18O。相反，提出Phalabowa杂岩岩浆的高δ18O值反映了地幔源的高δ。高δ18O值也是Bushveld杂岩勒斯滕堡层状岩组（2060至2055 Ma）的一个特征，该岩组可能具有类似的高δ18奥地幔源。卫星正长岩较高的δ18O值与交代围岩部分熔融的成因一致。
{"title":"A high-δ18O mantle source for the 2.06 Ga Phalaborwa Igneous Complex, South Africa?","authors":"Joshua T. Munro, C. Harris","doi":"10.1093/petrology/egad063","DOIUrl":"https://doi.org/10.1093/petrology/egad063","url":null,"abstract":"\u0000 The 2060 ± 2 Ma Phalaborwa Complex is a pipe-like, ultramafic to carbonatite intrusion formed from multiple magma pulses. The complex is made up of a main pipe consisting of clinopyroxenites, ultramafic pegmatoids, carbonatites and foskorite (olivine-apatite-magnetite-calcite assemblage), surrounded by many smaller syenite plugs. The range in mineral δ18O values for all rock types and minerals analysed in the Phalaborwa Complex is 2.24 to 18.3‰. However, 24 analyses of the most abundant and robust mineral, diopside, all have δ18O values between 6.2 and 7.7‰. The δ18O values of baddeleyite, olivine, diopside, magnetite, apatite and aegirine are thought to be magmatic. Most mineral pairs have differences in δ18O value that are consistent with magmatic equilibrium at high temperatures down to closure temperature. Alkali feldspar and phlogopite have more variable δ18O values, and both minerals may have undergone subsolidus O-exchange. The δD values for petrographically fresh Phalaborwa Complex phlogopite range from -77 to -48‰ with a mean of -64 ± 9‰ (1σ, n=19). The phlogopite δD values are consistent with subduction-related magmatic water. Despite petrographic evidence for fluid-rock interaction in the carbonatite-foskorite rocks, the carbonatite δ13C and δ18O range (δ18O, 8.13 to 12.00‰; δ13C, -3.19 to -5.60‰) overlaps with the unaltered, primary igneous carbonatite field.\u0000 Magma δ18O values estimated from silicate and oxide minerals are mostly higher than normal mantle magmas (pyroxenites, ~7.6‰; foskorite, 7.2‰). The δ18O value of syenite magma estimated from aegirine is 7.8 ± 0.9‰ (1σ, n=8), in equilibrium with whole-rock syenite δ18O values (8.7 ± 0.4‰, 1σ, n=5). Local basement rocks have average bulk δ18O values of 8.6‰, and realistic proportions of assimilation by a mantle-derived magma (δ18O, 5.7‰) could not have produced the δ18O values in the pyroxenites or foskorites. Instead, it is proposed that the high-δ18O values of Phalaborwa Complex magmas reflect that of the mantle source. High δ18O values are also a feature of the Rustenburg Layered Suite of the Bushveld Complex (2060 to 2055 Ma), which may have had a similar high-δ18O mantle source. The higher δ18O values of the satellite syenites are consistent with an origin by partial melting of metasomatised country rock.","PeriodicalId":16751,"journal":{"name":"Journal of Petrology","volume":" ","pages":""},"PeriodicalIF":3.9,"publicationDate":"2023-08-23","publicationTypes":"Journal Article","fieldsOfStudy":null,"isOpenAccess":false,"openAccessPdf":"","citationCount":null,"resultStr":null,"platform":"Semanticscholar","paperid":"42917336","PeriodicalName":null,"FirstCategoryId":null,"ListUrlMain":null,"RegionNum":2,"RegionCategory":"地球科学","ArticlePicture":[],"TitleCN":null,"AbstractTextCN":null,"PMCID":"","EPubDate":null,"PubModel":null,"JCR":null,"JCRName":null,"Score":null,"Total":0}
Pub Date : 2023-08-22DOI: 10.1093/petrology/egad062
E. Demonterova, A. Ivanov, V. Savatenkov, M. Chu, S. V. Panteeva, Hao-Yang Lee, I. Bindeman
Continental rifting is usually viewed in terms of two contrasting models of active and passive extension. The origin of the Baikal Rift, adjacent to the southern part of the Siberian Craton, has been described by both models in the past. It is expected that basaltic magmatism in an active model scenario should be primarily sourced from a mantle plume or plume-fed asthenosphere, whereas melting of the lithospheric mantle is expected to be a predominant source for magmatism in the passive model. In this paper, we focus on the Miocene volcanic rocks sampled along two 60 km-long profiles that cross the boundary between the Neoproterozoic Tuva-Mongolian massif and the Archean-Paleoproterozoic Siberian Craton. Most of the samples studied are trachybasalts. In terms of trace element concentrations normalized to primitive mantle, the lavas mimic OIB-like patterns with troughs at Rb, Th–U, Pb, and Y, and peaks at Ba, Nb, Ta, K, and Sr. Moreover, similar trace-element patterns to the studied samples are also observed for Miocene and Quaternary lavas located in the southwestern of the Baikal Rift, and adjacent regions of non-rifted Mongolia. According to the ratio of CaO to MgO, and TiO2/Al2O3 to SiO2, the compositions of the studied lavas coincide with experimental melts derived from mafic lithologies. Trace element data of samples suggest that garnet was a residual phase during partial melting. The Sr-Nd isotopic characteristics of the studied lavas are 87Sr/86Sr 0.70427–0.70469 and 143Nd/144Nd 0.51267–0.51284. They are identical to the coeval Miocene lavas of neighbouring volcanic fields, but they differ from the Quaternary lavas that extend to lower 87Sr/86Sr (0.7038–0.7044) with near identical 143Nd/144Nd. Isotopes of Hf for studied samples show values εHf = 6.0–7.7, except for the two samples taken within the boundary between two lithospheric blocks with εHf 4.6 and 4.8. The δ18O of olivine from lava samples is everywhere higher than that of the asthenospheric mantle and ranges from 5.5 to 6.4‰. Variations of δ18O versus Mg#, 87Sr/86Sr and εHf in the studied samples do not correlate, but do unequivocally rule out crustal assimilation. The isotopic variations are consistent with recycling of mafic crustal lithologies at mantle depths. Lavas from the Tuva-Mongolian massif and the Siberian Craton differ in lead isotopes by lower values of 206Pb/204Pb (< 17.785) and higher values of Δ8/4Pb (61–75) for on-cratonic samples and the reverse relationship for off-cratonic lava (> 17.785 and 55–61) respectively. The equation for Δ8/4Pb=[208Pb/204Pb-(1.209*(206Pb/204Pb) +15.627)] *100 is from (Hart, 1984). The correlation of lead isotopes with the mafic recycled component, the sharp change of lead isotopic values at the cratonic boundary and decoupling of lead isotope ratios from other isotopic ratios lead us to suggest that the values of 206Pb/204Pb and Δ8/4Pb are associated with an ancient accessory mineral phase such as sulphide confined within the lithosphe
{"title":"Miocene Volcanism of the Baikal Rift across the Boundary of the Siberian Craton: Evidence for Lithospheric Mantle Melting","authors":"E. Demonterova, A. Ivanov, V. Savatenkov, M. Chu, S. V. Panteeva, Hao-Yang Lee, I. Bindeman","doi":"10.1093/petrology/egad062","DOIUrl":"https://doi.org/10.1093/petrology/egad062","url":null,"abstract":"\u0000 Continental rifting is usually viewed in terms of two contrasting models of active and passive extension. The origin of the Baikal Rift, adjacent to the southern part of the Siberian Craton, has been described by both models in the past. It is expected that basaltic magmatism in an active model scenario should be primarily sourced from a mantle plume or plume-fed asthenosphere, whereas melting of the lithospheric mantle is expected to be a predominant source for magmatism in the passive model. In this paper, we focus on the Miocene volcanic rocks sampled along two 60 km-long profiles that cross the boundary between the Neoproterozoic Tuva-Mongolian massif and the Archean-Paleoproterozoic Siberian Craton. Most of the samples studied are trachybasalts. In terms of trace element concentrations normalized to primitive mantle, the lavas mimic OIB-like patterns with troughs at Rb, Th–U, Pb, and Y, and peaks at Ba, Nb, Ta, K, and Sr. Moreover, similar trace-element patterns to the studied samples are also observed for Miocene and Quaternary lavas located in the southwestern of the Baikal Rift, and adjacent regions of non-rifted Mongolia. According to the ratio of CaO to MgO, and TiO2/Al2O3 to SiO2, the compositions of the studied lavas coincide with experimental melts derived from mafic lithologies. Trace element data of samples suggest that garnet was a residual phase during partial melting. The Sr-Nd isotopic characteristics of the studied lavas are 87Sr/86Sr 0.70427–0.70469 and 143Nd/144Nd 0.51267–0.51284. They are identical to the coeval Miocene lavas of neighbouring volcanic fields, but they differ from the Quaternary lavas that extend to lower 87Sr/86Sr (0.7038–0.7044) with near identical 143Nd/144Nd. Isotopes of Hf for studied samples show values εHf = 6.0–7.7, except for the two samples taken within the boundary between two lithospheric blocks with εHf 4.6 and 4.8. The δ18O of olivine from lava samples is everywhere higher than that of the asthenospheric mantle and ranges from 5.5 to 6.4‰. Variations of δ18O versus Mg#, 87Sr/86Sr and εHf in the studied samples do not correlate, but do unequivocally rule out crustal assimilation. The isotopic variations are consistent with recycling of mafic crustal lithologies at mantle depths. Lavas from the Tuva-Mongolian massif and the Siberian Craton differ in lead isotopes by lower values of 206Pb/204Pb (< 17.785) and higher values of Δ8/4Pb (61–75) for on-cratonic samples and the reverse relationship for off-cratonic lava (> 17.785 and 55–61) respectively. The equation for Δ8/4Pb=[208Pb/204Pb-(1.209*(206Pb/204Pb) +15.627)] *100 is from (Hart, 1984). The correlation of lead isotopes with the mafic recycled component, the sharp change of lead isotopic values at the cratonic boundary and decoupling of lead isotope ratios from other isotopic ratios lead us to suggest that the values of 206Pb/204Pb and Δ8/4Pb are associated with an ancient accessory mineral phase such as sulphide confined within the lithosphe","PeriodicalId":16751,"journal":{"name":"Journal of Petrology","volume":" ","pages":""},"PeriodicalIF":3.9,"publicationDate":"2023-08-22","publicationTypes":"Journal Article","fieldsOfStudy":null,"isOpenAccess":false,"openAccessPdf":"","citationCount":null,"resultStr":null,"platform":"Semanticscholar","paperid":"44613787","PeriodicalName":null,"FirstCategoryId":null,"ListUrlMain":null,"RegionNum":2,"RegionCategory":"地球科学","ArticlePicture":[],"TitleCN":null,"AbstractTextCN":null,"PMCID":"","EPubDate":null,"PubModel":null,"JCR":null,"JCRName":null,"Score":null,"Total":0}
Pub Date : 2023-08-15DOI: 10.1093/petrology/egad060
Yasuhiro Hirai, Y. Tamura, Tomoki Sato, T. Miyazaki, Q. Chang, B. Vaglarov, J. Kimura, K. Hoernle, R. Werner, F. Hauff, C. Timm
Primary andesitic magmas could be an important component of arc magma genesis and might have played a key role in the advent of continents. Recent studies hypothesized that primary andesitic magmas occur in the oceanic arc, where the crust is thin. The Kermadec arc has the thinnest crust among all the studied oceanic arcs (<15 km in thickness); however, there are no studies that corroborate the formation of primary andesitic magmas in the arc. The aim of this study is to develop a better understanding of primary andesites in oceanic arcs through the petrology of the Kermadec arc. Here, we present the petrology of volcanic rocks dredged from the Kibblewhite Volcano in the Kermadec arc during the R/V SONNE SO-255 expedition in 2017. Magma types range from andesite to rhyolite at the Kibblewhite Volcano, but basalts dominate at the neibouring cones. This study focuses on magnesian andesites from the northeastern flank of this volcano. The magnesian andesites are nearly aphyric and plagioclase-free but contain microphenocrysts of olivine (Fo84–86) and clinopyroxene (Mg# = 81–87). Using olivine addition models, the primary magmas were estimated to contain 55–56 wt.% SiO2 and 10–12 wt.% MgO, similar to the high-Mg andesites observed in other convergent plate margins, indicating the generation of primary andesitic magma beneath the Kibblewhite Volcano. The trace element and isotopic characteristics of the magnesian andesites are typical of volcanic rocks from the Kermadec arc. This indicates that the subduction of a young plate or melting of a pyroxenitic source is not necessary to produce magnesian andesites. Instead, we propose that the magnesian andesites were produced by the direct melting of the uppermost mantle of the Kermadec arc. The thin crust of the Kermadec arc should yield low-pressure conditions in the uppermost mantle, allowing the sub-arc mantle to generate primary andesitic melts. This study supports the hypothesis that primary andesitic magmas generate in the arc where the crust is thin and provides a new insight into the magma genesis of the Kermadec arc.
{"title":"Magnesian Andesites from Kibblewhite Volcano in the Kermadec Arc, New Zealand","authors":"Yasuhiro Hirai, Y. Tamura, Tomoki Sato, T. Miyazaki, Q. Chang, B. Vaglarov, J. Kimura, K. Hoernle, R. Werner, F. Hauff, C. Timm","doi":"10.1093/petrology/egad060","DOIUrl":"https://doi.org/10.1093/petrology/egad060","url":null,"abstract":"\u0000 Primary andesitic magmas could be an important component of arc magma genesis and might have played a key role in the advent of continents. Recent studies hypothesized that primary andesitic magmas occur in the oceanic arc, where the crust is thin. The Kermadec arc has the thinnest crust among all the studied oceanic arcs (<15 km in thickness); however, there are no studies that corroborate the formation of primary andesitic magmas in the arc. The aim of this study is to develop a better understanding of primary andesites in oceanic arcs through the petrology of the Kermadec arc. Here, we present the petrology of volcanic rocks dredged from the Kibblewhite Volcano in the Kermadec arc during the R/V SONNE SO-255 expedition in 2017. Magma types range from andesite to rhyolite at the Kibblewhite Volcano, but basalts dominate at the neibouring cones. This study focuses on magnesian andesites from the northeastern flank of this volcano. The magnesian andesites are nearly aphyric and plagioclase-free but contain microphenocrysts of olivine (Fo84–86) and clinopyroxene (Mg# = 81–87). Using olivine addition models, the primary magmas were estimated to contain 55–56 wt.% SiO2 and 10–12 wt.% MgO, similar to the high-Mg andesites observed in other convergent plate margins, indicating the generation of primary andesitic magma beneath the Kibblewhite Volcano. The trace element and isotopic characteristics of the magnesian andesites are typical of volcanic rocks from the Kermadec arc. This indicates that the subduction of a young plate or melting of a pyroxenitic source is not necessary to produce magnesian andesites. Instead, we propose that the magnesian andesites were produced by the direct melting of the uppermost mantle of the Kermadec arc. The thin crust of the Kermadec arc should yield low-pressure conditions in the uppermost mantle, allowing the sub-arc mantle to generate primary andesitic melts. This study supports the hypothesis that primary andesitic magmas generate in the arc where the crust is thin and provides a new insight into the magma genesis of the Kermadec arc.","PeriodicalId":16751,"journal":{"name":"Journal of Petrology","volume":" ","pages":""},"PeriodicalIF":3.9,"publicationDate":"2023-08-15","publicationTypes":"Journal Article","fieldsOfStudy":null,"isOpenAccess":false,"openAccessPdf":"","citationCount":null,"resultStr":null,"platform":"Semanticscholar","paperid":"42482170","PeriodicalName":null,"FirstCategoryId":null,"ListUrlMain":null,"RegionNum":2,"RegionCategory":"地球科学","ArticlePicture":[],"TitleCN":null,"AbstractTextCN":null,"PMCID":"","EPubDate":null,"PubModel":null,"JCR":null,"JCRName":null,"Score":null,"Total":0}
Pub Date : 2023-08-14DOI: 10.1093/petrology/egad059
T. J. Matthews, M. Loader, J. Wilkinson, Y. Buret, S. Large, Elliott A Birt
Although the evolution of residual melts in magmatic systems controls their eruptability and ore-forming potential, their compositions are obscured in plutonic rocks by a protracted near-solidus evolution, and the absence of interstitial glass. Here, we trace the evolution of residual melt compositions in rocks from the Strontian Intrusive Complex, Scotland, using the trace element chemistry of amphiboles and titanites which are intergrown with amphibole rims. Laser ablation mapping reveals an abrupt change in certain trace elements in the amphibole rims, with sharp increases in Eu/Eu* and Sr/Y, and decreases in rare earth elements, Ta, Nb, and Ta/Nb ratios. Core-rim variations in these elements in titanite show the same variations as in amphibole, but are more gradual. By reconstructing the crystallisation sequence of the Strontian magmas using textural observations and thermobarometric estimates, we determine that amphibole cores crystallised prior to titanite saturation, but amphibole rims crystallised simultaneously with titanite. Using the trace element composition of the mineral phases and their modal abundance in the rock, with comparison to the whole-rock chemistry, we determine that titanite hosts the majority of the rare earth and high field strength element budget of the rocks. We therefore propose that the onset of titanite crystallisation had a profound effect on the trace element composition of late-stage residual melts at Strontian, which were inherited by the amphibole rims and subsequent titanites. This is supported by Rayleigh fractional crystallisation modelling which demonstrates that the composition of amphibole rims cannot be explained without the influence of titanite. We therefore show that the saturation of trace element rich phases in magmas represents a significant geochemical event in the petrogenesis of intermediate to silicic magmas. This has implications for provenance studies which attempt to reconstruct bulk rock compositions from mineral compositions, as the residual melts from which those minerals crystallise can be driven to significantly different compositions from the host magma by late-stage accessory phase crystallisation.
{"title":"The Strontian Intrusive Complex: Petrography, Thermobarometry and the Influence of Titanite on Residual Melt Chemistry","authors":"T. J. Matthews, M. Loader, J. Wilkinson, Y. Buret, S. Large, Elliott A Birt","doi":"10.1093/petrology/egad059","DOIUrl":"https://doi.org/10.1093/petrology/egad059","url":null,"abstract":"\u0000 Although the evolution of residual melts in magmatic systems controls their eruptability and ore-forming potential, their compositions are obscured in plutonic rocks by a protracted near-solidus evolution, and the absence of interstitial glass. Here, we trace the evolution of residual melt compositions in rocks from the Strontian Intrusive Complex, Scotland, using the trace element chemistry of amphiboles and titanites which are intergrown with amphibole rims. Laser ablation mapping reveals an abrupt change in certain trace elements in the amphibole rims, with sharp increases in Eu/Eu* and Sr/Y, and decreases in rare earth elements, Ta, Nb, and Ta/Nb ratios. Core-rim variations in these elements in titanite show the same variations as in amphibole, but are more gradual. By reconstructing the crystallisation sequence of the Strontian magmas using textural observations and thermobarometric estimates, we determine that amphibole cores crystallised prior to titanite saturation, but amphibole rims crystallised simultaneously with titanite. Using the trace element composition of the mineral phases and their modal abundance in the rock, with comparison to the whole-rock chemistry, we determine that titanite hosts the majority of the rare earth and high field strength element budget of the rocks. We therefore propose that the onset of titanite crystallisation had a profound effect on the trace element composition of late-stage residual melts at Strontian, which were inherited by the amphibole rims and subsequent titanites. This is supported by Rayleigh fractional crystallisation modelling which demonstrates that the composition of amphibole rims cannot be explained without the influence of titanite. We therefore show that the saturation of trace element rich phases in magmas represents a significant geochemical event in the petrogenesis of intermediate to silicic magmas. This has implications for provenance studies which attempt to reconstruct bulk rock compositions from mineral compositions, as the residual melts from which those minerals crystallise can be driven to significantly different compositions from the host magma by late-stage accessory phase crystallisation.","PeriodicalId":16751,"journal":{"name":"Journal of Petrology","volume":" ","pages":""},"PeriodicalIF":3.9,"publicationDate":"2023-08-14","publicationTypes":"Journal Article","fieldsOfStudy":null,"isOpenAccess":false,"openAccessPdf":"","citationCount":null,"resultStr":null,"platform":"Semanticscholar","paperid":"47377005","PeriodicalName":null,"FirstCategoryId":null,"ListUrlMain":null,"RegionNum":2,"RegionCategory":"地球科学","ArticlePicture":[],"TitleCN":null,"AbstractTextCN":null,"PMCID":"","EPubDate":null,"PubModel":null,"JCR":null,"JCRName":null,"Score":null,"Total":0}
Pub Date : 2023-08-08DOI: 10.1093/petrology/egad058
B. Pitcher, A. Grunder, A. Kent
Ignimbrite flare-ups are rare periods of intense silicic volcanism during which the pyroclastic volume and eruptive frequency is more than an order of magnitude higher than background activity. Investigating the compositional differences between flare-up and steady-state magmas provides critical constraints on the petrogenetic causes for the event and can offer unique opportunities to investigate the role of large-scale tectonic or geodynamic processes in arc magmatism. In this study, we focus on the bimodal Deschutes Formation ignimbrite flare-up of Central Oregon, which erupted unusually high volumes of pyroclastic material 6.25-5.45 Ma from a new axis of volcanism in the Cascades arc. This episode is marked by increased eruption rates and eruption of more silicic compositions relative to the Quaternary Cascade arc, which rarely erupts rhyolites. Ignimbrites are crystal-poor (< 10%) dacite to rhyolites (mostly 65-77 wt. % SiO2) with anhydrous mineral assemblages and higher FeO/MgO, Y, Eu/Eu*, MREE and Zr/Sr, indicating drier magmatic evolution compared to the Quaternary arc, and are more similar to those from the rear-arc High Lava Plains (HLP) province that lies to the east. Magnetite-ilmenite oxybarometry indicates that Deschutes Formation felsic magmas tend to be hotter and more reduced (NNO-1 to NNO) than the Quaternary arc (NNO to NNO+1.5). Rhyolite-MELTS geobarometry suggests complex storage of diverse Deschutes Formation magmas within the shallow crust (50 – 250 MPa), and the common co-eruption of multiple plagioclase populations, pumice compositions, and compositionally banded pumice suggest variable degrees of mixing and mingling of distinct magmas. Deschutes magmas also have low δ18Oplagioclase values that indicate partial melting and assimilation of hydrothermally altered shallow crust. Trace element systematics and rhyolite-MELTS modelling suggests that felsic pumice cannot be produced by simple fractionation of co-erupted mafic pumice or basaltic lavas, and requires a crustal melting origin, and trace elements and Pb isotopes suggest that young mafic crust may have been the primary protolith. We suggest that partial melting produced low-Si rhyolite melt (~72 wt. %) that acted as both a parent for the most evolved rhyolites, and as a mixing endmember to create the dacite to rhyodacite magmas with heterogenous plagioclase populations. Unlike the predominantly calc-alkaline basalts erupted in the Quaternary Cascade arc, Deschutes Formation primary basalts are mostly low-K tholeiites, indicative of decompression melting. These are similar to the compositions erupted during a contemporaneous pulse of low-K tholeiite volcanism across the whole HLP that reached into the Cascades rear-arc. We suggest that intra-arc extension focused decompression melts from the back-arc into the arc and that tensional stresses allowed this high flux of hot-dry-reduced basalt throughout the crustal column, causing partial melting of mafic protoliths and t
{"title":"Flare up of hot-dry-reduced ignimbrites related to extension in the Cascades Arc: the Deschutes Formation, central Oregon","authors":"B. Pitcher, A. Grunder, A. Kent","doi":"10.1093/petrology/egad058","DOIUrl":"https://doi.org/10.1093/petrology/egad058","url":null,"abstract":"\u0000 Ignimbrite flare-ups are rare periods of intense silicic volcanism during which the pyroclastic volume and eruptive frequency is more than an order of magnitude higher than background activity. Investigating the compositional differences between flare-up and steady-state magmas provides critical constraints on the petrogenetic causes for the event and can offer unique opportunities to investigate the role of large-scale tectonic or geodynamic processes in arc magmatism. In this study, we focus on the bimodal Deschutes Formation ignimbrite flare-up of Central Oregon, which erupted unusually high volumes of pyroclastic material 6.25-5.45 Ma from a new axis of volcanism in the Cascades arc. This episode is marked by increased eruption rates and eruption of more silicic compositions relative to the Quaternary Cascade arc, which rarely erupts rhyolites. Ignimbrites are crystal-poor (< 10%) dacite to rhyolites (mostly 65-77 wt. % SiO2) with anhydrous mineral assemblages and higher FeO/MgO, Y, Eu/Eu*, MREE and Zr/Sr, indicating drier magmatic evolution compared to the Quaternary arc, and are more similar to those from the rear-arc High Lava Plains (HLP) province that lies to the east. Magnetite-ilmenite oxybarometry indicates that Deschutes Formation felsic magmas tend to be hotter and more reduced (NNO-1 to NNO) than the Quaternary arc (NNO to NNO+1.5). Rhyolite-MELTS geobarometry suggests complex storage of diverse Deschutes Formation magmas within the shallow crust (50 – 250 MPa), and the common co-eruption of multiple plagioclase populations, pumice compositions, and compositionally banded pumice suggest variable degrees of mixing and mingling of distinct magmas. Deschutes magmas also have low δ18Oplagioclase values that indicate partial melting and assimilation of hydrothermally altered shallow crust. Trace element systematics and rhyolite-MELTS modelling suggests that felsic pumice cannot be produced by simple fractionation of co-erupted mafic pumice or basaltic lavas, and requires a crustal melting origin, and trace elements and Pb isotopes suggest that young mafic crust may have been the primary protolith. We suggest that partial melting produced low-Si rhyolite melt (~72 wt. %) that acted as both a parent for the most evolved rhyolites, and as a mixing endmember to create the dacite to rhyodacite magmas with heterogenous plagioclase populations. Unlike the predominantly calc-alkaline basalts erupted in the Quaternary Cascade arc, Deschutes Formation primary basalts are mostly low-K tholeiites, indicative of decompression melting. These are similar to the compositions erupted during a contemporaneous pulse of low-K tholeiite volcanism across the whole HLP that reached into the Cascades rear-arc. We suggest that intra-arc extension focused decompression melts from the back-arc into the arc and that tensional stresses allowed this high flux of hot-dry-reduced basalt throughout the crustal column, causing partial melting of mafic protoliths and t","PeriodicalId":16751,"journal":{"name":"Journal of Petrology","volume":" ","pages":""},"PeriodicalIF":3.9,"publicationDate":"2023-08-08","publicationTypes":"Journal Article","fieldsOfStudy":null,"isOpenAccess":false,"openAccessPdf":"","citationCount":null,"resultStr":null,"platform":"Semanticscholar","paperid":"44131812","PeriodicalName":null,"FirstCategoryId":null,"ListUrlMain":null,"RegionNum":2,"RegionCategory":"地球科学","ArticlePicture":[],"TitleCN":null,"AbstractTextCN":null,"PMCID":"","EPubDate":null,"PubModel":null,"JCR":null,"JCRName":null,"Score":null,"Total":0}
Pub Date : 2023-08-08DOI: 10.1093/petrology/egad054
M. Mangler, M. Humphreys, Eshbal Geifman, Alexander A. Iveson, F. Wadsworth, R. Brooker, A. Lindoo, K. Hammond
Crystal growth is often described as either interface-controlled or diffusion-controlled. Here, we study crystal growth in an intermediate scenario where reaction rates at the crystal-melt interface are similar to the rates of diffusive transport of ions through the melt to the advancing crystal surface. To this end, we experimentally investigated euhedral plagioclase crystal shapes in dry mafic (basaltic) and hydrous silicic (haplodacitic) melts. Aspect ratios and inferred relative growth rates of the 3D short (S) and intermediate (I) crystal dimensions vary significantly between mafic and silicic melts, with δS:δI = 1:6 – 1:20 in basalt and 1:2.5 – 1:8 in hydrous haplodacite. The lower aspect ratios of plagioclase grown in the silicic melt coincide with 10-100x lower melt diffusion rates than in the mafic melt. Using an anisotropic growth model, we show that such differences in melt diffusivity can explain the discrepancy in plagioclase aspect ratios: if interface reaction and melt diffusion rates are of similar magnitude, then the growth of a crystal facet with high interfacial reaction rates may be limited by melt diffusion while another facet of the same crystal with lower interfacial reaction rates may grow uninhibited by melt diffusivity. This selective control of melt diffusion on crystal growth rates results in progressively more equant crystal shapes as diffusivity decreases, consistent with our experimental observations. Importantly, crystals formed in this diffusion-moderated, intermediate growth regime may not show any classical diffusion-controlled growth features. The proposed model was developed for plagioclase microlites, but should be generalisable to all anisotropic microlite growth in volcanic rocks.
{"title":"Melt diffusion-moderated crystal growth and its effect on euhedral crystal shapes","authors":"M. Mangler, M. Humphreys, Eshbal Geifman, Alexander A. Iveson, F. Wadsworth, R. Brooker, A. Lindoo, K. Hammond","doi":"10.1093/petrology/egad054","DOIUrl":"https://doi.org/10.1093/petrology/egad054","url":null,"abstract":"\u0000 Crystal growth is often described as either interface-controlled or diffusion-controlled. Here, we study crystal growth in an intermediate scenario where reaction rates at the crystal-melt interface are similar to the rates of diffusive transport of ions through the melt to the advancing crystal surface. To this end, we experimentally investigated euhedral plagioclase crystal shapes in dry mafic (basaltic) and hydrous silicic (haplodacitic) melts. Aspect ratios and inferred relative growth rates of the 3D short (S) and intermediate (I) crystal dimensions vary significantly between mafic and silicic melts, with δS:δI = 1:6 – 1:20 in basalt and 1:2.5 – 1:8 in hydrous haplodacite. The lower aspect ratios of plagioclase grown in the silicic melt coincide with 10-100x lower melt diffusion rates than in the mafic melt. Using an anisotropic growth model, we show that such differences in melt diffusivity can explain the discrepancy in plagioclase aspect ratios: if interface reaction and melt diffusion rates are of similar magnitude, then the growth of a crystal facet with high interfacial reaction rates may be limited by melt diffusion while another facet of the same crystal with lower interfacial reaction rates may grow uninhibited by melt diffusivity. This selective control of melt diffusion on crystal growth rates results in progressively more equant crystal shapes as diffusivity decreases, consistent with our experimental observations. Importantly, crystals formed in this diffusion-moderated, intermediate growth regime may not show any classical diffusion-controlled growth features. The proposed model was developed for plagioclase microlites, but should be generalisable to all anisotropic microlite growth in volcanic rocks.","PeriodicalId":16751,"journal":{"name":"Journal of Petrology","volume":" ","pages":""},"PeriodicalIF":3.9,"publicationDate":"2023-08-08","publicationTypes":"Journal Article","fieldsOfStudy":null,"isOpenAccess":false,"openAccessPdf":"","citationCount":null,"resultStr":null,"platform":"Semanticscholar","paperid":"43656748","PeriodicalName":null,"FirstCategoryId":null,"ListUrlMain":null,"RegionNum":2,"RegionCategory":"地球科学","ArticlePicture":[],"TitleCN":null,"AbstractTextCN":null,"PMCID":"","EPubDate":null,"PubModel":null,"JCR":null,"JCRName":null,"Score":null,"Total":0}
Pub Date : 2023-08-01DOI: 10.1093/petrology/egad057
R. G. Cawthorn, T. McCarthy
The mafic layered Bushveld Complex, South Africa, contains numerous examples of monomineralic layers within its succession. The Upper Zone (UZ) contains approximately 24 magnetitite layers, the genesis of which have been extensively debated. The Main Magnetitite Layer (MML) is approximately 2 m thick and is traceable around its edge across >60,000 km2. Its basal contact with underlying anorthosite is planar and sharp, but the top contact grades upward with increasing plagioclase content. Sampling at a vertical spacing of a very few cm from seven profiles in the east over many tens of km, and one other 300 km to the west reveals concentrations of Cr in magnetite that decrease extremely rapidly upward (typically from >8,000 to 1,000 ppm within 30-60 cm from the base), punctuated by sharp concentration reversals and rare decreases. Here, we describe an outcrop where the MML splits into three sub-layers, separated by magnetite-plagioclase rocks. Twelve profiles across this zone of splitting have been similarly analysed. Lateral variations in Cr profiles across a few metres are observed at this locality. We offer the suggestion that magnetite formation may have been induced by shock-wave nucleation on the bottom of the chamber accounting for the abrupt appearance of magnetitite over such a wide area. Bottom growth of magnetite lowered the density and Cr content of the evolving magma, causing turbulence and convective overturn near the base of the chamber that created inhomogeneities in Cr on various scales, preserved in the ensuing magnetite compositions both vertically and laterally. Intermittent and abrupt (on a scale of 1-2 cm) upward increases in the Cr contents of up to 3,000 ppm in magnetite profiles resulted from convective overturn impinging on the floor. The tops of the magnetitite sub-layers grade into magnetite-plagioclase rocks and continue the upward decrease in Cr content in magnetite, typically at 900 ppm Cr, demonstrating upward continuity of fractionation. In contrast, there are reversals in Cr content between the magnetite-plagioclase rocks and the overlying magnetitite sub-layers that we attribute to convective overturn, with an increase in the Cr content. Two profiles through the MML show abrupt upward discontinuities to lower Cr contents that we attribute to physical erosional events. Anorthosite fragments in magnetitite, and magnetitite fragments in anorthosite layers further attest to such processes. The anorthite contents of plagioclase do not change across the MML, suggesting that magma addition was not responsible for the formation of magnetitite layers. Primary topographic variations at the base of the chamber also preclude addition of dense magma for the formation of magnetitite layers. Upward infiltration metasomatism, slightly resetting Cr contents, is limited to <3 cm.
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